The role of inputs of marine wrack and carrion in sandy‐beach ecosystems: a global review

ABSTRACT

Sandy beaches are iconic interfaces that functionally link the ocean with the land via the flow of organic matter from the sea. These cross‐ecosystem fluxes often comprise uprooted seagrass and dislodged macroalgae that can form substantial accumulations of detritus, termed ‘wrack’, on sandy beaches. In addition, the tissue of the carcasses of marine animals that regularly wash up on beaches form a rich food source (‘carrion’) for a diversity of scavenging animals. Here, we provide a global review of how wrack and carrion provide spatial subsidies that shape the structure and functioning of sandy‐beach ecosystems (sandy beaches and adjacent surf zones), which typically have little in situ primary production. We also examine the spatial scaling of the influence of these processes across the broader land‐ and seascape, and identify key gaps in our knowledge to guide future research directions and priorities. Large quantities of detrital kelp and seagrass can flow into sandy‐beach ecosystems, where microbial decomposers and animals process it. The rates of wrack supply and its retention are influenced by the oceanographic processes that transport it, the geomorphology and landscape context of the recipient beaches, and the condition, life history and morphological characteristics of the macrophyte taxa that are the ultimate source of wrack. When retained in beach ecosystems, wrack often creates hotspots of microbial metabolism, secondary productivity, biodiversity, and nutrient remineralization. Nutrients are produced during wrack breakdown, and these can return to coastal waters in surface flows (swash) and aquifers discharging into the subtidal surf. Beach‐cast kelp often plays a key trophic role, being an abundant and preferred food source for mobile, semi‐aquatic invertebrates that channel imported algal matter to predatory invertebrates, fish, and birds. The role of beach‐cast marine carrion is likely to be underestimated, as it can be consumed rapidly by highly mobile scavengers (e.g. foxes, coyotes, raptors, vultures). These consumers become important vectors in transferring marine productivity inland, thereby linking marine and terrestrial ecosystems. Whilst deposits of organic matter on sandy‐beach ecosystems underpin a range of ecosystem functions and services, they can be at variance with aesthetic perceptions resulting in widespread activities, such as ‘beach cleaning and grooming’. This practice diminishes the energetic base of food webs, intertidal fauna, and biodiversity. Global declines in seagrass beds and kelp forests (linked to global warming) are predicted to cause substantial reductions in the amounts of marine organic matter reaching many beach ecosystems, likely causing flow‐on effects for food webs and biodiversity. Similarly, future sea‐level rise and increased storm frequency are likely to alter profoundly the physical attributes of beaches, which in turn can change the rates at which beaches retain and process the influxes of wrack and animal carcasses. Conservation of the multi‐faceted ecosystem services that sandy beaches provide will increasingly need to encompass a greater societal appreciation and the safeguarding of ecological functions reliant on beach‐cast organic matter on innumerable ocean shores worldwide.

I. INTRODUCTION

Shorelines are formed by various coastal landforms, including sandy beaches, surf zones, different dune landscapes, subtidal and intertidal rocky reefs, and sea cliffs. Shorelines are also highly dynamic, being the interfaces between the ocean and the land (Pilkey et al., ²⁰¹¹). The world's coastlines have been widely transformed for human uses, providing valuable ecosystem services to society (Small & Nicholls, 2003; Bowen, Frankic & Davis, ²⁰⁰⁶). Iconic sandy beaches cover 31% of the world's ice‐free shoreline (Luijendijk et al., ²⁰¹⁸), and represent highly valued economic, ecological, recreational and cultural assets for societies worldwide (Barbier et al., ²⁰¹¹). Beaches are dynamic landforms that are often heavily altered by humans; rising sea levels and storm regimes can cause massive and widespread changes to beaches and dunes (Dugan et al., ²⁰¹⁰; Vousdoukas et al., ²⁰²⁰).

Beaches form an ecotone between the ocean and land, which is influenced by land and ocean inputs and processes. Beaches and adjacent surf zones along open coasts are often considered to have little in situ primary production, but their fauna can nevertheless be abundant and diverse (Brown & McLachlan, 1990). This paradox can be attributed, at least partly, to the flow of organic matter from productive to less‐productive ecosystems (i.e. ‘spatial subsidies’ sensu Polis et al., ¹⁹⁹⁷). Sandy beach systems are highly permeable, lacking significant barriers amongst habitats, whilst having strong transport mechanisms driven by tides, waves and currents; these forces can bring large organic matter (e.g. macroalgae, seagrass, dead animals) ashore to become stranded on beaches as wrack and carrion (Schlacher, Strydom & Connolly, ^2013a ; Hyndes et al., ²⁰¹⁴).

Organic matter stranded on beaches is typically derived from reefs and seagrass beds that supply detached macrophytes (e.g. kelp thalli and seagrass leaves). It can also be in the form of dead animals that wash ashore to become nutritious carrion (i.e. the tissues of animal carcasses) beach‐cast on sandy shores. Many studies have focused on the production and fate of organic matter in kelp forests (e.g. Krumhansl & Scheibling, ²⁰¹¹; de Bettignies et al., ²⁰¹³; Pedersen et al., ²⁰²⁰) and seagrass meadows (see Heck Jr. et al., ²⁰⁰⁸). Reviews show that these highly productive ecosystems can export large quantities of organic matter via dissolved organic nutrients or particulate organic matter to adjacent and more distant ecosystems, including beaches (Heck Jr. et al., ²⁰⁰⁸; Krumhansl & Scheibling, ²⁰¹²; Hyndes et al., ²⁰¹⁴). However, these reviews provide little detail on the fates of organic matter in beach ecosystems. Inputs of detached macrophytes (wrack) and carrion (dead animals) (Figs 1A and 2) may, however, provide critical ecosystem functions (physical, chemical and biological processes within ecosystems) on many sandy beaches. These functions include: (1) providing a food subsidy to support the high secondary production and biodiversity that underpin rich coastal food webs (e.g. Crawley et al., ²⁰⁰⁹); (2) recycling nutrients (e.g. Dugan et al., ²⁰¹¹); (3) enhancing key habitats, such as fish nursery grounds (surf zone) and bird nesting sites (e.g. Crawley, Hyndes & Ayvazian, ²⁰⁰⁶; Schlacher et al., ^2013a ); and (4) trapping wind‐blown sand, thereby limiting the extent of coastal erosion and facilitating beach accretion and dune formation (e.g. Dugan, Hubbard & Page, ²⁰⁰⁵).

Fig. 1.

Conceptual diagrams of wrack dynamics in beach ecosystems. (A) The principal sources, transport routes, and biological fates of marine organic material cast upon sandy beaches; (B) disruption of natural processes caused by beach grooming and coastal armouring; and (C) predicted consequence of climate change for the supply, type and biological fates of marine organic matter in sandy beach ecosystems. POM, particulate organic matter. Diagrams created using IAN Image Library (http://ian.umces.edu).

Fig. 2.

Wrack, scavengers and human disturbance on sandy beaches. Wrack on beaches in (A) Cape Town, South Africa (photograph L. Harris); (B) Santa Barbara County, USA (photograph J. Dugan); (C) Salina Bay, Malta (photograph M. Mateo); and (D) Perth, Australia (photograph G. Hyndes). Dingo (E) and white‐bellied sea eagle (F) scavenging on carrion on beaches near Brisbane, Australia (photographs A. Olds), and beach cleaning on beaches in (G) Brisbane, Australia (photograph A. Olds) and (H) Carpinteria, USA (photograph J. Dugan).

The organisms and functions linked to marine matter cast upon beaches were reviewed by Colombini & Chelazzi (2003). Since then, significantly more research on these functions has been undertaken in various regions across the globe. Importantly, the effects of global warming on coastal systems that supply organic material to beaches, and on beaches themselves, are now better understood (Vitousek, Barnard & Limber, 2017; Smale et al., ²⁰¹⁹). Indeed, marine heatwave events and subsequent poleward shifts in the distribution of tropical grazers have led to ecosystem shifts from kelp forests to barrens in parts of the world (Smale et al., ²⁰¹⁹), which has immense ramifications for ecosystems relying on kelp as subsidies. Changes to beach structure and habitat availability through erosion, sea‐level rise, storms, and coastal development (Lee et al., ²⁰¹⁸; Dugan et al., ²⁰⁰⁸) strongly affect the ability of drift material to flow onto and be retained in these dynamic ecosystems. In addition, management activities, such as grooming, that actively remove wrack from beaches used for tourism, impact wrack‐associated biota and processes (e.g. Schooler, Dugan & Hubbard, ²⁰¹⁹). Furthermore, introductions of invasive species through global warming or other human‐induced mechanisms (e.g. Jiménez et al., ²⁰¹⁵) are likely to influence the supply and form of organic inputs to beach ecosystems.

Here, we review and synthesise the existing knowledge on the ecosystem functions of wrack and carrion in sandy‐beach ecosystems. We also outline knowledge gaps and identify priorities for future research. We focus mainly on the dynamics and ecological pathways of marine matter becoming stranded on ocean sandy shores, including the supply, retention, and processing of macrophytes and carrion, and how matter can be transferred inland or back to sea with or without alteration. We reviewed the literature based on searches in Elsevier Scopus and Clarivate Web of Knowledge using the terms (‘beach*’ OR ‘shore’ OR ‘surf zone’) AND (‘wrack’ OR ‘carrion’ OR ‘allochthonous’ OR ‘beach‐cast’ OR ‘drift’ OR ‘detritus’ OR ‘kelp’ OR ‘seagrass’). Search results were supplemented with references cited in Colombini & Chelazzi (2003), and papers from the authors' personal collections. This process yielded 336 articles that report various aspects of wrack or carrion in sandy‐beach ecosystems, with 245 of those papers published after Colombini & Chelazzi (2003).

We first summarise the global distribution of the research effort and the focal themes. Secondly, we provide an overview of the main forms and amount of wrack and carrion deposited on beaches and adjacent surf zones. We then examine several topics concerning the dynamics and influence of wrack and carrion subsidies to beaches, specifically: (1) factors influencing the availability, deposition and retention of these subsidies; (2) the role of subsidies in regulating faunal abundance and biodiversity; (3) the fate of wrack and carrion, particularly their roles in beach food webs; (4) the spatial scaling of subsidy effects in sandy‐beach ecosystems and across the broader seascape and landscape; and (5) the influence of anthropogenic changes and management interventions on wrack and carrion dynamics. Finally, we identify key knowledge gaps to guide future research.

II. THE CURRENT STATE OF KNOWLEDGE

(1). Thematic areas and the geography of the global research effort

Of the 336 papers that examined various aspects of wrack or carrion in sandy‐beach ecosystems, 89% focused on the beach, with fewer studies from the surf zone or waters adjacent to sandy beaches (see Table S1). Three thematic areas dominate the research effort (Table 1, Fig. 3): (1) spatial or temporal patterns in the amount of wrack (‘wrack dynamics’, 36%); (2) spatial or temporal patterns in the abundance of fauna (‘fauna dynamics’, 33%); and (3) trophic ecology (28%). Themes with moderate research focus include population biology of fauna in wrack (16%), and chemical composition and processes or the nutrient dynamics of wrack (12%). Only 10% of studies examined distribution patterns of carcasses or carrion on beaches (‘carrion and carcass dynamics’). Similarly, only 9% of studies examined human use, such as harvesting kelp for food products or biofuel and human impacts such as the effects of beach grooming on wrack‐associated fauna. Few studies (7%) examined environmental processes and influences, including factors that affect the amount and composition of wrack on beach systems. Even fewer (only 5%) looked at decomposition and microbial processes in wrack, and a small number of studies (<2%) examined topics such as invasion biology, human health, or genetics (Table 1).

Table 1.

Themes addressed in the literature on beach‐cast organic matter (plant wrack and animal carrion). The total number of studies identified was 336, but many papers investigated more than one theme. See Table S1 for full list of publications and theme designations.

Theme	# studies	%
Wrack dynamics (temporal and spatial variability)	121	36.0
Faunal dynamics (temporal and spatial variability)	111	33.0
Trophic ecology	94	28.0
Population biology	54	16.1
Chemical composition and processes*	42	12.5
Carrion and carcass dynamics	32	9.5
Human use and impacts	31	9.2
Environmental processes and influences	23	6.8
Decomposition and microbial processes**	16	4.8
Dispersion of sources***	7	2.1
Human health	6	1.8
Invasion biology	5	1.5
Taxonomic inventories****	5	1.5
Hydrodynamics	4	1.2
Influence on dunes	4	1.2
Genetics (population genetics and phylogenetics)	3	0.9
Methods	3	0.9
Restoration	3	0.9
Fauna physiology	1	0.3
Movement of fauna	1	0.3
Sediment transport	1	0.3

^*Forms and concentrations of nutrients and nutrient cycling.

^**Biomass loss of wrack or carrion, and/or microbial processes leading to those losses.

^***Export mechanisms for the dispersal of wrack or carrion from donor systems.

^****Inventory of algal or faunal taxa, or new records.

Fig. 3.

Global distribution of studies (N = 336) classified by the main theme with respect to wrack and carrion on sandy beaches and in surf zones. Pie charts illustrate the different themes of published studies, with the size indicating the total number of studies for a region. Note that more than one theme could be covered by each paper, but the number of studies in each region reflects the total number of papers regardless of theme. See Table 1 for more detail on themes.

We found few studies (N = 10) published before the 1980s. The number of published studies almost doubled from 42 during the 1980s to 76 during the 2000s, doubling again to 165 articles during the 2010s. All but four studies examined aspects of sandy beach ecosystems rather than beaches with larger sediments such as cobbles and boulders – most of these studies sampled in higher latitude regions (>40°N or S). The majority of research on wrack or carrion has been carried out in the USA (58 studies), Australia (53), Spain (30), Canada (21), and UK and New Zealand (20 each) (Fig. 3). Countries with moderate research effort (7–15 studies) include Italy, South Africa, Chile, and Brazil. Notable areas with significant sandy‐beach shorelines with a paucity of studies on wrack and carrion inputs include most of the tropics, such as the Caribbean, central and north Africa, the Arabian Peninsula, southeast Asia and South America (except Chile and Brazil) (Fig. 3). While several studies have been carried out in the polar regions, most beaches in these studies comprised coarser sediment grain, e.g. shingles or boulders (Crafford & Scholtz, 1987; Chown, ¹⁹⁹⁶; Lastra et al., ²⁰¹⁴).

(2). Global patterns in standing stock of wrack on beaches

We found over 60 papers that quantified the amount or composition of wrack on sandy beaches or in adjacent surf zones. However, the metric used to report the amount of wrack was variable, including volume (l) or wet or dry mass (g or kg) of wrack within a linear metre or square metre of beach. Because the most common unit has been kg wet weight (WW) per linear metre of beach, we use this measure to examine global patterns in standing stock. However, even for this metric, the number of studies is low. Based on papers allowing data from multiple sites or seasons to be directly extracted, or by converting extracted dry mass estimates where conversion factors were available, the average biomass of wrack on beaches varies considerably from 0.1 to 325 kg WW m⁻¹ (Fig. 4). Exceptionally high biomass (>100 kg WW m⁻¹) has been recorded on beaches in the Mediterranean Sea, south‐western Australia, Argentina and western Canada. However, inconsistencies in the metric used to report wrack biomass reduce the pool of comparable data and make comparisons across regions challenging. To facilitate such comparisons, we recommend that future studies provide sufficient data (e.g. beach/surf zone width length, volume to biomass conversions) to allow standing stock (wet mass) per unit area to be calculated.

Fig. 4.

Global patterns in wrack composition (N = 43) and wet mass (WW; N = 27) of wrack (kg WW wrack m⁻¹ of coastline) on beaches and in surf zones based on published papers. Numbers in plain text to the right of each pie chart indicate the number of studies. The text in italics below the pie charts indicates the average biomass in each region, while the numbers in parentheses to the right of the text indicates the number of studies that provided the data. Note that for a study to be included, data needed to be provided for at least three sites or times. Wrack composition was based on wet and dry mass and volume data, while biomass data were based on studies where mass was either provided or data could be converted to wet mass per linear metre of coastline. Distribution of seagrass from UNEP‐WCMC seagrass maps based on Green & Short (2003), while kelp distributions are based on Filbee‐Dexter & Wernberg (2018).

The composition of wrack in sandy‐beach ecosystems varies geographically, but often is dominated by macroalgae, particularly kelp species, and seagrasses (Fig. 4). At a global scale, major factors that influence biomass and composition of wrack include the offshore benthic substrate type and the associated distribution, productivity rate and composition of coastal primary producers (seagrass, kelp, etc.) as potential sources of wrack macrophytes. For instance, kelp forests span temperate to arctic regions. Therefore, kelp is a major input of wrack along the coasts of cooler waters, particularly the western USA, Chile, southern Africa and New Zealand, and a substantial input in southern Australia (Fig. 4). Dominant taxa include Durvillea, Ecklonia, Lessonia, Macrocystis and Nereocystis. In comparison, seagrasses span boreal to tropical regions, and form a major component of wrack along coastlines in the Mediterranean and Caribbean Seas, East Africa, and Australia (Fig. 4), with Posidonia, Zostera, Cymodocea, Halodule, Halophila or Thalassia being the main genera. Indeed, seagrass is the only form of wrack on the beaches in the Mediterranean Sea, where Posidonia oceanica can form major ‘banquettes’ (wrack deposits). In the northern hemisphere (the Baltic Sea, Western Europe, Caribbean Sea and west coast of North America) and in Argentina, other forms of macroalgae (red, green and non‐kelp brown algae) also form significant components of wrack (Fig. 4).

Besides the inflow of macrophytes, carrion (dead invertebrates, fish, birds, and mammals as large as whales) can be numerous on beaches, when animals that die at sea wash ashore (Sikes & Slowik, 2010). However, few studies have focused on carrion or animal carcasses compared to beach‐cast macrophytes (Table 1, Fig. 3), possibly reflecting: (1) a lower proportion of carrion compared to wrack; (2) more episodic deposition and patchy distribution; (3) more rapid consumption and removal by scavengers; or (4) a reluctance by researchers to work on decomposing animal bodies.

(3). Factors influencing standing stocks of wrack

The standing stock of wrack on sandy‐beach systems is highly dynamic, both in space and time. Wrack biomass can range over 1–4 orders of magnitude across locations within a region (Tarr & Tarr, 1987; Dugan et al., ^{2003, 2011}; Barreiro et al., ²⁰¹¹; Liebowitz et al., ²⁰¹⁶; Holden et al., ^2018a ; Reimer et al., ²⁰¹⁸; Schooler et al., ²⁰¹⁹). Similarly, standing stock may show temporal variation from no visible accumulation to tens or thousands of grams dry mass per square metre, within hours to days (Ruiz‐Delgado et al., ^2016b ; Vieira et al., ²⁰¹⁶), and across years (Barreiro et al., ²⁰¹¹; Revell, Dugan & Hubbard, ²⁰¹¹; Jiménez et al., ²⁰¹⁵; Liebowitz et al., ²⁰¹⁶; Holden et al., ^2018a ). Wrack dynamics are principally influenced by the input, accumulation, and export of material. Several factors shape these dynamics, including: (1) the types and strength of physical forces that transport material; (2) the geomorphology of beaches; (3) the broader landscape context of beaches; and (4) the characteristics or traits of the donor system (Table 2).

Table 2.

Factors that either positively or negatively affect the deposition and retention of wrack on beaches.

Factor	Description	References
State of donor ecosystem
Standing stock	Biomass of subsidy and availability for export	de Bettignies et al., 2013; Krumhansl & Scheibling (2011); Pedersen et al. (2020)
Senescence/growth	Primary productivity, biomass availability/ turnover/nutritional condition and availability for export	Rodriguez et al. (2013, 2016)
Macrophyte phenology	Annual versus perennial, seasonal cycles of export	Hamilton et al. (2020)
Management	Direct harvest, grooming, fisheries, marine protected areas, biomass available for export	Dugan & Hubbard (2010)
Characteristic of beach
Proximity to donor ecosystems	Rocky reef, kelp forest, seagrass bed, estuary, rocky intertidal	Orr et al. (2005); Reimer et al. (2018); Liebowitz et al. (2016)
Beach morphology	Width of zones and slopes affect delivery and retention	Orr et al. (2005); Revell et al. (2011); Barreiro et al. (2011); Wickham et al. (2020)
Beach orientation	Shore orientation relative to prevailing swell and currents affects delivery and retention	Orr et al. (2005); Gómez et al. (2013)
Back beach type	Retention and fate of wrack varies among dune‐, cliff‐ and seawall‐backed shores	Dugan et al. (2008); Heerhartz et al. (2014)
Beach management	Grooming and armouring alter retention and fate	Dugan & Hubbard (2010); Schooler et al. (2019)
Sediment supply/budget	Availability and resilience of beach habitat affects retention	Zoulas & Orme (2007); Orme et al. (2011); Griggs & Patsch (2018)
Disturbances
Storm events	Changes in wave height affect donor and recipient ecosystems through removal and erosion	Barreiro et al. (2011); Reed et al. (2011)
Storm season	Wave‐driven removal of substrate and subsidies	Cavanaugh et al. (2011); Reed et al. (2011)
Climate events	El Nino, oceanographic oscillations, marine heatwaves, etc. change primary production, supply and dynamics of recipient ecosystem	Cavanaugh et al. (2011); Revell et al. (2011); Thomsen et al. (2019); Strydom et al. (2020)
Climate change	Increased ocean temperature and sea‐level rise: warming impacts kelp forests/seagrass beds and sea‐level rise causes loss of beach habitat	Jordà et al. (2012); Krumhansl et al. (2016); Bell et al. (2018); Cavanaugh et al. (2019)
Overgrazing	Effect on standing stock and resilience, urchins and urchin barrens, biomass available for export	Rose et al. (1999); Ling et al. (2015, 2019); Rogers‐Bennett & Catton (2019)
Invasive species	Outcompete natives, altered life cycles and biomass production/export and food quality	Marks, Reed & Holbrook (2020); Schiel et al. (2018)

Ocean processes	Drivers of exchanges
Tides	Daily and semi‐lunar tides affect delivery and retention on beaches	Zobell (1971); Revell et al. (2011); Orr et al. (2005)
Wave climate	Event‐, season‐ and climate‐driven wave dynamics affect donor (loss of biomass, whole plants) and recipient (wrack biomass dynamics and beach erosion/rotation/retention) ecosystems	Zobell (1971); Revell et al. (2011); Liebowitz et al. (2016)
Currents	Transport and delivery of macrophytes to beaches. Can move wrack along and on and off the beach	Orr et al. (2005); Gómez et al. (2013); Liebowitz et al. (2016)
Sea level rise	Erosion and long‐term loss of recipient beach habitat zones affects retention of wrack	Myers et al. (2019)
Wind	Surface currents and erosion processes affect both donor and recipient ecosystems, wrack burial and transport inland	Rossi & Underwood (2002); Hammann & Zimmer (2014); Liebowitz et al. (2016); Del Vecchio et al. (2017)

Various physical forces (e.g. tides, waves, currents, wind) transport material onshore. It follows that variation in physical forces results in differences in the deposition and resuspension of wrack on sandy beaches, operating at timescales from hours to weeks and over distances of hundreds of metres to hundreds of kilometres (Table 2, Fig. 5). Rising tides tend to remove wrack from beaches, while falling tides are more conducive to intertidal deposition of buoyant material (Fig. 1A; Zobell, ¹⁹⁷¹; Orr et al., ²⁰⁰⁵). Spring high tides also remove wrack (Zobell, 1971) or shift wrack deposits up‐shore towards the base of dunes or bluffs backing the beach. Strong seasonal patterns of kelp deposition are often related to changes in wave climate (Zobell, 1971; Revell et al., ²⁰¹¹). For example, storms generated at monthly to annual timescales (Fig. 5) uproot and transport large amounts of macrophytes to become drift material deposited on beaches (Zobell, 1971;Revell et al., ²⁰¹¹; Liebowitz et al., ²⁰¹⁶). However, the effects of storms on the supply can be modified by changes in the life‐history traits of potential wrack material in donor systems. For instance, early‐season storms can dislodge and transport large amounts of macrophytes, resulting in less material being available when storms occur later in the growing season (e.g. Zobell, ¹⁹⁷¹). However, wrack supply is further complicated by differences in the overall life histories among primary producers in the donor systems. Some kelp species are annuals (e.g. Nereocystis), while others are perennials (e.g. Macrocystis), and some macroalgae senesce as part of their life cycle (e.g. Sargassum). Supply is also influenced by ocean‐scale to global atmospheric and oceanic events, such as El Nino Southern Oscillation (ENSO) events (Seymour, 2003) or heatwave events and broader ocean warming (Wernberg et al., ²⁰¹⁹), occurring over timescales of years to thousands of years and spatial scales of tens to tens of thousands of kilometres (Fig. 5).

Fig. 5.

Coastal, oceanographic and atmospheric factors that influence wrack supply and retention on sandy beaches, and the spatial and temporal scales at which they operate. Sources of information are provided in Table 2. Supply = processes that influence the input of wrack to beach ecosystems; retention = processes that influence the ability of wrack to remain in beach ecosystems. The colour of the ellipse for each factor indicates whether the factor affects either supply or retention, or both supply and retention of wrack in beach systems. Beach management refers to management practices such as beach grooming and armouring that influence supply and retention of wrack, while overgrazing refers to impacts on donor systems such as kelp forests due to grazing pressure.

The geomorphology (e.g. slope, width) and aspect (orientation) of beaches influence standing stocks of wrack (Table 2), primarily by altering the capacity to retain drift material (Revell et al., ²⁰¹¹; Liebowitz et al., ²⁰¹⁶). For example, Barreiro et al. (2011) showed that small, wave‐sheltered beaches retain higher amounts of wrack than more exposed beaches in Spain, while deposition rates of buoyant material can differ substantially between windward and leeward shores (Lastra et al., ²⁰¹⁴). Wrack retention can also be influenced by the elevation, and the width and slope of the drier, upper part of beaches (Zobell, 1971; Revell et al., ²⁰¹¹; Liebowitz et al., ²⁰¹⁶). Similarly, substrate type influences the retention of wrack along shorelines. Consolidated rocky shores retain far less wrack than shores comprising sand, cobble or boulders (Wickham et al., ²⁰²⁰). Furthermore, cobble beaches appear to retain more wrack than gravel or sandy beaches (Orr et al., ²⁰⁰⁵). In addition, coastal armouring structures (seawalls, revetments) that reduce beach width and eliminate upper beach zones have been shown to have profound effects on the retention and standing stock of wrack (see Section III.1).

The proximity of beaches to donor ecosystems, and the form and condition of those systems, can strongly influence the supply of wrack to sandy shores. For example, in northern California, USA, the deposition of macrophytes on beaches varies depending on the source of wrack: subtidal reefs, rocky intertidal shores and estuaries (Liebowitz et al., ²⁰¹⁶). Moreover, beaches within 0.5–1 km of donor ecosystems were most strongly influenced by wrack from those sources, while the role of more distant (7 km) kelp beds was only detectable during storm events (Liebowitz et al., ²⁰¹⁶). Similarly, Reimer et al. (2018) showed that proximity of sources, ocean upwelling, estuarine outwelling, beach geomorphology and wave climate contributed to patterns of wrack on beaches in the US Pacific. Finally, wrack supply to beaches is influenced by the changing state of the donor systems. For example, excessive grazing by urchins can shift kelp forests to urchin barrens (Ling et al., ²⁰⁰⁹), reducing the availability of wrack material over timescales of years to tens of years and spatial scales up to tens of kilometres (Fig. 5).

(4). Links between wrack and fauna in beach ecosystems

(a). Invertebrates

Sandy‐beach ecosystems have traditionally been viewed as ‘harsh’ environments, characterised by low diversity and low abundance of intertidal invertebrates (Brown & McLachlan, 1990). This historical view, however, largely ignores the pivotal role of wrack and carrion for the sandy beach fauna as well as the associated biodiversity and productivity. The effect of wrack on overall intertidal biodiversity can be substantial, with wrack‐associated macrofauna (macroinvertebrates, >1 mm) making up, on average, >45% of the species present on beaches that receive inputs of giant kelp in California, USA (Dugan et al., ²⁰⁰³). In fact, intertidal species richness on Californian beaches is strongly correlated with kelp wrack abundance but not with beach morphodynamics (e.g. Dugan et al., ²⁰⁰³; Schooler et al., ²⁰¹⁷). Wrack supports invertebrate biodiversity (Fig. 1A) by providing a food source for populations of specialized intertidal invertebrates and a refuge from predation and environmentally stressful conditions, such as desiccation (Dugan et al., ²⁰⁰³; Crawley & Hyndes, ²⁰⁰⁷; Colombini et al., ²⁰⁰⁹). Interestingly, it has also been shown that wrack availability can modify the balance of biological interactions (Duarte et al., ^2010a ). However, in some cases, extensive wrack deposits can negatively impact some invertebrates through physical disturbance or anoxia, including meiofauna, (invertebrates <1 mm; McGwynne, McLachlan & Furstenburg, ¹⁹⁸⁸) and macrofauna (e.g. wedge clam Donax serra; Soares, Schlacher & McLachlan, ¹⁹⁹⁷), but few studies have examined these disturbance processes. Relatively few studies have examined meiofauna, yet they can be abundant in wrack (McGwynne et al., ¹⁹⁸⁸; Alkemade & van Rijswijk, ¹⁹⁹³; Jȩdrzejczak, ^2002a ; Urban‐Malinga et al., ²⁰⁰⁸) and appear to play a crucial role in the decomposition of detrital material (see Section II.5.a ).

Wrack contains specialized mobile intertidal macrofauna, highlighted by the greater species richness and density of macrofauna on beaches with wrack compared with those with little or no wrack both in south‐western Australia (Ince et al., ²⁰⁰⁷) and the west coast of the USA (Dugan et al., ²⁰⁰³). Similarly, the removal of wrack on beaches in USA, Brazil and Spain significantly reduced the diversity and density of these specialized arthropods (Dugan et al., ²⁰⁰³; Schooler et al., ^{2017, 2019}; Vieira et al., ²⁰¹⁶). By contrast, the experimental addition of wrack to the upper parts of beaches resulted in higher arthropod abundances, particularly amphipods, and increased abundances of several beetle species in southern Australia (Schlacher et al., ²⁰¹⁷). Correlation analyses of invertebrate abundance versus wrack biomass data extracted from the literature showed that beach‐cast macrophytes positively influence invertebrate species richness and abundance (N = 15, Fig. 6). The diverse suite of invertebrates that use wrack comprises three main trophic guilds: (1) ‘detritivores’ that feed directly on the detrital macrophytes; (2) bacterivores that feed mainly on wrack‐associated bacteria; and (3) ‘predators’ and ‘scavengers’ feeding on live prey or animal carcasses of the detritivores and bacterivores.

Fig. 6.

The numbers and proportions of papers with a focus on different aspects of total invertebrate assemblages (A) and amphipod populations (B) in wrack on beaches and in surf zones, and the relationship between amphipod abundance and wrack biomass (g wet mass m⁻²) (C) based on data extracted from the literature. In (A) and (B), circle quarters represent summaries of correlations of wrack biomass with invertebrate assemblages and amphipod populations (i.e. abundance, biomass, species richness, diversity). In each quadrant, the number of studies is displayed in parentheses, and the percentage of studies reporting a significant effect for each variable is illustrated by the coloured region in each quadrant (e.g. 83% of invertebrate studies reported positive effects on invertebrate abundance). In C, correlations are based on generalised additive models (GAMs) that assess the relationship between amphipod abundance and wrack biomass, with GAMS limited to four knots. See Table S2 for data sources.

Detritivores, represented almost exclusively by arthropods, form the most common and diverse trophic guild of macrofauna associated with beach‐cast wrack. Amphipods dominate this guild (Fig. 6), but other groups such as isopods and several beetle species (Tenebrionidae, Histeriidae and Curculionidae) are also common detritivores on stranded wrack in many areas (Griffiths & Stenton‐Dozey, 1981; Dugan et al., ²⁰⁰³; Jaramillo et al., ²⁰⁰⁶). Amphipods are almost ubiquitous in wrack deposits at temperate latitudes and are typically the most abundant taxon in fresh wrack deposits around the globe (Fig. 7). Amphipods in beach‐cast wrack mainly belong to the family Talitridae, including the genera Talitrus, Megalorchestia, and Orchestoidea. Importantly, although highly mobile and building a new burrow every day (Emery et al., ²⁰²²), amphipods and several other beach crustaceans brood their young, possess no planktonic life stages, and have limited dispersal as adults. This combination of traits makes these populations dependent on local reproduction and survival (Dugan et al., ²⁰⁰⁵; Hubbard et al., ²⁰¹⁴). Generalised additive models of amphipod abundance versus wrack biomass data extracted from the literature show that wrack biomass strongly influences the abundance and biomass of amphipods, revealing a unimodal response where peak abundance of amphipods occurs at moderate levels of wrack biomass, but decreases at very high levels of wrack biomass (Fig. 6). At very high levels of wrack standing stock, the reduced abundance of amphipods likely reflects compaction, anoxia, and other unfavourable environmental conditions created by large amounts of decomposing organic matter (e.g. McGwynne et al., ¹⁹⁸⁸). Combining their high consumer densities, which can reach 10,000–100,000 individuals per m beach (e.g. Lastra et al., ²⁰⁰⁸; Lowman et al., ²⁰¹⁹), and their high feeding rates on wrack (see Section II.5.a ), amphipods play a crucial role in linking wrack subsidies from donor ecosystems to higher trophic levels in sandy‐beach ecosystems (see Section II.6).

Fig. 7.

Abundance rank of the main invertebrate taxa found on the beach and in the surf zone based on extracted data from the literature from across the globe. Dots indicate the normalised ranks (i.e. taxa in order of abundance, e.g. 1, 2, 3, etc., converted to values between 0 and 1, where 1 indicates the most abundant) of taxa in individual studies, while the vertical lines denote the mean rank and horizontal lines the 95% confidence intervals. Sample sizes (number of studies) are provided in parentheses. See Table S3 for data sources.

Dipteran flies form one of the most abundant bacterivore groups in wrack deposits and are almost ubiquitous in wrack across the globe (Fig. 7). Even though diets show considerable variation between dipteran families, bacteria are likely the most common food source for Diptera in the beach‐cast wrack, as shown for Coelopa frigida (Cullen, Young & Day, 1987). Seaweed or kelp flies, belonging to several families within Sciomyzoidae, are the most common dipteran group in wrack (e.g. Egglishaw, ¹⁹⁶⁰; Dobson, ^1974b ), but taxa such as Fucellia spp. (Anthomyiidae), Ephydridae, and Sphaeroceridae are also common in wrack and are typically important bacterivores (Cole, 1969; Griffiths & Stenton‐Dozey, ¹⁹⁸¹). The larvae of these flies feed on kelp, fungi and bacteria (Cullen et al., ¹⁹⁸⁷), forming an important bacterivore compartment in the food web of beach‐cast wrack. Similar to amphipods, the abundance of kelp and seaweed flies is also strongly correlated with wrack biomass on beaches in California (Dugan et al., ²⁰⁰³). However, several much smaller fly species from several families are often overlooked in standard surveys (Cole, 1969) and would be worthy of future investigation.

The main groups of macrofauna feeding on detritivores and bacterivores in beach‐cast wrack deposits are beetles (particularly Staphylinidae, Carabidae and Histeriidae) and spiders (Moore & Legner, 1976; Griffiths & Griffiths, ¹⁹⁸³; Mellbrand et al., ²⁰¹¹). Wrack‐associated beetles include some flightless species that complete their entire life cycle in the intertidal zones of beaches. For example, the staphylinid species Thinopinus pictus and Hadrotes crassus in North America have a specialised diet, feeding throughout their life cycle on amphipods (Orchestoidea) in beach intertidal zones (Craig, 1970; Richards, ¹⁹⁸²), while another staphylinid genus (Aleochara spp.) parasitise seaweed fly larvae (Yamazaki, 2012). In general, only a few invertebrate species have been reported to predate on amphipods, with a wider range preying on the abundant dipteran larvae (Yamazaki, 2012). However, many species, such as most spiders, have a fairly broad diet (Verschut et al., ²⁰¹⁹).

Since wrack deposition on beaches is generally highly dynamic over time, macrofauna utilising this resource need high intertidal mobility, temporary burrows and rapid development to make the best use of an essentially ephemeral resource (Dugan, Hubbard & Quigley, 2013; Emery et al., ²⁰²²). Mobility is also critical to avoid being washed out to sea with the wrack during high tides, but some species are also reported to have very low alongshore mobility (Schooler et al., ²⁰¹⁷). Not surprisingly, stranded macroalgal deposits are often colonized by mobile macrofauna (particularly amphipods and dipterans) within a few hours of deposition, followed by predatory staphylinid beetles (Pelletier et al., ²⁰¹¹; Yanenik, ¹⁹⁸⁰). Studies from the UK suggest that the life cycle of kelp flies (Coelopa sp.) is completed within 1 month, depending on temperature (Dobson, ^1974b ). Rapid larval development is supported by the elevated temperatures within the masses of stranded wrack, particularly in larger, deeper wrack beds, where the temperature may be >10 °C above ambient levels (Crafford & Scholtz, 1987), and moisture levels are maintained (Kompfner, 1974), but not in smaller clumps where the temperature is more similar to air temperature (Dobson, ^1974b ). Development coordinated with wrack deposition has also been suggested for some Coelopa spp., where larvae develop in spring high‐tide deposits of wrack and emerge from pupae by the next spring tide (Kompfner, 1974). Development times for different kelp fly species may be linked to tidal elevation, with species at lower elevations developing faster (Kompfner, 1974). The initial burst in colonisation of freshly stranded wrack by selected macrofauna is followed by a succession of other species (Yanenik, 1980; Colombini et al., ²⁰⁰⁹), and a gradual increase in meiofauna such as nematodes, oligochaetes and mites (Jedrzejczak, ^2002b ), followed by coleopteran species (Griffiths & Stenton‐Dozey, 1981; Colombini et al., ²⁰⁰⁰).

In addition to quantity, the composition of the wrack input can influence invertebrate assemblages, but this effect seems to vary among consumer species (Mews, Zimmer & Jelinski, 2006; Olabarria et al., ²⁰¹⁰). For example, in south‐western Australia, densities of invertebrates were greater in experimental plots of stranded seagrass compared with Sargassum wrack on the beach (Mellbrand et al., ²⁰¹¹), while the amphipod Allorchestes compressa showed a preference for macroalgae over seagrass in the surf zone (Crawley & Hyndes, 2007). Similarly, in southern California, amphipods (Megalorchestia spp.) preferred experimental wrack patches comprising kelp rather than seagrass, and different Megalorchestia species varied in their colonisation rates in wrack patches, perhaps to avoid competition with congeners (Michaud et al., ²⁰¹⁹). The causes of these different affinities are likely to be complex when the macrophytes making up wrack vary in species, age, decomposition levels, and in terms of physical structure, nutrient content, palatability and bacterial community (Lowman et al., ²⁰²¹) (see Section II.5.a ).

(b). Fishes

Wrack in the surf zones of ocean beaches provides important feeding, sheltering, and nursery habitats for a diversity of fishes (Fig. 1A; Crawley et al., ²⁰⁰⁶; McLachlan & Defeo, ²⁰¹⁷; Ortodossi et al., ²⁰¹⁹). Drifting macrophytes are widely reported to influence the abundance, diversity, and species composition of surf fishes (e.g. Robertson & Lenanton, ¹⁹⁸⁴; van der Merwe & McLachlan, ¹⁹⁸⁷; Andrades et al., ²⁰¹⁴), but the ecological functions of wrack as a fish habitat remain poorly understood, largely because few studies (N = 20) have examined how macrophyte accumulations affect fish populations, assemblages, or food webs (Olds et al., ²⁰¹⁸). Studies that examine the links between wrack and surf fishes come mainly from Australia (N = 12), limiting their broader generality and highlighting the need to gain a more global understanding of the role of drifting macrophytes for fish in surf zones. Nevertheless, the biomass of drifting macrophytes in surf zones can shape the composition of fish assemblages, modify patterns in fish diversity, abundance, and biomass, and alter the structure of coastal food webs (Crawley et al., ²⁰⁰⁶; Clark, Bennett & Lamberth, ^1996b ; Baring, Fairweather & Lester, ²⁰¹⁴; Vargas‐Fonseca et al., ²⁰¹⁶). Our analyses showed that fish abundance (N = 15 studies) and biomass (N = 2 studies) are mostly positively correlated with standing stock of wrack, whereas fish diversity is greatest at moderate levels of wrack biomass (N = 9 studies) (Fig. 8).

Fig. 8.

Proportions of papers with a focus on different aspects of fish assemblages in the wrack in surf zones (A), and the relationship between fish abundance (B) and diversity (C) with wrack volume (litres per 100 m²) based on data extracted from the literature. In (A), circle quarters represent summaries of correlations with fish assemblages (i.e. abundance, biomass, species richness, diet). In each quadrant, the number of studies is displayed in parentheses, and percentage of studies reporting a significant effect for each variable is illustrated by the coloured region in each quadrant (e.g. 67% of studies on surf fish assemblages report positive effects of wrack biomass on fish abundance). In (B) and (C), correlations are based on generalised additive models (GAMs) assessing the relationship between fish abundance and diversity against wrack volume with GAMs limited to four knots. See Table S4 for data sources.

Most research on the role of surf‐zone wrack has focused on fish assemblages, whereas data on individual taxa are sparse (Clark, Bennett & Lamberth, ^1996a ; Lacerda, Barletta & Dantas, ²⁰¹⁴; Baring, Lester & Fairweather, ²⁰¹⁶). Numerous fish species have been reported to prey on epifaunal amphipods that accompany drifting macrophytes (Crawley et al., ²⁰⁰⁶; Baring, Lester & Fairweather, ^2018b ). A greater biomass of drift macrophytes has been reported to have positive effects on the abundance of several fish species (Robertson & Lenanton, 1984; Andrades et al., ²⁰¹⁴; Baring, Lester & Fairweather, ²⁰¹⁹). Generalised additive models of fish diversity versus surf‐zone wrack biomass extracted from the literature showed that, at least at low to moderate levels of wrack biomass there is a positive effect on fish diversity in surf zones by providing enhanced feeding opportunities (N = 9) (Fig. 8C). However, some species show either a neutral (Robertson & Lenanton, 1984) or negative (Crawley et al., ²⁰⁰⁶) diversity response to increasing wrack biomass in surf zones, presumably due to high densities of wrack impeding the movement and foraging of fish with particular morphological and feeding traits. Variation in the type of drifting macrophytes can also affect fish abundance, which is likely to reflect primarily the different affinities of their prey to different macrophytes as habitat and food. For example, the amphipod A. compressa displays a strong preference for brown algae as food and habitat (Crawley & Hyndes, 2007) and forms the main diet of juvenile fish with strong affinities to drifting wrack (Crawley et al., ²⁰⁰⁶). The types of drifting macrophytes in surf zones are therefore likely to influence the bottom‐up control of food webs in this ecosystem.

(c). Birds, mammals and reptiles

While highly mobile birds, mammals, and reptiles can use various resources across the landscape, many species show consistent associations with wrack or carrion as food resources on sandy beaches. Birds are often the most abundant vertebrates on sandy beaches, including shorebirds, waders, gulls, and a wide variety of terrestrial birds (from vultures to flycatchers) (Dugan et al., ²⁰⁰³). Many waders and passerines feed on wrack‐associated invertebrate prey (Lopez‐Uriarte et al., ¹⁹⁹⁷; Dugan et al., ²⁰⁰³), while gulls, raptors, ravens, crows and vultures feed on carrion (Table 3). Wintering waders or shorebirds can reach high abundances (>95 individuals km⁻¹) on wrack‐strewn open coast beaches and their numbers can be strongly and positively correlated with the abundance of stranded wrack and wrack‐feeding invertebrates on those beaches (Tarr & Tarr, 1987; Dugan et al., ²⁰⁰³; Hubbard & Dugan, ²⁰⁰³), reflecting their use of wrack‐associated invertebrates as prey (Griffiths, Stenton‐Dozey & Koop, 1983; Dugan et al., ²⁰⁰³; Schlacher et al., ²⁰¹⁷). Wrack is particularly important for plovers, which are visual surface feeders; this group includes IUCN red‐listed species in many parts of the world (e.g. the Western snowy plover Charadrius nivosus nivosus on the California coast; Dugan et al., ²⁰⁰³).

Table 3.

Examples of scavengers and the carrion they consume on ocean‐exposed sandy beaches reported in the literature.

Scavenger(s)	Carrion type	Location	Reference
Invertebrates
Whelks (Bullia rhodostoma, B. digitalis)	Jellyfish	South Africa	Brown (1961, 1971)
Gastropod (Reticunassa festiva)	Fish	Hong Kong	Morton & Yuen (2000)
Isopods (cirolanids)	Various drift organisms	USA (California)	Dugan et al. (2003); Quilter (1987)
Polychaetes	Various drift organisms	USA (California)	Dugan et al. (2003)
Beetles	Various drift organisms	USA (California)	Dugan et al. (2003)
Ghost crabs (Ocypode spp.)	Diverse/mixed carrion	multiple countries and locations	Wolcott (1978); Lucrezi & Schlacher (2014); Rae et al. (2019)
Reptiles
Lace monitor (Varanus varius)	Fish	Australia (East Coast)	Schlacher et al. (2013a , b ); Bingham et al. (2018)
Cottonmouth snake (Agkistrodon piscivorus conanti)	Fish	USA (Florida, Gulf Coast Islands)	Lillywhite et al. (2008)
Birds
Corvidae (crows and ravens)	Fish	Australia (East & South Coast)	Huijbers et al. (2013, 2015, 2016); Schlacher et al. (2013a , b ); Brown et al. (2015); Bingham et al. (2018)
Gulls (several spp.)	Fish	Australia (East & South Coast)	Huijbers et al. (2013, 2015, 2016); Schlacher et al. (2013a , b ); Bingham et al. (2018); Brown et al. (2015)
White‐bellied sea eagle (Haliaeetus leucogaster)	Various mammals, other birds, fish, and crabs	Australia (various coastal areas)	Smith (1985); Huijbers et al. (2013, 2015, 2016); Schlacher et al. (2013a , b ); Brown et al. (2015); Bingham et al. (2018)
Whistling kite (Haliastur sphenurus)	Various terrestrial and marine animals (rodents, reptiles, fish)	Australia (woodland and coastal areas)	Gosper (1983); Huijbers et al. (2013, 2015, 2016); Schlacher et al. (2013a , b ); Brown et al. (2015); Bingham et al. (2018)
Brahminy kite (Haliastur indus)	marine carrion such as fish and crabs	Australia (various coastal areas)	Smith et al. (1978); Lutter et al. (2006); Huijbers et al. (2013, 2015, 2016); Schlacher et al. (2013a , b ); Brown et al. (2015); Bingham et al. (2018)
Mammals
Coyote (Canis latrans)	Seals, birds, sea turtles, fish, marine arthropods	CA, USA	Rose & Polis (1998)
Lion (Panthera leo)	Cape fur seal carcasses	Namibia (Skeleton Coast)	Bridgeford (1985)
Tasmanian devil (Sarcophilus harrisii)	Fish	Australia (Tasmania)	Moore (2002); T.A. Schlacher, personal observations
Brown hyena (Parahyaena brunnea)	Cape fur seal carcasses	Namibia (Skeleton Coast)	Skinner et al. (1995); Kuhn et al. (2008)
Black‐backed jackal (Canus mesomelas)	Mammals (seal pups), birds, and fish	Namibia (Skeleton Coast)	Oosthuizen et al. (1997); Avery et al. (1987)
Dingo (Canis lupus dingo)	Broad range of stranded material including dugong and whale carcasses	Australia (East Coast, Fraser Island)	Moore (2002); Behrendorff et al. (2016, 2018b )
Red fox (Vulpes vulpes)	Fish	Australia (East & South)	Huijbers et al. (2013, 2015, 2016); Brown et al. (2015); Bingham et al. (2018)
Dogs and cats (feral and domestic)	Fish	Australia (East & South)	Huijbers et al. (2013)

Amongst mammals, many terrestrial species occasionally forage on living macrophytes or invertebrates in the lower intertidal zone during low tides (Carlton & Hodder, 2003). Populations of the coyote Canis latrans (Rose & Polis, 1998) and foxes (Cypher et al., ²⁰¹⁴; Bingham et al., ²⁰¹⁸; Schlacher et al., ²⁰²⁰) have been linked directly to the use of either marine‐derived carrion or wrack‐associated invertebrates as food sources (Table 3). Deer species have been observed foraging directly on beach‐cast macroalgae (Conradt, 2000). The side‐blotched lizard (Uta stansburiana) consumes marine algae (Barrett et al., ²⁰⁰⁵), and the brown tree lizard (Anolis sagrei) is strongly attracted to wrack (Spiller et al., ²⁰¹⁰).

Far less is known about the use of wrack by birds, marine mammals and reptiles in the surf zone, although many species [e.g. ducks (Neff, Page & Boehm, 2011); otters and sea lions (Somers, 2000; Osterrieder, Salgado Kent & Robinson, ²⁰¹⁷); sea turtles, penguins and sea lions (Tershy, Breese & Croll, 1997; Witherington, Hirama & Mosier, ²⁰¹¹; Colombelli‐Négrel, ²⁰¹⁹)] occur regularly in the waters adjacent to beaches. Some birds, such as geese and swans (Percival & Evans, 1997; Choney et al., ²⁰¹⁴), are known to feed on subtidal or intertidal seagrass in meadows along the shoreline of sheltered coastal systems. However, there is a paucity of studies linking birds, mammals, and reptiles to surf‐zone wrack as a food source or habitat, except for the cormorant Phalacrocorax varius which has been observed actively foraging for fish in surf‐zone wrack in Australia (Robertson & Lenanton, 1984). Clearly, there is a need to gain knowledge on the surf zone to understand the overall influence of wrack on higher‐order consumers.

(5). Biological processing and fate of wrack

(a). Decomposition and consumption processes

Wrack on sandy beaches represents a rich source of organic matter. The principal mechanisms breaking down this organic matter pool on sandy beaches are microbial decomposition, physical processing and consumption by intertidal meiofauna and macrofauna (Fig. 9A; Jȩdrzejczak, ^2002b ; Lomstein et al., ²⁰⁰⁶; Lastra et al., ²⁰⁰⁸; Lastra, López & Neves, ²⁰¹⁵; Rodil et al., ^2015c ). All these mechanisms play a key role in determining wrack residence time on the beach (e.g. Mateo, ²⁰¹⁰).

Fig. 9.

Conceptual diagram of: (A) decomposition and nutrient cycling; (B) the grazer/detritivore food web; and (C) scavenging pathways as key processes for the fate of stranded organic material in beach ecosystems. Diagrams created using IAN Image Library (http://ian.umces.edu).

Stranded macrophytes on beaches may have undergone decomposition for hours to days before stranding (e.g. Griffiths & Stenton‐Dozey, ¹⁹⁸¹; Colombini et al., ²⁰⁰⁰; Jaramillo et al., ²⁰⁰⁶). Once stranded, bacterial concentrations increase dramatically (Koop, Newell & Lucas, ^1982a ; Cullen et al., ¹⁹⁸⁷; Urban‐Malinga & Burska, ²⁰⁰⁹). For example, bacterial biomass increased 12‐fold on kelp (Ecklonia maxima) stranded for 8 days (Koop et al., ^1982a ). The microbial communities associated with wrack likely develop from the biofilm of the macrophytes transported onto beaches rather than from the microbial community in the surrounding water (Singh et al., ²⁰²¹). Macroalgae and seagrasses are covered in diverse microbial communities that include bacteria, microalgae, fungi, and protists, and are highly specific and distinct from the seawater microbiome (Wahl et al., ²⁰¹²; Tarquinio et al., ²⁰¹⁹). For example, Bacteriodetes and Proteobacteria are dominant phyla in the biofilm of macroalgae and seagrasses (Wahl et al., ²⁰¹²; Tarquinio et al., ²⁰¹⁹). Bacteriodetes may comprise up to 25–50% of the biofilm (Berdan et al., ²⁰²¹), whereas this phylum makes up only a small portion of the seawater microbiome (Sunagawa et al., ²⁰¹⁵). Bacterial assemblages vary somewhat among species of algae in wrack (Rodil, Fernandes & Mucha, ^2015a ), similar to variation in biofilms across living macroalgae species (e.g. Staufenberger et al., ²⁰⁰⁸; Trias et al., ²⁰¹²). Furthermore, the microbiome is likely to shift during the decomposition process, as occurs on kelp detritus on the seafloor (Brunet et al., ²⁰²¹). The analysis of these microbial communities is still in its infancy, and there is a clear knowledge gap about microbial communities and their role in wrack and the broader sandy‐beach ecosystem.

As part of the decomposition process, heterotrophic bacteria digest macrophytes, converting organic carbon and nitrogen into simpler forms of dissolved organic and inorganic nutrients, which are released into the water column or incorporated into bacterial biomass (Säwström et al., ²⁰¹⁶) and flow into higher trophic levels in wrack accumulations (Fig. 9A). The rates of decomposition can be influenced by chemical and morphological traits of the macrophytes forming the wrack (Duggins & Eckman, 1997; Bucholc et al., ²⁰¹⁴). For example, the kelps M. pyrifera, Saccorhiza polyschides and Undaria pinnatifida are morphologically simple algae with soft, long, strap‐like blades that stack in layers on the sand (Lastra et al., ²⁰⁰⁸; Rodil et al., ²⁰¹⁹). These algae can decompose rapidly through the joint action of microbes and detritivores (Rodil et al., ²⁰¹⁹). By contrast, Sargassum muticum and Cystoseira baccata are morphologically more complex, with tough thalli bearing secondary and tertiary branches, and decompose much more slowly (e.g. Olabarria et al., ²⁰¹⁰). The structural molecules in seagrasses, which are vascular marine plants, are refractory and slow to break down (Trevathan‐Tackett et al., ²⁰¹⁷). These reduce both decomposition and consumption rates (see below) and allow large banquettes of seagrass to accumulate in some regions, such as the Mediterranean Sea (e.g. Mateo, Sánchez‐Lizaso & Romero, ²⁰⁰³) (see Figs 2C and 4).

Macroinvertebrate detritivores can rapidly process large quantities of wrack in sandy‐beach ecosystems. This can result in fragmentation which can enhance decomposition of wrack (Jȩdrzejczak, ^2002b ; but see Catenazzi & Donnelly, ^2007a ) and drive the food web (see Section II.4.b ). Talitrid amphipods can eat >50% of macroalgal wrack standing stock, thus playing a pivotal role in wrack turnover (Colombini et al., ²⁰⁰⁰; Lastra et al., ²⁰⁰⁸). Feeding preferences due to differences in palatability influence consumption rates in wrack accumulations (e.g. Michaud et al., ²⁰¹⁹). Based on a range of studies examining feeding preferences, kelp and other species of brown algae are generally the preferred food source for talitrid amphipods, whereas seagrass species (e.g. Phyllospadix) and green algae (e.g. Ulva spp.) are generally the least preferred (see Tables 4 and 5). Similarly, analyses of published consumption data indicate that consumption rates by amphipods are greater for kelp, particularly Durvillaea and Egregia, than other macroalgae and seagrasses (Fig. 10). Differences in preference and consumption rates can vary among regions and life stages of wrack detritivores. For example, Duarte, Jaramillo & Contreras (2008) and Duarte et al. (2010b ) showed that adults of the amphipod Orchestoidea tuberculata preferred D. antarctica over the algae M. pyrifera and L. nigrescens, whereas juveniles preferred L. nigrescens (Duarte et al., ^2010b ). Interestingly, the stark contrast in consumption rates between Durvillaea and Macrocystis reported from Chile (Duarte et al., ^2010b ) was not found for Bellorchestia quoyana in New Zealand, where both kelp species were consumed at similar rates (Suárez‐Jiménez et al., ^2017a ). These differences may reflect varying feeding preferences among amphipod species or differences in the nutritional quality of food sources between regions, making it difficult to generalise regarding consumption and resultant turnover rates of beach wrack, at least within brown algae (Emery et al., ²⁰²¹; Lowman et al., ²⁰²¹). However, the far lower consumption rates of seagrass (Crawley & Hyndes, 2007; Michaud et al., ²⁰¹⁹) indicate that this type of wrack has much lower dietary benefits despite its high biomass in wrack in sandy‐beach systems in many regions (Fig. 4).

Table 4.

Ranking of different wrack components in terms of preference, growth and survival rates of amphipods and ghost crabs. Food sources that a consumer species preferred equally are given the same rank, where 1 is the highest/most preferred rank and 4 the lowest/least preferred.

Variable	Taxa	Consumer species	Food source							Reference
			Kelp	Other brown algae	Red algae	Green algae	Seagrass	Other	Carrion
Food preference	Amphipod	Allorchestes compressa	1	1	2	2	4			Crawley & Hyndes (2007)
	Amphipod	Allorchestes compressa	1	2	3	3	3			Robertson & Lucas (1983)
	Amphipod	Bellorchestia quoyana	1			2				Suárez‐Jiménez et al. (2017a )
	Amphipod	Megalorchestia corniculata	1				2			Lastra et al. (2008)
	Ghost crab	Ocypode convexa	2				2	2 +	1	Rae et al. (2019)
Growth	Amphipod	Allorchestes compressa	1		3		4	2*		Robertson & Lucas (1983)
	Amphipod	Megalorchestia corniculata	1				2			Lastra et al. (2008)
Survival	Amphipod	Allorchestes compressa	1		3		4	2*		Robertson & Lucas (1983)
	Amphipod	Notorchestia sp.		1		2	3			Poore & Gallagher (2013)

⁺ Dune vegetation.

^*Fine particles.

Table 5.

Median percentage contribution of different types of food sources to the diets of consumers in wrack based on mixing model outputs of stable isotopes for consumers and potential food sources extracted from the literature.

Taxon	Consumer species	Food source (%)							Region	Reference
		Brown algae	Red algae	Green algae	Seagrass	Dune plants	POM	Carrion
Beach
Amphipod	Talitrus saltator	16	—	5	—	32	29	16	Atlantic (East)	Bessa et al. (2014)
Amphipod	Talorchestia brito	31	—	12	—	17	22	19	Atlantic (East)	Bessa et al. (2014)
Amphipod	Tylos europaeus	22	—	18	—	13	31	13	Atlantic (East)	Bessa et al. (2014)
Amphipod	Allorchestes compressa	5	12	—	25	21	—	—	Eastern Indian Ocean	Ince et al. (2007)
Amphipod	Talorchestia capensis	8	9	8	—	—	9	—	Southern Arica	Porri et al. (2011)
Ghost crab	Ocypode convexa	17	9	—	9	15	—	10	Eastern Indian Ocean	Rae et al. (2019)
Surf zone
Amphipod	Allorchestes compressa	32	14	—	54	—	—	—	Eastern Indian Ocean	Crawley et al. (2009)

POM, fine particulate organic matter in sediment or water column.

Fig. 10.

Mean + SE consumption rates by amphipods feeding on different sources of wrack in beach and surf zone habitats. WW, wet mass. Data extracted from Duarte et al. (2008, 2010b ) (Chile), Lastra et al. (2008) (Spain), Gomes Veloso et al. (2012) (Brazil), MacMillan & Quijón (2012) (Canada), Poore & Gallagher (2013) (Australia), Michaud et al. (2019) (USA) and Suárez‐Jiménez et al. (2017a ) (New Zealand).

The nutritional quality of wrack obviously plays a critical role in determining feeding choices in macroinvertebrate detritivores on beaches (Lastra et al., ²⁰⁰⁸; Duarte et al., ^{2010b , 2011, 2014}; Lowman et al., ²⁰²¹), similar to mesograzers (macrofauna grazing on living macrophytes) in subtidal ecosystems (Duffy & Hay, 1991). The protein content of algae is considered a key trait in determining their nutritional quality as food sources (Cruz‐Rivera & Hay, 2000), similar to the case for plant consumers in other systems (White, 1993), and appears selectively to influence feeding by detritivores on sandy beaches (e.g. Duarte et al., ^{2011, 2014}). Physical structure (shape and toughness) and the presence of chemical defences (secondary metabolites) have also been shown to influence food selection in these taxa (e.g. Pennings et al., ²⁰⁰⁰; Cruz‐Rivera & Hay, ²⁰⁰³; Duarte et al., ²⁰¹¹). As expected, growth rates and survival of detritivores are strongly linked to food preferences and nutritional quality (Table 4). For example, Lastra et al. (2008) showed that the preferred macroalgae species sustained the highest growth rates in M. corniculata, and this pattern also was observed for other amphipods (Duarte et al., ^{2010b , 2011}).

Nutritional quality and palatability are affected by environmental factors and decomposition. Moisture and solar radiation influence the nutritional quality and palatability of wrack (Lastra et al., ²⁰¹⁵; Rodil et al., ^2015c ). Even small changes in ambient ultraviolet (UV) radiation may modify the structural and nutritional status of wrack (Rodil et al., ^2015c ), while ocean acidification induces changes in algal palatability and protein concentration of macrophytes (Benítez et al., ²⁰¹⁶; Duarte et al., ²⁰¹⁶). However, the age of the detritus (related to the level of decomposition) and algal structure seem more important for food choice by detritivores in these systems (Pennings et al., ²⁰⁰⁰; Duarte et al., ^{2010b , 2011, 2014}). Decomposition affects the nutritional quality (Rothäusler et al., ²⁰⁰⁵; Rothäusler & Thiel, ²⁰⁰⁶) and palatability of macrophytes for consumers, but the interactions between detritus, microbes, and detritivores make it difficult to disentangle the interactive effects of microbes and detritivores in the decomposition process.

In terrestrial systems, microbes are known to improve the quality and appeal of detritus as a food source (Zimmer & Topp, 1997), but they can also be consumed directly by detritivores (Thompson, Abreu & Cavalli, 1999). For sandy beaches, we know that nematodes and many dipteran larvae feed directly on bacteria (Porri, Hill & McQuaid, 2011; Urban‐Malinga & Burska, ²⁰⁰⁹; Cullen et al., ¹⁹⁸⁷). Thus, their role is likely to be similar to that seen in saltmarsh systems, where meiofauna influence the decomposition of saltmarsh detritus and fluxes of nutrients via their influence on detritus‐associated microbes (e.g. Alkemade, Wielemaker & Hemminga, ¹⁹⁹²; Lillebø et al., ¹⁹⁹⁹), although their influence on decomposition can vary across taxa (De Mesel et al., ²⁰⁰³). Bacteria provide a more nutritious food source than the macrophytes themselves, as they have a lower C:N ratio (Fukuda et al., ¹⁹⁹⁸), and higher levels of lipids and polyunsaturated fatty acids (de Carvalho & Caramujo, ²⁰¹²). Indeed, growth and reproductive rates for the amphipod A. compressa were shown to be enhanced when bacteria were abundant on the kelp Ecklonia radiata (Singh et al., ²⁰²¹). Clearly, a more comprehensive understanding of these interactive effects is required.

(b). Nutrient fluxes and chemical transformation

Sandy beaches have long been considered ‘biogeochemical hotspots’, ‘reactors’, or ‘digestors’, reflecting their high levels of nutrient fluxes and transformations (e.g. Pearse, Humm & Wharton, ¹⁹⁴²). Wrack accumulations represent peak hotspots for biogeochemical processes as indicated by high metabolic rates that release high levels of CO₂ (Coupland, Duarte & Walker, 2007; Gómez et al., ²⁰¹⁸). Indeed, CO₂ production by wrack accumulations on beaches can surpass the most active soils on Earth (Gómez et al., ²⁰¹⁸). Similarly, wrack on beaches can be three times more metabolically active than subtidal seagrass or macroalgal beds (e.g. Coupland et al., ²⁰⁰⁷; Lastra, López & Rodil, ²⁰¹⁸; Liu et al., ²⁰¹⁹). By contrast, methane emissions from wrack appear to be negligible, at least under dry conditions (Liu et al., ²⁰¹⁹), despite wrack having a demonstrated potential for biogas production (Kaspersen et al., ²⁰¹⁶; Misson et al., ²⁰²⁰).

During consumption and decomposition of wrack in sandy‐beach systems, complex biomolecules are transformed to simpler organic forms and mineralised to inorganic nutrients (e.g. NH₄, NO₃ and PO₄) (Coupland et al., ²⁰⁰⁷; Dugan et al., ²⁰¹¹; Lowman et al., ²⁰¹⁹). As in other systems, heterotrophic bacteria likely play a major role in this conversion, but little work has been carried out in sandy‐beach systems. Rodil et al. (2019) is one of a few studies demonstrating a strong positive relationship between bacterial diversity and dissolved inorganic nitrogen in wrack deposits.

Since nitrogen is often the limiting nutrient in coastal marine ecosystems (Howarth & Marino, 2006), much of the focus of nutrient processes in beach ecosystems has been on N cycling, including standing stocks and fluxes (Mateo, 2010; Goodridge & Melack, ²⁰¹⁴). Nitrogen produced during mineralisation of wrack can enter several pathways: (1) incorporation into the food web, primarily via uptake by surf‐zone phytoplankton and possibly by benthic microalgae; (2) flushing back to the sea; (3) loss to the atmosphere via denitrification; (4) incorporation into dune vegetation; and/or (5) immobilisation in recalcitrant pools after burial in the long‐term sedimentary sink (Fig. 9A) (McLachlan & McGwynne, 1986; McLachlan & Romer, ¹⁹⁹⁰). Nitrogen fixation in bare sands, decaying wrack, or surf zones is probably a negligible component of the N cycle of these systems, while denitrification rates on exposed beaches are low and restricted to the few top centimetres of the sand (McLachlan & Romer, 1990). Denitrification only accounted for 2% of the N supplied by kelp or 5–10% of the overall N inputs to the beach (McLachlan & McGwynne, 1986). Both nitrification (oxidation of ammonia to nitrate to nitrite) and denitrification (reduction of nitrate to nitrogen gas) are processes mainly associated with the groundwater, likely reflecting N inputs from the land rather than from the wrack itself (e.g. Santoro, Boehm & Francis, ²⁰⁰⁶).

While the role of sandy beaches in the processing of organic matter and nutrient cycling has been repeatedly demonstrated, the question of whether beaches function as sources or sinks of nutrients remains largely unanswered, and there are differences among nutrient species and locations (e.g. Cockcroft & McLachlan, ¹⁹⁹³; Goodridge & Melack, ²⁰¹⁴; Prasad et al., ²⁰¹⁹). Supporting the ‘sink’ hypothesis, buried beach‐cast kelp (Fucus spp.) has been shown to enhance the growth of pioneer dune vegetation in the Netherlands (van Egmond et al., ²⁰¹⁹). The seagrass Posidonia oceanica provides an important nutrient source for adjacent dune vegetation in the Mediterranean Sea (Cardona & García, 2008; Jiménez et al., ²⁰¹⁷), where wrack is dominated by seagrass (Fig. 4). However, supporting the ‘source’ hypothesis, field studies show that kelp decaying on the beach can rapidly leach high concentrations of dissolved nutrients [dissolved inorganic nitrogen (DIN), dissolved organic nitrogen (DON), dissolved organic carbon (DOC), total dissolved nitrogen (TDN) and total dissolved phosphorus (TDP); see Fig. 9A] for plant uptake and export (Koop & Lucas, 1983; McGwynne et al., ¹⁹⁸⁸; Dugan et al., ²⁰¹¹). The concept of wrack‐covered beaches functioning as nutrient sources is supported by the correlation between wrack biomass and DIN concentration in surf‐zone water reported in California (Dugan et al., ²⁰¹¹) and by the increased productivity of local or invasive macroalgal species and phytoplankton blooms in waters adjacent to shorelines with increased wrack biomass load (Cockcroft & McLachlan, 1993). The dominance of a source or sink role may vary over time – leaching is likely to be greater in late summer and autumn when TDN fluxes from the beach are greater (Dugan et al., ²⁰¹¹), or during beach erosion episodes that release stored N in intertidal porewater to the ocean.

Our capacity to reject either hypothesis is limited by the lack of comprehensive studies examining nutrient dynamics in sandy‐beach ecosystems characterised by wrack input, and their ability to return nutrients to adjacent coastal ecosystems. For example, the outflow of DON may contribute to production in adjacent coastal systems as decomposing kelp is known to leach large amounts of DON (Hyndes, Lavery & Doropoulos, 2012). Exported DON is likely to be remineralised by highly abundant heterotrophic bacteria in the biofilm of living macrophytes (Egan et al., ²⁰¹³; Tarquinio et al., ²⁰¹⁹) in adjacent ecosystems (e.g. kelp forests or seagrass beds) and provide DIN to their hosts (e.g. Tarquinio et al., ²⁰¹⁸). In addition, nutrient budgets need to consider all sources of nutrients, including groundwater. In some regions, the discharge of groundwater derived from further inland can contribute 50–99% of the total submarine groundwater discharge (Urish & McKenna, 2004; Li et al., ²⁰¹¹) and supply nutrients to beach ecosystems (Santoro et al., ²⁰⁰⁶; Loveless & Oldham, ²⁰¹⁰).

(c). Scavenging

Carrion forms a highly nutritious and widespread food resource that is exploited by a rich diversity of scavengers (Table 3; Wilson & Wolkovich, ²⁰¹¹). Like detrital macrophytes, carrion supply is often highly variable in time and space, and is likely to be a nutritionally and energetically important resource in many beach food webs. There is generally no physical barrier to prevent animal carcasses from becoming stranded on beaches, or for scavengers to reach those carcasses, making them an easily accessible food source for consumers on ocean‐exposed shores (Schlacher et al., ^2013a ). Scavengers can be categorised into two functional groups: (1) facultative scavengers, which feed on live prey as predators and on dead animals as scavengers (e.g. foxes, birds of prey), and (2) obligate scavengers that rely exclusively on carrion as their sole source of nutrition (e.g. vultures; Ruxton, ²⁰⁰⁴). It is, however, increasingly recognised that most predators will readily scavenge, and many marine carnivores will also consume dead animal matter (Britton & Morton, 1994).

A wide variety of scavenging species is found on sandy beaches worldwide, encompassing both invertebrates (e.g. whelks, isopods, polychaetes, beetles, ghost crabs, dipterans, etc.), and vertebrates (e.g. reptiles, raptors, and carnivorous mammals) (Table 3). Some of the best‐studied invertebrate scavengers on sandy beaches are gastropods, particularly the genus Bullia in South Africa (Brown, 1961), which rapidly detect and consume a wide range of carrion (Brown & McLachlan, 1990). Crustaceans (e.g. isopods, amphipods, decapods) contain many taxa that consume carrion (Brown & McLachlan, 1990). For example, Tylos latrielle shifts its diet ontogenetically, with juveniles preferring carrion, whereas adults prefer seaweed (Kensley, 1974; Brown & McLachlan, ¹⁹⁹⁰). Hippid crabs (Hippa spp.) are important scavengers on tropical island beaches, catching and consuming Portuguese man of war jellyfish (Physalia physalis) washed onto the beach (e.g. Wenner, Ricard & Dugan, ¹⁹⁸⁷; Lastra et al., ²⁰¹⁶). Ghost crabs (Ocypode spp.) are the largest invertebrates on many sandy beaches, occupying a range of trophic levels formed by a diverse diet that regularly includes dead animal flesh (Lucrezi & Schlacher, 2014). In fact, when given a choice, ghost crabs strongly prefer carrion over algae and plants (Rae, Hyndes & Schlacher, 2019). The strandline of beaches also harbours a rich fauna of insect carrion feeders, including a high diversity of Coleoptera (Rozen, Engelmoer & Smiseth, 2008; Irmler, ²⁰¹²). However, despite insects being considered to be important scavengers of animal carcasses in other terrestrial systems (Redondo‐Gómez et al., ²⁰²²), little is known about their role as scavengers on sandy beaches (Blandford et al., ²⁰¹⁹).

Reptiles, birds and mammals are functionally important scavengers in many ecosystems, and their role in sandy‐beach ecosystems is becoming more apparent. In Australia, lace monitors (Varanus spp.) are widespread consumers of birds, fish, mammals, amphibians, eggs, and insects, and carrion can at times dominate their diet (Guarino, 2001). Indeed, lace monitors regularly consume fish carcasses (Schlacher et al., ^2013b ; Bingham et al., ²⁰¹⁸). Similarly, in the USA, cottonmouth snakes (Agkistrodon piscivorus conanti) traverse areas of vegetation at the beach's edge and feed on fish that have been discarded, or regurgitated, by colonial waterbirds (Lillywhite, Sheehy III & Zaidan III, 2008). Also, birds such as raptors can dominate carrion consumption in landscapes not strongly altered by urbanisation, followed by corvids and gulls (e.g. Huijbers et al., ²⁰¹⁶). Similarly, many mammals are attracted to, and feed on, stranded dead animals on sandy beaches, including hyenas (Parahyaena brunnea), black‐backed jackals (Canus mesomelas), coyotes, dingos, foxes (Vulpes vulpes), feral pigs, and even lions (Table 3). Strandings of cetaceans and dead seals are prominent examples of carrion providing intermittent bounties for carnivores (Behrendorff, Belonje & Allen, ^2018a ).

(6). Connectivity with adjacent ecosystems

The transfer of drift macrophytes and carrion from the sea to the beach can provide a significant energy subsidy to beach ecosystems. Such ‘spatial subsidies’ across highly permeable ecosystem boundaries increase secondary productivity and biodiversity in systems that contain otherwise low in situ primary productivity. In the preceding sections, we have highlighted that seagrasses, and particularly brown algae (mainly kelp), provide the main vectors for this subsidy (sensu Hyndes et al., ²⁰¹⁴) by supporting food webs as well as creating habitats for a diversity of microbes, invertebrates, fish, birds, reptiles and mammals in beach ecosystems (Fig. 9B,C). Biological and physical processes can recycle this imported material through consumption, fragmentation and decomposition, releasing dissolved nutrients back into the water or transferring nutrients through the food web and ultimately exporting those nutrients into other coastal ecosystems on land or in the sea (Fig. 9B,C). However, as discussed in Section II.5.b , the role of these systems as a source or sink of nutrients is equivocal, and the spatial extent and magnitude of any export of nutrients into other ecosystems is generally not well quantified for most settings.

Wrack deposits on the beach are often concentrated close to the waterline but regularly extend higher on the shore through the action of high tides, storm surges, and wind‐driven transport. Further inland transport of wrack‐ or carrion‐derived material depends on its direct or indirect consumption or transport by more mobile animals and their subsequent inland movement. For instance, Mellbrand et al. (2011) showed that seaweed flies feeding on wrack, and predators such as spiders feeding on detritivores in wrack, may move marine carbon many metres inland. However, this movement was not detectable beyond the primary dune, most likely due to the dilution of marine‐derived material as other land‐based food sources become more available, or the limited movement of those invertebrates feeding directly or indirectly on marine‐derived material, or a combination of both.

Larger and more mobile consumers are likely to provide a greater role as vectors for the inland transport of marine‐derived material. Since invertebrates associated with beach‐cast wrack provide an important food source for a variety of birds, such as plovers, swallows, and flycatchers (e.g. Dugan et al., ²⁰⁰³; Schlacher et al., ²⁰¹⁷), it logically follows that this marine‐derived material may enhance the breeding success and productivity of these birds. Such bottom‐up control of populations is also likely for other mobile animals that assimilate nutrients from invertebrates associated with beach‐cast wrack, such as lizards (Barrett et al., ²⁰⁰⁵; Spiller et al., ²⁰¹⁰), rodents, foxes, and bears (Ricci et al., ¹⁹⁹⁸; Stapp & Polis, ²⁰⁰³; Fox et al., ²⁰¹⁴; Davidson et al., ²⁰²¹; Page et al., ²⁰²¹) as well as feral pigs (M.A. Mateo & J.E. Dugan, personal observations). Also, deer can forage directly on wrack (Conradt, 2000), while domestic livestock, such as pigs, sheep and cattle, can be common on beaches in some regions (J.E. Dugan, C. Duarte & M.A. Mateo, personal observations), where they may graze on wrack. However, this mechanism has not been tested, and the magnitude of the subsidy for these mobile consumers will depend on: (1) the proportion of their food derived from the wrack; (2) the inland extent of their movement; (3) the extent to which they contribute to higher trophic levels; and (4) the release of nutrients in inland areas through their faeces or carcasses.

The consumption of carrion by scavengers on sandy beaches illustrates a broader functional role concerning the transfer of nutrients and organic matter across ecosystem boundaries, linking food webs at the landscape scale and creating meta‐population dynamics in the consumers. The mobility of birds makes them pivotal vectors for transferring nutrients and energy across ecotones, including consumption and transfer of carrion‐derived matter across surf–beach–dune landscapes (Fig. 9C; Whelan, Wenny & Marquis, ²⁰⁰⁸). Flying enables birds to search over large areas and detect patchy resources (i.e. carcases) in ways generally not possible for other consumers. The spatial extent of other mobile scavengers, such as rodents and foxes, is also likely to be substantial. One of the most seminal contributions to understanding the pivotal role of carrion subsidies on sandy shores comes from Rose & Polis (1998). They showed that coyote populations were 2–14 times higher on the coast compared to upland areas. In their study system, food supply that included carcasses washed ashore was greater on the coast.

Wrack stranded high on the shore may also influence both vegetation and the landscape structure of this dynamic zone. For example, seasonal pulses of wrack, primarily Sargassum, deposited by storms have been shown to enhance foliage growth of native shrubs and trees (Spiller et al., ²⁰¹⁰). Such wrack deposits may provide nutrients, propagules and a favourable microhabitat for terrestrial plants, particularly the salt‐tolerant pioneering species typical of coastal strand and foredune habitats (Dugan & Hubbard, 2010). The propagules of many dune plants can be transported and delivered with wrack and other drift material (e.g. Hesp, ²⁰⁰²). Similar to pioneering dune plant species, wrack deposits can act as ecosystem engineers that influence the geomorphology of shorelines by trapping wind‐blown sand to form hummocks and embryo dunes, at least in the short term (Nordstrom, Jackson & Koroty, ^2011a ). These features can then buffer beaches and dunes from erosion during storms.

The return of wrack‐derived nutrients back into coastal waters is likely to occur through two main processes. Firstly, the breakdown and decomposition of wrack release large quantities of dissolved nutrients (e.g. Dugan et al., ²⁰¹¹) and fine particulates (Soares et al., ¹⁹⁹⁷) that provide vectors for the return of nutrients back to the sea. However, the spatial extent of this return of nutrients is largely unquantified. It is likely that dissolved nutrients are rapidly diluted, but they may be utilised by intertidal and shallow subtidal primary producers, such as surf‐grasses and kelps (Dugan et al., ²⁰¹¹). However, it does appear that organic matter derived from detrital kelp along the beaches of South Africa supports populations of the subtidal filter‐feeding bivalve Donax serra (Soares et al., ¹⁹⁹⁷). Furthermore, while some studies have concluded that fine particulate kelp supports filter and suspension feeders in other coastal systems (e.g. Stuart, Field & Newell, ¹⁹⁸²; Duggins, Simenstad & Estes, ¹⁹⁸⁹), the evidence is equivocal (Miller & Page, 2012; Yorke et al., ²⁰¹³). Thus, the supply rate of particulate kelp from beaches may be important for suspension feeders in coasts across the globe, a predictive hypothesis that requires testing. Secondly, surf‐zone wrack can provide an important feeding and sheltering habitat for fish, particularly for juveniles. For example, some fish feed almost exclusively on wrack‐associated amphipods in the surf zones of south‐western Australia (Crawley et al., ²⁰⁰⁶). Through the ontogenetic movement of those juveniles towards their more offshore spawning grounds, they form vectors for the return of wrack‐derived nutrients into other marine ecosystems across coastal seascapes. However, the spatial extent and magnitude of this mechanism remain unknown. The degree to which the return of nutrients from wrack on the beach and in the surf‐zone to adjacent coastal systems, regardless of the vector, occurs is likely to differ according to: (1) different types and periodicities of subsidies (e.g. kelp, seagrass, carrion) supplied to sandy‐beach ecosystems; (2) the residence time and dominant processes (e.g. decomposition, grazing, transport) acting on the wrack in those systems; (3) the type of vectors (e.g. fish, bird, reptile, mammal, invertebrate); (4) physical processes (e.g. tide and storm surges) that erode beaches and export nutrients and wrack; and (5) the seascape and landscape contexts.

III. HUMAN USE, IMPACTS AND MANAGEMENT

(1). Beach grooming and harvesting

Once stranded on beaches, humans can impact wrack deposits by grooming and harvesting (Fig. 1B). Beach grooming intentionally removes macrophyte wrack, litter and other debris from beaches, usually through raking and sieving the sand using specialised heavy equipment often on beaches in populated or urban areas (Fig. 2G,H; Dugan et al., ²⁰⁰³; Fanini, Cantarino & Scapini, ²⁰⁰⁵; Dugan & Hubbard, ²⁰¹⁰). Grooming can be intensive and frequent (daily to weekly). It can have substantial adverse environmental effects, impacting the habitat quality, biodiversity, geomorphology, and functioning of beach ecosystems. Physical disturbance caused by grooming stymies dune formation and plant colonisation (Dugan & Hubbard, 2010). It also reduces the species richness, abundance, and biomass of wrack‐associated fauna, such as amphipods, isopods, beetles and flies (Dugan et al., ²⁰⁰³; Gilburn, ²⁰¹²; Schooler et al., ²⁰¹⁹). In southern California, impacts of widespread grooming have contributed to local and regional losses of populations of vulnerable wrack‐dependent taxa, such as isopod species occurring only on beaches (Hubbard et al., ²⁰¹⁴). Although meiofaunal communities can recover quickly (within 24 h) from a single, short‐term grooming event (Gheskiere et al., ²⁰⁰⁶), the consequences of repeated, regular beach cleaning may be significant for these intertidal communities. These direct impacts are likely to extend through the food web and to affect mobile predators, such as shorebirds that rely on wrack‐associated prey as food. Also, since wrack can supply nutrients to the adjacent sea (see Section II.5.b ), its removal from beaches may impact the donor and other neighbouring marine ecosystems by depriving them of a substantial part of their recycled production (e.g. Mateo et al., ²⁰⁰³).

The practice of beach grooming or raking is widespread globally and is often a component of well‐established management regimes for beaches used for tourism and recreation (Davenport & Davenport, 2006). For example, around 45% (>150 km) of sandy beaches are groomed at least seasonally in densely populated southern California, USA (Dugan et al., ²⁰⁰³), while >106,000 m³ of Posidonia wrack are estimated to be removed in 1 year from 44 beaches on the island of Sardinia (de Falco, Simeone & Baroli, ²⁰⁰⁸). Beach rating systems and ‘ecolabels’ often contain criteria that encourage wrack removal (Zielinski, Botero & Yanes, 2019). The Blue Flag Program is the largest of these ecolabels and includes over 4000 beaches in 47 countries (Boevers, 2008). Klein & Dodds (2018) note that this program emphasises tourism promotion over environmental protection and conservation of beach ecosystems. Indeed, many beaches rated under the Blue Flag scheme are very likely to have management regimes that remove wrack, thereby causing environmental harm (Mir‐Gual et al., ²⁰¹⁵; Gilburn, ²⁰¹²). Thus, the concept of ‘cleaning’ the beach ignores the ecological and conservation value of wrack for the beach ecosystems and broader seascapes/landscapes.

Macroalgal wrack is regularly harvested in some regions of the world, including Chile, North America, Ireland and Australia (Kirkman & Kendrick, 1997; Holden et al., ^2018a ). Native macroalgal species are harvested for various uses, including production of alginate and agar, cattle feed, soil fertilizers and conditioners, and feed for abalone hatcheries; seagrasses are harvested for insulation and soil fertilizer (Kirkman & Kendrick, 1997). Methods and equipment used in harvesting macrophytes from beaches vary from artisanal hand picking (a few tonnes y⁻¹) to commercial harvesting using heavy equipment, such as bulldozers (hundreds to thousands of tonnes y⁻¹, Holden et al., ^2018a ). Harvesting can remove large proportions of beach wrack. For example, harvests of drift bull kelp (D. potatorum) on Australia's King Island (average harvests of 2500 tonnes year⁻¹; Holden et al., ^2018a ) account for approximately 50% of the kelp deposited annually on those beaches (Kirkman & Kendrick, 1997). Non‐native species may also be harvested. For example, quotas of 900–1500 tonnes year⁻¹ for the invasive red alga (Mazzaella japonica) accounts for <16% of the available biomass along a shoreline section of Canada's Vancouver Island (Holden et al., ^2018a ). Overall, the harvest methods and their associated impacts, and the relative magnitude of wrack harvest are not well documented (see Kirkman & Kendrick, ¹⁹⁹⁷; Holden et al., ^2018a ), suggesting this is a significant gap in the information needed to manage wrack harvest on sandy beaches. A report on impacts of wrack harvest in Ireland recommended using non‐mechanical harvest methods, prohibiting removal of sediment or substrate and minimising disturbance to surrounding environments by commercial wrack harvest (McLaughlin et al., 2006 cited in Holden et al., ^2018a ).

(2). Shoreline armouring and coastal development

Many shorelines, including those formed by sandy beaches, have been profoundly altered through the construction of coastal armouring structures and extensive shoreline development (Charlier, Chaineux & Morcos, 2005) (Fig. 1B). Coastal development (e.g. ports, marinas, resorts) can cover, or remove sandy beaches, thereby profoundly impacting the habitat value and function of beach ecosystems (see Dugan et al., ²⁰¹²; Hubbard et al., ²⁰¹⁴; Jaramillo et al., ²⁰²¹). Shoreline armouring, such as seawalls and revetments, is a common practice around the world to protect coastal development and infrastructure from erosion and coastal hazards (Airoldi et al., ²⁰⁰⁵; Dugan et al., ²⁰¹²), and has received the most attention with regard to environmental impacts (Dugan et al., ^{2008, 2012, 2018}). Seawalls have been shown to reduce the overall width of sandy beaches, with the most significant impacts in the upper part of the intertidal zone (Dugan et al., ²⁰⁰⁸; Jaramillo et al., ²⁰²¹). The resulting losses in the upper beach zone, suitable for the retention of wrack, disrupt the trophic subsidy provided by donor ecosystems and significantly reduce the diversity and abundance of wrack‐associated invertebrates (e.g. Dugan & Hubbard, ²⁰⁰⁶; Dugan et al., ²⁰⁰⁸; Jaramillo, Dugan & Hubbard, ²⁰¹²; Jaramillo et al., ²⁰²¹; Dethier et al., ²⁰¹⁶). Importantly, these impacts extend up the food web, where armouring can significantly reduce the use of beaches by shorebirds and seabirds (Dugan & Hubbard, 2006; Dugan et al., ²⁰⁰⁸). Similar impacts may also extend to surf zone fishes, but more research is needed to evaluate this. Other forms of armouring, such as groynes and detached breakwaters, may increase or decrease the standing stock of wrack on beaches and in surf zones, depending on the scale, orientation and design of those structures (e.g. Airoldi et al., ²⁰⁰⁵; Dugan et al., ²⁰¹²; Martin et al., ²⁰⁰⁵).

Although small‐scale beach‐restoration efforts suggest that biodiversity and ecosystem functions of beaches can be restored through the removal of armouring structures (Lee et al., ²⁰¹⁸), increased armouring of shorelines is the expected global trend as coastal hazards increase with climate change. As sea level rises, the effects of coastal squeeze exerted by existing armouring structures on beaches are also expected to increase as structures interact more frequently with waves and tides (Dugan et al., ²⁰¹⁸). Robust evaluations of the ecosystem services provided by intact dune–beach–surf‐zone systems, and the protocols for quantifying the often dynamic indicators of those services, are generally not available for proposed armouring projects (King et al., ²⁰¹⁸). As a result, other than recreation and storm buffering, values of the ecological functions and services of beaches are rarely applied to assess the impacts of these projects. New approaches to evaluating beaches as ecosystems, based on restoration or replacement costs, are urgently needed for mitigating the impacts of shoreline armouring, especially as pressures from sea level rise and coastal squeeze intensify (e.g. King et al., ²⁰¹⁸).

(3). Invasive species

Numerous species of non‐native algae have been introduced to coastal waters, either via human activities (e.g. shipping) or as a result of species range shifts due to ocean warming (Hurd et al., ²⁰¹⁴). For example, the green alga Codium fragile and the kelp Undaria pinnatifida, both originally from the north Pacific Ocean, now have near‐global distributions (Hurd et al., ²⁰¹⁴). The ecological impacts of invasive species on subtidal habitats are equivocal [Suárez‐Jiménez et al. (2017b ) and references therein], but we know far less about the impacts of these invasive species as beach‐cast wrack (see Quijon, Tummon & Duarte, ²⁰¹⁷) even though they can be commonly cast ashore (e.g. Piriz, Eyras & Rostagno, ²⁰⁰³; Rodil et al., ²⁰⁰⁸). Certainly, the composition, quality and perhaps the biomass, of the beach‐cast material could be affected by the displacement of native reef algae with invasive species, thereby altering wrack inputs and food value. However, this effect will depend on the life‐cycle patterns and buoyancy of the invasive species, which alter the ability of algae to be transported to the beach and the timing of the detrital input. For example, U. pinnatifida is now a major component of subtidal reefs in New Zealand, but contributes little to beach wrack, which likely reflects its lack of buoyant structures (e.g. pneumatocysts or buoyant blades) which are present in the native M. pyrifera and D. antarctica that are common in beach wrack in the region (Suárez‐Jiménez et al., ^2017a ). However, the invasive brown alga Sargassum horneri is an annual species that can outcompete native kelp species and can seasonally dominate the wrack deposited on some Californian beaches (Marks, Reed & Holbrook, 2020). Beach‐cast of its congener S. muticum appears either to alter or to have no effect on invertebrate composition and densities compared to native macroalgal species (Rodil et al., ²⁰⁰⁸; Cacabelos et al., ²⁰¹⁰). While S. muticum appears to contribute to the food web on some beaches (Olabarria et al., ²⁰⁰⁹), the magnitude of an invasive species' influence on beach food webs will depend on the ability of native consumers to utilise the new resource and its nutritional quality and palatability. For example, while the invasive U. pinnatifida has similar nutrient characteristics to native species in New Zealand, the consumption of this invasive species by the amphipod B. quoyana appears to be hindered by its physical properties such as toughness (Suárez‐Jiménez et al., ^2017b ). With few studies focusing on the impacts of invasive species on the food webs and ecosystem structure of beach systems, it is presently not possible to make generalisations regarding this potential impact.

(4). Climate change impacts to recipient and donor ecosystems

Arguably, the greatest threat to sandy‐beach ecosystems is the effect of climate change, especially via sea‐level rise, more intense and frequent storms, and ocean warming (Fig. 1C). Sea‐level rise and storm frequency are projected to intensify beach loss, through increased erosion rates, inundation, and coastal squeeze (Vitousek et al., ²⁰¹⁷). These processes will significantly impact beach ecosystems and their functioning, including the loss of habitat available for wrack deposition and retention and the survival of associated biota (see Schlacher et al., ²⁰⁰⁸; Myers et al., ²⁰¹⁹). The impacts of sea‐level rise on beaches are projected to manifest earliest in the upper zones of beaches where wrack deposits support biodiversity and coastal food webs (Dugan et al., ^{2012, 2013}). These vulnerable zones have already been widely impacted by coastal armouring and development, sediment starvation, and beach management practices (Dugan et al., ^{2008, 2013}; Myers et al., ²⁰¹⁹). A case study of several Californian beaches projected that a 0.5 m increase in sea level would result in a 75% loss of the upper beach zone where wrack accumulates and is processed, eliminating around 50% of intertidal biodiversity and numerous vital ecosystem functions (Myers et al., ²⁰¹⁹). Furthermore, recent ENSO events that cause a combination of warming, storms and sea‐level rise (i.e. are a proxy for climate change), combined with a multi‐year drought, have caused historically high levels of beach erosion and loss, with beaches retreating beyond previous extremes along the southern portion of the north‐eastern Pacific coast (Barnard et al., ²⁰¹⁷). While beach nourishment may be seen as a mitigation measure against this impact (de Schipper et al., ²⁰²¹), burial of wrack and carrion, and changes to the deposition dynamics of the shoreline are likely to impact ecological processes in sandy‐beach ecosystems.

Ocean warming and related marine heatwave events will also strongly affect key donor ecosystems that supply organic material in the form of drift macrophytes to beaches and surf zones (Fig. 1C). Important donor ecosystems, such as kelp forests, are already being affected by ocean warming worldwide (Wernberg et al., ²⁰¹⁹). Heatwave events have led to local extinctions of the kelp E. radiata (Wernberg et al., ²⁰¹⁶) and severe reductions in the biomass of seagrass meadows (Arias‐Ortiz et al., ²⁰¹⁸) along the west coast of Australia. Climate change and other human impacts have already led to declines in seagrasses and kelp, with declines of 7 and 2% year⁻¹ in recent years, respectively (Waycott et al., ²⁰⁰⁹; Krumhansl et al., ²⁰¹⁶). Furthermore, increasing sea temperatures have either led to, or are predicted to, extend the ranges of tropical macrophytes and consumers into higher latitudes (temperate regions) (Vergés et al., ²⁰¹⁴; Hyndes et al., ²⁰¹⁶). Ultimately, these climate‐driven effects will limit the supply, or alter the form, of macrophytes and alter the ecosystem functioning of sandy‐beach ecosystems in those regions. For example, ocean warming is projected to reduce the body size and fecundity of intertidal biota such as talitrid amphipods that are key consumers of wrack (Jaramillo et al., ²⁰¹⁷), which is likely to alter the rates of wrack processing by detritivores on beaches. Another global impact, ocean acidification, is predicted to alter the nutritional quality of algae, possibly translating to altered trophic dynamics of consumers processing this wrack material (Benítez et al., 2016; Duarte et al., ²⁰¹⁶).

To understand these processes, we need studies conducted at larger spatial and temporal scales, involving the dynamics of wrack inputs and connectivity of beaches to source ecosystems, presumably using data from remote sensing and other synoptic resources. For example, wrack on tropical beaches have essentially been ignored, yet they are experiencing increased deposition of Sargassum (Maurer, de Neef & Stapleton, 2015; Schell, Goodwin & Siuda, ²⁰¹⁵), and other macroalgal inputs as coral reefs transition to turf macroalgae (Sura et al., ²⁰¹⁹). Quantifying the biomass and composition of wrack will play a critical role in determining the shifts in supply over these timescales, and particularly in relation to the impact of the shifting state of donor systems due to climate change and invasive species.

IV. FUTURE DIRECTIONS

Some of the priority research directions emerging from our review include, but are not limited to, five broad themes:

1. Quantifying the rates of processing of wrack inputs by the entire suite of organisms from microbes to higher order consumers and how their interactions influence wrack persistence and turnover rates.

2. Developing comprehensive nutrient budgets in beach ecosystems, including the export of nutrients to inland and other coastal systems, which will ultimately develop a predictive understanding of the magnitude of the role of wrack in seascape connectivity.

3. Quantifying the rates of input and turnover of animal carcasses over a broad range of species and regions that encompass the diversity of vertebrate carnivores/scavengers.

4. Examining the role of microbes and meiofauna in driving secondary production and food webs in the beach ecosystem.

5. Determining the magnitude of flows, and their spatial footprint, for nutrients originating from wrack and carrion through coastal food webs over a range of different seascape settings, including the movement of predators (e.g. fish, birds, mammals) and scavengers (e.g. birds, mammals) ultimately linked to marine energy stranded on sandy beaches.

6. Evaluating the effects of human impacts, including climate change and ocean acidification, through shifts in the quantity and quality of wrack inputs affecting ecosystem functioning of sandy beach systems.

V. CONCLUSIONS

(1) Sandy beaches are iconic features of our coastlines, globally prized for providing valuable ecosystem services such as coastal protection, support of wildlife, fisheries, unique biodiversity, and the creation of tourism and recreation opportunities (Barbier et al., ²⁰¹¹). We show that large quantities of detrital macrophytes can flow into and be processed in this shoreline ecotone worldwide, albeit highly variable in amount and composition. Supply and retention of wrack are influenced by the oceanographic processes that transport it, the geomorphology, orientation and landscape context of beaches, and the condition, life history and morphological characteristics of species that produce the wrack in the marine donor ecosystems.

(2) Wrack deposits on beaches often create hotspots of microbial metabolism, secondary productivity, biodiversity and nutrient remineralisation. Decomposing wrack produces dissolved organic and inorganic nutrients that can return to coastal waters. However, the magnitude and spatial extent of the return of nutrients to other coastal ecosystems is largely unknown. Furthermore, there is a clear need to understand the role of microbes in driving secondary production and biodiversity of macrofauna, and to a lesser extent meiofauna.

(3) Seagrass and kelp are typically the main components of wrack. Many mobile invertebrates of the intertidal zone, particularly amphipods, prefer kelp as a food source, making kelp the most energetically important carbon source in food webs compared to seagrasses.

(4) The invertebrate consumers of wrack and associated microbes, especially amphipods and dipterans, channel energy up the food chain to predatory invertebrates (beetles), fish, birds and occasionally mammals. Large and mobile consumers can become vectors that transport wrack‐derived nutrients across habitat boundaries and more broadly across coastal seascapes. The magnitude and spatial scale of such transfers is currently not quantified.

(5) Increasing wrack biomass in surf zones increases fish diversity and abundance, although diversity declines at very high wrack biomass. Surf‐zone wrack appears to play an important nursery role for some fish species despite its transient nature, but it remains unclear how important this habitat is compared to other potential nursery habitats, and their relative importance to the spawning biomass of those species.

(6) The energetic role of beach‐cast marine carrion in sandy‐beach ecosystems is likely widely underestimated, as it can be removed very rapidly by highly mobile scavengers. These scavengers are predicted to be pivotal biological vectors that transfer marine productivity inland, thereby linking marine and terrestrial ecosystems. However, we currently have very little data on the quantity of carrion on beaches, and the magnitude, frequency, geographic distribution, and spatial scales of these sea‐land couplings performed by scavengers.

(7) Wrack is frequently removed to ‘improve’ the aesthetic value of beaches used intensively for recreation. However, beach grooming also removes the multiple ecological values created by wrack (food, habitat, dune formation). Similarly, shoreline structures such as seawalls impact the supply and retention of wrack and its ecological value. The juxtaposition of conserving wrack whilst clearing wrack or protecting shorelines provides a major management challenge. Consequently, conservation efforts should consider improving societal awareness on the ecological importance of stranded organic matter, which is currently viewed in the same light as anthropogenic debris (rubbish, litter).

(8) Rising ocean temperatures and sea levels due to climate change will alter the supply, deposition and retention of wrack and carrion in sandy‐beach ecosystems and alter the processing of organic material in these systems. Losses of donor material and the ability of beaches to retain material provide the greatest and most widespread threat to the ecosystem functions and services provided by subsidies of wrack and carrion in sandy‐beach ecosystems. This impact will be more pronounced in temperate regions around the globe where kelp subsidies are a prominent component of wrack. To understand the potential level of this impact, we need more data on the quantities and forms of detritus exported from donor systems, the distances that these forms of detritus travel to recipient systems, the retention and fate of detritus in those systems, and the likely loss of ecosystem functions related to reduced detritus in sandy beaches and connected ecosystems.

Supporting information

Table S1. All publications identified in our search on various aspects of wrack or carrion in sandy‐beach ecosystems, and their locations, habitats and research themes.

Table S2. References used to produce Fig. 6.

Table S3. References used to produce Fig. 7, including details of the country (region), habitat sampled and type of material in wrack (mixed = seagrass and macroalgae).

Table S4. References used to produce Fig. 8.