Victim of changes? Marine macroalgae in a changing world

Abstract

Background

Marine macroalgae (‘seaweeds’) are a diverse and globally distributed group of photosynthetic organisms that together generate considerable primary productivity, provide an array of different habitats for other organisms, and contribute many important ecosystem functions and services. As a result of continued anthropogenic stress on marine systems, many macroalgal species and habitats face an uncertain future, risking their vital contribution to global productivity and ecosystem service provision.

Scope

After briefly considering the remarkable taxonomy and ecological distribution of marine macroalgae, we review how the threats posed by a combination of anthropogenically induced stressors affect seaweed species and communities. From there we highlight five critical avenues for further research to explore (long-term monitoring, use of functional traits, focus on early ontogeny, biotic interactions and impact of marine litter on coastal vegetation).

Conclusions

Although there are considerable parallels with terrestrial vascular plant responses to the many threats posed by anthropogenic stressors, we note that the impacts of some (e.g. habitat loss) are much less keenly felt in the oceans than on land. Nevertheless, and in common with terrestrial plant communities, the impact of climate change will inevitably be the most pernicious threat to the future persistence of seaweed species, communities and service provision. While understanding macroalgal responses to simultaneous environmental stressors is inevitably a complex exercise, our attempt to highlight synergies with terrestrial systems, and provide five future research priorities to elucidate some of the important trends and mechanisms of response, may yet offer some small contribution to this goal.

INTRODUCTION

Whether they are small, rock-like turfs encrusting intertidal rocks or tide pools, or huge subtidal kelps washed up onto an exposed beach, marine macroalgae (‘seaweeds’) are some of the most obvious, familiar and perhaps totemic organisms of the marine coastal shelf environment. Only recently, however, have we begun to appreciate the remarkable evolutionary history and biology of ‘red’, ‘green’ and ‘brown’ seaweeds and perhaps, more importantly, the array of essential ecosystem services they contribute to the global biosphere. Studies on ‘seaweed forests’, for example, reveal their wide-ranging distribution (25 % of the world’s coastlines), potential individual size (>30 m length and 42 kg biomass), exceptional net primary productivity (NPP; 536 gC m⁻² year⁻¹), and vital contribution to marine habitat and ecosystem service provision (ESP; Wernberg et al., 2019; Pessarrodona et al., 2022; Eger et al., 2023). Like many terrestrial habitats, however, the impact of human activity on kelp forests has multiplied over recent decades leading to an estimated global decline in abundance of around 2 % per year (Wernberg et al., 2019). Similarly, many red, green and brown algal bed and turf communities have been harmed by environmental changes associated with human activity (Pessarrodona et al., 2018, 2022; Krause-Jensen et al., 2020; Piñeiro-Corbeira et al., 2024).

The realization that, globally, macroalgae face increasing environmental stress provides the impetus for understanding how pollution, overexploitation, invasive species, habitat modification and anthropogenic climate change (ACC) affect coastal vegetation and the critical habitats and ecosystem services they support. With this in mind, and following a short summary of marine macroalgal biology, we synthesize how and why these incredibly diverse and important primary producers contribute to global ESP and explore some of the major threats they, and the services they deliver, face. Our review takes advantage of contributions from within this Special Issue to contextualize these threats, but also identifies opportunities for macroalgal research. In so doing, our aim is not only to highlight how important marine macroalgae are for the global system, but also to underline the many parallels between coastal marine and terrestrial primary producer species and communities, and through this synthesis, provide an enhanced appreciation of seaweed biology, ecology and conservation in a rapidly changing world.

MARINE ‘MACROALGAE’ – SIMPLE ORGANISMS WITH A COMPLEX TAXONOMY

In common with the more colloquial ‘seaweed’, the term ‘macroalgae’ lacks a formal definition, but broadly refers to marine photosynthetic organisms visible to the naked eye. While the bluegreen algae (Cyanobacteria) and some other algal groups can thus be included under these umbrella terms (Lobban and Harrison, 1994), we follow the common practice of considering only multicellular red (Rhodophyta), green (Chlorophyta) and brown (Phaeophyta) algae species. From here, however, things get considerably more complicated, since the common red and green, and brown algae actually belong to different taxonomic ‘supergroups’: the Archaeplastida (‘Plantae’ sensu lato) and the SAR (Stramenopiles, Alveolates and Rhizarians). Members of the Chlorophyta (specifically the Ulvophyceae, which includes common intertidal species such as Ulva, Cladophora and Codium), are a separate class within the clade Viridiplantae (Bowles et al., 2022). Although also within the Archaeplastida, the Rhodophyta belong to an entirely different group (Bowles et al., 2022) and appear red/purple because the pigments phycoerythrin and phycocyanin mask green chlorophyll. Most of the common and widely distributed intertidal and subtidal Rhodophytes (Gelidium, Mastocarpus, Chondrus and the calcifying or ‘coralline’ reds) belong to the class Florideophyceae. By contrast, the ‘brown algae’ (class Phaeophyceae), a diverse assemblage that includes kelps (Laminariales) and wracks (Fucales), are in the Stramenopile (also called Heterokont) clade of the highly variable SAR supergroup (Bringloe et al., 2020). They appear brown because fucoxanthin masks the green chlorophyll.

Given their complex taxonomic affiliations, it is perhaps unsurprising that the traditional red, green and brown seaweed groups present a complex evolutionary history. A full exploration of this topic is beyond the scope of this review (see Bringloe et al., 2020; Bowles et al., 2022). Nonetheless, there is some consensus that endosymbiotic engulfment of a green or red member of the ‘cyanobacteria’ to form chloroplasts or rhodoplasts, probably around a billion years ago, facilitated the evolution of the Chlorophyta and Rhodophyta respectively (Bowles et al., 2022). The Phaeophyceae are distinguished from Archaeplastid macroalgae by their different cell membrane structure and possession of plastids, putatively formed when ancestral photosynthetic heterokonts endosymbiotically engulfed a red alga. It remains unclear exactly when the Phaeophyte lineage emerged, but estimates using molecular phylogenies indicate this happened in the Late Palaeozoic (<300 Mya), with significant diversification 200 million years later during the Cretaceous (Bringloe et al., 2020). Despite their later emergence and diversification, the Phaeophyta now dominate the macroalgal assemblages of temperate and polar shelf seas and include in their number some of the largest and fastest growing autotrophs on the planet.

DISTRIBUTION AND ECOLOGY

Notwithstanding the Fucoid- and Laminariale-domination of the world’s temperate oceans (one quarter of all coastlines; Wernberg et al., 2019), macroalgae of all kinds have a global distribution and critical ecological role as primary producers in shallow coastal seas wherever they establish. Although, like the other seaweed groups, red algae are found from Pole to Equator, they tend to be most numerous and diverse in tropical regions (van den Hoek, 1984). As with many marine and intertidal organisms, the current biogeographical distribution of the 4000 or so Rhodophyta species is linked to changes in temperature and shifts in oceanic currents associated with continental movements, opening and closing of channels between major waterbodies, and more recent waxing and waning of polar ice sheets during the Pleistocene (van den Hoek, 1984; Aguirre et al., 2000). Red seaweeds are more tolerant of low light conditions than brown or green species and are, consequently, often associated with deeper waters and or below the canopies of larger kelps and fucoids. In such habitats, relatively large, canopy-forming species exist, including those like Gelidium and Chondrus (Fig. 1D) with a significant economic value to the food industry. By virtue of their relative tolerance of desiccation, high salinity and grazing (Paine and Vadas, 1969; Steneck, 1985; Firth et al., 2023), red crustose and coralline species often dominate intertidal rock pools and inter- and subtidal habitats, particularly at lower latitudes (Fig. 1C; Steneck, 1986).

Fig. 1.

A montage illustrating the global taxonomic and morphological diversity of marine macroalgae. (A) Hormosira banksia – Fucales (Phaeophyta) – Hobart, Tasmania; (B) Nereocystis luetkeana – Laminariales (Phaeophyta) – Bandon Bay, Oregon, USA; (C) Corallina officinalis – Corallinales (Rhodophyta) – Sennen Cove, UK; (D) Chondrus crispus – Gigartinales (Rhodophyta) – Sennen Cove, UK; (E) Ecklonia maxima – Laminariales (Phaeophyta) – Cape Town, South Africa; (F) Ascophyllum nodosum – Fucales (Phaeophyta) – Wembury, UK (algae growing on concrete defence blocks); (G) Halimeda spp. – Bryopsidales (Chlorophyta) – East Kalimantan, Indonesia (algae growing on mangrove roots); (H) Ulva latuca – Ulvales (Chlorophyta) Plymouth Breakwater, UK. All photos by the authors.

Green algae are highly tolerant of different environments and are thus found in a variety of brackish and highly saline systems, ranging from the subtidal right up into the intertidal splash zone. They are both morphologically diverse and globally distributed, although their relatively small stature (typically <200 mm) often makes the Chlorophyta less obvious than the Phaeophyta with which they commonly share habitat space. Species in the genus Ulva (Fig. 1H) are typical of this widespread global habitat distribution, although many are especially common early-successional macroalgae, occupying, most classically, the highly disturbed boulder habitats described by Sousa (1979).

With over 2000 highly diverse species, including those attaining the largest stature of all marine primary producers, the Phaeophyta not only dominate temperate and polar shallow marine habitats (Dayton, 1985; Steinberg, 1989), but also the attention of the scientific community; 78 % of the papers in this Special Issue, for example, focus on just ten kelp genera. To some extent perhaps, the strong research bias on the Phaeophyta reflects their importance in dictating the habitat structure and productivity of coastal environments. The kelps (Laminariales), for example (Fig. 1B, E), are not only present in over 40 % of the Earth’s marine ecoregions but are amongst the most productive primary producers anywhere on the planet (Krumhansl et al., 2016). The NE Pacific bull kelp (Nereocystis luetkeana) is perhaps the best known simply for its ability to grow to excess of 36 m in a single season (Abbott and Hollenberg, 1976). Nonetheless, like other species within its lineage, Nereocystis also has considerable importance as a ‘foundation’ species in supporting higher trophic levels and represents a biogenic structure that provides structural habitat for a host of other organisms (Dayton, 1985; Pessarrodona et al., 2022; see Teagle et al., 2017 for a review on habitat provisioning role for all kelp taxa). The (generally) subtidal kelps also contribute to a distinct pattern of distribution where they are progressively replaced in the intertidal by various Fucoid species (Fig. 1A, F). Although this zonation is by no means unique to the Phaeophyta, the stature and productivity of the Fucoids, along with their moderating effect on the distribution of green and red species in temperate and polar regions, has traditionally seen this group dominate our understanding of intertidal rocky shore macroalgal community ecology (Raffaelli and Hawkins, 1999; Hawkins et al., 2019).

Although green and red macroalgae are generally considerably smaller than their brown counterparts, turf-forming Chlorophyta, along with the foliose and crustose Rhodophytes, nonetheless play critical ecological roles, especially in those habitats from which brown species are ecophysiologically excluded (Raffaelli and Hawkins, 1999). In terms of NPP, for example, seasonal pelagic Ulva blooms (average 394 gC m⁻² year⁻¹) and mixed-assemblage algal turf (321 gC m⁻² year⁻¹) habitats were ranked second and third respectively, behind ‘forest-forming’ kelp and fucoid species (536 gC m⁻² year⁻¹) (Pessarrodona et al., 2022). Even the apparently diminutive coralline and rhodolithic (coral-like, benthic crustose algae, Fig. 1) Rhodophyte communities accounted for over 200 gC m⁻² year⁻¹ NPP.

The ecological impact of this extremely high NPP extends well beyond the marine environment. Hyndes et al. (2022), for example, reviewed how accumulation of detritus from dislodged seagrass and macroalgae (‘wrack’) washed onto sandy beaches can shape the structure and functioning of often otherwise low-productivity sandy littoral habitats. The amount of wrack can be enormous, running into several thousands of kilograms per metre of coastline each year (see Colombini and Chelazzi, 2003). Importantly, and unlike terrestrial plants, detached and even fragmented phaeophyte wrack can remain photosynthetically viable and extend its structural integrity and availability as a food source for months after detachment from the substrate (Frontier et al., 2021; Wright et al., 2024). While Hyndes et al. (2022) noted how wrack supply and retention are influenced by local oceanographic processes and the geomorphology of recipient beaches, the age, condition, life history, and physiological and morphological characteristics of the macrophytes that constitute wrack can also be important (i.e. kelps, again, play a key trophic role). In the sandy littoral zone, wrack is often the only abundant, even preferred, food source for mobile invertebrates (e.g. amphipods and Coelopid seaweed flies) that are themselves are a critical resource for other predatory invertebrates and birds that transport marine productivity inland (Colombini and Chelazzi, 2003; Hyndes et al., 2022). A similar transfer of marine-derived productivity into terrestrial systems occurs when mammals, including ungulates (Conradt, 2000; Hansen and Aanes, 2012), and even foxes (Hersteinsson and Macdonald, 1996), forage on fresh wrack or in situ intertidal fucoids. Evidently, what happens in, and to, the marine environment does not stay there.

ECOSYSTEM SERVICES

The term ‘ecosystem service’ refers to any benefit provided by ecosystems to humans which contribute to making human life both possible and worth living (MEA, 2005). At the most basic level, macroalgae play a critical role in primary production (Bruno et al., 2005; Miller et al., 2011), providing the oxygen essential for all respiring organisms. Even when detached, kelp ‘detritus’ can make significant contributions to NPP and nutrient cycling (de Bettignies et al., 2020; Brunet et al., 2021; Filbee-Dexter et al., 2022). In a study comparing Laminaria ochroleuca and L. hyperborea, Frontier et al. (2021) found that whilst decaying, the photosynthetic performance of the two species responded differently to simulated depths, but fragments of both species continued to produce oxygen for up to 56 d and sustain positive NPP. In the context of climate change, decline and miniaturization of kelp forests, shifts in their species composition and increased lability of kelp detritus are likely to have far-reaching impacts on the marine carbon cycle. This is demonstrated in this Special Issue by Wright et al. (2024). Working on detrital photosynthesis in the Australian kelp Ecklonia radiata, they show that temperatures in excess of 15 °C reduce the potential for detritus to remain photosynthetically viable, and thus render it more prone to decomposition. While Wright et al. (2024) suggest physiological integrity at cooler depths close to potential carbon sinks, the breakdown of detrital photosynthesis at elevated temperatures has the potential to reduce carbon sequestration.

The recent review by Ross et al. (2023) comprehensively summarizes the range of climate mitigation services by macroalgae, and the key problems and research issues associated with understanding and facilitating these services. These potential climate benefits of macroalgae can be divided into direct carbon sequestration (currently estimated at 4.9 megatons of carbon per year; Eger et al., 2023), offsetting of CO₂ emission using products derived from kelp and seaweeds, and reduction of methane emissions by livestock. Rather than repeat Ross et al. (2023), we synthesize some of the key contributions and issues they highlighted in Table 1.

Table 1.

A summary of the potential ecosystem service role of marine macroalgae for climate change mitigation through carbon sequestration, CO₂ offsetting and methane reduction in agriculture. See Ross et al. (2023) for full details.

Mitigation service	How it is done	Description	Critical issues
Carbon sequestration	Protect/restore natural habitats	Capacity for fast-growing species such as kelps and Sargassum to sequester CO2 through primary production and release dissolved and particulate organic matter into, and beyond, their habitat (‘lateral export’)	Limited quantitative data on actual global primary productivity or amount of lateral export of organic materials
			Need to better understand how ACC and other stressors affect macroalgae distribution and primary productivity
			Enhanced focus on restoration and management techniques
	Expand nearshore macroalgae aquaculture	Carbon capture into sediments beneath, and possible ‘lateral export’ from, seaweed farms	Limited quantitative data on lateral export of organic materials to sink sites
			Impact of seaweed aquaculture on nearby natural carbon sinks
			Expand beyond Asian focus (97 % of all seaweed aquaculture)
			Ramifications of using non-native or genetically modified seaweeds on local natural populations and ecosystems
	Sink macroalgae into deep-sea storage	Transport (or grow in situ on surface) coastal seaweeds and deposit the assimilated carbon into deep-sea environment	Limited evidence that technique sequesters enough carbon to justify carbon inputs or mitigate later release
			Ethics of using human/animal food stuffs to sink to seabed
			Incomplete understanding of potential impact on nutrient-deprived deep-sea ecosystems
CO2 offsetting	Algae products for emission abatement	Aquaculture products have a lower carbon footprint than the products they replace (e.g. seaweed-based bioplastics and biodiesel)	Existing products release CO2 and CH4 back to atmosphere on decomposition after use
			Large areas needed for cultivation to compete with existing products and minimize carbon footprint
			Limited quantitative data on carbon footprint of aquaculture activities
Methane reduction	Algal feedstuffs mitigate CH4 emission from ruminant livestock	With relatively high bromoform content, Rhodophytes, specifically Asparagopsis sp., greatly reduce enteric CH4 emissions from cattle	Bromoform may have toxicological effects on food items and the environment
			Asparagopsis sp. must be fed daily to have any effect
			Need to screen species other than Asparagopsis sp. for methane mitigation potential

Typically for marine macroalgae, the literature on ESP and valuation tends to focus on kelp. Eger et al. (2023), for example, estimated the total global annual value of all ESPs provided by kelp forests to average $500 billion, considering services such as fisheries production, nutrient cycling and climate mitigation. Even here, the estimate was based on just six kelp genera (Ecklonia, Laminaria, Lessonia, Macrocystis, Nereocystis and Saccharina). It is becoming clear however that many different seaweeds can contribute to climate mitigation. Using a combination of environmental (eDNA) metabarcoding and surficial sediment sampling, Bellgrove et al. (2023) explored the presence and distribution of macroalgal carbon in near-shore coastal sediments in south-eastern Australia. A total of 68 taxa from all three major macroalgal groups were identified at over 100 locations, illustrating how seaweed biomass and organic carbon from a host of different species is exported and deposited into near-shore sediments. With a similar aim, Quieros et al. (2023) used eDNA and a particle tracking model to study the effects of hydrodynamics, algal density and decay state upon transport of algal detrital fragments to deep benthic sediments. They reported species- and condition-specific idiosyncrasy in the likelihood of fragments reaching environments where burial and long-term storage were possible. In the face of such unexpected differences, much more research is required on the burial and sequestration mechanics of the contribution of algal productivity to climate change mitigation.

Nonetheless, Eger et al. (2023) underline the contribution that marine macroalgae make to global ESP beyond climate change mitigation, valuing fisheries production and nitrogen removal more highly than carbon sequestration. Estimates for fisheries, moreover, considered only the economically exploited species associated with kelp forests, Eger et al. (2023) noting that there were many other species that probably contribute indirectly to commercial operations via food webs or provide additional value via recreational fishing. Inevitably when one considers the full range of ESPs gained from Chlorophyta, Rhodophyta and more than a small sub-set of the Phaeophyta, a host of important potential benefits are evident.

Cotas et al. (2023) reviewed the many provisioning, regulating and cultural services provided by seaweeds, and in addition to those already discussed, highlighting important contributions to coastal protection, production of atmospheric oxygen, pharmaceuticals and education. We focus briefly here on just one of those, coastal protection. There is little doubt that a combination of increased sea surface temperatures (SSTs), sea level rise (SLR) and increased incidence and severity of extreme weather events are together likely to pose a major environmental threat to our coasts and continental shelf systems (Oppenheimer et al., 2019). Consequently, the wave energies and sediment disturbance associated with intense storm activity will almost certainly impact macroalgal communities but also bring into focus their contribution to coastal protection (Hanley et al., 2020). In comparison to other biogenic coastal defences, little is known about the role of kelps in coastal protection (Smale et al., 2013). Despite a general perception that they have a positive role (Mork, 1996; Rosman et al., 2007; Cotas et al., 2023), it is actually unclear whether macroalgal communities play any significant part in wave attenuation. Morris et al., (2020), for example, note how only a limited number of studies have investigated coastal protection by seaweeds, and in their own study with the kelp Ecklonia radiata in SE Australia, found that beneficial effects were restricted to a small subset of environmental conditions. The degree of wave attenuation is likely to be strongly influenced by the architecture of the dominant kelp species (i.e. prostate, stipitate, canopy) and the community structure of the understorey canopy (Eckman et al., 1989; Türker et al., 2006; Gaylord et al., 2007) and, as such, will vary between biogeographical regions (Smale et al., 2013). Far less attention has been given to the role of fucoids in coastal protection on rocky reefs, although Tyrrell et al. (2015) described how fucoid algae in saltmarshes can attenuate wave energy and play a significant role in sediment deposition and accretion. In this Special Issue, Elsmore et al. (2024) compare the wave attenuation capacity of the Californian kelp, Macrocystis pyrifera, between fully established and uncolonized restoration sites. Having quantified changes in energy as waves propagated from outside and into the habitat, the authors show how fully established Macrocystis reduced seabed wave energy flux by 12 % over 180 m of travel, with reductions slightly greater for waves of longer periods and smaller heights. Although ‘significant’, this magnitude of wave attenuation is small compared to the demonstrated buffering capacity of mangroves and seagrasses (Hanley et al., 2020).

Of greater significance to the dynamics of coastal processes and protection, macroalgae are responsible for trapping detritus and sediments in coastal shelf and intertidal environments and are even capable of considerable autochthonous sediment supply (Cotas et al., 2023). Attached to vascular plants, embedded in sediment and present as free-living individuals, macroalgae are an abundant, if often overlooked, component of saltmarsh communities (Adam, 1993). One of the most well known, the rhodophyte Bostrychia scorpiodes, is representative of a widespread genus found in both saltmarshes and mangrove communities throughout the northern hemisphere. Bostrychia species are anecdotally thought to contribute to sediment accumulation in both habitats, but empirical evidence is lacking. Indeed, in general the role of macroalgae in sediment trapping and accumulation in situ appears more assumed than evidenced. Nonetheless, it is important to remember that coastal habitats are strongly inter-connected, such that sediment loss and erosion from a subtidal habitat can have repercussions for sediment transport to supra-tidal areas such as sand dunes many kilometres distant (Hanley et al., 2014). The loss of seaweeds from subtidal kelps to intertidal Bostrychia species could have important ramifications for vascular plant communities along our coasts.

It is much clearer that the movement of dislodged or free-floating seaweeds is responsible for sediment capture on beaches or even significant sediment transport to them. Garden and Smith (2015) showed how in some regions over one-third of floating macroalgae carried a variety of sediment types including smaller carbonate materials (shells) and sand, and even large boulders. They estimated that around 350 tonnes of sediments may be drifting with macroalgae in the Southern Ocean at any one time, a potentially significant source of supply to littoral systems. We have discussed already the important contribution that intertidal seaweeds and beach-washed wrack make to nutrient budgets in terrestrial coastal environments. Wrack is also critically important in facilitating sediment accumulation in the sandy littoral zone, classically often forming the nucleus upon which embryo dunes initiate and grow in sand dune systems (Hesp, 2002; Colombini and Chelazzi, 2003). In addition to a role in building these vital ‘soft defences’ along our coastlines, the combined influence of sedimental transport and wrack accumulation provided by macroalgae may be vital to maintain them. In a flume experiment, Innocenti et al. (2018) describe how at the largest densities of Sargassum wrack used, the estimated reduction of sand dune erosion potential exceeded 100 % (this compares with a wave attenuation capacity of only 12 %). Even at the lowest densities, Sargassum wrack yielded probable reductions in dune erosion. Field studies are needed, but the common management practice of removing wrack deposited on tourist and residential beaches for aesthetic reasons may be counter-productive to the geomorphological stability of our coastlines.

THREATS TO MACROALGAE

The loss of global biodiversity and the ecosystem services it supports has many interacting drivers, but the principal causes are habitat loss, invasive species, overexploitation, climate change and pollution (MEA, 2005). Although the Millennium Ecosystem Assessment (MEA, 2005) does not consider macroalgal assemblages specifically, they identified habitat loss, pollution and over-exploitation as the most significant historical causes of biodiversity loss in coastal and marine systems. All five of the main drivers of biodiversity loss were, however, predicted to increase in importance over coming decades (with the sole exception of ‘invasives’ in marine systems). Inevitably, these drivers affect all coastal and marine species and habitats differently. For example, by virtue of the impacts of urbanization and agriculture, coastal habitat loss is most keenly felt in terrestrial environments. Rather than attempt an exhaustive account, we focus on one or two critical issues under each of the five drivers, although by virtue of its increasing importance, we summarize a wider range of threats associated with climate change, with examples, in Table 2.

Table 2.

A summary of some of the principal acute environmental threats posed by anthropogenic climate change (for information on marine heatwaves see main text), with recent example responses reported for marine macroalgal species (‘seaweeds’) and communities.

Threat	Species/community examined	Response	Example studies
Sediment deposition, increased turbidity and rapid salinity change	Alaskan Fucus distichus populations	In laboratory experiments, Fucus distichus was unaffected by changes in light environment suggesting a degree of tolerance to sediment loading associated with glacial melt. The authors note the potential importance of multiple stressors, however, including reduced salinity as well as increased temperature	Umanzor et al. (2023)
	NW Atlantic kelp species (Alaria esculenta and Saccharina latissimi)	Increased sedimentation reduced spore development in both species, but spore attachment and gametophyte density increased with sediment concentration for Saccharina only. At the maximum sediment level examined, spore attachment and gametophyte densities were similar for both Alaria and Saccharina	Picard et al. (2022)
	NE Atlantic kelp (Laminaria) species	Under reduced light conditions, heatwaves reduced biomass, blade surface area and photosynthetic efficiency in the cool-water L. digitata and L. hyperborea, but not the warm-adapted L. ochroleuca. Such species-specific responses indicate that kelp communities will undergo shifts in species composition, depending on interacting environmental drivers	Bass et al. (2023)
	East Australian kelp (Ecklonia radiata) forests	Coastal shelf systems experienced freshwater influx following severe storms; widespread Ecklonia mortality resulted. Experiments confirmed that reduced salinity, comparable to those during floods, caused reductions in sporophyte photosynthetic yield and tissue strength. Gametophytes, by contrast, were resilient to short-term reduced salinity, explaining the rapid kelp recovery observed after the floods	Davis et al. (2022)
Elevated CO2 and ‘ocean acidification (OA)’	Japanese marine CO2 seep communities	Sites exposed to natural OA (near volcanic vents) were monopolized by structurally simple, fast-growing turf algae. When these communities also experienced seasonal typhoon disturbance, they were much less resistant than nearby macroalgal-dominated communities	Hudson et al. (2023)
	East Australian kelp (Ecklonia radiata) and Rhodophyte species (Lenormandia marginata and Plocamium cirrhosum)	Elevated CO2 had no effect on the physiology for any species and suggests photosynthetic CO2 fixation rates will not increase irrespective of whether seaweeds operate CO2-concentrating mechanisms to facilitate the use of bicarbonate,(Ecklonia and Lenormandia) or simply use only CO2 for photosynthesis (Plocamium)	Paine et al. (2023)
	North Atlantic Fucus vesiculosus	Exposure to simulated OA corresponding to predictions for 2100 resulted in reduced apical thallus strength (through lower Ca and Mg content) and loss of exposed individuals in the field. If so weakened by the effects of OA, F. vesiculosus will be at greater risk of physical damage and detachment	Kinnby et al. (2023)
	Chilean kelp (Lessonia spicata)	Simulated ocean acidification and warming induced change in Lessonia phenolic content, and with this, acceptability to herbivorous snails. Disruption of trophic interactions may have profound effects on populations of both algae and herbivores and with this, ecosystem stability.	Benitez et al. (2023)

Invasives

The problem of invasive species is especially pernicious in the highly connected marine environment. Once non-natives have been transported into a new area, often in the ballast water of transoceanic ships (Costello et al., 2022), they tend to establish preferentially in degraded or disturbed natural habitats (see Byers et al., 2023 for a review). These degraded sites include the abundant ‘bare space’ available on coastal infrastructure in marinas and ports, artificial habitat that frequently provides ‘steppingstones’ for further spread (O’Shaughnessy et al., 2020).

Macroalgal communities are affected by competition from other invasive seaweed species, but also by the negative ecological interactions exerted by other trophic levels (see Enge et al., 2017 for a review of herbivore effects). Perhaps one of the most well known and most severe impacts of an invasive animal species on native macroalgal assembles is that of the urchin Centrostephanus rogersii on the kelp forests of Tasmania. In response to recent warming and a strengthening of the East Australia Current, the urchin has undergone southern range extension from mainland Australia across the Bass Strait to Tasmania (Johnson et al., 2011). More than 95 % of Tasmania’s surface canopy-forming giant kelp (Macrocystis pyrifera) forests have been lost over recent decades, to be replaced by Ecklonia radiata or urchin barrens (Johnson et al., 2011), although efforts are now being made to restore and rehabilitate these vulnerable ecosystems (see Layton et al., 2020 for a review).

Native to the northwest Pacific, the kelp Undaria pinnatifida is a global invader that has dispersed to the northeast and southwest Atlantic, southwest and east Pacific, and the Tasman Sea (Epstein and Smale, 2017). Although in New Zealand invasion by U. pinnatifida had limited effects on the diversity and abundance of native algae and invertebrates, it caused NPP to more than double in recipient communities (South et al., 2015). Similarly in the northeast Atlantic, studies on U. pinnatifida on rocky reef communities have suggested negligible impacts on native biodiversity (Heiser et al., 2014; Epstein and Smale, 2017), although Epstein et al. (2019) found that experimental removal of U. pinnatifida led to increased abundance and biomass of the native annual kelp, Saccorhiza polyschides. Interestingly, however, Epstein et al. (2019) reported limited impact on two perennial native kelps (Laminaria digitata and Saccharina latissimi) which may indicate that Undaria’s annual and opportunistic life-history (it is associated with disturbance in its native range) makes it less competitive than long-lived native kelp species. Nonetheless, given that U. pinnatifida is a relatively recent invader in the northeast Atlantic, it is possible that its impacts are being underestimated and further research is required to assess the role of this species in this ecoregion.

In their contribution to this Special Issue, Firth et al. (2024) raise an intriguing perspective on the spread and impact of non-native seaweeds in coastal communities. A common and problematic invasive in the intertidal zone around the southern part of the British Isles, Japanese wireweed (Sargassum muticum) was long assumed to require hard surfaces for attachment, thus limiting its ability to invade the soft sediments demanded by seagrass beds. Working in SW England, Firth et al. (2024) demonstrated that Sargassum was able to invade Zostera marina seagrass beds via attachment to (unoccupied) limpet shells. Moreover, having established in seagrass communities, the presence of Sargassum was associated with a reduction in Zostera marina density and phenolic content. While the mechanistic relationship between Sargassum competition and Zostera phenolics requires further work, the reduction in seagrass defensive capabilities may imbue Sargassum with a competitive advantage where herbivores and/or epibionts are common, facilitating further proliferation of the invasive.

Pollution

Like most terrestrial plants, marine macroalgae are susceptible to the impacts of toxic metals (Contreras-Porcia et al., 2017), organic pesticides (Zokm et al., 2022) and, more specific to the marine environment, oil spills (Keesling et al., 2018). In addition, there has been tremendous emphasis recently on the accumulation of plastics in our seas. It is now well established that substantial and ever-increasing quantities of plastic pollution are entering the marine environment from many different sources (Thompson et al., 2004; Geyer et al., 2017; Courtene-Jones et al., 2021) of which ‘microplastics’ under 5 mm (Frias and Nash, 2019) in size are perhaps the most pernicious. The negative consequences of this pollution on marine systems are beyond debate (Bucci et al., 2020; Napper and Thompson, 2020; Courtene-Jones et al., 2021). Whilst countless studies have described the presence and impacts of microplastics in animals (Browne et al., 2008; Steer et al., 2017; Carlin et al., 2020; Salerno et al., 2021), research on macroalgae is only really beginning to emerge.

Working in China, Li et al. (2022) characterized the occurrence of plastic debris in five macroalgae species and showed that accumulation (entanglement, adherence, wrapping, embedment and entrapment by epibionts) varied according to the size of the plastic and between species, although unsurprisingly microplastics were by far the most common (by mass). A recent review by Huang et al. (2023) found that marine benthic vegetation (seaweeds and seagrasses) typically supports greater abundances of microplastics compared to unvegetated sites. Moreover, there was notable variation in microplastic accumulation between species; filamentous algae in particular contained more microplastics than other species largely due to entanglement (see also Ng et al., 2022). The impacts of microplastics on algal fitness are less clear. Feng et al. (2020) reported that microplastics did not affect growth rate, effective photochemical efficiency of photosystem II (PSII) or saturating irradiance of Ulva prolifera until reaching an extremely high concentration (100 mg L^–1). Whilst direct impacts on macroalgae may be limited, the important role of macroalgae in marine food webs means that they can potentially transfer microplastics from the surrounding environments to fishes, invertebrates and epibionts via herbivory (Gutow et al., 2016). There is also evidence that microplastics can accumulate in commercially grown seaweeds (Li et al., 2020). This is clearly a research area that requires further attention.

Of the many instances of eutrophication affecting coastal shelf systems, so-called ‘Green tides’ and ‘Golden tides’ are noteworthy, particularly where large-scale aquaculture, practised widely in SE Asia, causes the release of nutrients to the local environment. Resulting population explosions of Ulva sp. and Sargassum sp. (‘Green’ and ‘Golden’ respectively) lead to mass blooms of drifting algae mats. Rather than a threat to other macroalgae, however, these blooms are more often damaging to sandy littoral habitats and the species they support when the resulting accumulations of wrack wash up on beaches (McLachlan and Defeo, 2017).

Overexploitation

Forty per cent of humanity lives within 100 km of the sea (Firth et al., 2016) and as the global population rises, the pressure upon coastal environments to contribute to the provisioning of food, shelter, fuel and employment is ever-increasing. Macroalgae have long been collected by different indigenous human populations as food, fodder, fuel and fertilizer and for medicine, and the rising human population makes their further exploitation on industrial scales an inevitability. Buschmann et al. (2017) reviewed the many uses of macroalgae by humans and it is not our intent to repeat that exercise here. Nonetheless, as the range of uses for macroalgae and their derivatives grows, so does the threat of over-exploitation. To avoid such over-exploitation whilst supplying the burgeoning seaweed products marketplace, advances in aquaculture of marine macroalgae and adoption of integrated approaches to mariculture (Chopin et al., 2001) in site-specific schemes (Visch et al., 2023) are required.

Whilst extractive industries no doubt pose a threat to macroalgal stocks there have as yet been few confirmed macroalgal population extirpations as a result of overharvesting. However, changes in population density and age structure have been observed, for example in the case of Laminaria digitata in northwest France (Davoult et al., 2011) and Lessonia nigrescens in northern Chile (Vásquez et al., 2012). The threat of macroalgal exploitation and the consequences of declining macroalgal density and local extinction for their associated biota are more well known. Macroalgae serve as crucial nursery and refuge habitats for many fish and invertebrates, and the bottom-up influence they have upon associated food chains is exemplified by the classic examples of urchin over-grazing of kelps described in the Atlantic by Dayton et al. (1998) and in the Pacific by Estes and Duggins (1995). In both cases mesopredator release via anthropogenic over-exploitation of higher-level predators (halibut, wolfish and cod in the Atlantic, sea otters in the Pacific) triggered the trophic cascade that resulted in the decline of the kelp population. However, such trophic cascades result in even greater effects as a consequence of the loss of the kelps themselves. Estes et al. (2011) described the impact of kelp forest decline and the fish they support, on the diet of bald eagles and gulls, but even filter feeders such as mussels suffered as a result of the lack of kelp fragments in the water column (see also Lorentsen et al., 2010).

Habitat loss

Although habitat loss in terms of gross destruction or modification of the substratum upon which seaweeds grow is arguably a much lesser threat in marine than terrestrial habitats, increased urbanization of coastlines poses an existential threat to intertidal habitats such as saltmarshes and mangroves (Mangialajo et al., 2008; Ferrario et al., 2016) where many specialist macroalgae are found. While there have been few, if any, documented instances of extirpation of macroalgal species as a result of coastal urban development, the small size and taxonomic intractability of many algal species that associate with mangrove and saltmarshes makes them prime candidates for being overlooked.

Unlike habitat loss per se, there are many instances of marine habitat degradation, but changes in local environmental conditions (e.g. temperature, oxygen content, seawater pH, sediment load/turbidity, nutrient status) are frequently associated with ACC and are dealt with in the next section. Nonetheless, increased sediment and nutrient loads via eutrophication and increased runoff (‘coastal darkening’) from degraded terrestrial systems do happen independently of ACC (Aksnes et al., 2009; Filbee-Dexter and Wernberg, 2018). Changes in water quality, particularly where it influences the competitive balance between large, canopy-forming macroalgae and small, often opportunistic turf-forming species, can effect significant changes in seaweed community composition (Eriksson et al., 2011; Kraufvelin et al., 2020). It is also worth noting that where the canopy cover of kelp species is reduced by ACC-linked factors, there will be an inevitable loss of habitat for epibiotic seaweeds to colonize (Teagle et al., 2017; Smale et al., 2022), and an important change in the seabed community beneath the canopy (Brodie et al., 2014; Ferrario et al., 2016).

Climate change

Although threats from pollution, habitat change, overexploitation and invasives are important, and certain to continue into the future, there seems little doubt that the greatest risk to marine macroalgae stems from ACC (MEA, 2005). As is the case with terrestrial plant communities (Parmesan and Hanley, 2015), elevated atmospheric CO₂, and associated shifts in temperature, and precipitation (via salinity change) will have profound effects on marine macroalgae. Seaweeds also face the added pressures imposed by the combination of SLR and increased SSTs causing an increase in storm activity (Oppenheimer et al., 2019). With the realization that extreme events pose a more severe threat to ecosystem function than more gradual change in average conditions (see Parmesan and Hanley, 2015), and as the phenomenon becomes more commonplace and extreme, by far the most widely researched impact of ACC on macroalgae is, however, marine heatwaves (MHWs; Filbee-Dexter et al., 2020; Smith et al., 2023). We focus briefly on this phenomenon here, but additionally summarize many of the other critical threats to marine macroalgae associated with ACC in Table 2. Critical points to note in Table 2 are that changes in macroalgae growth, morphology and biochemistry will have far-reaching repercussions for interactions with seaweed species and across trophic levels. It is also worth pointing out that not all studies report the expected seaweed responses to ACC-linked factors.

MHWs are defined as discrete periods of anomalously high ocean temperatures (Hobday et al., 2016), the frequency of which has increased by over 50 % in recent decades (Cooley et al., 2022). Between 2014 and 2016, the Northeast Pacific experienced the longest sustained MHW yet recorded, the so-called ‘Blob’. Anomalously high sea temperatures associated with this event exacerbated further the sea star wasting disease epidemic and resulted in the functional extinction of Pycnopodia helianthoides from the region (Hamilton et al., 2021). The loss of this important predator resulted in a large increase in sea urchin populations and even where kelps were buffered from the direct effects of increased water temperatures, overgrazing by sea urchins caused the local extirpation of kelp forests (Starko et al., 2022).

MHWs are now widely known to exert major negative impacts on seaweed communities and the ecological functions and services they provide (Smale et al., 2019; Smith et al., 2023). The temperature extremes associated with MHWs are particularly harmful for individuals located at the equatorial edge of their species’ range distributions (Filbee-Dexter et al., 2020; Smith et al., 2023), simply because temperatures rise above thermal tolerances, causing widespread mortality, tissue damage, dislodgement, reduced photosynthetic performance and reduced reproductive fitness (Wernberg et al., 2013; Smale et al., 2019; Hollarsmith et al., 2020). There is an assumption therefore that the ranges of ‘cold water’ species will contract poleward, to be replaced by more ‘warm-water’ species.

This assumption was the basis for Leathers et al. (2024) to explore the response of congeneric Laminaria species to simulated MHWs at different levels of intensity (Control, 14 °C; Moderate, 18 °C; and Extreme, 22 °C) and duration (14 or 28 d). In SW England, (‘cold water’) Laminaria digitata reaches its trailing edge and is undergoing a range contraction whereas (‘warm water’) L. ochroleuca reaches its leading edge and is undergoing a range expansion. However, for both kelps, simulated MHWs across different intensities caused sub-lethal stress (moderate intensity) or dramatic declines in growth and photosynthesis (extreme intensity), and responses were always exacerbated by longer MHW duration. These results suggest that anticipated MHW events in the region will have major deleterious effects on both kelp species, with major impacts on associated communities and ecosystems.

Globally, many warm range margins of normally cold-water-adapted species such as kelps have witnessed accelerating loss of habitat cover with a concomitant increase and persistence of degraded, sediment-laden algal ‘turfs’ with the negative implications for ESP and kelp recovery that ensues (Filbee-Dexter and Wernberg, 2018; Filbee-Dexter et al., 2020; Smith et al., 2023).The study by Leathers et al. (2024) is concerning, since it suggests that cold water kelp replacement by structurally different turf communities instead of warm water kelps will be a much more common occurrence than expected. In many ways these trends mirror unexpected range shift patterns and community responses in terrestrial vegetation (see Parmesan and Hanley, 2015). Indeed, the more we investigate ACC-driven changes in seaweed communities, the more departures from expected responses we find.

A recent series of multi-year heatwaves in the northeast Pacific Ocean was the spur for Csordas et al. (2024) to use a combination of in situ and remote sensing survey approaches conducted at three spatial scales (region, shoreline segment and individual patch), to examine changes in the distribution, population structure and morphometrics of an intertidal kelp, Postelsia palmaeformis. While the authors reported changes in Postelsia morphology (increased blade lengths), and a shift to advanced reproductive phenology over the 15-year study, they also found little evidence of change in population stability, and only slight distributional shifts. Csordas et al. (2024) concluded that this apparent long-term constancy may be due to a combination of thermal buffering and wave exposure at the northern range edge, acting to limit the impact of MHWs on Postelsia. Their study underlines the importance of multi-scale assessment, and especially local conditions, when investigating and attempting to understand species responses to climate-linked stressors (see also Hollarsmith et al., 2024).

Similarly, Krumhansl et al. (2024) highlight how dramatic changes in kelp community composition over a 40-year period along the Nova Scotia coastline vary according to location and whether communities were growing in exposed or thermally buffered sites. There was a significant loss of cold-tolerant species (Alaria esculenta, Saccorhiza dermatodea and Agarum clathratum) concomitant with an increased representation of species adapted to warmer water (Saccharina latissima and Laminaria digitata). Most notable, however, was that despite rapid warming and broad-scale change in community assemblages, the predicted shift to turf dominance did not occur. Indeed, in some instances, kelp recovered from turf dominance following initial losses during a warm period from 2010 to 2012. Although the studies by Csordas et al. (2024) and Krumhansl et al. (2024) offer hope that climate refugia may exist for some marine algae species in the face of gradual and extreme temperature rise, they also confound our ability to make general predictions about wider global changes.

SYNTHESIS AND FUTURE STUDIES

Inevitably this Review and the Special Issue in which it sits, can only deal with a small fraction of the many issues confronting marine macroalgae ecosystems at the present time. Not only are we dealing with a complex, global issue, but there are many interacting stressors and potential mitigations that confound our ability to make predictions about species or community-level responses to the myriad threats. Nonetheless, this makes for an exciting research landscape (although one born out of the grim necessity to understand how and why our species is so disruptive to the natural world) and we highlight (Fig. 2) just a few of the potential research avenues available to marine biologists and ecologists over coming decades.

Fig. 2.

A summary of the principal research priorities (I–IV) and avenues for future study needed to understand the response of marine macroalgae to continued anthropogenic disturbances and how macroalgae contribute to mitigating these threats.

Research priority I – long-term monitoring of macroalgae communities using appropriate biodiversity, population and phase shift metrics

Any threat to the macroalgae inhabiting shallow marine environments inevitably impacts the multitude of species that depend on them. Our ability to properly monitor how global change affects ecosystem health depends on the correct use and application of biodiversity metrics. Piñeiro-Corbeira et al. (2024), for example, show that traditional measures of species richness (α-diversity) failed to identify important changes highlighted by more spatially explicit (intra- and inter-reef scale) β-diversity approaches. Comparing the macroalgal understorey of healthy reefs (with a Laminaria ochroleuca canopy) and degraded reefs (where the Laminaria canopy collapsed following excessive fish herbivory) in northern Spain, Piñeiro-Corbeira et al. (2024) report how none of the conventional indicators of α-diversity detected negative shifts in composition of the macroalgal understorey in degraded reefs. By contrast, small-scale spatial β-diversity decreased significantly due to kelp canopy loss. This critical shift in community spatial arrangement signposts how and why more studies must utilize a variety of different diversity metrics at different spatio-temporal scales to properly understand how global change impacts seaweed canopies and the habitat they provide (see also O’Shaughnessy et al., 2023).

As a consequence of anthropogenic activities and associated stressors, many marine ecosystems have inevitably experienced the kind of perturbation described by Piñeiro-Corbeira et al. (2024). One of the biggest challenges facing marine biologists is to monitor habitats which are frequently remote, are sub-tidal (and many metres deep) and where any detectable changes occur over many decades. Hollarsmith et al. (2024) used the novel approach of combining historical and modern surveys to detect an overall increase in the spatial extent of canopy kelp throughout the Gulf of Alaska during the period 1913–2010. Warm-adapted kelp species (e.g. the giant kelp, Macrocystis pyrifera) increased in distribution more than cold-adapted species (e.g. the dragon kelp, Eualaria fistulosa), suggesting an important response to increasing sea temperatures. Importantly, however, some of these changes were closely associated with the reintroduction of sea otters to the region in the late 1960s, highlighting the potential importance of the well-known trophic cascade between kelp, sea urchins and otters (see Research Priority IV). Hollarsmith et al. (2024) emphasize the need for further monitoring to detect kelp range shifts, to detect changes in community structure and to identify possible refugia for cold-adapted species. To do so at a global level and especially in remote locations such as Alaska, they propose the establishment of strategically distributed ‘sentinel’ kelp beds which can be monitored at regular intervals using a combination of remote sensing techniques and citizen science collaboration with coastal communities.

Hollarsmith et al.’s (2024) contribution to this Special Issue not only highlights the potential importance of trophic interactions in understanding and predicting macroalgal community responses to global change (and a volcanic eruption) but also emphasizes why our understanding of macroalgal community responses to (multiple) environmental stressors must include an understanding of population-level changes and processes. This point is made by Salland et al. (2024) in their ‘Research in Context’ article exploring the population demography of the ‘warm water’ kelp Saccorhiza polyschides in southwest England. A series of intertidal surveys along a gradient of wave exposure in Plymouth Sound were undertaken over a 15-month period and while they showed pronounced seasonality, there were remarkably consistent demographic patterns across sites and depths. These demographic trends may explain why Saccorhiza was a dominant habitat-former on both intertidal and subtidal reefs, and why the species has proliferated in recent decades across the southern UK in response to increasing temperatures, physical disturbance and reduced competition. Salland et al. (2024) argue that only by initiating high-resolution cross-habitat surveys of this kind can we then generate robust baselines of seaweed population demography from which we might then detect the ecological impacts of climate change and other stressors.

The value of long-term population and community monitoring can best be realized if marine biologists effectively model and understand how anthropogenic activities affect macroalgae communities. One particular problem is being able to distinguish naturally occurring stochastic changes from shifts in ecological states caused by anthropogenic stressors. Dudgeon and Johnson (2024) review the difficulties that emerge when translating between ecological state change theory and empirical study at this time of unprecedented environmental change. Using ‘real-world’ examples of phase shifts in marine macroalgal communities, they review a variety of approaches and argue that the application of so-called Characteristic Length Scales best facilitates the identification of phase shifts caused by anthropogenic perturbation, from natural transient changes.

Research priority II – monitoring and understanding macroalgae species and community responses to environmental stressors using functional traits

Ever since the pioneering work of Phil Grime (e.g. Grime, 1974), the use of plant functional traits has been promoted as a useful ‘shorthand’ to explore terrestrial plant species and community responses to environmental change. Application of the concept to marine algae has, however, generally lagged (Griffin et al., 2023) and only recently have there been attempts to build trait-based frameworks for marine macro-algae (Mauffrey et al., 2020; Fong et al., 2023). Amstutz et al. (2024) underscore why an understanding of life history traits is important in understanding the responses of intertidal macroalgae to extreme temperatures. Using aspect as a surrogate for increased annual and extreme air temperatures in the temperate intertidal zone, they showed that Pole-facing (PF) macroalgal communities were consistently more taxon-rich and possessed higher functional richness and dispersion than adjacent Equator-facing (EF) slopes. Moreover, PF-slopes were dominated by algae with traits linked to rapid resource capture and utilization, but low desiccation tolerance, while less diverse EF-surfaces were dominated by desiccation-tolerant fucoids. Amstutz et al. (2024) argue that these differences may imbue cooler (PF-aspect) habitats with resilience against environmental perturbation, but if predicted increases in global temperatures are realized, these relatively species-diverse and functionally diverse intertidal habitats may shift to a depauperate, desiccation-tolerant seaweed community with a concomitant loss of functional diversity and redundancy. Although clear patterns of trait variation were apparent between PF- and EF-slopes, Amstutz et al. (2024) note that environmental selection is likely to be more severe for algae at the recruitment stage. Consequently, there is a pressing need to focus on trait expression and characterization during early algal ontogeny (specifically gametophytes and newly settled sporophytes) if we are to understand algal–environment interactions (see Veenhof et al., 2024).

Research priority III – macroalgae reproductive performance and recruitment

Despite the long-held assumption that early ontogeny is pivotal to subsequent terrestrial plant demography and phytosociology (Harper, 1977), there has been limited focus on how environmental perturbation during recruitment and establishment can disrupt communities. This is not only true of terrestrial plant population and community responses to ACC (Parmesan and Hanley, 2015), but also holds for marine macroalgae, being a critical knowledge gap that limits predictions about seaweed resilience and response to the multitude of threats they face. This knowledge gap is made even more apparent because of the diversity of life history and reproductive strategies macroalgae present. The often heavily modified basic pattern of alternation between diploid sporophyte and haploid gametophyte results in species with effectively only a single life phase through to others with highly complex multiphasic development. Of critical importance is the fact that developmental stages and the processes of gametogenesis and sporogenesis are all under the influence of abiotic environmental factors such as temperature, photoenvironment and salinity. In Laminariales for example, a diploid sporophyte gives rise to haploid spores which settle and develop into microscopic gametophytes that produce gametes which fuse to give rise to a new sporophyte, completing the life cycle. The gametophytes in this instance may have a completely different set of environmental optima and limitations compared to the much more readily studied sporophyte, and the influence of abiotic variables upon survival and performance of the less easily studied stages of the life cycles may hold the key to predicting the species’ responses to a changing environment.

Veenhof et al. (2024) highlight how important the distinction between life history stages is for understanding and predicting kelp responses to ocean warming. Working on a latitudinal gradient spanning 13 degrees of latitude along Australia’s east coast, they show how Ecklonia radiata gametophytes exhibited much greater thermal tolerance of local temperature extremes than the established sporophyte stage from the same population. Although the authors report some latitudinal variation in gametophyte resilience (high-latitude populations being more resilient), Veenhof et al. (2024) argue that this critical difference between life history stages may offer hope for the species in a warming world. Gametophyte resilience may not only buffer local populations against increasing temperature but facilitate persistence a species across its entire range through dispersal of heat-tolerant gametophytes. Nonetheless, they also note that mismatch between peak spore availability and how environmental stress influences the transition from the gametophyte to sporophyte stages is critical in understanding how ocean warming affects kelp persistence.

Research priority IV – anthropogenic disruption of biotic interactions involving macroalgae

Several papers in this Special Issue highlight how and why biotic interactions are fundamental to the understanding of macroalgal community responses to environmental stressors (e.g. Amstutz et al., 2024; Dudgeon and Johnson, 2024; Hollarsmith et al., 2024). In parallel with terrestrial plant communities (Klironomos, 2002; Hannula et al., 2021), marine biologists have recognized the pivotal role that microbiomes play in seaweed species persistence and stress response. Pearman et al. (2024), for example, report how the biogeography of the macroalgae Durvillaea and its associated microbiome has been shaped by shifting environmental conditions. Using a combination of environmental data collected from the coast of South Island, New Zealand, complemented by genotyping-by-sequencing (seaweed host) and 16S rRNA amplicon sequencing (microbiome) techniques, they attempted to determine what external factors might shape the algal/microbiome interaction. Although as expected, host and microbiome shared biogeographical structure, unlike Durvillaea, where distribution was limited by propagule spread (geographical distance decay), the biogeography of the microbiome was dictated by environmental selection (environmental distance decay). In essence, Pearman et al. (2024) show that while the host macroalgae exhibited the expected reduction in similarity with increasing geographical distance, the microbiome was more variable according to local environment. The upshot is that where environmental conditions change due to anthropogenic activity, the host–microbe interaction will become decoupled across present-day biogeographical ranges and with it the possible loss of microbial associates that provide essential services to the host macroalgae algae such as vitamin production and deterrence of epiphytic microeukaryotes and microalgae.

Research priority V – interactions between macroalgal litter and vascular plant habitats

We have highlighted already the important and highly remarkable interconnectivity between marine seaweed productivity and several geomorphological and ecological processes that occur in terrestrial ecosystems. Nonetheless, there is more to explore. Hyndes et al. (2022) note how expected loss and decline in kelp forests and seagrass beds associated with ACC will inevitably reduce the amount of marine-derived organic matter reaching beaches, but other intertidal ecosystems such as mangrove forests, sand dunes and saltmarshes will doubtless experience the same deficits. We actually understand little about how marine-derived litter (specifically from macro-algae) contributes to the nutrient budgets, sediment and propagule trapping, and vascular plant establishment in these habitats, but seaweed litter is almost certainly vital to these processes (see Adam, 1993).

Equally important perhaps is the potential impact of increased macroalgae (‘wrack’) deposition onto our coasts. Largely through a combination of rising sea levels and especially an increase in the frequency and intensity of storm activity, the deposition of debris onto coastal ecosystems is becoming a significant issue. Platt et al. (2015) highlighted how wrack deposited by the storm surges associated with hurricanes along the northern Gulf of Mexico can lead to partial or even complete mortality of groundcover vegetation in coastal savannas in Mississippi, USA. Wrack deposited in this way can reduce native seed germination (Castillo et al., 2023) and even limit the amount of rainfall that reaches the soil surface several years after the material was first deposited (van Stan et al., 2020). A potentially positive effect of wrack deposition, however, may be the reduced germination and establishment of invasive plant species (Abbas et al., 2014). Clearly the intriguing interaction between marine litter and terrestrial plant ecology has considerable resonance in our understanding of the long-term stability of inter- and supra-tidal coastal ecosystems and, with this, our understanding of their long-term contribution to the natural defence of our coastlines.

CONCLUSIONS

These five (of many potential) potential research priorities underline the complexity inherent in all biological systems, but complexities that we believe must be addressed in marine macroalgae (and terrestrial plant) systems if we hope to understand and mitigate how anthropogenic activity threatens our planet’s species and the ecosystem services they provide. Long-term monitoring has seldom been a priority for any funding agency, yet one cannot expect to quantify human impacts, or assess the efficacy of mitigation strategies, without it. A cynic might argue that this is perhaps why it is seldom funded by governments. Even with more financial support, it is also obvious that we cannot understand the response of all macroalgae species to all anthropogenic stressors, even in isolation, let alone in combination. The autecological review of the sugar kelp Saccharina latissima in this Special Issue (Diehl et al. 2024) is one of the few comprehensive syntheses of the ecophysiological and biochemical responses of any seaweed species to environmental stressors. Although an invaluable source of information about kelp survival in nature (with the potential to inform kelp cultivation methods for the many important sustainable applications), generally speaking, biologists have to be selective in what they study and, where possible, use short-cuts to elucidate global trends.

Sentinel species and habitats offer one such approach, but as shown by Hollarsmith et al. (2024), complications associated with unexpected events (volcanic eruptions) and biological interactions (sea otters) can confound matters. Similarly, understanding variation in algal functional traits, and how environmental filters select and shape those traits, is gaining traction, but as we highlight, not only is it vital to understand trait (and species) responses to multiple and simultaneous physio-chemical factors (heatwaves, elevated CO₂, microplastics, etc.), biotic interactions are also important. Thus, interactions between macroalgae, their herbivores, symbionts and competitors (and including invasive species and interactions throughout the ecosystem), and even how these changes impact coastal terrestrial systems, are essential to the story. Similarly, one cannot simply focus on the biology of established algae and expect to understand the very different selective processes that operate on functional traits, and by extension species, at the regeneration stage. These myriad complexities must be better understood if we are to have any hope of predicting the effects of global change on seaweeds and the many species and services they support. Time is running out if we are to achieve this goal and use the information to effect positive change in our marine environment.