Good news and bad news of blue carbon

Traditionally, ocean acidification researchers have focused on how secular changes in carbon dioxide (CO₂) or pH will impact organisms. Global mean pH is estimated to have decreased by 0.1 pH units (representing a 28% increase in acidity) since the preindustrial age and may drop another 0.3 pH units by the end of this century (1). Several recent papers, however, have highlighted the importance of understanding changes in the short-term variability in carbon parameters in addition to the secular trends (2–4). An article in PNAS by Pacella et al. (5) examines how net community metabolism (NCM) in a coastal seagrass bed can help slow the long-term secular change in ocean acidification but exacerbates the short-term variations in carbon system parameters. These short-term variations can drive the pH or the saturation state of the waters with respect to aragonite below a threshold for certain organisms that may prevent them from ever benefitting from the long-term relief.

Of course, the driver of the secular ocean acidification trend is the accumulation of anthropogenic (human-derived) carbon in the surface ocean. This accumulation occurs everywhere (coastal and open ocean) as atmospheric CO₂ increases, even in areas where seawater CO₂ values are higher than atmospheric values. Whether it is low CO₂ waters absorbing more CO₂ from the atmosphere over time or high CO₂ waters releasing less CO₂ back into the atmosphere over time, the net effect of carbon accumulation and acidification of the waters is the same.

The seasonal cycle of CO₂ has been appreciated for its impact on our ability to detect when a secular trend can be distinguished from the natural variability (6). This so-called “time of emergence” varies across ocean basins but is generally earlier in the open ocean, where natural variability is much smaller than in coastal regions. Larger coastal variability has also been used to suggest that coastal organisms may be more tolerant to the relatively small secular changes in seawater chemistry (7). However, the concept of thresholds (limits beyond which organisms cannot tolerate environmental conditions), as well as a recognition that many organisms are already living in waters very close to their threshold limits, implies that small changes in the environment can be significant, even in areas with large short-term variability (8–10).

This latest work finds that, in areas with high NCM, the amplitude of the seasonal cycles of CO₂ and pH will increase with time as anthropogenic carbon accumulates in coastal waters (5). This occurs because the buffering capacity (the ability of the waters to buffer against changes in pH) is decreased as carbon accumulates in the waters. Furthermore, because the buffering capacity of the waters is nonlinear, the low pH values will change more than the high pH values, leading to an asymmetrical impact on local organisms.

Paradoxically, while the seasonal cycle of pH increases with time, the seasonal amplitude of the saturation state of the waters decreases with time. This is explained by the fact that temporal variations in the saturation state are primarily controlled by carbonate ions that are being consumed as CO₂ is added to the waters, while the hydrogen ions that lower pH are growing with time. Pacella et al. (5) note, however, that the change in median saturation state values outweighs the decrease in seasonal amplitude, so there is little relief from the small reduction in short-term variations.

An increase in seasonal CO₂ amplitude was recently demonstrated in the open ocean by Landschützer et al. (11), but larger swings in coastal variability, as demonstrated by Pacella et al. (5), have implications for sessile organisms or other coastal species with a particularly sensitive growth stage because they cannot move away and will have longer exposures to suboptimal water chemistry. This short-term amplification in CO₂ and pH happens sooner than the long-term slowing of acidification associated with the incorporation of carbon into the seagrass bed or other organic reservoirs (5).

The model of Pacella et al. (5) suggests that a seagrass bed in Puget Sound could delay the median saturation state of the waters with respect to aragonite from crossing an established shellfish threshold by 26 y. However, the NCM produced by the seagrasses drives daily variations in saturation state that cross that same threshold 44 y earlier than in the absence of the NCM.

One might infer that a similar pattern might be experienced in coastal waters with respect to the relatively low NCM in open ocean waters, as illustrated in Fig. 1. The dashed line and darker shaded area represent the changes in the median and range, respectively, of open ocean pH from the preindustrial age to the end of this century. The solid line and lighter shading represent the median and range of high NCM coastal waters. The dotted line represents a nominal threshold pH value for an organism.

Fig. 1.

Conceptual illustration of the changes in median open ocean (dashed line) and coastal ocean (solid line) pH values with time. The shaded regions represent the short-term variability in open ocean (dark gray) and coastal (light gray) waters. The dotted line represents a nominal pH threshold to demonstrate that short-term variability in coastal waters will cross that threshold years before any open ocean values cross it even though the median values in the coastal ocean acidify more slowly than those in open ocean waters.

Note that the median pH of the open ocean in 2015 is lower (more acidified) than that of the coastal waters; however, given the larger short-term variability, the lowest values in the coastal waters cross the threshold well before any values in the open ocean. The ranges in both areas increase with time, but the lower pH values change faster than the higher pH values, resulting in an asymmetrical growth in the range.

The question then is which is more important to organism survival: the median conditions they are exposed to or the time that they spend below that critical threshold? Additional research is needed to fully address this for a range of coastal organisms.

The study by Pacella et al. (5) is focused on a highly productive seagrass bed during the dry season when fresh water complications were minimized. Many different processes can drive variability in the coastal zone. Fresh water inputs from rivers or ground water discharge can drive variability (12). If these waters have high organic carbon content and low alkalinity, they could further reduce the buffer capacity of the waters, exacerbating the problem. Variable coastal upwelling of high CO₂ waters can also produce large pH swings (13). These upwelled waters may also bring up waters with relatively high alkalinity that could moderate the anthropogenic changes, further complicating the interpretation in areas more complicated than the relatively simple system examined here.

This work (5) and the question posed above do have societal implications. As the article by Pacella et al. (5) points out, the current Environmental Protection Agency rules that determine whether a body of water is considered outside of US water quality standards are not clear on whether they refer to the naturally occurring range in median values or the range in the total short-term variability (14).

This also has implications for the “blue carbon” movement to promote the expansion of salt marsh, mangrove, and seagrass beds to sequester anthropogenic carbon in the coastal zone (15). If the seagrass beds are located in areas with organisms sensitive to the high-frequency periods of acidified conditions, then these beds may have unintended impacts on some species.

Clearly, an improved understanding of the consequences of both long-term exposure to acidified waters and the implications of more frequent low pH excursions for a range of coastal organisms needs to be developed. Despite the limited circumstances characterized by this study, it does highlight an important aspect of coastal carbon chemistry that needs careful consideration.