Will Warming and “Acidification” Create Chaos in Coastal Ecosystems?

One of the key concerns about climate change is ecosystem resilience. This is particularly true for those that are anchored over large locations with little ability to move. Ecological communities in the Chesapeake Bay come to mind.

According to the U.S. National Climate Assessment report published in 2014 (Melillo et al., 2014), there is “very high confidence that coastal ecosystems are particularly vulnerable to climate change because they have already been dramatically altered by human stresses, as documented in extensive and conclusive evidence” (Moser et al., 2014). Additionally, the report claims there is “very high confidence that climate change will result in further reduction or loss of the services that these ecosystems provide, as there is extensive and conclusive evidence related to this vulnerability” (Moser et al., 2014).

That Assessment has been criticized as being far too alarmist, too political, and very incomplete with regard to its summarization of important scientific literature. It didn’t help that when it was released, the National Oceanic and Atmospheric Administration (whose bailiwick includes coastal ecosystems), called the report “a key deliverable in President Obama’s Climate Action Plan” in the press release for its rollout.

It’s important to quantify claims like the ones made above, and one type of ecosystem that has received considerable attention in this regard is the seagrass biome. These dense underwater meadows are found in numerous coastal waters, including those of the United States. They are a foundational basis for an ecosystem as diverse and variegated as those associated with coral reefs, but they get little public attention because they aren’t nearly as showy. But they are important. Their presence helps to reduce coastal erosion, improve water quality and mediate ocean chemistry, as which adds economic value. Given the important functions that they perform within their coastal ecosystems, it should come as no surprise, therefore, that concerns have arisen over the current and future ability of seagrass ecosystems to withstand rising atmospheric CO₂ concentrations – i.e. global warming and ocean acidification.

A new study by Shelton et al. (2017) sheds some important light in this regard. Working with over 160,000 observations from Puget Sound, Washington, USA, the team of six researchers created a database of eelgrass, a common constituent of seagrass ecosystems worldwide. They surveyed data along hundreds of kilometers of shoreline over the 41-year period 1972-2012 in the Puget Sound, home to millions of people as well as tourism, transportation and recreation. It’s fair to call it the Chesapeake Bay of the Northwest, and there’s all kinds of pressures to keep it healthy. Their long survey period includes rapid economic development as well as increases in dissolved carbon dioxide as atmospheric concentrations rose. Their hope was to quantify the natural and anthropogenic factors contributing to eelgrass change across various spatial and temporal scales.

Shelton et al. indeed did report there were “substantial changes” in eelgrass populations over the four decades of study. But a look at the smaller spatial scales yielded “no obvious geographic coherence in [the] trends,” noting that adjacent eelgrass sites sometimes had opposite trends. This lack of geographic coherence, according to Shelton et al., “would [not] be expected if shared oceanographic or climate drivers controlled eelgrass trends,”.  Those would include, especially, climate change or ocean acidification.

Scaling up to the regional level and covering the entire estuary, Shelton et al. report, as illustrated in the figure below, that “over the past 40 years, eelgrass in Puget Sound has proven resilient to large-scale climatic and anthropogenic change,” confirming once again that “we do not see coincident changes in eelgrass populations that would indicate a major shared climatic driver across sites.”

This large-scale stability of eelgrass populations observed in the Puget Sound estuary over the past four decades has endured despite (1) a more than doubling of the human population in the area and (2) multiple major oceanographic anomalies (including several major El Niño and La Niña events), which is a testament to the adaptability and resistance of this keystone marine ecosystem species to human influence.

Perhaps more important, this undermine the “very high confidence” the U.S. National Climate Assessment assigns to predictions of future coastal ecosystem demise in response to CO₂-induced global warming and ocean acidification. The reality is that estimates of such vulnerability are largely overstated. One can only hope that the forthcoming 2018 U.S. National Climate Assessment will temper such projections by incorporating the realism observed in nature from studies like that of Shelton et al.