Source: Lefcheck, J. S., Wilcox, D. J., Murphy, R. R., Marion, S. R. and Orth, R. J. (2017), Multiple stressors threaten the imperiled coastal foundation species eelgrass (Zostera marina) in Chesapeake Bay, USA. Glob Change Biol, 23: 3474–3483. doi:10.1111/gcb.13623
Why we care?
While many people prefer to spend their summer vacations soaking up the sun on sandy beaches, I prefer to explore what lies beneath the surface of the ocean. Fellow snorkelers, scuba divers, and boaters have undoubtedly seen the beautiful underwater seagrass meadows that populate nearshore ocean waters. But did you know that these seagrass meadows—commonly referred to as seagrass beds—are critically important to our economy? Not only do seagrass beds serve as a global carbon sink, they also provide a habitat to young blue crabs and many fishes that we love to eat. After all, our summer vacations wouldn’t be complete without delicious crab legs and fish and chips.
How seagrass beds are changing
Unfortunately, seagrass beds have been declining worldwide in response to multiple threats including increasing coastal development and the warming of ocean waters due to global climate change. When coastlines are developed, the amount of sediment in nearshore waters increases, leading to cloudy water. Decreased water clarity is bad news for seagrass, which need light to photosynthesize.
In Chesapeake Bay, eelgrass (Zostera marina) is the dominant species of seagrass. Recent research on eelgrass beds in Chesapeake Bay has shown that decreasing water clarity and increasing water temperature from climate change are interacting to drive the loss of economically important eelgrass beds. Researchers analyzed 31 years of records on water quality in addition to aerial photographs of seagrass beds from 1984-2014 to determine that seagrass beds have declined 29% since 1991 in Chesapeake Bay.
It’s not just the decline that is startling to scientist though; it’s the movement of eelgrass to shallower waters. Lefcheck et al. reported that eelgrass beds have shifted over 500 feet closer to shore because of reduced water clarity, resulting in a dramatic reduction (50%) of deeper eelgrass beds. Warming temperatures have also had a negative impact on eelgrass beds, especially in shallow waters where light does not limit seagrass growth.
Most importantly, Lefcheck et al. show that the effects of decreasing water clarity and increasing temperature interact to reduce eelgrass beds more than either driver alone. The study reports that projected increases in temperature over the next 30 years would reduce eelgrass beds by 38%, while projected decreases in water clarity over the same time period would lead to a 84% decrease in eelgrass beds. However, when changes in water temperature and clarity are considered simultaneously, the result is a loss of 95% of current eelgrass in Chesapeake Bay. The potential economic loss as a result of declining seagrass is estimated at $1.5-2.5 billion, although these estimates are based on small-scale data and should be interpreted with caution. Still there is no doubt that these dramatic changes in eelgrass communities come with a significant ecological and economic cost.
Despite the significant decline of eelgrass beds in Chesapeake Bay, Lefcheck et al. also show that eelgrass has the potential to recover from episodic warming temperatures. In both 2005 and 2010, Chesapeake Bay experienced high average summer temperatures, which led to decreases in eelgrass beds. However, because these events did not persist for multiple years, eelgrass beds were able to recover during the next summer.
What can we do?
So what can coastal managers do to aid in the recovery of eelgrass beds? While managers have little control over global phenomena (e.g. warming ocean temperatures), they can implement further management strategies to improve water clarity in coastal watersheds. Lefcheck et al. provide further evidence that if water clarity improves in Chesapeake Bay, eelgrass could survive moderate increases in temperature in the future. Management efforts that can improve water clarity include reducing nutrient release from wastewater treatment facilities, investing in stormwater treatment, and promoting coastal management that decreases the loss of sediment from coastal habitats.