Multiple stressors threaten the imperiled coastal foundation species eelgrass (Zostera marina) in Chesapeake Bay, USA

Abstract

Interactions among global change stressors and their effects at large scales are often proposed, but seldom evaluated. This situation is primarily due to lack of comprehensive, sufficiently long-term, and spatially-extensive datasets. Seagrasses, which provide nursery habitat, improve water quality, and constitute a globally-important carbon sink, are among the most vulnerable habitats on the planet. Here, we unite 31-years of high-resolution aerial monitoring and water quality data to elucidate the patterns and drivers of eelgrass (Zostera marina) abundance in Chesapeake Bay, USA, one of the largest and most valuable estuaries in the world with an unparalleled history of regulatory efforts. We show that eelgrass area has declined 29% in total since 1991, with wide-ranging and severe ecological and economic consequences. We go on to identify an interaction between decreasing water clarity and warming temperatures as the primary driver of this trend. Declining clarity has gradually reduced eelgrass over the past two decades, primarily in deeper beds where light is already limiting. In shallow beds, however, reduced visibility exacerbates the physiological stress of acute warming, leading to recent instances of decline approaching 80%. While degraded water quality has long been known to influence underwater grasses worldwide, we demonstrate a clear and rapidly emerging interaction with climate change. We highlight the urgent need to integrate a broader perspective into local water quality management, in the Chesapeake Bay and in the many other coastal systems facing similar stressors.

An invasive foundation species enhances multifunctionality in a coastal ecosystem

“…Carolina alone, estimates suggest that 97% of seagrasses (36), 90% of oyster reefs (4,37), and 12% of salt marshes (7) have been lost relative to their historical extent, a pattern similar to losses of native coastal foundation species throughout the biogeographical region that Gracilaria has invaded (e.g., refs. 4,12,14,36,37) and, more generally, to global losses (6-8). Gracilaria's invasion of North Carolina mudflats provides a unique system for testing how a nonnative foundation species affects ecosystem functioning where native analogs are absent or have declined significantly in recent times (6,38).…”

“…Because a significant proportion of society depends on these coastal ecosystems to generate >10 14 USD in services each year through storm protection, food production, and tourism (1,9), conservation resources of >10 9 USD are spent globally each year in an attempt to reverse the decline of these coastal foundation species (10,11). Yet many efforts to restore coastal habitats have achieved only limited success despite high investment per hectare (12)(13)(14). It is therefore pertinent that research examine different means by which the loss of marine foundation species can be offset in order to provide society with the goods and services needed to prosper.…”

“…5,8,14,28,29), recognizing that nonnative species may enhance ecosystem functions could influence management decisions (17,18). For example, in coastal areas that have experienced degradation of native foundation species, such as on coral reefs (5,30), seagrass beds (6), and oyster reefs (8,12), invasive foundation species could have significant positive effects, particularly if they replace and substitute ecological functions that have otherwise been lost (18,21,22,31).…”

An assessment of the extent and social-ecological impacts of salt marsh mosquito ditching

“…Yet mangroves and salt marshes have been extensively cleared and filled to make room for coastal development (Teal et al 1986, Bromberg and Bertness 2005, Polidoro et al 2010, Thomas et al 2017, while mangroves and oysters have been over-exploited and harvested for timber and food, leading to dramatic declines in their abundances (Newell 1988, Alongi 2002, Grabowski and Peterson 2007. Exacerbating these direct reductions in biomass, indirect impacts such as runoff-induced water quality changes have also been linked to declines in oysters, salt marsh grasses, seagrasses, and kelp (Deegan et al 2012, Lefcheck, Wilcox, et al 2017, Filbee-Dexter and Wernberg 2018, Jeppesen et al 2018.…”