Deoxygenation, Acidification and Warming in Waquoit Bay, USA, and a Shift to Pelagic Dominance

Coastal nutrient pollution, or eutrophication, is commonly linked to anthropogenic influences in terrestrial watersheds, where land-use changes often degrade water quality over time. Due to gradual changes, the management and monitoring of estuarine systems often lag environmental degradation. One example can be found at the Waquoit Bay National Estuarine Research Reserve, where we developed an analysis framework to standardize and analyze long-term trends in water quality and submerged vegetation data from monitoring programs that began in the 1990s. These programs started after the nearly complete loss of historically extensive Zostera marina (eelgrass) meadows throughout the estuary. Recently, eelgrass only persisted in small, undeveloped sub-embayments of the estuary, with conservative declines of over 97% in areal coverage. Over the past 2 decades, the average deoxygenation, acidification, and warming were −24.7 µmol O kg (−11%), 0.006 µmol H kg (+ 34%), and 1.0 °C (+ 4%), respectively. Along with the loss of eelgrass, there was also a decline in macroalgal biomass over 3 decades, resulting in a system dominated by pelagic metabolism, indicated by a 71% increase in water column chlorophyll a concentrations since 2009. This recent increase in phytoplankton biomass, which is highly mobile and transported throughout the estuary by tides, has resulted in recent degradation of isolated embayments despite their lower nutrient loads. This shift toward pelagic dominance in Waquoit Bay may indicate that other eutrophic and warming estuaries may also shift toward pelagic dominance in the future, as the Northeastern US is one of the fastest warming regions across the world.