Eutrophication, Harmful Algae, Oxygen Depletion, and Acidification

Eutrophication is among the most widespread and deleterious anthropogenic impacts to coastal marine ecosystems. The Chesapeake Bay (CB) and Northern Adriatic Sea (NAS) have long histories of nutrient‐fueled eutrophication. We compare the susceptibility and symptoms of eutrophication in both systems and discuss recent reversals of eutrophication (oligotrophication) and future considerations. Differences in the residence time of water (CB > NAS), volume relative to the riverine transport of nutrient‐rich water (CB << NAS), bathymetry, freshwater‐driven seasonal stratification, and density‐driven estuarine circulation make CB more susceptible to eutrophication than the NAS. Thus, CB has experienced large accumulations of phytoplankton biomass each spring, widespread summertime bottom oxygen depletion, and long‐term loss of submerged aquatic vegetation. Despite its lower susceptibility, the NAS has also exhibited numerous symptoms of eutrophication, including elevated phytoplankton biomass in coastal river plumes, frequent harmful algal events involving a diversity of potentially toxic phytoplankton, unique mucilage events, and episodic oxygen depletion dependent on vertical stratification and current patterns. In recent decades, some of these symptoms have begun to lessen, at least in part due to reductions in nutrient loading. The interaction of oligotrophication with changes in other stressors will become increasingly important, particularly climate‐related changes to hydrology, temperature, and acidity.