Climatic control on eutrophication of the Hudson River estuary

Eutrophication is arguably the biggest pollution problem facing estuaries globally, with extensive consequences including anoxic and hypoxic waters, reduced fishery harvests, toxic algal blooms, and loss of biotic diversity. However, estuaries vary greatly in their susceptibility to eutrophication. The Hudson River estuary receives very high levels of nutrient inputs yet in the past has shown relatively low rates of phytoplankcon productivity and is generally considered to be only moderately susceptible to eutrophication. Here, we show that eutrophication and primary production in the Hudson estuary can increase dramatically in response to climatic variation and lowered freshwater discharge from the watershed. During dry summer periods in 1995 and 1997, rates of primary production were substantially higher than those measured during the 1970s, when freshwater discharge tended to be high. In the Hudson, low freshwater discharge increases water residence times and stratification and deepens the photic zone, all of which (alone or in combination) could lead to the observed increase in primary production. Our data, along with the prediction of most climate change models that freshwater discharge will be lower in the future during the summer in the northeastern US, suggest that the Hudson will become more susceptible to eutrophication. Eutrophication in an estuary is a complex process, and climate change is likely to affect each estuary differently due to interactions with nutrient loadings and physical circulation. Hence, it is essential to consider the effects of climate change in the context of individual estuarine functioning to successfully manage eutruphication in the future.