Non-linear Responses of a Coastal Aquatic Ecosystem to Large Decreases in Nutrient and Organic Loadings

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Between 1991 and 2000, Boston Harbor, a bay-estuary in the northeast USA, experienced a decrease in loadings of total nitrogen (TN), total phosphorus (TP), and particulate organic carbon (PC) of between ~80% and ~90%. The average concentrations of TN and TP in the harbor water column were decreased in linear proportion to the loadings. The changes to the chlorophyll-a (chl-a), PC, and bottom water DO concentrations were curvilinear relative to the loadings, with larger changes at low than high loadings. For TN and TP, the starts of the decreases in concentrations coincided with the starts of the decreases in loadings. For the three variables that showed curvilinear responses, the starts of the changes lagged by 2 to 3 years the starts of the decreases in TN loadings. Total suspended solid concentrations and water clarity in the harbor were unchanged. The study shows that for systems such as Boston Harbor, decreases in nutrient loadings will have quite different effects depending on the base loadings to the system. © 2010 Coastal and Estuarine Research Federation.

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Estuaries and Coasts