Coupled and De‐Coupled Watershed Nutrient Loading and Estuarine Nutrient Concentrations: A Case Study of the Chesapeake Bay

ABSTRACT A persistent challenge for coastal zone eutrophication management is the influence of site‐specific factors in moderating water quality responses to nutrient loading. Although conceptual models of the eutrophication process have existed for decades, site‐specific differences in physical and biological factors complicate our ability to predict changes in estuarine conditions resulting from nutrient loading. Furthermore, differences in the magnitude and speed of nutrient load reductions among estuaries with contrasting land uses and nutrient sources will impact estuarine responses. We analyzed three decades of monitoring data in Chesapeake Bay within the context of a simple framework to examine the spatial and temporal patterns of water quality change in response to watershed load reductions across diverse sites. We found that the majority of sites we analyzed fit the conceptual model if load reductions primarily resulted from wastewater treatment plant upgrades. In contrast, coastal plain watersheds with high‐intensity agricultural uses continued to yield high amounts of nutrients, and water quality has not improved in the receiving waters in some of these tributaries. A small subset of systems had unexpected responses (e.g., a long‐term load decline but concentration increase). This approach is useful for application in other well‐monitored systems to enhance our understanding of factors involved in the spatial and temporal variation in recovery from eutrophication.