Dynamic modeling of nutrient inputs and ecosystem responses in the Waquoit Bay estuarine system (Massachusetts)

Waquoit Bay (Massachusetts) consists of several embayments under varying degrees of eutrophication, primarily due to increased nitrogen loads. In order to address the issue of critical nitrogen loading to this estuary, a watershed nitrogen loading model and an estuarine ecosystem model have been applied. Three subestuaries (Childs River, Quashnet River, and Sage Lot Pond) were chosen for direct comparison.; In conjunction with historical patterns of land use and groundwater travel in the Waquoit Bay watershed, an existing steady-state watershed nitrogen loading model (NLM) was modified to estimate historical and future rates of total dissolved nitrogen (TDN) to the coastal margins of Waquoit Bay and its subestuaries. The model simulations indicated an increased from approximately 5000 to 23,000 kg N yr−1 (0.8 to 3.7 g N m−2 yr−1) from 1930 to 2000. The model was also compared to steady-state simulations where the lag effect of groundwater travel time was not considered. These results indicate occasional significant differences (up to 44%) between the two modeling methods, especially between 1950 and 1990 when large areas of naturally vegetated and agricultural land within the watershed was converted to unsewered residential housing.; An estuarine ecosystem model was revised to explore the adaptation of new formulations associated with phytoplankton and benthic macroalgal production, community respiration, benthic-pelagic coupling, flushing, hypsography, and ecological management indicators (eelgrass and anoxia). Specifically, an alternative phytoplankton production formulation was adapted for shallow systems but proved to be incompatible with observed Waquoit Bay data. A sensitivity analysis was performed and highlights directional and magnitudinal responses to changes in several key model coefficients.; Based on a series of model comparisons, we conclude that the critical nitrogen load to the Waquoit Bay estuarine system, based on Z. marina habitat, is approximately 4 g TN m−2 yr −1. This estimate is consistent with observed and historical conditions and is likely the upper threshold associated with the decline in Z. marina habitat. Management options that include the sewering of all existing wastewater sources and significant reductions (perhaps 50%) of watershed fertilizer use, can result in nitrogen loads below this threshold in all but the Childs River and Quashnet River subestuaries where background loading exceeds estimated critical load.