A time-dependent model of nitrogen loading to estuaries from coastal watersheds

We have developed a dynamic nitrogen loading model (NLM) that incorporates temporal and spatial trends in land use with a three-dimensional ground water model. In conjunction with historical patterns of land use in the Waquoit Bay (USA) watershed, we have modified an existing steady-state watershed NLM 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 a significant increase in nitrogen loading to these systems in recent decades, We estimated that the TDN loading rate to Waquoit Bay increased from approximately 5000 to 23 000 kg yr(-1) (0.8 to 3.7 g N m(-2) yr(-1)) from 1930 to 1990, We also compared the dynamic model with steady-state simulations where the lag effect of ground water travel time was not considered, These results indicate occasional significant differences (up to 37%) between the two modeling methods, especially between 1950 and 1990, when large areas of naturally vegetated and agricultural land within the watershed were converted to unsewered residential housing. Although all subestuaries experienced similar temporal trends in nitrogen load, heterogeneity in the timing, source, and magnitude indicates that these factors are dependent upon watershed size, shape, and spatio-temporal trends in land use.