Effect of water residence time on annual export and denitrification of nitrogen in estuaries: A model analysis

A simple model of annual average response of an estuary to mean nitrogen lo ading rate and freshwater residence time was developed and tested. It uses nitrogen inputs from land, deposition from the atmosphere, and first-order calculations of internal loss rate and net export to perform a steady-state analysis over a yearly cycle. The model calculates the fraction of total n itrogen input from land and the atmosphere that is exported and the fractio n that is denitrified or lost to other processes within the estuary. The mo del was tested against data from the literature for I I North American and European estuaries having a wide range of physical characteristics, nitroge n loading rates, and geographical and climatic settings. The model shows th at the fraction of nitrogen entering an estuary that is exported or denitri fied can be predicted from the freshwater residence time. The first-order r ate constant for nitrogen loss within an estuary, as a fraction of total ni trogen in the water column, is 0.30 mo (1). Denitrification typically accou nts for 69-75% of the total annual net nitrogen removal from the water colu mn by processes within the estuary. The model makes explicit the dependence of nitrogen concentration in the water column on the loading rate of nitro gen, water residence time, estuary volume, and the rate constant for loss w ithin the estuary.