Impacts of Nutrients on Narragansett Bay Productivity: A Gradient Approach

Abstract

A goal of this study was to use ecological data collected previously in Narragansett Bay as a basis for predicting the impact of an imposed nitrogen reduction at sewage treatment facilities. Nitrogen and rates of primary production declined exponentially with dilution and distance toward the mouth of the bay, resulting in the nutrient and productivity gradient according to a previous study. An exponential regression was fit to total nitrogen (TN) data along the north-south axis of the bay. Since this type of regression reflects the down-bay dilution gradient and the nitrogen levels of productivity gradient, it has been assumed that lower trophic levels, but perhaps also higher trophic levels, might respond to the same gradient for secondary productivity. The ecological variables were regressed as functions of TN at the various distances south along the gradient. New plots were calculated as regression functions of 80 and 50% TN concentration decreases and re-plotted as distance south. The assumptions were that factors such as changes in nutrient inputs, temperature, mixing depth, water residence time, currents, sediment type, turbidity, and oxygen were uniform along the gradient to have no effect on the ecological variables. These analyses have indicated that a decrease of TN into Narragansett Bay due to tertiary treatment by 20 to 50% may decrease phytoplankton primary production and biomass, as well as zooplankton biomass and benthic infauna abundance in the same proportion. The decrease may be detectable at a 50% reduction in TN, but not at a 20% reduction based on the current survey data. Shellfish and fish changes will not be detectable along a nutrient gradient in Narragansett Bay due to exploitation, responses to climate trends, and the habitat preferences of species in these populations. The decrease in primary production may alleviate hypoxia associated with summer neap tide periods.