Prehistoric Nutrient Inputs and Productivity in Narragansett Bay

Abstract

Calculations by others of the preindustrial deposition of inorganic nitrogen from the atmosphere in the area of Narragansett Bay compared with recent measurements suggest that this flux has increased almost 15 times over natural background. On the basis of modern studies of the export of nitrogen and phosphorus from temperate forests, the prehistoric watershed also probably contributed very little reactive N or P to the bay. New information from undisturbed old-growth forests suggests that most of the N that was exported from the watershed was probably associated with refractory dissolved organic matter and thus contributed little to the fertility of the bay. The largest source of reactive dissolved inorganic nitrogen (DIN) and phosphorus (DIP) for Narragansett Bay under prehistoric conditions was the coastal ocean water entrained in the bay in estuarine circulation. The total input of DIN to this estuary has increased about five-fold and the input of total DIP has approximately doubled as a result of human activities. Recent ecosystem-level experiments using large (13 m3, 5 m deep) mesocosms designed as living models of Narragansett Bay showed that the primary production of phytoplankton in the bay is limited by the supply of DIN and that annual phytoplankton production is strongly correlated with the rate of input of DIN. The relationship between DIN input and annual phytoplankton production in the mesocosms is consistent with observations published by others working in 10 different natural marine systems, and a functional regression of the field and experimental data provides a tool to calculate the rate of prehistoric phytoplankton production that would have been associated with the prehistoric DIN input estimates. The result of this calculation suggests that phytoplankton production in the bay has approximately doubled (from about 130 g C m−2 yr−1 to 290 g C m−2 yr−1 for a baywide average) since the time of European contact. It also seems likely that seagrasses and macroalgae once made a much larger contribution to total system production than they do today.