PREHISTORIC NUTRIENT INPUTS AND PRODUCTIVITY IN NARRAGANSETT BAY

Calculations by others of the preindustrial deposition of inorganic ni trogen from the atmosphere in the area of Narragansett Bay compared wi th recent measurements suggest that this flux has increased almost 15 times over natural background. On the basis of modern studies of the e xport of nitrogen and phosphorus from temperate forests, the prehistor ic 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 a ssociated with refractory dissolved organic matter and thus contribute d little to the fertility of the bay. The largest source of reactive d issolved inorganic nitrogen (DIN) and phosphorus (DIP) for Narraganset t Bay under prehistoric conditions was the coastal ocean water entrain ed 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 ecosyste m-level experiments using large (13 m(3), 5 m deep) mesocosms designed as living models of Narragansett Bay showed that the primary producti on of phytoplankton in the bay is limited by the supply of DIN and tha t annual phytoplankton production is strongly correlated with the rate of input of DIN. The relationship between DIN input and annual phytop lankton production in the mesocosms is consistent with observations pu blished 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 tha t 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 Euro pean contact. It also seems likely that seagrasses and macroalgae once made a much larger contribution to total system production than they do today.