Seasonal and hydrological control of phytoplankton nutrient limitation in the lower Neuse River Estuary, North Carolina

Abstract

Nutrient limitation of phytoplankton production was assessed monthly from 1987 through 1990 in the lower Neuse River Estuary, North Carolina, USA, a well-mixed, mesotrophic system. Nutrient addition bioassays indicated that the lower estuary experienced a general state of nitrogen limitation, with especially pronounced limitation during summer months, a period of high phytoplankton productivity. Bioassays conducted during spring months showed significantly greater stimulation of algal productivity with the addition of nitrogen and phosphorus than that found with nitrogen addition alone. This CO-stimulation occurred during periods when surface-water dissolved inorganic nitrogen : dissolved inorganic phosphorus ratios were elevated above typical values of < 5. Seasonal patterns in ambient nutrient concentrations revealed nitrogen maxima associated with spring, fall, and winter runoff events, with summer minima Hydrologically driven nitrogen loading exerted a strong, yearround influence on primary production and nutrient limitation characteristics. High-flow events acted to oversaturate the upper estuarine nutrient filtering capacity, resulting in increased delivery of nitrogen to the lower estuarine environment. The phytoplankton community responded to increased flow and concomitant nutrient loadings by increasing production and b~omass levels, often very rapidly. In this regard, hydrologic factors influencing nitrogen loading (terrigenous runoff, point source inputs, and wet and dry atmospheric deposition) are key determinants of the trophic state of this estuary.