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

Abstract

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 N and P than that found with N addition alone. This co-stimulation occurred during periods when surface-water dissolved inorganic N:dissolved inorganic P ratios were elevated above typical values of <5. Seasonal patterns in ambient nutrient concentrations revealed N maxima associated with spring, fall, and winter runoff events, with summer minima. Hydrologically driven N loading exerted a strong, year-round influence on primary production and nutrient limitation characteristics. High-flow events acted to oversaturate the upper estuarine nutrient filtering capacity, resulting in increased N delivery to the lower estuarine environment. The phytoplankton community responded to increased flow and nutrient loadings by increasing production and biomass levels, often very rapidly. 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. -from Authors