Predicting benthic microalgal oxygen and nutrient flux responses to a nutrient reduction management strategy for the eutrophic Neuse River Estuary, North Carolina, USA

Abstract

In response to recent water quality declines, caused by excessive nitrogen (N) loading, a 30% reduction of N inputs into the Neuse River Estuary (NRE) has been mandated by the North Carolina State Legislature. Water quality model predictions as well as nutrient bioassays indicate that a 30% reduction in N will result in a 15% reduction in phytoplankton biomass (as chlorophyll a) in the NRE. Using previously published NRE light extinction coefficient component data and NRE irradiance data, we calculated that the average NRE compensation depth (<1% surface irradiance) would deepen by 13 cm following a 15% reduction in phytoplankton biomass. Hydrographic and bathymetry data were used in a Geographical Information System to plot the resulting increase in euphotic sediment surface area based on the predicted change in the compensation depth. The newly created euphotic sediment surface area represents 4.47 × 10 6 m 2 which is 20% larger than the average sediment surface area in the euphotic zone during the study period (1998–2000). Previous NRE work revealed that euphotic sediment in the NRE support autotrophic benthic microalgal communities (BMC) that alter oxygen and nutrient fluxes. To further quantify this effect, we conducted a series of light versus dark incubations of NRE sediments collected from above (shallow euphotic areas < 1 m water depth) and below (deep aphotic areas > 3.5 m water depth) the compensation depth. Sediment oxygen demand (SOD), nutrient flux and organic matter content were significantly lower in shallow water cores compared to their deep-water counterparts. Furthermore, the illuminated shallow cores demonstrated a 45% decrease in SOD compared to shallow cores incubated in the dark. The combined effect of the projected increase in BMC habitat coupled with the reduction in SOD and nutrient flux associated with BMC represents an overlooked and potentially important benefit of reduced N inputs that could accelerate water quality recovery in the NRE.