Nitrogen Loading Sources and Eutrophication of the Neuse River Estuary, North Carolina: Direct and Indirect Roles of Atmospheric Deposition

Abstract

A multi-year (1990-1993) field survey and in situ bioassay study was undertaken to examine trophic and biogeochemical impacts of nutrient loading events at 3 representative oligohaline and mesohaline locations in the Neuse River Estuary, North Carolina.Additional data were evaluated from an earlier study (1987)(1988)(1989)(1990)) at a mesohaline location Previous studies showed the estuary as being nitrogen-limited throughout much of the year.In addition there is evidence and concern that recent increases in nitrogen loading have led to spatial and temporal expansion of phytoplankton blooms, indicating accelerating eutrophication.Accordingly, we examined the roles of significant nitrogen (N) inputs on the eutrophication process.Emphasis was placed on external or "new" N loading sources such as riverine discharge and direct atmospheric deposition (rainfall), as well as internal or regenerated N.The amounts and timing of N loading events, coupled to water residence time, salinity, and advection proved to be overriding "forcing features" controlling phytoplankton productivity, biomass, species composition and bloom formation.In addition, bottom water oxygen depletion and release of internal N (as ammonium) and P (as orthophosphate) supplies were indirectly controlled by hydrological events.Large contrasts in phytoplankton production dynarnics were observed between low discharge and high discharge years.Irradiance (light intensity and flux) and water temperature fluctuations played additional roles in determining seasonal production and bloom patterns.Field and biossay studies strongly implicated N loading as an overriding determinant of primary production.Phosphorus played a co-limiting role only during high N discharge and loading events in late-winter and spring, when estuarine N:P ratios exceeded 151.Rainfall-based N proved to be an important "new" N source, both budgetarily and as a direct stimulant of primary production.This was most evident between late spring and fall, when ambient dissolved inorganic nitrogen concentrations were low.Internal N loading events, such as sediment ammonium (NHJ release during N-limited summer and fall months, were of considerable importance in sustaining primary production.The maintenance of acceptable phytoplankton production and bloom conditions in the Neuse River Estuary should focus on controlling (reducing) N loading.A reduction to approximately 70 % of current levels is recommended.This recommendation is based on both nutrient addition bioassay results and documented increases in N loading over the past 2 decades.Both the amounts and timing of terrigenous and atmospheric sources of nitrogen loading should be targeted when considering cutbacks.More than half the annual dissolved inorganic nitrogen (DIN) loading can occur during the winter-spring (January-April) high rainfall / runoff periods.Recommended actions are timely and prudent applications of N-based fertilizers, construction of vegetation "buffer zones", no-till farming practices, close scrutiny and control of N waste discharge from poultry, hog and cattle operations and adherence to wastewater treatment plant effluent standards will be economically attainable and environmentally effective.Reduction of atmospheric N inputs must rely on statewide and national efforts aimed at controlling N emissions (NO,-) from fossil fuel combustion and agricultural emissions.Relatively high ambient P 46 .......