9629314 Zehr Productivity in estuaries is often controlled by nitrogen(N) availability,even though there is a vast reservoir of gaseous N2 in the atmosphere. N2 fixation, the conversion of atmospheric N2 into cellular nitrogen, can occur at high rates in the plankton of freshwater ecosystems, due to dense blooms of N2 fixing cyanobacteria. These blooms can be toxic and cause water taste and odor problems. Such blooms are scarce in marine systems, even those undergoing eutrophication (becoming more productive as a result of increased nutrient inputs). The contrast between the predominance of N2 fixing organisms in freshwater and the apparent dearth of N2 fixing organisms in estuaries and oceans, provides the basis for the proposed research. Estuaries define the transition from freshwater to the marine environment, and thus provide a basis for investigating the factors that control N2 fixing blooms in both freshwater and marine environments. This objective of this research is to define how N2 fixing organisms are distributed spatially and temporally in the freshwater, mesohaline and euhaline reaches of the Neuse River estuary. The conceptual framework developed in this research project will provide a new perspective for understanding N2 fixation potentials in marine, estuarine and freshwater environments, which will help to understand when and where nitrogen-fixing blooms are likely to occur in response to the threat of cultural eutrophication. A particularly novel aspect of this investigation is that experimental manipulations will be used to determine how salinity and nutrients affect the distribution, growth and activity of specific N2 fixing populations using a molecular biology approach. The results of these experiments will provide new information on the regulation of productivity and algal bloom-formation in estuaries, freshwaters, and t he marine environment.
