Biologically Mediated Nitrogen Dynamics in Eutrophying Estuaries. Assessing Denitrification, N2 Fixation and Primary Productivity Responses to Proposed N Loading Reductions in the Neuse River Estuary

摘要

Experimental manipulations were used to predict the effects of reducing the ratio of dissolved inorganic nitrogen (DIN) to dissolved inorganic phosphorous (KIP) (DIN:DIP ratio) in the Neuse River Estuary (NRE) on the abundance and activity of N2 fixing cyanobacteria. Changes in primary productivity, N2 fixation (nitrogenase activity), genetic potential for N2 fixation (presence of nifH), phytoplankton taxonomic composition (based on diagnostic photopigment concentrations) and numbers of N2 fixing cyanobacteria (microscopy) were determined. Results from these experiments indicate that if reduction of N loading leads to a reduced ambient DIN:DIP ratio in the NRE, rates of N2 fixation may be higher when diazotropic cyanobacteria are present. We did not, however, detect an increase in either the diversity or abundance of N2 fixers resulting from experimental manipulations of DIN:DIP ratio. A 30% reduction in nitrogen (N), phosphorus (P) or both N and P concentrations caused a reduction in phytoplankton assimilation number at the estuarine bioassay site. There was no reduction in assimilation number at the riverine site when assessing the data for the entire experimental period. When analyzing individual experiments, N dilution reduced assimilation number as often as dilution of both N and P concentration, while P reduction along reduced assimilation number only once. Phytoplankton taxonomic composition as measured by HPLC diagnostic photopigment analysis was not altered by any dilution at any location in this work.