Network analysis of nitrogen inputs and cycling in the Neuse River Estuary, North Carolina, USA

The Neuse River estuary, North Carolina, United States, has demonstrated various symptoms of eutrophication during the past 20 yr. We contributed to an environmental assessment program, through ecological network analysis, a group of algorithms to evaluate networks of material flows within a structured system. Networks of nitrogen (N) cycling for 16 consecutive seasons were constructed based on previous field and laboratory studies. Network analysis provided understanding of the relationship between N loading and recycling, the fates of N and the expected interseasonal variation of both model inputs and outputs. Various indices indicated that recycling of imported N was very high, supporting measured observations. There was little correlation between estimates of loading and N uptake by phytoplankton, although loading of total and particulate N did correlate positively with export. Because of the high degree of recycling of N, the rate of loading of new N is a small fraction of the total processing of N or of the needs for primary production alone. We predict that on a short-term basis the controls on primary production tend to be associated with conditions in the estuary rather than import. This condition is likely to postpone easily observable responses to loading reduction over the entire estuary and in the short term, although improvements in water quality should occur over time.