Seasonal and long-term nutrient trend decomposition along a spatial gradient in the Neuse River watershed

The Neuse River Estuary in North Carolina has recently received considerable public attention for severe algal blooms, large fish kills, and outbreaks of toxic microorganisms. To investigate the belief that nutrient enrichment has worsened in recent years, we analyzed long-term and seasonal trends in nutrient concentrations along the river and estuary employing seasonal trend decomposition using local regression analysis (STL). The nonparametric nature of the STL approach makes it possible to identify nonlinear trends and seasonal interactions that would be missed by traditional trend detection methods. The results indicate that while there may have been minor increases in nitrogen concentrations at upstream locations over the past twenty years, those changes are not reflected in the lower river and estuary. However,the pronounced decreases in phosphorus concentrations that occurred upstream, corresponding to a phosphorus detergent ban in 1988, do persist downstream. The net result is that the ratio of nitrogen to phosphorus concentrations in the estuary has increased considerably in the last 10 years. When compared with the Redfield value, ambient nutrient ratios suggest that phytoplankton growth in the estuary may be experiencing a shift from nitrogen to phosphorus limitation during much of the year. This shift may be inducing a change in the biotic Community that would help explain the perception of worsening eutrophication, despite an overall reduction in nutrient concentrations.