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 considerabl e public attention for severe algal blooms, large fish kills, and outbreaks of toxic microorganisms. To investigate the belief that nutrient enrichmen t has worsened in recent years, we analyzed long-term and seasonal trends i n nutrient concentrations along the river and estuary employing seasonal tr end decomposition using local regression analysis (STL). The nonparametric nature of the STL approach makes it possible to identify nonlinear trends a nd seasonal interactions that would be missed by traditional trend detectio n methods. The results indicate that while there may have been minor increa ses in nitrogen concentrations at upstream locations over the past twenty y ears, those changes are not reflected in the lower river and estuary. Howev er,the pronounced decreases in phosphorus concentrations that occurred upst ream, corresponding to a phosphorus detergent ban in 1988, do persist downs tream. The net result is that the ratio of nitrogen to phosphorus concentra tions in the estuary has increased considerably in the last 10 years. When compared with the Redfield value, ambient nutrient ratios suggest that phyt oplankton growth in the estuary may be experiencing a shift from nitrogen t o 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 concent rations.