nutrient loading based on areal loading alone may be improved (Nixon 1992).
Scientists and managers are equally concerned about reversing the eutrophication trend observed in many of the nation’s estuaries. Whether the same classification schemes being developed to predict effects of increasing nutrients will work equally well in predicting reversals, if nutrient inputs are reduced, remains to be determined. As some studies have indicated, there are non-linearities and thresholds in eutrophication response to increased nutrient loading. There are undoubtedly additional non-linearities and thresholds governing the response of estuarine systems to reductions in nutrient loading. While loading, dilution, mixing, flushing, circulation, and stratification may be the key parameters governing the initial response of a system to nutrient increases, internal stores of organic matter and internal nutrient cycling processes might be important factors governing eutrophication reversal. There are few locations where nitrogen loading reductions have occurred and even fewer where resultant changes in estuarine trophic status have been chronicled.
The Himmerfjärden, an estuary of the Swedish central Baltic coast, is one example where eutrophication has been reversed following reductions in nitrogen loading. Present nitrogen loads are less than 10 percent of pre-treatment input levels. Primary production, chlorophyll concentrations, and water transparency have all improved since treatment began. In this system, the reversal showed little lag in response following source reductions, presumably reflecting the rocky nature of the coastal zone and small internal stores of organic nutrients (Granéli et al. 1990; Elmgren and Larsson 1997).
Tampa Bay, Florida, is another coastal system where local managers are reversing eutrophication. In this seagrass-dominated estuary, however, significant nutrient reductions have not resulted in eutrophication reversals to the extent observed in the Himmerfjärden. Time lags in recovery are hypothesized to result from large internal sources of detrital nitrogen accumulated over years of earlier eutrophic conditions. Recovery is likely to be slow until these historic stores of nitrogen are reprocessed and either flushed from the system or denitrified. Box 6-5 describes the Tampa Bay reversal in greater detail.