Trophic state responses of Onondaga Lake, New York to reductions in phopshorus loading from advanced wastewater treatment

This study evaluated the response of previously culturally hypereutrophic Onondaga Lake, New York, to major reductions in phosphorus (P) loading from a municipal wastewater treatment plant achieved over a 35-year period. Analysis of the response was based on long-term monitoring of effluent total P concentration (TP Metro ) and a robust array of in-lake metrics of trophic state, including (1) TP, (2) chlorophyll a (Chl- a ), (3) Secchi disk depth, (4) areal hypolimnetic oxygen deficit (AHOD), (5) the downward flux of volatile suspended solids, and (6) the minimum dissolved oxygen concentration in the upper waters during fall turnover. Substantial positive responses, which were significantly linearly related to the decreases in TP Metro , were documented for all of the metrics over the monitoring period. For example, a 91% reduction in TP Metro resulted in reductions of 84% and 59% in Chl- a and AHOD, respectively. The changes depict a transformation from hypereutrophy to upper mesotrophy, with the exception of AHOD values that remain in the eutrophic range. The delayed response of AHOD reflects slower sediment diagenesis processes. The relationships among the metrics of trophic state are considered in the context of literature expressions, accepted paradigms, and limitations related to variations in food web effects. Empirical models that include the effects of multiple drivers on interannual variations in contemporary trophic state metrics were developed and applied to forecast variability under existing conditions. This case study contributes to the scientific literature describing lake rehabilitation through reductions in P loading.