Original paper

A long-term record of epilimnetic phosphorus patterns in recovering Onondaga Lake, New York

Effler, Steven W. O'Donnell

Abstract

A retrospective analysis for a 23 year (1987-2009) record of spectrophotometically determined forms of phosphorus (P) is presented for the epilimnion of Onondaga Lake, New York, a period over which it was transformed from severely culturally eutrophic to mesotrophic through reductions in point source inputs. Patterns of five forms of P were evaluated, total P (TPe), total dissolved P (TDPe), soluble reactive P (SRPe), particulate P (PPe = TPe - TDPe) and dissolved organic P (DOPe = TDPe - SRPe) based on weekly observations from multiple epilimnetic depths for the April (spring turnover) - October (fall turnover) interval of each year. A 10-fold reduction in the effluent concentration of the dominant source (TPMetro), made in several steps, caused dramatic decreases in summer average epilimnetic P concentrations, 6.8-fold for TPe, 4.8-fold for PPe, 14.9-fold for TDPe, 8.4-fold for DOPe, and 70-fold for SRPe, as well as improvements in common metrics of trophic state. Long-term patterns of summer average concentrations of each of these forms tracked the progression of TPMetro at an annual time step. Both short- and long-term patterns of in-lake concentrations, particularly for SRPe, support the position that the lake remained often nutrient-saturated through much of the record, but shifted to distinctly P limited in response to the most recent (2005) reduction in TPMetro. The diagnostic value of monitoring the dynamics of these forms of P at multiple time steps to track lake metabolism and features of the P cycle is demonstrated, including uptake of dissolved forms, effects of Daphnia metabolism, dynamics of phytoplankton uptake, and long-term responses to reductions in external loading. Evidence is presented that a noteworthy fraction of the PPe pool is associated with non-phytoplankton particulates.

Keywords

phosphorusepilimnionsoluble reactive phosphorusdaphniaretrospective analysesphosphorus cycle