Changes in primary production in Onondaga Lake, NY: Magnitude, metrics, and drivers

Changes in phytoplankton primary production and common metrics and drivers of trophic state are documented and critically evaluated for a 35 year (1968-2002) period for polluted, culturally eutrophic Onondaga Lake, NY. The lake is presently the focus of an on-going rehabilitation program to abate its cultural eutrophication problems. The analysis is supported by measurements of primary production and long-term monitoring of trophic state metrics and other common limnological parameters. Measurements of primary production and community respiration were made in 1978, 2002, and portions of 2000 and 2001, utilizing dissolved oxygen-based isolated community (light/dark bottle) and non-isolated community (diel measurements) protocols. Limnological and discharge monitoring of the following metrics supported the analysis: epilimnetic total phosphorus, epilimnetic chlorophyll a (Chl(c)), Secchi disc (SD), areal hypolimnetic oxygen deficit (AHOD), salinity (S), downward flux of particulate organic carbon (POCdf), and concentration of TP in the effluent of a wastewater treatment plant (TPMetro) that dominates loading. Direct measurements establish the areal daily average gross primary production has decreased similar to 30 to 40% since 1978. The magnitude of this decrease corresponds well with decreases observed in the surrogate metrics of primary production of POCdf (37%) and AHOD (49%). Detailed time series of POCdf and AHOD indicate the decrease occurred abruptly in the late 1980s. This decrease was primarily a result of the reduction in S that attended cessation of production by an industry, rather than a response to the 30-fold reduction in TPMetro achieved over the last 32 years. Magnitudes of Chl(c) and SD are not presently reliable metrics of trophic state in this lake because of increases, and year-to-year variations, in top-down effects since closure of the industry.