Decreases in primary production in Onondaga Lake from reductions in point source inputs of phosphorus

Patterns of planktonic primary production (net and gross) and community respiration are documented for culturally eutrophic Onondaga Lake, NY, USA. for 2005, based on sixty-eight in situ experiments that utilized the dissolved oxygen light/dark bottle methodology. This analysis was supported by common measures of drivers and manifestations of lake metabolism, including phosphorus (P) loading, chlorophyll-a concentrations, and Secchi disk transparency. The production and respiration observations for 2005 are evaluated in the context of previous results of metabolic measurements for this system, particularly those made in 1978 with the same protocols, and reductions in P loading from an ongoing management program. Approximately three-fold reductions in areal gross primary production (PP_g/a) and areal community respiration (R _c/a) are documented from 1978 to 2005, primarily in response to a fourteen-fold decrease in P loading from the primary point source. Average values for PP_g/a, and R_c/a in 2005 were 4.47 g O ₂ m^-2d^-1 and -4.43 g O₂ m ^-2d^-1, respectively. Coupled improvements in related features of water quality are reported, including decreases in chlorophyll-a concentrations and increases in Secchi disk transparency. A rectangular hyperbola type dependence of PP_g/a on P loading is indicated when the results from the two years are considered along with those from two intervening studies, depicting a shift from nearly nutrient-saturated production to a distinctly P-limited state. We found that primary production and respiration have been approximately in balance in this productive lake over the intervals of the metabolic studies.