Changes in deposition of phytoplankton constituents in a Ca2+ polluted lake

Systematic reductions in the deposition rate of organic C, N, P, and chloro phyll (Chi) are documented for Ca2+ polluted, culturally eutrophic, Onondag a Lake, NY, based on analyses of weekly sediment trap collections over the May-October interval for 10 years of the 1980-1992 period. Inputs of both n utrient-rich domestic waste and industrial salt waste (including Ca2+) decr eased over this period. Constituent ratios of the collected sediment indica te phytoplankton biomass was the dominant source of the deposited organic C , N, and Chi. Substantial decreases in downward fluxes of these constituent s occurred starting in 1987: 37, 42, 25, and 54%, on average, for organic C , N, P, and Chi, respectively. These reductions were driven primarily by th e decreases in the lake's salinity and Ca2+ concentration, that resulted fr om the closure of a soda ash manufacturing facility (1986), rather than dec reases in water column P concentrations from reductions in domestic waste l oading. Three different mechanisms for the decreased deposition, related to the reductions in salinity and Ca2+ concentration, are considered: (i) dec rease in coating of phytoplankton with CaCO3 precipitate, (ii) increased gr azing of phytoplankton by large cladocerans, and (iii) decreases in coagula tion of phytoplankton. The greater loss of phytoplankton biomass through de position, driven by salt waste inputs from the industry, exacerbated the la ke's problem of high primary production. This response is consistent with e cological theory for nutrient saturated phytoplankton growth but has not pr eviously been demonstrated on a whole-lake basis.