CHANGES IN PRIMARY PRODUCTIVITY ASSOCIATED WITH LIMING AND REACIDIFICATION OF AN ADIRONDACK LAKE

The addition of calcium carbonate to a clearwater acidic lake had a significant effect on phytoplankton abundance and on the biomass-specific rate of production. Chlorophyll concentrations and productivity : biomass ratios were strongly correlated with changes in lake acidity associated with liming and reacidification (r2 = 0.89 and 0.73, respectively). Summer mean chlorophyll increased by 0.9 Mg m-3 for each unit increase in lake pH, whereas productivity : biomass ratios decreased from >2 to <1, with an increase in pH from 5-5 to 7.0. Higher phytoplankton standing crops after liming were attributed to lower grazing rates following the decline of the dominant zooplankton taxa. A model relating biomass-specific productivity to incident light levels was found to be useful in interpreting treatment effects on productivity. Incident light and biomass (based on chlorophyll) accounted for 81% of the variability in productivity at the treated site. Comparisons of photosynthesis-irradiance curves indicated that the maximum photosynthetic capacity was higher following the watershed liming, compared to data collected after lake liming and during reacidification.