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Daphnia grazing, the clear water phase, and implications of minerogenic particles in Onondaga Lake

Abstract We evaluated the influence of Daphnia grazing on water clarity in Onondaga Lake, New York, by testing 2 related hypotheses: (1) that the high clarity (Secchi disk depth [SD]) events observed in 18 years of a 27-year record were occurrences of the clear water phase (CWP) associated with elevated levels of Daphnia grazing, and (2) that reductions in minerogenic (i.e., inorganic) particle concentrations and the accompanying reductions in light scattering (quantified by bm) due to grazing contributed importantly to the SD signatures of the CWPs. Our analysis is based on a 27-year record of the Daphnia population, SD, and the concentration of chlorophyll a (Chl-a), as well as shorter-term characterizations of minerogenic particle concentrations in the lake and application of an optical theory-based SD model previously tested for the lake. The first hypothesis is supported by (a) the consistency of seasonal patterns of Chl-a, SD, and Daphnia biomass, (b) the consistency of Daphnia biomass maxima with literature values associated with the CWP, (c) the positive relationships demonstrated between interannual differences in Daphnia biomass (and estimated grazing rates) and peak SD and duration of high clarity events, and (d) because during CWPs, estimated grazing rates exceed by a wide margin reasonable maximum values of phytoplankton growth rates. The second hypothesis is supported by (a) demonstration that the minerogenic particle sizes responsible for bm (and so its influence on SD) are within the size range subject to Daphnia grazing, and (b) application of the SD model, in the context of SD and Chl-a observations, showing that a reduction in bm is necessary to explain the high SD values observed during CWPs. Finally, a review of the growing literature on the contribution of bm to the overall particle scattering coefficient in various lakes suggests that minerogenic particles are often important to the SD signature of CWPs.