Patterns and modeling of the long-term optics record of Onondaga Lake, New York

Abstract

A retrospective analysis of long-term data sets of Secchi disc depth (SD, 40 years), the diffuse light attenuation coefficient for downwelling irradiance for photosynthetically available radiation (k d (PAR), 23 years), and chlorophyll-a ([Chl], 28 years), is presented for culturally eutrophic and industrially polluted Onondaga Lake, New York. The effects of changes in multiple drivers on SD and k d (PAR) are resolved in the record, including: (1) salinity decreases from closure of an industry, (2) changes in the grazing by exotic and native Daphnia, (3) long-term variations in populations of planktivorous fish, (4) runoff events, and (5) progressive decreases in phosphorus (P) loading. Four time intervals, or regimes, are identified for the record that had unifying features with respect to drivers and SD observations. The most dramatic changes in clarity conditions were the abrupt transition to annual occurrences of a clear-water phase (CWP) for 16 years following a coupled decrease in salinity and return of native Daphnia, and the subsequent loss of the CWP (for 5 years) from the loss of these cladocerans. The CWP was characterized by coincident dramatic increases in SD (annual maxima ≥ 4.55 m) and decreases in [Chl] and high densities of Daphnia. The benefits of P management were most clearly manifested by a 3-fold decrease in average [Chl] and a 2-fold increase in SD between two of the regimes, separated by 25 years, which both lacked CWPs. A probabilistic mechanistic model for SD and k d (PAR), that represents the effects of multiple constituents on the regulating processes of absorption and scattering, is developed, tested for the long-term record. The model is applied to partition the scattering coefficient (b) according to contributions of phytoplankton and tripton (b NAP ). Good model performance is demonstrated through: (1) closure of estimates of b with independent measurements, (2) consistencies of b NAP predictions with independent estimates based on an individual particle analysis technique, and the timing of runoff events, and (3) closure of predictions of k d (PAR) with observations. Broadly applicable nomographs are formed with the model that represent the dependence of SD and k d (PAR) on phytoplankton ([Chl]) and inorganic tripton (b NAP ) levels, that can be used to predict responses of these optical metrics to changes in the levels of these attenuating constituents. Inorganic tripton in the size range 1 to 10 μm, has played a critical role in influencing important features of the lake's optical regime by: (1) contributing importantly to b throughout the record (30 to 40 %, on average), (2) regulating optical signatures from runoff and CWP events, and (3) limiting increases in SD that can be achieved from decreases in phytoplankton biomass.