Quantitative paleolimnological inference models applied to a high-resolution biostratigraphic study of lake degradation and recovery, Onondaga Lake, New York (USA)

Once considered one of North America’s most polluted lakes, today Onondaga Lake, New York, USA has surface-water quality last seen prior to 1900. Paleolimnological inference models based on the remains of diatoms and chironomids in an annually dated sediment core were used to reconstruct quantitatively the post-1700 history of phosphorus, specific conductance, and volume-weighted hypolimnetic oxygen levels in the lake. Ostracode and sediment chemical analyses contributed to a detailed interpretation of past lake conditions. Thirteen biostratigraphic intervals are described. Discernible aspects of water quality include climate variability, trophic state, duration of bottom water anoxia, abundance of seasonal algal blooms, trends in specific conductance (a proxy for salinity), nineteenth and twentieth century industrial influences, pollution control efforts back to the late nineteenth century, and the timing of biological response to physical changes in the lake. Before 1822, freshwater, oligotrophic to borderline mesotrophic (~10 µg/l TP) conditions existed in Onondaga Lake and seasonal anoxia occurred in the hypolimnion. After the lowering of its water level in 1822, the lake became mesotrophic (10–20 µg/l TP). It became eutrophic (>20 µg/l) at mid-century, but between 1900 and 1919 there were sporadic returns to mesotrophic conditions. Hypereutrophy (>100 µg/l TP) prevailed from 1944 into the 1980s. Highest TP levels in the lake occurred during the 1950s through 1970s. Hypolimnetic anoxia increased after 1822 and the lake’s profundal benthos declined markedly, disappearing entirely early in the twentieth century. The lake became progressively more saline during the mid-nineteenth century, and was brackish (specific conductance >500 µS/cm) by the late nineteenth century. The highest salinity levels in the lake occurred from 1972 to 1980. Since the 1970s, inferences from paleolimnological analysis of the lake sediments reflect the well documented, steady decline of phosphorus concentration and salinity in Onondaga Lake, although chironomid-based evidence for abatement of deepwater anoxia is still lacking. This study demonstrates that quantitative paleolimnological inference models can be a valuable, complementary addition to lake management mechanistic modeling, as well as a key part of detailed historical water quality assessments.