Past trophic status and hypolimnetic anoxia during eutrophicaton and remediation of Gravenhurst Bay, Ontario: comparison of diatoms, chironomids, and historical records

Abstract

Quantitative paleolimnological inferences of diatom-inferred total phosphorus and chironomid-inferred hypolimnetic oxygen levels (measured as the anoxic factor) were compared along with historical records for Gravenhurst Bay, Ontario, prior to and following sewage treatment. Water quality declined dramatically following European settlement in the mid-1800s and reached its highest inferred nutrient concentrations during the first half of the twentieth century. After treatment of sewage began in 1972, surface water total phosphorus rapidly returned to near oligotrophic conditions. Diatom assemblages reflected the period of nutrient enrichment, as well as the subsequent recovery. Chironomid assemblages exhibited trends consistent with decreased availability of dissolved oxygen to deepwater habitats since ca. 1886, with profundal taxa being largely absent since ca. 1958 when deepwater anoxia became more severe. Despite remediation efforts, Gravenhurst Bay still experiences long periods of anoxia, and chironomid assemblages have shown no sign of recovery to the improved surface water quality. We suggest that chironomid assemblages responded more strongly to changes in deepwater oxygen availability than to epilimnetic nutrient concentrations, especially during periods of pronounced hypoxia. This study demonstrates the advantages of using both chironomids and diatoms in paleolimnological assessments of eutrophication, as the indicators track changes in different lake strata.