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

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