Algal distribution patterns in Lake Erie: Implications for oxygen balances in the eastern basin

Depleted oxygen levels in Lake Erie once resulted in massive fish die-offs and changes in the lake's gross chemistry and biology. Given the reductions in phosphorus loads to the lake and recent invasion by Dreissenid mussels, it is likely that the sources of primary production within the lake will shift (surface, subsurface, or benthic), and that these changes may influence the lake's oxygen balance. From 1997-98, the biomass and taxonomic composition of major algal assemblages was measured (surface, subsurface, and benthic) at 16 sites throughout the entire lake (n = 100). Following thermal stratification, a subsurface phytoplankton assemblage developed in the deep waters (15-40 m depth) of all three basins in Lake Erie, and its biomass was significantly greater than surface phytoplankton (2.70 versus 1.91 mg chl/m³). In the eastern basin, all samples collected from lake sediments contained considerable concentrations of phaeopigments, while only 32% of the samples contained chlorophyll (8 of 25 samples). Microscopic analysis confirmed that viable benthic algae were present in all samples analyzed (range from 6.8 to 35.2 mg C/m²); however because most were pelagic diatom species, it is likely they settled out the water column. Dissolved oxygen concentrations varied 10-fold among the 57 samples taken in Lake Erie from July-October (range 1.35 to 13.26 mg/L). Most of this variation was related to changes in concentration with water column depth, with the lower values being measured in bottom waters. In the eastern basin, hypolimnetic depletion rates were similar to those measured during the 1970-80s (1.6 to 2.2 mg O₂/L/month). Therefore, despite the occurrence of both subsurface phytoplankton and benthic algal assemblages, the rate of hypolimnetic oxygen depletion in the eastern basin of Lake Erie has not changed, suggesting that physical factors play a major role in mediating this process.