Year: 1995
Abstract:
Profundal (15 to 25 m) macroinvertebrate responses to hypolimnetic oxygenation in Amisk Lake, Alberta, were dominated by Chironomus spp. Mean densities and biomasses of C. anthracinus at 25 m increased 55- and 109-fold, respectively, in the treated basin, but major changes in abundance were not apparent until 2 yrs after treatment commenced. Densities of C. anthracinus increased similarly in the reference basin, but mean larval weights and biomasses were significantly lower than in the treated basin (paired t-test, P $<$ 0.001). Anoxic conditions, rather than low temperatures, appeared to limit profundal C. anthracinus distribution in Amisk Lake. Densities and biomasses of C-cucini were higher in the reference basin than in the treated basin of Amisk Lake. Densities of C. decorus and C. plumosus group increased in the treated basin but remained of minor importance. In comparison, densities of C. cucini and C. plumosus group in Baptiste Lake, a reference site, declined over the study period. Densities of other macroinvertebrate taxa remained low in the profundal region of Amisk Lake. Shannon-Weaver indices of diversity decreased as oxygenation processed, in contrast to aeration studies in other locations. Increased Chironomus spp. abundance in Amisk Lake suggested an increase in fish food, but fish foraging below 15 m was restricted by low DO concentrations and water temperatures.
The relationship between profundal macroinvertebrate biomass (PMB) and DO concentrations has never been empirically investigated. PMB estimates from 32 Alberta lakes of moderate to high primary productivity (mean summer total phosphorus concentration 57 $\pm$ 9 $\mu$g$\cdot$L$\sp{-1}$) were regressed against DO concentrations and additional trophic, morphometric, and water chemistry variables. Minimum open-water DO concentrations explained 38% of the variance in PMB at sites where minimum DO was $<$4 mg$\rm\cdot L\sp{-1}$, but accounted for no detectable variation when minimum DO was $>$4 mg$\rm\cdot L\sp{-1}$. Bottom slope (12%) and specific conductivity (4%) increased to 55% the amount of variance explained for PMB at sites where minimum DO was $<$4 mg$\rm\cdot L\sp{-1}$. Variables indicative of phytoplankton biomass (chlorophyll a, total phosphorus, and Secchi transparency) were nonsignificant in Alberta lakes, suggesting that food was non-limiting for profundal macroinvertebrates in these predominantly eutrophic lakes. These results suggest that DO concentrations hold potential as a predictor of PMB in eutrophic lakes.
