Impact of hypolimnetic oxygenation on profundal macroinvertebrates in a eutrophic lake in central Alberta. II. Changes in Chironomus spp. abundance and biomass

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Abstract
Effects of hypolimnetic oxygenation on profundal (15-25 m depth) Chironomus spp. in double-basined Amisk Lake were studied from 1988 to 1991. Responses were species specific. Mean densities and biomasses of C. anthracinus at 25 m increased 55- and 109-fold, respectively, in the treated north basin over the study period. Densities also increased in the reference south basin, but mean larval weights and biomasses were significantly lower than in the treated basin (P < 0.001). Anoxic conditions in the hypolimnion (dissolved oxygen concentrations -1), rather than low hypolimnetic temperatures, appeared to limit C. anthracinus distribution in Amisk Lake. Major changes in C. anthracinus abundance were not apparent until 2 years after the treatment commenced. Densities and biomasses of C. cucini were higher in the reference basin than in the treated basin of Amisk Lake. Densities of the C. decorus and C. plumosus groups increased in the treated basin but remained of minor importance. In comparison, densities of the C. cucini and C. plumosus groups at 25 m in the reference south basin of nearby Baptiste Lake declined over the study period. Increased profundal Chironomus spp. abundance in Amisk Lake suggested an increase in potential fish food.