Size Distribution of the Macroinvertebrate Community in a Freshwater Lake

Abstract

Macroinvertebrates were collected every 2 wk for 18 wk from three depth zones in a deep lake in Alberta. Unionid clams comprised 80% of the total macroinvertebrate biomass and were excluded from initial analyses. The seasonal average size-spectrum for the littoral zone community was bimodal with peaks in the 8–16 and 256–512 mg weight-classes; size-spectra for the sublittoral and profundal communities were unimodal with peaks in the 8–16 and 16–32 mg weight-classes, respectively. Slopes of the normalized size-spectra for the littoral, sublittoral, profundal and whole-lake communities were not significantly different from −1.0, −1.0, 0.0, and −1.0, respectively. These results suggest that biomass is evenly distributed across logarithmically even size-classes for the average macroinvertebrate community in the littoral zone, sublittoral zone, and on a whole-lake basis. The biomass peak for unionids (16.4–32.8 g weight-class) was 10–30 times greater than biomass peaks for the remaining macroinvertebrates. The slopes of normalized size-spectra for the littoral zone and whole-lake were changed significantly when unionids were included; however, unionids presumably play a minor role in the macroinvertebrate community because they are an energy sink in the present context. Despite wide seasonal variation, average normalized size-spectra based on samples collected at 4- and 6-wk intervals were very similar to those based on nine biweekly collections.