Density and microdistribution of chironomid larvae in an Alberta brown-water stream

Abstract

Seasonal changes in density of chironomid larvae were examined in nine microhabitats in a small, slow-flowing stream draining muskeg terrain. During July and August aquatic macrophytes increased the area available for colonization by chironomids 5- to 10-fold, but harbored only 30–40% of the total density of 20 000–52 200 larvae m−2 of stream surface. In the sediment mean density was 19 900 larvae m−2 and varied 17-fold over a 23-month period. Minimum and maximum densities were reached in June and March, respectively. During May–August mean densities on Sparganium, Potamogeton, Hippuris, submerged moss, sponge colonies, and filamentous algae were 93, 171, 466, 978, 32, and 35 larvae per gram dry weight, respectively. Larval densities on annual aquatic macrophytes increased during the course of the summer and reached highest values on those macrophytes with the greatest surface area: weight ratio. Mean density on submerged wood was 16 larvae cm−2. Two-thirds of the 32 most abundant species were found primarily on only one of the nine microhabitats. The number of species found specifically in one microhabitat was 13 for sediment, 3 for moss, 2 for Sparganium, 1 each for Potamogeton, Hippuris, filamentous algae, and wood, and none for sponge and autumn-shed leaves.