Current velocity as a factor in determining macroinvertebrate assemblages on wood surfaces

Abstract

The velocity of moving water near submerged wood surfaces may be an important factor for shaping benthic communities in sandy-bottomed streams. As wood can be suspended above the streambed and be kept free of inundating sediments, wood substrates may provide the primary hard surface for macroinvertebrate colonization. To examine this, we sampled macroinvertebrate assemblages from the surface of 20 pieces of submerged wood in a sandy-bottomed stream in the west-central Wisconsin. Near-surface velocity was characterized at the wood surface and the streambed macroinvertebrate assemblage was sampled near each wood piece for comparison. We found that the density of macroinvertebrates was five times greater on wood than the sandy streambed and the average richness on wood was twice as great. Macroinvertebrate abundance and richness on wood surfaces increased with velocity; however, abundance increased as a consequence of adding taxa rather than adding individuals of the dominant taxa. These data suggest that near-surface current velocity on wood may be an important determinant of benthic community composition when the availability of other hard surfaces is limited.