SUBSTRATE ROUGHNESS, VELOCITY REFUGES, AND MACROINVERTEBRATE ABUNDANCE ON ARTIFICIAL SUBSTRATES IN THE LOWER MISSISSIPPI RIVER

Articulated concrete mattress blocks (ACM) are now used almost exclusi vely for erosion control of banks along the lower Mississippi River (L MR) and provide habitat for macroinvertebrates and fishes which could not otherwise colonize the unstable substrate of the natural river cha nnel. We studied the distribution and abundance of macroinvertebrates in the LMR on ACM blocks with smooth, rough, and grooved surfaces. Add itionally, the effects of block surface modifications on substrate vel ocity profiles were determined at three flow velocities in a laborator y flume using a thermistor-based current meter. Total density of macro invertebrates and the densities of individual taxa were significantly different on the three surface types after 3 mo of colonization; rough blocks had 1.4x greater densities than smooth blocks, and densities o n grooved blocks were 2.3x higher than smooth and 1.6x higher than rou gh blocks. The caddisfly Hydropsyche orris and the amphipod Corophium lacustre were significantly denser on grooved than on the other block types. In a flume, velocities in groove bottoms were 7-24x lower than ambient water column velocities (0.12, 0.23, and 0.44 m/s) and increas ed significantly from 1 mm to 65 mm above the block surface at all flu me velocities. For all surface types there was a negative relationship between height above the block surface and measured variance in veloc ities and a positive relationship between flume velocities and the mea sured variance in velocity readings which could be attributed to incre ased turbulence over the block surfaces. In the field H. orris in groo ves built retreats with the anterior opening containing the feeding ne ts extending up into the near-surface flows over the grooves. Low velo city eddies in the grooves provided a velocity refuge while still allo wing the feeding structures to contact feeding flows over the block su rface. Grooved blocks in the LMR supported, on average, more than twic e the invertebrate density as smooth blocks, and are recommended for f uture deployment of mattress blocks.