Stream fauna responses to decreasing streambed complexity: An experimental laboratory study.

摘要

A new approach to studying sediment pollution and rocky streambed fauna is presented. It concentrated on sedimentation induced reductions of streambed complexity and relief, and eliminated negative impacts of fine sediments on animals bodies. Artificial substrates (20 cm x 60 cm with 'rocks' made from tiles), used without sediment, modeled geometric changes that would occur in streambed patch at four levels of sediment accumulation. Beginning with the most complex substrate (S0), each subsequent substrate (S1, S2, and S3) modeled an increase of about 1 cm of sediment. Only S0 and S1 had crevices under artificial rocks. Thirty-two laboratory trials using these artificial substrates, a flow tank, Hydropsyche and Cheumatopsyche , and Etheostoma flabellare, were run, with one type of substrate used per trial. A blocked design was used. Ten caddisflies were allowed 1.5 hours to build retreats, then were located and their positions mapped. One darter was introduced and left there for 19 hours. The first two hours were videotaped. Surviving caddisflies were located and their locations were mapped. Substrate perimeter use by caddisflies was considered to be a rejection of the substrate. The two hypotheses this study tested were: (1) Decreased substrate complexity would lead to caddisflies using the perimeter more than the interior (p 0.001, Chi-square); (2) Decreased substrate complexity would lead to increased darter predation success (1st 2 hours: p = 0.001 and 19 hours: p = 0.016, Cochran-Armitage Trend Test). In all, 79 caddisflies used the perimeter. Of these, 52 were near the tank sides instead on or near the downstream screen. Caddisfly relocation by movement towards the stream edges seemed to exceed relocation by drift. Also, the time from a darter's introduction to the tank to its first predation action (3 to >120 minutes) increased with substrate complexity (p 0.01, Tarone and Ware Trend Test). These effects of substrate complexity were observed independent of abrasion, smothering, and burial effects of sedimentation.