Effects of fish, water depth, and predation risk on patch dynamics in a north-temperate river ecosystem

Spatial and temporal variation in water depth affected habitat use by prey fish and their predators, generating a dynamic mosaic of patches with different benthic ecosystem properties. Twelve 4.6-m(2) pens (5-mm mesh), closed or open to fish, were built in two pools of an upland river near Tahlequah, Oklahoma, USA. Fish effects were attributed primarily to the benthic algivores Campostoma anomahum, C. oligolepis and Notropis nubilus, which comprised 60% of fish abundance in the reach and 83% of benthic fish. Furthermore, mean hourly removal of benthic organic matter (4.4 g m(-2) h(-1) ash-free dry mass) by these fish exceeded the mean daily accumulation (3.7 g m(-2) d(-1)) in closed pens. Fish maintained a relatively silt-free grazed epilithon in open pens and unenclosed river, until large fish that predominated in one pool abandoned habitats where their predation risk became critical as water became shallow (< 28 cm). In closed pens and areas abandoned by fish, benthic algal biomass initially increased, but then visibly accumulated silt in an algal matrix that eventually sloughed and regrew in patches of substrata. Blue-green and green algae Like that in areas with fish predominated in regrown parches. Closed pens with regrown algae had higher biomass-specific net primary productivity and higher percentage of organic matter than ones with an intact matrix. invertebrate abundance was higher in closed pens with few sloughed patches than in open pens with fish, but similar in closed pens with regrown patches and open pens abandoned by fish. Invertebrate predators and collector-gatherers predominated in closed pens with an intact matrix and higher percentage of medium (157-507 mu m) benthic particulate organic matter (BPOM): collector-filterers and scrapers predominated in open pens, which had higher percentage of ultrafine (0.44-40 mu m) BPOM. Assemblages in closed pens with patches of regrown algae and open pens abandoned by fish were dominated by scrapers and collector-gatherers.