EFFECTS OF FISH, WATER DEPTH, AND PREDATION RISK ON PATCH DYNAMICS INA 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-m m mesh), closed or open to fish, were built in two pools of an upland river near Tahlequah, Oklahoma, USA. Fish effects were attributed prim arily to the benthic algivores Campostoma anomahum, C. oligolepis and Notropis nubilus, which comprised 60% of fish abundance in the reach a nd 83% of benthic fish. Furthermore, mean hourly removal of benthic or ganic matter (4.4 g m(-2) h(-1) ash-free dry mass) by these fish excee ded the mean daily accumulation (3.7 g m(-2) d(-1)) in closed pens. Fi sh maintained a relatively silt-free grazed epilithon in open pens and unenclosed river, until large fish that predominated in one pool aban doned habitats where their predation risk became critical as water bec ame shallow (< 28 cm). In closed pens and areas abandoned by fish, ben thic algal biomass initially increased, but then visibly accumulated s ilt 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 hi gher biomass-specific net primary productivity and higher percentage o f organic matter than ones with an intact matrix. invertebrate abundan ce was higher in closed pens with few sloughed patches than in open pe ns with fish, but similar in closed pens with regrown patches and open pens abandoned by fish. Invertebrate predators and collector-gatherer s predominated in closed pens with an intact matrix and higher percent age of medium (157-507 mu m) benthic particulate organic matter (BPOM) : collector-filterers and scrapers predominated in open pens, which ha d higher percentage of ultrafine (0.44-40 mu m) BPOM. Assemblages in c losed pens with patches of regrown algae and open pens abandoned by fi sh were dominated by scrapers and collector-gatherers.