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We examined how biotic factors (natural assemblages of omnivorous fishes) control the community response of primary producers to physical disturbance (frequent high discharge events) in a lowland tropical stream. As a secondary objective, we assessed effects of fishes on benthic insects. We used electric ’’fences’’ to exclude access of fishes to benthic communities in a riffle of a fourth-order stream draining the foothills on Costa Rica’s Caribbean slope. Benthic algae and aquatic insects accessible to fishes were subject to intense omnivory by a diverse assemblage of fishes. Over a 7-wk period in the rainy season, three large discharge events occurred in which water levels rose up to 3 m above base flow, discharge increased by up to 160-fold, and suspended gravel and small cobbles caused major scouring. Fishes significantly affected algal succession, community composition, and response to high discharge events. Algal assemblages that developed in the presence of fishes were dominated by filamentous blue-green algae (Lyngbya sp.), whereas diatom assemblages prevailed where fishes were excluded. Fishes significantly reduced numbers of larval Chironomidae (Diptera) and total insects.

Algal biovolume was higher and more variable in the absence of fishes than in their presence: significant (P < 0.05) reductions in algal biovolume occurred in the diatom-dominated fish exclusion treatment in response to high discharge. In contrast, in the presence of fishes, algal biovolume increased steadily through time and was not significantly reduced by storms. The third and most severe storm event (39-40 d) resulted in decreases in total algal biovolume, diatom biovolume, algal taxon diversity, and richness in the fish exclusion treatment. However, these parameters increased (despite the storm) in the presence of fishes. Our results show that fishes play a key role in maintaining the stability of benthic algal assemblages and their resistance to storm events. Moreover, results suggest that in the absence of omnivorous fishes, high discharge events would play a major role in structuring benthic algal assemblages, resulting in extreme fluctuations in algal biomass.