Complex interactions of fish, snails, and littoral zone periphyton

This study examines the interaction of predation and resource-based effects of fish on freshwater periphyton communities. Current theory predicts that fish primarily influence periphyton communities by controlling herbivore densities. But fish may also influence algal communities by increasing available nutrients via nutrient recycling. To separate these effects and to determine their relative importance, we made a number of specific predictions based on the literature and tested these predictions by manipulating the presence and absence of the molluscivorous redear sunfish, Lepomis microlophus, and snails, Physella heterostropha, in a replicated factorial experiment. We examined the effects of fish on nutrient concentrations in the water and on snail grazing activity and life history traits. We also evaluated the effect of fish and snails on cell number, biovolume, cell size, and growth form of all periphyton, green algae, diatoms, and blue-green algae. Concentrations of phosphorus and nitrogen in the water were significantly higher in the presence of fish, but this had little effect on total periphyton. Although fish had only visual and waterborne (olfactory) contact with snails, their presence inhibited snail reproduction and/or increased mortality of small snails such that twice as many snails were produced in the absence of fish. Snail grazing activity was six times higher in the absence of caged fish than in their presence. In the absence of fish, snails reduced periphyton cell number and increased the average size of the periphyton cells, primarily through effects on green algae. Snails reduced the biovolume of diatoms and blue-green algae. Fish also reduced diatom biovolume by decreasing the average cell size of diatoms. Snails increased the proportion of gelatinous colonies in the periphyton. Previous research suggests that fish have a positive indirect effect on algae by removing grazers. Fish can augment this effect by negatively affecting snail life history traits and by reducing grazing activity. In addition, fish may have an important but less obvious direct effect on algae via changing nutrient concentrations and possibly altering competitive outcomes among taxa and growth forms.