Indirect effects of brook trout (Salvelinus fontinalis) on the structure of epilithic algal communities in an oligotrophic boreal forest stream

Abstract

The impact of fish on epilithic algae community structure was studied in a replicated series of artificial outdoor channels fed by a boreal forest stream (Canada) to test the hypotheses that brook trout (Salvelinus fontinalis) induce a three-trophic chain top-down change in epilithic community structure, at both taxonomic and physiognomic levels. Fish were introduced into five of ten channels and the epilithic algae growing on tiles were sampled four times during summer to determine biomass, algal physiognomy and species composition in relation to presence of fish and invertebrate community structure. Diatom biomass showed distinctive development patterns, increasing at the beginning of fish introduction and decreasing towards the end of the experiment. The responses were the reverse in the absence of fish. Chrysophyceae increased in the absence of fish, particularly at the end of the experiment. From the seven algal physiognomic types considered, epilithon overstory, attached erected algae and loosely attached algae were generally more abundant in the presence of fish. Species' biomass was markedly different in both treatments and followed contrasted temporal patterns. In the presence of fish, attached erected diatoms species rapidly reached a complex spatial overstory structure, while in the absence of fish the epilithic community remained at a younger successional stage. Partial redundancy analysis showed that those changes were mediated by large grazing invertebrates, which had lower abundance over the bottom surfaces in the presence of fish. Variance partitioning analysis reinforced the hypothesis that most of the variability in epilithic community structure was explained by fish-invertebrate interactions and that the effect of fish presence alone was not significant. The results show that reduction in total epilithic algal biomass was a transient top-down effect, but the three-trophic level control on major taxa, species composition and community physiognomy remained along the summer experiment.