Direct and indirect effects in flowhabitatbiota relationships in the littoral zone of a large lowland river

Abstract

An overall lack of generality still exists in relationships between degree of fl ow modifi cation and eco- logical/geomorphological response, important for better understanding and management of freshwater resources. Here, we model direct and indirect effects in fl ow-habitat-biota relationships in the littoral zone of a 230 km stretch of the lower River Murray (Victoria, Australia), a large and highly-regulated lowland river. We rely on a novel (to lotic ecology) statistical method called the 'd-sep test'. A number of theory-laden (including PHABSIM-based) competitive models were assessed, and model reliability further evaluated against alternative models. The model of choice was one with fl ow directly affecting all abiotic (substratum composition) and biotic (aquatic vegetation, zooplankton abundance, fi sh abundance) components. The latter were in turn linked in a bottom-up series, with inclusion of a direct effect of aquatic vegetation on fi sh abundance. However, an alternative model with a direct ef- fect of substratum composition on fi sh abundance was also partly supported by the data. We discuss the ecological signifi cance of the proposed model(s), along with their value, relevance, and implications in (fi sh) habitat assess- ment studies.