Measuring the ecological impact of long-term flow disturbance on the macroinvertebrate community in a large Mediterranean climate river

The Feather River is a large flow-regulated river in the Central Valley of California. The Mediterranean climate of the area imposes a natural flow regime for the region that is characterized by predictable high flows in the winter and spring and low flows in the summer and fall. The Oroville Dam complex on the Feather River has created a permanent low-flow section of the river where a base flow is continuous year round but the natural annual variability of flow has been completely eliminated. We used this modified section of the river to examine the ecological impact that removing natural flow variability has on the macroinvertebrate assemblage and how we might measure such a change if it is present. Specifically, we examined whether biodiversity and community similarity differed between the low- and high-flow sections of the river for both benthic and drifting aquatic invertebrates. Using a modified Surber sampler we collected samples at three distinct time periods within a year (January, April, and July) for both drift and benthic fauna. Our results showed little difference between the low- and high-flow assemblages using common measures of diversity (i.e., species richness and Shannon diversity) and a measure of environmental tolerance (Hilsenhoff biotic index). Yet when we employed a multivariate measure of community similarity (i.e., non-metric multidimensional scaling) and associated statistical tests, we found significant assemblage differences between the low- and high-flow sections of the river. This study suggests that flow disturbance of this sort is likely to alter the macroinvertebrate community in ways that are not easily observed using common ecological metrics.