Invertebrate responses to flow: trait-velocity relationships during low and moderate flows

Abstract

The mechanisms underlying responses of stream invertebrates to low flows are poorly understood. To clarify the strategies used to tolerate and survive low flow periods, we tested associations between patch-scale invertebrate densities and velocity within riffle habitats during moderate and low flows. We focused on filter-feeding and respiration traits because they have a mechanistic link to invertebrate low flow responses. 228 samples were collected from 3 riffles at 2 low and 2 moderate flow periods. Maximum densities of filter-feeders were positively associated with the highest velocities during low flows, but not moderate flows, partly consistent with our prediction that they would be structured along velocity gradients to meet nutritional requirements. Gill-respiring invertebrate densities were inversely related to velocity during both flows, probably attributable to organic matter accumulated in low velocity areas. O2 requirements during low flows did not constrain tegument-respirers to higher velocity patches, inconsistent with perceived views of their response to low flows. We found high velocity areas could act as refuges for some invertebrates during low flows, while low velocity patches, which are vulnerable to rapid flow recession, are important habitats for other invertebrates. Environmental flow management should focus on strategies that preserve these habitats during low flows.