Mechanisms of low flow-induced community change: Effects on invertebrate development and mortality

Low-flows are a major disturbance in streams and affect invertebrate community structure potentially through both abiotic and biotic processes. However, the relative importance of the different mechanisms in structuring low flow-affected communities is not well understood. Invertebrate responses to low-flows are likely to depend on their life-history strategy and susceptibility to predation. We investigated how the abundance of primary consumers and predatory invertebrates in streams changed when subject to low-flow disturbance. Low-flow sections of streams were generally characterised by higher average water temperatures, lower average stage heights, and lower dissolved oxygen levels relative to their equivalent permanent sites. The abundance of mayflies and caddisflies decreased, and the number of predators relative to prey increased in the low-flow sections. A mesocosm experiment was also conducted to determine how low-flows changed predation pressure, and how invertebrates with different life histories (a hemimetabolous insect the mayfly Deleatidium sp., and a holometabolous insect the cased caddisfly Pycnocentrodes aeris) responded to these conditions. Low flows and presence of predators affected the development and emergence of both mayflies and caddisflies in the mesocosms. Low-flow conditions initially increased the number of invertebrates emerging, but emergence was impeded with an extended period of low-flow. Low-flows had the greatest negative impact on Pycncentrodes because they responded slower, and were more vulnerable to predation in low-flows. The results from this study highlight the importance of both abiotic and biotic (particularly predation) processes in structuring communities in streams with low-flows.