Seasonal changes in invertebrate drift: effects of declining summer flows on prey abundance for drift-feeding fishes

To evaluate the consequences of declining summer discharge for drift abundance and energy flux to drift-feeding fish, we collected monthly drift samples from April to September in three small British Columbia salmonid streams. We complemented this with an analysis of published studies to test for broader-scale effects of summer low flows across multiple streams. This analysis indicated that a reduction in drift biomass concentration (mg m −3 ) over the summer low-flow recession appears to be typical in most temperate and Mediterranean climate streams. However, there was also some evidence for a behavioral increase in drift at flows below 5% of mean annual discharge (MAD), although this threshold should be treated as provisional because of low sample size. A general decline in drift biomass per m 3 over the summer low-flow recession suggests that energy flow and therefore habitat capacity for drift-feeding salmonids likely decreases much more quickly than discharge, or with predictions from traditional instream flow models that only consider flow-related changes in available physical habitat and neglect changes in prey abundance.