Macroinvertebrate Drift as a Habitat Quality Assessment Tool

The principal food supply for rearing salmonids is drifting macroinvertebrates.  Drift is measured with a range of approaches, and its use as a tool in water quality evaluation has been limited.  We hypothesized that juvenile rearing potential was constrained, particularly at baseflow, by reduced food availability and physical habitat and high stream temperatures, thus creating an energetic bottleneck.  We examined macroinvertebrate drift in the Calapooia River in western Oregon  and found that drift biomass was dominated by five taxa groups:  Baetis tricaudatus, Calineuria californica, Hesperoperla pacifica, Simulium spp., and Chironomidae.  These comprised 65% of total biomass, indicating that much of the energy transfer to higher trophic levels is via a few dominant taxa.  Macroinvertebrate drift densities ranged from 0.7 to 13.7 ind/m³ and biomasses from 0.02 to 1.23 mg/m³, which is comparable to other efforts.  This suggests that the available energy in the drift can support rearing salmonids.  However, drift was non-uniform, being higher upstream (P<0.0001) and declining (P<0.001) during the study.  While this is revealing and perhaps valuable information in this river’s restoration effort, routine use of drift as a water quality tool is likely to be limited without standardized methodologies in both the field and the laboratory.