Caddisflies as biomonitors identifying thresholds of toxic metal bioavailability that affect the stream benthos

It has been proposed that bioaccumulated concentrations of toxic metals in tolerant biomonitors be used as indicators of metal bioavailability that could be calibrated against the ecological response to metals of sensitive biotic assemblages. Our hypothesis was that metal concentrations in caddisfly larvae Hydropsyche siltalai and Plectrocnemia conspersa, as tolerant biomonitors, indicate metal bioavailability in contaminated streams, and can be calibrated against metal-specific ecological responses of mayflies. Bioaccumulated concentrations of Cu, As, Zn and Pb in H. siltalai from SW English streams were related to the mayfly assemblage. Mayflies were always sparse where bioavailabilities were high and were abundant and diverse where bioavailabilities of all metals were low, a pattern particularly evident when the combined abundance of heptageniid and ephemerellid mayflies was the response variable. The results offer promise that bioaccumulated concentrations of metals in tolerant biomonitors can be used to diagnose ecological impacts on stream benthos from metal stressors.

► Metal concentrations in caddisfly larvae can be calibrated against mayfly ecological responses.
► Cu, As, Zn and Pb concentrations in Hydropsyche siltalai were related to stream mayfly assemblages.
► Mayflies were sparse in high metal bioavailabilities, and abundant in low bioavailabilities.
►  Joint heptageniid and ephemerellid mayfly abundance was the most sensitive response variable.
► Copper, arsenic and, in one catchment, lead were the primary stressors limiting mayfly abundance.