Short-Term Effects of High Suspended Sediments on Six Common New Zealand Stream Invertebrates

Effects of high suspended-sediment (SS) levels (up to c. 20,000 NTU) from fine clay on 5 species of New Zealand stream insects and the crayfish Paranephrops planifrons were assessed by short-term (24 h) exposures to different SS concentrations. There were no patterns of increasing mortality over 24 h with increasing SS concentrations. Effects of longer-term exposure (14 days) of repeated 4 h exposures to lower SS concentrations (c. 1000 NTU) on the common leptophlebiid mayfly Deleatidium were also investigated. No difference in mortality was found between the test and control solutions. We suggest that so-called ‚sensitive’ invertebrates are tolerant of high SS concentrations, and that even repeated exposures to high SS concentrations have little effect on survival. Absence of these animals from catchments with high SS concentrations thus most likely reflects adverse changes to instream habitat conditions, such as filling of interstitial spaces, contamination of food sources, or both.