The effect of trout farm effluent on the taxa richness of benthic macroinvertebrates

Abstract

Benthic macroinvertebrates were collected using an intensive multi-habitat qualitative technique similar to that used by the North Carolina Division of Environmental Management to assess water quality in streams and rivers. Comparisons of the taxa richness of stream insect groups sensitive to water quality [i.e., mayflies (Ephemeroptera), stoneflies (Plecoptera) and caddisflies (Trichoptera) = EPT] were made in May, June, August and October of 1990 and in February/March of 1991 above three commercial trout farms in western North Carolina, just below their outfalls and approximately 1.5 km further downstream. EPT taxa richness was significantly lower just below their outfalls compared to stream sites above their intakes indicating reduced water quality. Differences in taxa richness were greatest in June, August and October, periods of low flow and high temperature. Higher abundances of pollution-tolerant forms [Chironomidae, Simuliidae, Oligochaeta and Pisidium (Bivalvia: Sphaeriidae)] were noted just below the outfalls for all sampling dates. Taxa richness was significantly higher 1.5 km further downstream compared to sites just below the outfall, however, richness here was also significantly lower compared to sites above the farms indicating that water quality had not fully recovered. Significantly greater percentages of collectors and percent Hydropsychidae to Total Trichoptera were found at both sites below the outfalls suggesting utilization of organic solids coming from the farms and a continuum shift in the EPT communities below trout farms. The presence of sewage fungus just below the outfalls resulting from these increased organics suggests that anaerobic conditions existed within the microhabitats of the benthos. This may have been primarily responsible for the decrease of the EPT taxa richness. This study also provides a basis for assessing the carrying capacity of catchments for fish farms.