Benthic Macroinvertebrate Susceptibility to Trout Farm Effluents
Roberts, Lenn Darrell
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The direct effects of a Virginia trout farm on benthic macroinvertebrates were examined using multiple approaches. Static laboratory tests with the amphipod, Hyallela azteca, were conducted with exposures to water taken from a spring, effluent above a sedimentation basin, and effluent below a sedimentation basin. On-site mesocosms were constructed to expose previously colonized artificial substrates to the same treatments as the laboratory tests. Flat-headed mayflies were also collected from a nearby stream and transported to the mesocosms for a 10 day exposure. There was no significant difference between treatments in the laboratory tests after 20 days, but after 28 days the control was significantly lower than the above sedimentation basin treatment in one test. In the multispecies field tests, a clear decrease in total invertebrate abundance and EPT abundance was seen in the effluent treatments compared to the spring water treatments, with a slight improvement in survival in the treatment below the sedimentation basin. However, only total invertebrate abundance after 21 days produced statistically significant differences. A significant difference was detected between the effluent and the spring treatments in the flat-headed mayfly field test. We suggest that the effects seen in this study do not explain the lack of taxa richness in the receiving stream. The main cause of mortality from trout effluents appears to be solids accumulating upon the organisms, and sedimentation basins should be effective best management practices for protecting macroinvertebrates.
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