Do agricultural pollutants affect competition between filter-feeding caddis fly larvae? Results of laboratory microcosm experiments

1. Predictable changes in the composition of macroinvertebrate communities in response to non-point source pollution are the basis of macroinvertebrate community indices. However, other factors, such as biotic interactions, also affect the composition of macroinvertebrate communities and may complicate the interpretation of such indices by interacting with abiotic stressors. 2. The effect of agricultural pollutants on competition between two filter-feeding caddisflies, Brachycentrus appalachia and Hydropsyche spp. was studied in the laboratory using recirculating flow tanks. 3. Experiments were conducted at two densities of Hydropsyche: high (300 per tank) and low (80 per tank). Brachycentrus density was kept constant at 40 per tank in all experiments. Experiments assessed the effect of an agricultural pollutant mix (sand, cow-manure compost mix) and the presence of the competitor on both taxa. 4. The only negative response to pollutants was a larger number of Hydropsyche drifting in high-density tanks. In fact, at high densities, mortality of Hydropsyche was higher in controls than in tanks receiving pollutants. 5. Hydropsyche and B. appalachia competed for resources and the presence of pollution did not affect the outcome of this competition. The presence of high densities of Hydropsyche resulted in higher drift and lower condition of B. appalachia. At low densities of Hydropsyche, B. appalachia larvae suffered greater mortality and tended to be in poorer condition. Brachycentrus appalachia may be less successful at even moderate densities of Hydropsyche due to competition between these taxa, most likely for prime feeding positions. 6. Streams receiving non-point source pollution from agriculture may be able to support higher densities of Hydropsyche, which may further reduce the suitability of these habitats for B. appalachia. 7. This experimental study suggests that competition may complicate interpretation of biotic indices, as changes in benthic invertebrate community structure normally ascribed to pollution may arise from intra-specific competition.