THE EFFECTS OF STREAM ACIDITY ON BENTHIC INVERTEBRATE COMMUNITIES IN THE SOUTH-EASTERN UNITED-STATES

1. Patterns in invertebrate community structure were determined from analysis of benthic samples taken quarterly for 1 year from four sites in the Great Smoky Mountains National Park (GSMNP). Sites ranged in baseflow pH from 4.5 to 6.8, with a corresponding range in inorganic monomeric aluminium concentrations from 3 to 197-mu-g l-1. 2. Total invertebrate density (excluding Chironomidae) was correlated with pH (R2 = 0.90, P < 0.05). Density of Ephemeroptera was more highly correlated with pH than density of invertebrates in other taxonomic orders. Invertebrate density (total and density of different orders) was generally more strongly correlated with pH than to inorganic monomeric aluminium concentrations. 3. Species richness declined from sixty-nine species at the highest pH site to thirty-three species at the lowest pH site. Richness of Ephemeroptera and Trichoptera was positively correlated with pH (R2 = 0.96 in both cases). Trichopteran richness was also negatively correlated with inorganic monomeric Al concentrations (R2 = 0.96, P < 0.05). Plecopteran richness was not related to stream acidity. 4. Not all species of Ephemeroptera were negatively affected by low stream water pH. At low pH sites, Ameletus lineatus Traver (Family: Siphonoluridae) was relatively more abundant compared to other species of Ephemeroptera, and had greater biomass and greater weight per individual length than at higher pH sites. 5. Differences in invertebrate community parameters among sites were not based on changes in food abundance. Although grazer densities were positively correlated with pH (R2 = 0.93, P < 0.05), differences in density were not related to differences in food. To the contrary, lower grazer densities occurred at the lowest pH site where measured food abundance (algae) was highest. Likewise, shredder density increased with increasing pH but was correlated with standing stocks of benthic organic matter (BOM), which had generally higher standing stocks at lower pH sites. 6. Transplants of two species of Ephemeroptera between sites differing in pH (6.4 v 5.0) resulted in increased mortality of organisms placed at lower pH sites. In another transplant experiment, differences in mortality were not observed, although surviving invertebrates at low pH sites had x 10 higher body burdens of aluminium following the transplant than invertebrates from the reference site. 7. These results indicate that direct effects of pH and Al (affecting survival) were more important than indirect effects of food availability in determining changes in invertebrate community structure between the study sites.