Mesohabitat current velocity effects on Didymosphenia geminata and macroinvertebrates in a SE USA hypolimnetic tailwater

The diatom Didymosphenia geminata is known to alter benthic habitat and macroinvertebrate diversity and community structure. Associations between macroinvertebrate communities and D. geminata biomass in riffle and run mesohabitats were investigated in the South Fork Holston River in Tennessee and Virginia, USA. We found that low current velocity, low turbidity, and high dissolved oxygen (DO) were strong predictors of D. geminata mat presence. Didymosphenia geminata ash-free dry mass was significantly higher in run mesohabitats with low current velocity (CV) than in riffle mesohabitats with higher CV. Macroinvertebrate alpha diversity (Shannon Diversity H’ ) was only marginally significantly different between riffle and runs, while beta diversity (community composition) was highly significantly different between these mesohabitats. NMDS analyses found that D. geminata was a relatively unimportant predictor of changes in community structure relative to specific conductance, CV, DO, and turbidity. However, effects of D. geminata on macroinvertebrates appear to be very taxon specific with effects on individual taxa potentially masked by tailwater effects on general macroinvertebrate diversity in global analyses. We observed that taxon-specific effects include, but are not limited to, (1) reduction of bryophyte microhabitat utilized by dominant ephemeropterans, trichopterans, amphipods, coleopterans, and some chironomid genera in run mesohabitats from competition with D. geminata for substrate attachment space; and (2) differences in utilization of D. geminata mat biomass as a food resource and microhabitat for chironomids. Our insights into taxon-specific effects of D. geminata on macroinvertebrates open up multiple avenues for experimentation in which to validate our observational findings.