Effects of the nuisance alga Didymosphenia geminata on benthic food web structure and resource use
Didymosphenia geminata (didymo) is a benthic, freshwater diatom that has reached nuisance biomass levels in streams throughout the world. The massive stalks it forms, typically in oligotrophic waters, can completely cover stream substrata, which can alter benthic community structure. It is unclear whether structural changes in benthic communities influence resource use by benthic macroinvertebrates. Changes in resource use by macroinvertebrates may affect energy flow in riverine food webs. We investigated the effects of didymo mat proliferation on benthic community structure and function in the Clinch, South Holston, and Watauga Rivers in Tennessee. We sampled each river seasonally along a spatial gradient downstream of dams to better isolate the impacts of didymo from other benthic changes caused by dams. We assessed macroinvertebrate communities, identifying invertebrates to the lowest practical taxonomic class, and used ordination analyses to explore relationships between community structure and didymo abundance. We measured stable carbon and nitrogen isotope ratios of major food web components [including particulate organic matter (suspended, coarse, and benthic), algae, macrophytes, and invertebrates from multiple functional feeding groups] to assess potential changes in macroinvertebrate resource use due to didymo mat proliferation.
Consistent with previous studies, total invertebrate abundance increased with increasing didymo abundance. Nonmetric Multidimensional Scaling (NMDS) analysis of macroinvertebrate communities suggested that abiotic factors (e.g., water quality, hydrology) were more important than biotic factors (didymo abundance and chlorophyll a) in determining invertebrate community structure. Whereas structural changes in macroinvertebrate communities were inconsistent among rivers in the spring, resource use varied with didymo presence. Didymo coverage was consistently lowest in the Clinch River, and invertebrate isotopic signatures did not indicate a shift in resource use. However, in the other two rivers, didymo mats shifted consumption from rock biofilms to epiphytic biofilms. Overall, our analyses suggest that didymo mats have a stronger influence on macroinvertebrate resource use than on community structure.