Consequences of nuisance algal blooms of Didymosphenia geminata on invertebrate communities in Rocky Mountain streams

As climate change accelerates, low summer stream flows are becoming increasingly common in the Colorado Rocky Mountains, USA. The diatom Didymosphenia geminata (Lyngb.) M. Schmidt, typically observed under low-flow and low-P conditions, produces nuisance growth-persistent and extensive proliferation, covering the bottoms of streams in thick algal mats. Nuisance blooms of this diatom physically alter the benthic environment and thereby affect freshwater invertebrates directly and indirectly by altering stream food webs. We compared 9 y of survey data (2013-2021) of D. geminata proliferation with the composition of the macroinvertebrate communities at 8 stream sites near the Rocky Mountain Biological Laboratory in western Colorado. We counted and identified samples of benthic macroinvertebrates and used a glass-bottomed viewing box to estimate D. geminata biovolume at 2 scales: macrohabitat conditions (site level: 2013-2021) and microhabitat conditions (sample level: 2020-2021). At both scales, increases in D. geminata proliferation were associated with shifts in macroinvertebrate community composition that could be explained by altered abundances of focal taxa, specifically declines in Heptageniidae (Ephemeroptera) and increases in Chironomidae (Diptera). Abundances of Baetidae (Ephemeroptera) were unaffected by increased D. geminata biovolume. These changes indicate degradation of stream habitat for some sensitive groups of macroinvertebrates, which may affect higher trophic levels, such as trout, in these mountain stream ecosystems. As climate change trends toward lower summer streamflow, understanding the effects of proliferation of this ecosystem engineer is key to predicting the impact of climate change on stream food webs.