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The perils of unpalatable periphyton: Didymosphenia and other mucilaginous stalked diatoms as food for tadpoles

Proliferations of Didymosphenia geminata are becoming prevalent in rivers around the globe. In the Sierra Nevada of California, Didymosphenia and other taxa that produce mucopolysaccharide stalks (e.g., Gomphoneis, Cymbella) can dominate benthic environments, particularly in the altered hydrologic and thermal regimes downstream of dams. We compared the prevalence of stalked diatoms in paired reaches, one free-flowing and the other regulated, within two Sierran river systems, the American and Feather Rivers. In the regulated reaches, short-term power generation caused daily flow fluctuations and periphyton biovolume was dominated by either Didymosphenia (where hypolimnetic releases created cool summer temperatures) or other stalked diatom taxa (where temperatures were warm). Periphyton assemblages from the unregulated sites were significantly different from the regulated reaches based on biovolume, with Gomphonema being the genus at unregulated sites contributing to the dissimilarities after accounting for the stalked genera from the regulated reaches. We evaluated the consequences of mucopolysaccharides for a large-bodied grazer, tadpoles of the foothill yellow-legged frog (Rana boylii), in a factorial experiment manipulating diet and thermal regime. At 16.6°C mean daily temperature, tadpoles lost weight (72 h relative change of−16.1±7.2%) when grazing on periphyton from a Didymosphenia-dominated site. At 19.9°C (similar to unregulated river conditions), tadpoles grazed Didymosphenia at a rate similar to tadpoles consuming higher protein control periphyton, but the former tadpoles did not grow (relative change of 4.3±5.4% vs 30.7±3.4% for control periphyton). When tadpoles were fed periphyton dominated by mucilaginous stalked diatoms other than Didymosphenia, tadpole weight loss was 21.0±9.2% (cool) and 16.6±5.6% (warm). The results illustrate that hydrologically or thermally mediated shifts in periphyton composition can have significant implications for the energy transferred to grazers.