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

Abstract

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.

Keywords:

• dams
• Didymosphenia
• Epithemia
• food quality
• power-peaking
• Rana boylii
• stalked diatoms
• tadpole

Acknowledgements

We thank A. Catenazzi for help collecting eggs and algae; the East Bay Regional Park District for access to Alameda Creek; and the Public Interest Energy Research Program of the California Energy Commission for funding [CEC 500-08-031]; Pacific Gas & Electric for sharing data; M. E. Power and W. E. Dietrich for support and access to the Richmond Field Station. The project was approved by the UC Berkeley Animal Care and Use Committee (Protocol #R132) and the California Department of Fish and Wildlife (Scientific Collecting Permit # 10716). PCF received partial support from the National Science Foundation National Center for Earth-surface Dynamics [NCED; NSF OIA-0120914], a National Science Foundation grant awarded to Jill Welter [NSF-DEB 0950016] of St. Catherine University, and a St. Catherine University Academic Professional Development Committee grant awarded to PCF.