Consumer growth linked to diet and RNA-P stoichiometry: Response of Bosmina to variation in riverine food resources

Feeding experiments were performed with riverine Bosmina to investigate how their somatic growth rate responded to variation in food resources arising from changing hydrodynamic conditions. Experimental manipulations of food quality and quantity were achieved by diluting riverine suspended particulate matter (seston) to obtain a range of food concentrations and by amending natural seston with laboratory-grown Scenedesmus acutus. Bosmina experienced food limitation during periods of elevated discharge when the P content of seston was low (C : P . 600). Growth rates covaried with body RNA and P contents (R2 5 0.96 and 0.86, respectively; content expressed as percent of dry mass), which is consistent with the hypothesis that high growth rates require increased allocation to P-rich ribosomal RNA and that high seston C : P induces P limitation in riverine Bosmina. P limitation arises in riverine consumers of riverine seston when hydrologic conditions favor terrestrial inputs, sediment resuspension, and low algal productivity, thereby resulting in seston fractions dominated by P-poor materials. During low discharge, riverine seston was P-rich relative to Bosmina requirements and growth rates were decoupled from body P content. Bosmina RNA content was strongly and linearly related to growth over a broad range of resource conditions, suggesting that it may be a useful surrogate to assess dietary sufficiency of food resources in natural settings. Hydrodynamic environments range from relatively still (wind-driven mixing) in lentic waters to turbulent (fluvialand tidal-driven mixing) in streams, rivers, and estuaries. Cross-system differences and seasonal variation in physical forces are important determinants of the quantity and composition of suspended particulate matter available for consumers. Food quality (edibility, digestibility, nutritional sufficiency) is determined in large part by the relative contributions from allochthonous sources (terrestrial inputs) and autochthonous production (Findlay et al. 2001; Sobczak et al. 2005). The former is detrital material of low nutritive value, whereas the latter (algal fraction) is enriched in mineral nutrients and important biochemicals (fatty acids, amino acids, etc.) whose concentrations vary with algal species composition and nutritional status (Brett and Muller-Navarra 1997; DeMott et al. 1998; Von Elert and Wolffrom 2001). Consumers in advective systems (rivers, estuaries) are thought to benefit from allochthonous subsidies to a greater extent than their lentic counterparts as a result of the physical forces that transport and maintain particulate matter in suspension. More recent work, however, has shown that the importance of allochthonous inputs to lakes may be underappreciated (Pace et al. 2004), and concurrent studies in rivers and estuaries suggest that autochthonous sources may be nutritionally important even in systems dominated by allochthonous inputs (Thorp and DeLong 2002; Guelda et al. 2005; Sobczak et al. 2005). Zooplankton frequently experience reduced growth from low food quantity or poor food quality (Boersma et al. 2001; Acharya et al. 2004; DeMott et al. 2004). Recent 1 To whom correspondence should be addressed. Present address: Division of Hydrologic Sciences, Desert Research Institute, Las Vegas, Nevada 89119. Acknowledgments We acknowledge the assistance of the following people on this project. Derek Taylor of SUNY, Buffalo, helped with Bosmina genotype DNA identification. Allison Smith, Richard Schultz, Stacy Pritchard, and Mike Hanington helped with experiment preparation and water analysis. Two anonymous reviewers made helpful comments and suggestions that greatly improved the manuscript. K.A. was supported by a University of Louisville Wallace Research Fellowship. Research was supported by the Center for Environmental Science at the University of Louisville. Limnol. Oceanogr., 51(4), 2006, 1859–1869 E 2006, by the American Society of Limnology and Oceanography, Inc.