THE STABLE-ISOTOPE RELATIONSHIP BETWEEN A SCRAPER-GRAZER, GLOSSOSOMA NIGRIOR (GLOSSOSOMATIDAE:TRICHOPTERA), AND ITS ALGAL DIET

R. R. Doucett¹, D. J. Giberson², G. Power¹, and R. A. Cunjak³. ¹Department of Biology, University of Waterloo, 200 University Ave. , Waterloo, ON, Canada, N2L 3G1, ²Department of Biology, University of Prince Edward Island, 550 University Ave. , Charlottetown, PEI, Canada, C1A 4P3, ³Department of Biology, University of New Brunswick, Bag Service 45111, Fredericton, NB, Canada, E3B 6E1

Epilithic algae, collected as biofilm scraped from rock surfaces, exhibited distinct d¹³C and d¹⁵N values in Catamaran Brook and the LSW Miramichi River, New Brunswick, in August 1997. General patterns in algal d¹³C were related to the d¹³C of DIC at each site. The grazer, Glossosoma nigrior, showed markedly different d¹³C and d¹⁵N values among sites, and values correlated well with those of algae. Water velocity affected the d¹³C and d¹⁵N values in algae at only 1 of 4 sites, suggesting that other factors (growth rate, carbon-concentrating mechanisms, HCO₃^- utilization) may be more important in the determination of stable-isotope ratios in lotic microalgae. Glossosoma nigrior were more ¹³C-depleted and more ¹⁵N-enriched than other grazers (e.g., Rhithrogena impersonata), and differences may be related to feeding mode or microhabitat preference. Because of potential terrestrial and heterotrophic contamination in the epilithic matrix, invertebrate grazers may serve as better indicators of algal d¹³C than biofilm. However, care should be given to the selection of primary consumers as isotopic surrogates of original food sources. Extensive variation in algal and grazer d¹³C and d¹⁵N values in this study emphasizes a need for more comprehensive sampling of autochthonous resources when stable-isotope ratios are used for food-web interpretation.