The influence of nitrogen limitation on d15N and carbon : nitrogen ratios in sediments from sockeye salmon nursery lakes in British Columbia, Canada

Abstract

Carbon (C) and nitrogen (N) elemental and nitrogen isotopic ratios were determined for bulk organic matter in surface sediments of 11 sockeye salmon (Onchorynchus nerka) nursery lakes in British Columbia, Canada. Chitinous exoskeletons of Bosmina spp. were also picked from the sediments and analyzed for Δ15N. The correlation between salmon escapement (spawner abundance) and bulk organic Δ15N confirms the importance of marine-derived nitrogen to the study lakes. The relation between sediment Δ15N and escapement, however, suggests that simple mixing models may significantly underestimate spawner abundance in nitrogen-limited lakes. In addition, the use of sediment and chitin Δ15N and C:N data to characterize lake-specific differences demonstrates that sediment isotope signatures are dependent on attendant nitrogen deficiency. The sediment C:N ratio, on its own, provides a sensitive measure of nutrient status in these lakes. This finding provides an alternate additional interpretation of C:N ratios that may be more appropriate in large lakes where terrestrial contributions to the central basin are insignificant.