Chemistry and isotopic composition of resource pools in lakes of the Sierra Nevada, CA

The extent to which terrestrial organic matter supports aquatic consumers remains uncertain because factors regulating resource flows are poorly understood. We sampled 12 lakes throughout the Sierra Nevada (California, USA) spanning large gradients in elevation and size to evaluate how watershed attributes and lake morphometry influence resource flows to lake carbon pools and zooplankton. We found that the size and composition of carbon pools in lakes were often more strongly determined by watershed or lake features rather than by elevational position. Using three different tracers of resource origin (δ13C, Δ14C, C:N ratio), we found terrestrial contributions to most lake resource pools (DOC, POM, sediments) and pelagic consumers (zooplankton) were more strongly related to local-scale watershed features such as vegetation cover or watershed area:lake area rather than to elevation. Landscape patterns in multiple tracers indicated consistent contribution of within-lake C sources to bulk resource pools across elevations (POM, sediments, zooplankton). δ13C-enrichment of lake C pools and overlap with δ13C of terrestrial resources can arise due to reduced fractionation of 13C by phytoplankton under CO2 limitation, therefore we recommend careful consideration of potential environmental drivers when interpreting among-lake patterns in δ13C. Our findings emphasize the importance of local-scale variation in mediating terrestrial contributions to lake food webs.