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 (δ 13 C,
Δ 14 C, 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). δ 13 C-enrichment of lake C
pools and overlap with δ 13 C of terrestrial
resources can arise due to reduced fractionation of 13 C by phytoplankton under
CO 2 limitation, therefore we recommend careful
consideration of potential environmental drivers when interpreting
among-lake patterns in δ 13 C. Our findings
emphasize the importance of local-scale variation in mediating
terrestrial contributions to lake food webs.