Relative importance of CO2 recycling and CH4 pathways in lake food webs along a dissolved organic carbon gradient

Abstract

Terrestrial ecosystems export large quantities of dissolved organic carbon (DOC) to aquatic ecosystems. This DOC can serve as a resource for heterotrophic bacteria and influence whether lakes function as sources or sinks of atmospheric CO2. However, it remains unclear as to how terrestrial carbon moves through lake food webs. We addressed this topic by conducting a comparative lake survey in the northeastern U.S. along a gradient of terrestrial‐derived DOC. We used naturally occurring carbon stable isotopes of CO2, particulate organic matter (POM), and crustacean zooplankton, as well as gas measurements and culture‐independent assessments of microbial community composition to make inferences about the flow of terrestrial carbon in lake food webs. Stable isotope ratios of POM and zooplankton decreased with DOC and were often depleted in 13C relative to terrestrial carbon, suggesting the importance of an isotopically light carbon source. It has been proposed that the incorporation of biogenic methane (CH4) into plankton food webs would account for such trends in stable isotope ratios, but we found weak evidence for this hypothesis, on the basis of relationships of CH4, methanogenic archaebacteria, and methanotrophic bacteria in our lakes. Instead, our results are consistent with the view that phytoplankton increase their use of heterotrophically respired CO2 with increasing concentrations of terrestrialderived DOC. The effect of this CO2 recycling can be detected in the stable isotope composition of crustacean zooplankton, suggesting that the direct transfer of terrestrial DOC inputs to higher trophic levels may be relatively inefficient.