Effects of pelagic food web structure and nutrient concentration on anaerobic methane metabolism in lake sediments

Although anaerobic metabolism in lake sediments is strongly related to organic material from the trophogenic layer, little is known about the role of food web structure in this respect. We tested the influence of planktivorous fish (treatments with or without fish, ±F) and nutrients (treatments with or without fertilization, ±N) on chlorophyll a, zooplankton, sedimentation of particulate organic carbon (POC), and methane accumulation in large enclosures with anoxic hypolimnia (10 m diameter, 8 m deep, 2 × 2 factorial design). Additionally, methane production potential from settled material was estimated in laboratory experiments. In the enclosures, methane accumulation rate increased in the order +F/−N, −F/+N, −F/−N, and +F/+N, while POC sedimentation was similar in all treatments. Settled POC was more efficiently transformed into methane in −F/−N than in +F/−N treatments. However, an opposite effect was observed between −F/+N and +F/+N treatments. In the laboratory, methane production potential was higher when (1) POC content in settled matter increased, and (2) no fish were present. This corresponded well only to field results obtained in −F/−N and +F/−N treatments. The unexpectedly high methane accumulation in the +F/+N treatment was very likely related to abrupt depletion of other electron acceptors in the hypolimnion, which attenuated effects of food web structure. In conclusion, our results indicate that food web structure indirectly affects anaerobic microbial activity primarily due to changes in the decomposition potential of settled organic matter.