Recent studies have shown that larval chironomids assimilate 13 C-depleted carbon derived from biogenic methane by feeding on methane-oxidizing bacteria (MOB). The dietary contribution of MOB is known to be maximized in the autumn overturn period or winter in eutrophic dimictic lakes due to the increase of MOB biomass following the supply of oxygenated water, but in polymictic lakes, such seasonal variability has not been revealed. We investigated the seasonal patterns of larval d 13 C and methane concentrations in the sediment of a eutrophic polymictic lake, Izunuma, Japan. Larval d 13 C decreased in late summer and autumn. Methane concentrations above a 6 cm depth peaked in late summer or autumn, while those in the 10-11-and 20-21-cm layers peaked in October. Negative correlations between methane concentrations in the 5-6/10-11-cm layers and larval d 13 C were found. This suggests that an increase in the supply of methane stimulated the activity of MOB in a polymictic lake, where water above the lake bottom rarely became anoxic because of frequent overturn, thus increasing the dietary contribution of MOB to larval chironomids.

Piscivorous largemouth bass (Micropterus salmoides) have been introduced in several regions outside of their native range in North America, resulting in significant disturbance to native fish communities. This species exhibits an ontogenetic diet shift from zooplanktivory to piscivory as juveniles. An early switch to piscivory allows 0+ bass to increase their growth rate prior to winter, resulting in reduced mortality. However, little is known about the dietary switch at the population level during the first year. We used carbon stable isotope analyses to examine the diets of age 0+ individuals in Lake Izunuma, Japan. The onset of the shift to piscivory occurred at a smaller size than in native or other nonnative areas [>40 mm total length (TL)]. We found a positive correlation between TL and δ 13 C throughout summer and autumn. Small individuals had δ 13 C values that were similar to those of zooplankton, whereas large individuals had δ 13 C values that were similar to those of cyprinid prey species. This suggests that the smaller 0+ individuals remain zooplanktivorous until late autumn, whereas the larger individuals shift to piscivory as early as June, soon after the breeding season ends. Our results also suggest that a significant number of 0+ bass failed to switch to piscivory until the winter of their first year, despite the smaller size threshold for the onset of piscivory.

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