Potential role of methane-derived carbon as a food source for Daphnia in a North Carolina reservoir

Herbivorous zooplankton feed on seston, including algal, detrital, and bacterial components. However, many studies have found that Daphnia del13C is more depleted than seston del13C, and selective feeding on seston algal components has been hypothesized. We hypothesized that low zooplankton del13C was due to utilization of methane-derived carbon (MDC). Lake Brandt data indicated a strong seasonal effect on the del13C of Daphnia when compared to seston. During summer stratification, carbon signatures of Daphnia were more depleted than bulk seston, while during winter mixing Daphnia were more enriched. We conducted an experiment to assess utilization of MDC as a possible mechanism of depleted Daphnia del13C. At low methane concentrations, methane del13C was more enriched than at high concentrations, indicating isotopic fractionation by methanotrophs. Daphnia del13C was slightly enriched compared to seston at low methane concentrations, but was depleted compared to seston at high methane concentrations, consistent with incorporation of MDC. An antibiotic appeared to limit methanotrophs in the water column resulting in enriched Daphnia del13C. MDC contribution to the diet of Daphnia was estimated as almost 32% of assimilation. Our study provides strong evidence that assimilation of MDC resulted in depleted Daphnia del13C relative to seston. We suggest that this phenomenon is widespread in freshwater ecosystems, explaining the consistently light del13C of grazing zooplankton relative to bulk seston.