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Biogenic methane as an alternative carbon and energy source for freshwater organisms has been receiving increasing attention, but the phenomenon is still poorly understood for shallow lakes. We measured the carbon and nitrogen stable isotope signatures (δ13C, δ15N) for key groups of pelagic and benthic organisms, including crustacean zooplankton, chironomid larvae, young-of-the-year and adult fish, to assess whether biogenic methane contributes to pelagic and benthic food webs in a large, shallow lake, Lake Võrtsjärv, Estonia., In the southern part of the lake, covered by macrophytes, crustacean zooplankton showed strong seasonal variation of δ13C, with the lowest values occurring in the period of oxygen depletion. Chironomid ( Chironomus plumosus) larvae showed high isotopic variability within the lake, with strongly 13C-depleted and 15N-depleted signatures (down to δ13C −64.0‰ and δ15N −2.6‰) in the macrophyte-covered area., Our results indicate that carbon derived from biogenic methane can contribute seasonally to the benthic food web and, to some extent, also to the pelagic food web, where the lake is covered by macrophytes. Moreover, δ13C values for roach ( Rutilus rutilus), perch ( Perca fluviatilis) and pike ( Esox lucius) from the macrophyte-dominated area were on average 3.5‰ lower than those for the same fish from the plankton-dominated lake area, suggesting some carbon derived from methane is transferred up through the food web., Although no direct evidence is available, our results, together with previous studies of the lake, suggest that protozoans could be a potentially important link from methane-oxidising bacteria to animals higher in the web. [ABSTRACT FROM AUTHOR]

1. As grazers on picoplankton and nanoplankton, planktonic ciliates form an important link in pelagic food webs. Ciliate communities may be controlled by predation by metazooplankton. In eutrophic systems, however, where the number of large crustaceans is often low, the mechanisms that regulate ciliate dynamics have rarely been described. 2. We conducted an enclosure experiment with natural and screened (145 μm) summer plankton communities to investigate the effect of the small-sized crustacean zooplankton on ciliate community structure and the microbial loop in a shallow eutrophic lake. 3. The removal of the larger fraction of crustaceans initiated a decrease in total ciliate abundance. At the community level, we observed a substantial increase in large-sized predacious ciliates (>100 μm) and a simultaneous decrease in the abundance of smaller ciliates (<20-40 μm) that were mostly bacterivores and bacterio-herbivores. The compositional shift in the ciliate community, however, did not cascade down to the level of bacteria and edible phytoplankton. [ABSTRACT FROM AUTHOR]

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Stratified lakes often receive more attention from limnologists although most of the world's lakes are indeed shallow. This study describes ecological processes in three shallow lakes, one dominated by macrophytes, one by phytoplankton, and one with both of them. Investigations focused on abiotic (water chemistry) as well as biotic factors such as phytoplankton, bacteria, zooplankton, and macrophytes. The macrophyte- and plankton-dominated lakes differed clearly in water transparency, which was far greater in macrophyte-dominated lakes due to phytoplankton setback in summer. Similar nutrient and zooplankton dynamics occurred in both types of lakes, suggesting that an alternative mechanism may account for the observed phytoplankton decline in macrophyte lakes. In contrast, herbivorous ciliates dominated only in the macrophyte lake. More intensive ciliate grazing could be one factor causing a decrease in phytoplankton numbers. In the plankton-dominated lake bacteria were mostly related to phytoplankton production but that was not the case in the macrophyte-dominated lake. In the plankton-dominated lake the grazing rate of ciliates on bacteria was higher than in the macrophyte-dominated one. However, when grazing on small algae was considered, the highest grazing rates were found in the macrophyte-dominated lake. This suggests that the microbial loop was weaker in the macrophyte-dominated lake than in the plankton-dominated lake. [ABSTRACT FROM AUTHOR]