Methane as a source of carbon and energy for chironomid larvae

Food webs are an important component of community and ecosystem structure. Early descriptions of food webs were usually simple and consisted of few taxa and limited numbers of links. Until recently, photosynthetic phytoplankton, algae and macrophytes were considered to be the only primary producers and thus the major carbon source in aquatic systems. Chemosynthesis was considered quantitatively less important. When the hydrothermal vent communities were first discovered, biologists were astonished by the density of biomass associated with the active vents, where photosynthesis was not possible. These findings led towards a new area of research. Today, it is accepted that in these special environments, microbial chemoautotrophic primary production can replace photosynthesis as the dominant source of ecosystem energy production. Recent work in freshwater ecosystems also indicated that chemosynthetically fixed carbon sources (i.e. methane) could be of importance. Based upon high variation in chironomid larval δ¹³C values from eutrophic lakes with different mixing regimes, limnologists proposed differences in the methane cycle that provide benthic organisms with carbon to be responsible. In this thesis, I particularly focus on interactions between chironomid larvae, as a model organism for benthic macroinvertebrates, and the methane carbon cycle in freshwater lakes. By comparing lakes with different morphologies and mixing regimes, it could be shown that the variation in chironomid larval δ¹³C signatures previously found indeed relate to variations in conditions for methane production and oxidation in the sediment. The contribution of methane-derived carbon was higher in the dimictic Holzsee compared to the polymictic Großer Binnensee with its well-oxygenated water column, irrespective of an identical methanogenic and methanotrophic bacterial community (chapter I). By sampling both lakes during the year, I found seasonality in methane turnover activities and chironomid larval δ¹³C values only in the stratifyed and not in the polymictic lake. Thus, both the methane oxidation activity and the availability of MOB biomass determined the importance of methane-derived carbon for the larval diet (chapter II). The analysis of larval δ¹³C values and methane production from sediments at different water column depth provides additional evidence for a site-specific methanotrophic contribution to larval biomass within one lake. Based on larval distribution, the contribution of methane-derived carbon via chironomids to pelagic and terrestrial predators was estimated to be substantial (chapter III). Depleted δ¹³C values of chironomid larvae can also be a consequence of larvae feeding upon other chemoautotrophic bacteria, apart from using methane as a carbon source. Sulphur stable isotopes were used to demonstrate that within a single lake, one of two congeneric chironomid species relies more upon methanotrophic biomass whereas the other depends on chemoautotrophic biomass (chapter IV). The application of hydrogen isotopes provided further evidence for the importance of methane-derived carbon in the nutrition of chironomid larvae and gave insight into the dominant methanogenic pathways operating in different sediment strata (chapter V). By manipulating the isotopic signature of methane, I collected for the first time experimental evidence that chironomid larvae assimilate methane-derived carbon by feeding on MOB that are present in both, the sediment and the water column (chapter VI).Summary 1 Zusammenfassung 3 Introduction Zoobenthos 5 Sediment bacteria as a food source for zoobenthos 6 Sediment bacteria as key players in the carbon cycle in freshwater lakes 7 Stable isotopes as a tool to analyse food webs 8 Stable isotopes reveal evidence for chemosynthesis in marine environments 9 Stable isotopes reveal evidence for chemosynthesis in freshwater environments 10 Chironomid larvae, a model organism for lake zoobenthos 11 Thesis outline 14 Chapters I Methane cycling in lake sediments and its influence on chironomid δ¹³C 17 II Are seasonal changes of δ¹³C signatures in chironomid larval biomass 41 related to changes in the microbial methane cycle? III Site-specific methane production and subsequent midge mediation 61 within Esthwaite Water, UK IV Differential assimilation of methanotrophic and chemoautotrophic 79 bacteria by lake chironomid larvae V Can stable hydrogen and carbon isotope analyses be used to unravel 89 complexities in benthic food webs? VI Experimental evidence of chironomid larvae feeding on benthic as 103 well as pelagic methane-derived bacterial carbon Conclusion 124 Danksagung 126 References 128 Curriculum vitae 142 Erklärung 14