Effects of an anthropogenic stressor on carbon biogeochemical processes and fluxes in water to land transition zones of ponds

More than 2.4 % of the continental surface area is covered by shallow aquatic systems such as ponds. Despite occupying only a tiny fraction of the earth's surface area, ponds are globally significant sites of carbon cycling. They receive carbon, process it and emit large amounts of greenhouse gases into the atmosphere, the most potent among others are carbon dioxide (CO2) and methane (CH4). Tube-dwelling macroinvertebrates, such as chironomid larvae (Diptera: Chironomidae) change biogeochemical functions, particularly in shallow aquatic systems. Through bioturbation involving burrow ventilation and sediment particle reworking, tube-dwelling macroinvertebrates enhance solute exchange between sediment and water. Stimulate the benthic microbial community, and regulate organic matter decomposition. This doctoral project integrates aquatic carbon biogeochemical processes with the research field of ecology to relate knowledge of biogeochemical reaction dynamics upon application of the mosquito control biocide Bacillus thuringiensis israelensis (Bti), which is an entomopathogen that kills mosquitos larvae, but also reduces the abundance of chironomids. The interdisciplinary approach combines field measurements and laboratory experiments. First, an experiment was conducted in 12 outdoor floodplain ponds mesocosms (FPMs), where the effect of Bti application on carbon transformations, carbon pools, and carbon fluxes was monitored for one year. Half of the FPMs were Bti-treated and the remaining half were controls. The study revealed that seasonal variations governed changes in transformations, pools, and fluxes on the carbon components. Treated FPMs, for which a 26 % and 41% reduction in emerging merolimnic insects and macroinvertebrates abundance, respectively was reported (in companion studies) were higher CH4emitters (137% higher than in control mesocosms). The higher CH4 emissions occurred specifically in the shallow zone where the macroinvertebrate reduction was also significant. In the same treated FPMs, a tendency towards less dissolved organic carbon in porewater (33% lower than in control mesocosms), was potentially caused by the reduction in bioturbation activities of chironomids, whereas the remaining measured components of the carbon budget were not affected by the treatment with Bti. Second, laboratory microcosm (LMs) experiments that excluded environmental constraints were developed, to clarify the findings of the FPMs experiment. Out of the 15 microcosms, 3 were treated (each set) with standard Bti dose, 5 times standard Bti dose, chironomid larvae with low and high areal density, and control. The findings demonstrated that bioturbation increased CH4 and CO2 efflux and sediment oxygen (O2) consumption, while it did not affect the net production of CH4 and CO2. The negligible effect on net production rates in treatments with chironomids indicates that the increase in emissions rate was predominantly caused by bioturbation, which reduced the gas accumulation in the sediment. In the absence of chironomids, the application of any dose of Bti led to a three-fold higher net production rate of CH4 and CO2 (by up to 2.7 times than in control), due to the high addition of bioavailable carbon through the Bti excipients. However, the sole addition of carbon through the Bti excipients could not justify the high net production rate suggesting that the addition of Bti triggered a more robust carbon metabolism process. Both FPMs and LMs results suggested that the application of Bti may have functional implications on carbon biogeochemistry in affected aquatic systems beyond those mediated by changes in macroinvertebrate communities