Does biocide treatment for mosquito control alter carbon dynamics in floodplain ponds?

Shallow lentic aquatic ecosystems, such as ponds, are important repositories of carbon (C) and hotspots of C cycling and greenhouse gas emission. Tube-dwelling benthic invertebrates, such as chironomids, may be key players in C dynamics in these water bodies, yet their role in the C-budget at ecosystem level remains unclear. We tested whether a 41 % reduction in chironomid abundance after application of the mosquito control biocide Bacillus thuringiensis israelensis (Bti) had implications for the C-fluxes to the atmosphere, C-pools, and C-transformation (i.e. organic matter decomposition) in ponds. Data were collected over one year in the shallow, deep and riparian zones of 12 experimental floodplain pond mesocosms (FPMs), half of them treated with Bti. C-fluxes were measured as CO2 and CH4 emissions, atmospheric deposition, and emerging insects. C-pools were measured as dissolved inorganic and organic C in surface and porewater, sediment organic C, C in plant and in macroinvertebrate biomass. Despite seasonal variability, treated FPMs, for which higher CH4 emissions have been reported, showed a trend towards less dissolved organic C in porewater, while no effect was observed for all remaining components of the C-budget. We attribute the effect of Bti on the C-budget to the reduction in macroinvertebrates biomass, the increase in CH4 emissions, and the input of C from the Bti excipients. This finding suggests that changes in tube-dwelling macroinvertebrates have a weak influence on C cycling in ponds and confirms the existence of long-lasting effects of Bti on specific components of C-budgets.