Effects of simulated CO₂ escape from sediments on the development of midge Chironomus riparius

The acidification of freshwater caused by potential CO2 gas seepage from sediment layers has not previously been studied, although freshwater is likely to be affected by the accidental escape of this gas from onshore storage facilities. In this study, two riparian sediments with different contamination levels were subjected to acidification via direct injection of CO2 gas, simulating the potential leak of CO2. Tests with the midge Chironomus riparius larvae were used to assess metal fluxes and vulnerability of benthic invertebrates to the effects of acidification. The midges were grown in whole sediments over 28d after which midge emergence and ability to reproduce were tested. The results revealed that acidification is an important factor controlling the development of the midges. Although larval development and emergence were affected by neither acidity (pH 7.5 and 7.0) nor contamination levels, none of the eggs, laid during the exposure, hatched. In less contaminated sediment, Chironomus larvae succumbed to the impact of pH 6.5 and 6.0, showing suppressed growth and metamorphosis and consequently, no emergence. In highly contaminated sediment, pH 6.5 level retarded larval growth and inhibited emergence, while pH 6.0 caused mortality. The latter could also be attributed to metal mobilization effects facilitated by an increase in the ambient acidity. Experimentation on clean sediments at pH 6.5 and 6.0 also showed absence of emergence during the experimental period. These results suggest that acidity factor could seriously diminish the vitality of midge larvae and ability of midge to produce offspring, challenging the commonly known tolerance of midges to high acidity (up to pH 3.5). Whilst larval midges can survive acidity and/or metal exposure, stimulated by creeping CO2 gas, and even emerge as adults, physiological impairments may take place which may seriously threaten the resilience of C. riparius population and subsequently lead to the decline of the population size and disruption of their function in ecosystems. Suggestions on using more sensitive endpoints than emergence when testing acidification effects are made.