Exploration of the influence of global warming on the chironomid community in a manipulated shallow groundwater system

Uncertainty about predicted effects of global warming on freshwater ecosystems led us to manipulate the thermal regime of a shallow groundwater ecosystem. The study area was separated into a control and treatment block using a sheet-metal groundwater divide to a depth of 1 m. Temperatures were increased according to General Circulation Model (GCM) projections for Southern Ontario, Canada. We examined the response of the groundwater chironomid community during pre-manipulation, manipulation and recovery periods. We found that warming decreased the total abundance of chironomids whereas no significant change in taxonomic richness was apparent. Interestingly, taxon composition changed markedly during both the manipulation and the recovery period. Whereas Heterotrissocladius disappeared during the manipulation in the treatment block, other coldstenothermal taxa such as Micropsectra, Parametriocnemus and Heleniella remained unaffected. Conversely, Corynoneura, Polypedilum and Thienemannia gracilis disappeared but were not reported as coldstenothermal. The chironomid community composition in the system changed from a Heterotrissocladius, Brillia, and Tanytarsini-dominated community during the pre-manipulation towards one dominated by Parametriocnemus, Polypedilum, Orthocladius/Cricotopus and Corynoneura during the recovery. Although increased temperature had a strong effect, chironomid occurrence was also influenced by a number of other abiotic variables, such as dissolved oxygen, depth, ammonia concentration and TDS (Total dissolved solids).