Elevated pCO2 affects behavioural patterns and mechano-sensation in predatory phantom midge larvae Chaoborus obscuripes.

Aquatic acidification is a major consequence of fossil fuel combustion. In marine ecosystems it was shown, that increasing pCO2 levels significantly affect behavioural and sensory capacities in a diversity of species. This can result in altered predator and prey interactions and thereby change community structures. Just recently also CO2 dependent acidification of freshwater habitats has been shown. Also here, increased levels of pCO2 change organisms' behaviour and sensory capacities. For example, the freshwater crustacean Daphnia's ability to detect predators and accurately develop morphological defences was significantly reduced, rendering Daphnia more susceptible to predation. It was speculated that this may have cascading effects on freshwater food webs. However, for a comprehensive understanding of how increased levels of CO2 affect trophic interactions, it is also important to study how CO2 affects predators. We tested this using the dipeteran phantom midge larva Chaoborus obscuripes, which is a world-wide abundant inhabitant of freshwater impoundments. We monitored activity parameters, predation parameters, and predation rate. Chaoborus larvae are affected by increased levels of pCO2 as we observed an increase in undirected movements and at the same time, reduced sensory abilities to detect prey items. This is likely to affect the larvae's energy budgets. Chaoborus is a central component of many freshwater food-webs. Therefore, CO2 effects on predator and prey levels will likely have consequences for community structures.

Porewater methane transport within the gas vesicles of diurnally migrating Chaoborus spp.: An energetic advantage.

Diurnally-migrating Chaoborus spp. reach populations of up to 130,000 individuals m-2 in lakes up to 70 meters deep on all continents except Antarctica. Linked to eutrophication, migrating Chaoborus spp. dwell in the anoxic sediment during daytime and feed in the oxic surface layer at night. Our experiments show that by burrowing into the sediment, Chaoborus spp. utilize the high dissolved gas partial pressure of sediment methane to inflate their tracheal sacs. This mechanism provides a significant energetic advantage that allows the larvae to migrate via passive buoyancy rather than more energy-costly swimming. The Chaoborus spp. larvae, in addition to potentially releasing sediment methane bubbles twice a day by entering and leaving the sediment, also transport porewater methane within their gas vesicles into the water column, resulting in a flux of 0.01-2 mol m-2 yr-1 depending on population density and water depth. Chaoborus spp. emerging annually as flies also result in 0.1-6 mol m-2 yr-1 of carbon export from the system. Finding the tipping point in lake eutrophication enabling this methane-powered migration mechanism is crucial for ultimately reconstructing the geographical expansion of Chaoborus spp., and the corresponding shifts in the lake's biogeochemistry, carbon cycling and food web structure.