Direct and indirect effects of the increase in atmospheric CO2 and temperature on groundwater organisms

Atmospherically rising temperature and CO2 impact all freshwater systems, including groundwater. Increasing CO2 leads to more intense weathering of silicate rocks. Here, we tested whether the increased levels, the weathering, or rather the increasing temperature, impacted on fauna and prokaryotes in the groundwater ecosystem. We conducted the analyses separately for deep, i.e. secluded, and shallow, quaternary aquifers which exchange with the surface more intensely. Organism abundances and relative composition did not correlate with temperature or CO2 levels. While many organisms rely on silica, in contrast, we found negative correlations between silica and fauna. The increases in silica over time, i.e. temporal trends, also partly correlated negatively with organisms. We hypothesize that the unexpected negative correlations are not direct effects, but indirectly indicate that groundwater communities do not adapt rapidly enough to changes. This jeopardizes future drinking water production which relies on the self-cleaning ecosystem services in groundwater. ### Competing Interest Statement The authors have declared no competing interest.