Rising pCO2 in Freshwater Ecosystems Has the Potential to Negatively Affect Predator-Induced Defenses in Daphnia.

Anthropogenically released CO2 accumulates in the global carbon cycle and is anticipated to imbalance global carbon fluxes [1]. For example, increased atmospheric CO2 induces a net air-to-sea flux where the oceans take up large amounts of atmospheric CO2 (i.e., ocean acidification [2-5]). Research on ocean acidification is ongoing, and studies have demonstrated the consequences for ecosystems and organismal biology with major impacts on marine food webs, nutrient cycles, overall productivity, and biodiversity [6-9]. Yet, surprisingly little is known about the impact of anthropogenically caused CO2 on freshwater systems due to their more complex biogeochemistry. The current consensus, yet lacking data evidence, is that anthropogenic CO2 does indeed affect freshwater carbon hydrogeochemistry, causing increased pCO2 in freshwater bodies [10-13]. We analyzed long-term data from four freshwater reservoirs and observed a continuous pCO2 increase associated with a decrease in pH, indicating that not only the oceans but also inland waters are accumulating CO2. We tested the effect of pCO2-dependent freshwater acidification using the cosmopolite crustacean Daphnia. For general validity, control pCO2-levels were based on the present global pCO2 average. Treatments were selected with very high pCO2 levels, assuming a continuous non-linear increase of pCO2, reflecting worst-case-scenario future pCO2 levels. Such levels of elevated pCO2 reduced the ability of Daphnia to sense its predators and form adequate inducible defenses. We furthermore determined that pCO2 and not the resulting reduction in pH impairs predator perception. If pCO2 alters chemical communication between freshwater species, this perturbs intra- and interspecific information transfer, which may affect all trophic levels.