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

Anthropogenically released CO accumulates in the global carbon cycle and is anticipated to imbalance global carbon fluxes [1]. For example, increased atmospheric CO induces a net air-to-sea flux where the oceans take up large amounts of atmospheric CO (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 CO on freshwater systems due to their more complex biogeochemistry. The current consensus, yet lacking data evidence, is that anthropogenic CO does indeed affect freshwater carbon hydrogeochemistry, causing increased pCO in freshwater bodies [10-13]. We analyzed long-term data from four freshwater reservoirs and observed a continuous pCO increase associated with a decrease in pH, indicating that not only the oceans but also inland waters are accumulating CO. We tested the effect of pCO-dependent freshwater acidification using the cosmopolite crustacean Daphnia. For general validity, control pCO-levels were based on the present global pCO average. Treatments were selected with very high pCO levels, assuming a continuous non-linear increase of pCO, reflecting worst-case-scenario future pCO levels. Such levels of elevated pCO reduced the ability of Daphnia to sense its predators and form adequate inducible defenses. We furthermore determined that pCO and not the resulting reduction in pH impairs predator perception. If pCO alters chemical communication between freshwater species, this perturbs intra- and interspecific information transfer, which may affect all trophic levels.