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.