Long-term effects of simulated ocean acidification on the calanoid copepod Acartia tonsa: results from a laboratory CO2 selection experiment, link to supplementary material

The ongoing acidification process of the oceans is predicted to have consequences for many marine biota. Although evolutionary responses are expected during persisting environmental change, little is known about the evolutionary potential of populations to counter direct or indirect effects of elevated CO2. Therefore, we investigated the impacts of elevated CO2 on the fitness of the cosmopolitan calanoid copepod Acartia tonsa and its adaptive potential. We set up a 4 years long CO2 selection study, culturing A. tonsa under 200 and 800 µatm pCO2 conditions. We performed reciprocal transplant experiments and subsequently measured the carbon to nutrient ratios of the copepods and different fitness parameter. We found a significantly decreased developmental rate of the copepods (both selection lines) under high CO2 conditions, most likely caused by stoichiometric discrepancies between the requirements of A. tonsa and its food. We did not detect any beneficial evolutionary adaptions of the high CO2 selection line copepods. However, our A. tonsa populations showed a high plasticity level in the regulation of its body stoichiometry as both CO2 selection lines became more homeostatic over time. Furthermore, our results indicate plastic phenotypic adaptions of the high CO2 selection line populations, to ensure their reproductive output. Nevertheless, the nutritional quality of the high CO2 algae (high energy, low nutrients) presumably led to an increased body mass and decreased prosoma length of the high CO2 selection line copepods. Therefore, we conclude that ocean acidification might have fundamental long term consequences for marine copepods and thereby for ecosystem functions.