insights from a long-term laboratory selection study, link to supplementary material

The ongoing acidification process of the oceans is likely to have consequences for many marine biota. Although evolutionary responses are expected during persisting environmental change, little is known about the adaptability of copepods. Therefore, we set up a 3 ½ years long selection experiment, culturing Acartia tonsa populations in water treated with 200 and 800 µatm pCO2, feeding them with algae grown in f/2 medium under 200 µatm pCO2 and in f/2 N P medium under 800 µatm pCO2. After three reciprocal transplant experiments we measured copepods' developmental rates, carbon to nutrient ratios, egg production and hatching rates. Under high CO2 conditions, stoichiometric discrepancies between the requirements of A. tonsa and its food resulted in a significantly decreased developmental rate independent from the selective history. After one year, these discrepancies appeared alleviated by an optimised homeostasis regulation of the copepods, indicating a high body stoichiometry regulation plasticity. Egg production and hatching success were unaffected by the experimental conditions, however, results indicated a premature hatching of eggs from females with a high CO2 selective history. Over the experimental period we did not detect any beneficial adaptations of the copepods cultured under high CO2 conditions of elevated seawater pCO2 and associated food quality reduction. Towards the end of the experiment, copepods cultured under elevated pCO2 and fed with high CO2 algae, showed an increased body mass and decreased prosome length. Such physiological changes could have profound long term consequences for marine copepods, food web interactions, and ultimately ecosystem structures and functions.