Rapid, but limited, zooplankton adaptation to simultaneous warming and acidification

Predicting the response of marine metazoans to climate change is hampered by a lack of studies on evolutionary adaptation, particularly to combined warming and acidification. To test whether the ubiquitous marine copepod Acartia tonsa can adapt to warmer and acidified conditions, we tracked five fitness-relevant life-history traits for 25 generations (~ 1 year) with a 2 x 2 factorial design of temperature (18 C, 22 C) and pCO2 (400, 2000 μatm). Initially, combined warm, acidic conditions decreased egg production and hatching frequency, resulting in a 56% reduction in population fitness (net reproductive rate). However, both traits recovered after three generations and average fitness was reduced by only 9% thereafter, indicating rapid adaptation. Antagonistic interactions between warming and acidification in later generations decreased survival, thereby limiting full evolutionary rescue. Our results suggest that interactions between warming and acidification constrain evolutionary rescue and add complexity to predictions of metazoan populations responses to climate change.