Description: Concerns about increasing atmospheric CO2 concentrations and global warming have initiated studies on the consequences of multiple-stressor interactions on marine organisms and ecosystems. We present a fully-crossed factorial mesocosm study and assess how warming and acidification affect the abundance, body size, and fatty acid composition of copepods as a measure of nutritional quality. The experimental set-up allowed us to determine whether the effects of warming and acidification act additively, synergistically, or antagonistically on the abundance, body size, and fatty acid content of copepods, a major group of lower level consumers in marine food webs. Copepodite (developmental stages 1-5) and nauplii abundance were antagonistically affected by warming and acidification. Higher temperature decreased copepodite and nauplii abundance, while acidification partially compensated for the temperature effect. The abundance of adult copepods was negatively affected by warming. The prosome length of copepods was significantly reduced by warming, and the interaction of warming and CO2 antagonistically affected prosome length. Fatty acid composition was also significantly affected by warming. The content of saturated fatty acids increased, and the ratios of the polyunsaturated essential fatty acids docosahexaenoic- (DHA) and arachidonic acid (ARA) to total fatty acid content increased with higher temperatures. Additionally, here was a significant additive interaction effect of both parameters on arachidonic acid. Our results indicate that in a future ocean scenario, acidification might partially counteract some observed effects of increased temperature on zooplankton, while adding to others. These may be results of a fertilizing effect on phytoplankton as a copepod food source. In summary, copepod populations will be more strongly affected by warming rather than by acidifying oceans, but ocean acidification effects can modify some temperature impacts

Description: Quantifying effects of Ocean Acidification (OA) on marine primary and secondary producers is of acute interest, as they could translate up to higher trophic levels and ultimately may alter ecosystem services including fishery yields. A mesocosm approach was used to investigate the effects of OA on a natural plankton community in coastal waters off Norway by manipulating CO2 partial pressure (pCO2). Eight enclosures were deployed in the Raunefjord near Bergen. Treatment levels were ambient and elevated pCO2 of ~ 2000 µatm each in four replicate enclosures. The experiment lasted for 53 days in early summer of 2015. To assess impacts of OA on the plankton community, we measured phytoplankton and protozooplankton biomass and total seston fatty acid (FA) content. In both the control and the elevated pCO2 treatment, the plankton community was dominated by the dinoflagellate Ceratium longipes. In the elevated pCO2 treatment, however, this species as well as other dinoflagellates were strongly negatively impacted: At the end of the experiment, total dinoflagellate biomass was fourfold higher in the control group than under elevated pCO2 treatment. In a size comparison of C. longipes, individuals in the high pCO2 treatment were significantly larger. Fatty acid analysis revealed a decreased ratio of polyunsaturated fatty acids (PUFA) to saturated fatty acids (SFA) at elevated pCO2. Further, docosahexaenoic acid (DHA, C 22:6n3c), essential for development and reproduction of copepods and higher trophic levels, was lower in the high pCO2 treatment. Both in quality and quantity of their food, higher trophic levels thus experienced worse conditions in a community exposed to elevated pCO2, with potentially severe consequences for higher trophic levels.