No evidence of altered relationship between diet and consumer fatty acid composition in a natural plankton community under combined climate drivers

Fatty acids (FA), especially polyunsaturated fatty acids (PUFA), are key biomolecules involved in immune responses, reproduction, and membrane fluidity. PUFA in marine environments are synthesized exclusively by primary producers. Therefore the FA composition of these organisms at the base of the food web (i.e., phytoplankton) and their primary consumers (i.e., zooplankton) are important determinants of the health and productivity of entire ecosystems as they are transferred to higher trophic levels. However, environmental conditions such as seawater pH and temperature, which are already changing in response to climate change and predicted to continue to change in the future, can affect the FA composition of phytoplankton and zooplankton at both the organismal and community level. During a 20 day mesocosm experiment, we tested the effect of ocean acidification alone and in combination with ocean warming on 1) the fatty acid composition of a natural prey community for zooplankton (i.e. phytoplankton and microzooplankton), 2) the fatty acid composition of zooplankton, and 3) the relationship between prey and consumer fatty acid compositions in coastal waters. Significant effects of the climate stressors were not detected in the fatty acid composition of the prey or the relationship between diet and consumer fatty acids. A significant decrease in C18:4n-3 (stearidonic acid) was observed in the zooplankton but not their diet, but understanding the mechanism behind this decrease and its potential biological implications requires further investigation. These results highlight the importance of multi-stressor investigations on dynamics and variability contained within natural coastal plankton communities.