Influence of ocean acidification on plankton community structure during a winter-to-summer succession: An imaging approach indicates that copepods can benefit from elevated CO2 via indirect food web effects

Planktoncommunities play a key role in the marine food weband are expected to be highly sensitive to ongoing environmental change. Oceanic uptake of anthropogenic carbon dioxide (CO2) causes pronounced shifts in marine carbonate chemistry and a decrease in seawater pH. These changes–summarized by the term ocean acidification(OA)–can significantly affect the physiology of planktonicorganisms. However, studies on the response of entire planktoncommunities to OA, which also include indirect effects via food-webinteractions, are still relatively rare. Thus, it is presently unclear how OA could affect the functioning of entire ecosystems and biogeochemical element cycles. In this study, we report from a long-term in situ mesocosmexperiment, where we investigated the response of natural planktoncommunities in temperate waters (Gullmarfjord, Sweden) to elevated CO2 concentrations and OA as expected for the end of the century (~760 μatm pCO2). Based on a plankton-imaging approach, we examined size structure, communitycomposition and food webcharacteristics of the whole planktonassemblage, ranging from picoplanktonto mesozooplankton, during an entire winter-to-summer succession. The planktonimaging system revealed pronounced temporal changes in the size structure of the copepodcommunity over the course of the planktonbloom. The observed shift towards smaller individuals resulted in an overall decrease of copepodbiomass by 25%, despite increasing numerical abundances. Furthermore, we observed distinct effects of elevated CO2 on biomass and size structure of the entire planktoncommunity. Notably, the biomass of copepods, dominated by Pseudocalanus acuspes, displayed a tendency towards elevated biomass by up to 30–40% under simulated ocean acidification. This effect was significant for certain copepodsize classes and was most likely driven by CO2-stimulated responses of primary producersand a complex interplay of trophic interactions that allowed this CO2 effect to propagate up the food web. Such OA-induced shifts in planktoncommunity structure could have far-reaching consequences for food-webinteractions, biomass transfer to higher trophic levels and biogeochemical cyclingof marine ecosystems.