MEPS 486:37-46 (2013)  -  DOI: https://doi.org/10.3354/meps10352

ABSTRACT: Changing seawater chemistry towards reduced pH as a result of increasing atmospheric carbon dioxide (CO₂) is affecting oceanic organisms, particularly calcifying species. Responses of non-calcifying consumers are highly variable and mainly mediated through indirect ocean acidification effects induced by changing the biochemical content of their prey, as shown within single species and simple 2-trophic level systems. However, it can be expected that indirect CO₂ impacts observed at the single species level are compensated at the ecosystem level by species richness and complex trophic interactions. A dampening of CO₂-effects can be further expected for coastal communities adapted to strong natural fluctuations in pCO₂, typical for productive coastal habitats. Here we show that a plankton community of the Kiel Fjord was tolerant to CO₂ partial pressure (pCO₂) levels projected for the end of this century (<1400 µatm), and only subtle differences were observed at the extremely high value of 4000 µatm. We found similar phyto- and microzooplankton biomass and copepod abundance and egg production across all CO₂ treatment levels. Stoichiometric phytoplankton food quality was minimally different at the highest pCO₂ treatment, but was far from being potentially limiting for copepods. These results are in contrast to studies that include only a single species, which observe strong indirect CO₂ effects for herbivores and suggest limitations of biological responses at the level of organism to community. Although this coastal plankton community was highly tolerant to high fluctuations in pCO₂, increase in hypoxia and CO₂ uptake by the ocean can aggravate acidification and may lead to pH changes outside the range presently experienced by coastal organisms.

KEY WORDS: Ocean acidification · Copepods · Phytoplankton · Mesocosm · Plankton community · Microzooplankton · Reproduction