Abstract
The oceans absorb ~25% of the annual anthropogenic CO2 emissions. This causes a shift in the marine carbonate chemistry termed ocean acidification (OA). OA is expected to influence metabolic processes in phytoplankton species but it is unclear how the combination of individual physiological changes alters the structure of entire phytoplankton communities. To investigate this, we deployed ten pelagic mesocosms (volume ~50 m3) for 113 days at the west coast of Sweden and simulated OA (pCO2 = 760 μatm) in five of them while the other five served as controls (380 μatm). We found: (1) Bulk chlorophyll a concentration and 10 out of 16 investigated phytoplankton groups were significantly and mostly positively affected by elevated CO2 concentrations. However, CO2 effects on abundance or biomass were generally subtle and present only during certain succession stages. (2) Some of the CO2-affected phytoplankton groups seemed to respond directly to altered carbonate chemistry (e.g. diatoms) while others (e.g. Synechococcus) were more likely to be indirectly affected through CO2 sensitive competitors or grazers. (3) Picoeukaryotic phytoplankton (0.2–2 μm) showed the clearest and relatively strong positive CO2 responses during several succession stages. We attribute this not only to a CO2 fertilization of their photosynthetic apparatus but also to an increased nutrient competitiveness under acidified (i.e. low pH) conditions. The stimulating influence of high CO2/low pH on picoeukaryote abundance observed in this experiment is strikingly consistent with results from previous studies, suggesting that picoeukaryotes are among the winners in a future ocean.
Highlights
The seasonal succession of plankton involves the occurrence and disappearance of plankton taxonomic and functional groups in an annually repeated pattern [1]
CO2-outgassing at the air-sea interface in the high CO2 mesocosms was countered by regular additions of CO2-aerated water while pCO2 was not manipulated in the control treatment
The pCO2 levels averaged over the entire experimental period were 759 (±11) and 384 (±19) μatm in the high CO2 and control environments, respectively [12]
Summary
The seasonal succession of plankton involves the occurrence and disappearance of plankton taxonomic and functional groups in an annually repeated pattern [1]. The major biomass build-up during the spring bloom is traditionally seen as the starting point of the succession in temperate regions, the initiation of the bloom already takes place in early winter [2,3]. Influence of high CO2 on coastal phytoplankton succession data collection and analysis, decision to publish, or preparation of the manuscript
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