Simulated ocean acidification reveals winners and losers in coastal phytoplankton

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.

High CO2 Under Nutrient Fertilization Increases Primary Production and Biomass in Subtropical Phytoplankton Communities: A Mesocosm Approach

“…A number of experimental and modeling studies have suggested the existence of potential winners and losers in a future acidified ocean (Dutkiewicz et al, 2013;Kroeker et al, 2013;Bach et al, 2017). In our mesocosm experiment, the shift from a cyanobacteria-dominated community to a large diatom-dominated community in the high CO 2 scenarios after nutrient fertilization, as well as the distinct size-related relative change observed among size-fractions, seems to support this hypothesis.…”

The Influence of Plankton Community Structure on Sinking Velocity and Remineralization Rate of Marine Aggregates

“…Thus, accounting for the dependency of SV on P/NM could improve our ability to estimate export fluxes with optical tools and in biogeochemical models (Guidi et al, ; Siegel et al, ). Ultimately, this may be a significant step toward a better predictability of BCP strength and efficiency in a future ocean where picophytoplankton could become more dominant (Bach et al, ; Boyd, ).…”

CO<sub>2</sub> effects on diatoms: a synthesis of more than a decade of ocean acidification experiments with natural communities

“…From that point of view it is desirable to include all important ecosystem components because when trophic cascades are represented incompletely then the observed response in an experiment may not reflect the response that would occur in nature, which is what we are ultimately interested in (Carpenter, 1996). Clearly, investigating OA effects on diatoms or any other group in complex communities has the disadvantage that the actual cause for an observed response can hardly ever be determined with high certainty (Bach et al, , 2019. However, experiments compiled herein investigated the development of initially similar plankton communities over time with the only difference being carbonate chemistry conditions between control and the treatments.…”