Abstract
While the isolated responses of marine phytoplankton to climate warming and to ocean acidification have been studied intensively, studies on the combined effect of both aspects of Global Change are still scarce. Therefore, we performed a mesocosm experiment with a factorial combination of temperature (9 and 15°C) and pCO2 (means: 439 ppm and 1040 ppm) with a natural autumn plankton community from the western Baltic Sea. Temporal trajectories of total biomass and of the biomass of the most important higher taxa followed similar patterns in all treatments. When averaging over the entire time course, phytoplankton biomass decreased with warming and increased with CO2 under warm conditions. The contribution of the two dominant higher phytoplankton taxa (diatoms and cryptophytes) and of the 4 most important species (3 diatoms, 1 cryptophyte) did not respond to the experimental treatments. Taxonomic composition of phytoplankton showed only responses at the level of subdominant and rare species. Phytoplankton cell sizes increased with CO2 addition and decreased with warming. Both effects were stronger for larger species. Warming effects were stronger than CO2 effects and tended to counteract each other. Phytoplankton communities without calcifying species and exposed to short-term variation of CO2 seem to be rather resistant to ocean acidification.
Highlights
The well known increase of atmospheric CO2 does lead to climate warming because of the greenhouse effect and to “ocean acidification”, i.e. an increase of dissolved CO2, a decrease of water pH and a decrease in the saturation state of calcium carbonates in the ocean
At low CO2, warm and cold mesocosms began to diverge during the period from day 10 to 14when growth was maintained in the cold mesocosms but began to slow down in the warm ones
In the high CO2 mesocosm divergence between cold and warm ones was observed towards the end of the experiment and the differences between peak values of warm and cold mesocosms were less pronounced than in the low CO2 treatments
Summary
The well known increase of atmospheric CO2 does lead to climate warming because of the greenhouse effect and to “ocean acidification”, i.e. an increase of dissolved CO2, a decrease of water pH and a decrease in the saturation state of calcium carbonates in the ocean. Current predictions for atmospheric CO2 assume an increase from approximately 390 ppm to 700 ppm by the end of 21st Century (RCP8.5 scenario of the IPCC report 2013 [1]). The predicted increase in CO2 will lead to a further pH decrease by 0.3–0.4 units [2], while until today, ocean pH has declined by PLOS ONE | DOI:10.1371/journal.pone.0125239. 0.1 units from pre-industrial level and carbonate ion concentration have decreased by 30% [3]. The shift in carbonate chemistry is primarily a stressor for organisms with skeletal calcium carbonate structures [4], i.e. among phytoplankton coccolithophores should be affected most strongly [5,6,7]
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