With that in mind, a team from Germany has found that rising ocean temperatures speed up respiration of organic carbon by a community of plankton more than they accelerate photosynthesis (or organic carbon production).
"The plankton organisms are central components of the pelagic ecosystem, because they provide the base of the marine food web and are important mediators of biogeochemical cycling," Julia Wohlers of Kiel University told environmentalresearchweb. "Climate change effects on these organisms may hence be of particular importance for the functioning and productivity of the pelagic ecosystem and ultimately for the ocean's regulating role in Earth's climate system."
Wohlers and colleagues from the Leibniz Institute of Marine Sciences at Kiel University, Alfred Wegener Institute for Polar and Marine Research, and Leibniz Institute for Baltic Sea Research in Rostock carried out a laboratory study, replicating a spring phytoplankton bloom in waters from Kiel Bight in the Baltic Sea at temperatures of 2, 4 and 6 °C above normal.
A temperature increase of 2–6 °C decreased the biological drawdown of dissolved inorganic carbon in the surface layer of the water by up to 31%. Warming also tended to see production of relatively more dissolved organic carbon compared to particulate organic carbon, reducing the amount of particulate carbon sinking to the ocean floor.
"Increasing temperature had a notably stronger effect on the consumption of biomass through enhanced bacterial degradation and a higher respiratory activity of the plankton community relative to its photosynthetic production by algal cells," said Wohlers. "Surprising to us was that these shifts in the interplay of biological processes directly translated into a significant reduction of up to one-third in the net carbon dioxide uptake by the plankton community, thereby providing a potential feedback mechanism to climate change."
According to Wohlers, the study clearly demonstrated a shift in the cycling of carbon through a natural plankton system, due to differential responses of biological processes to surface ocean warming.
"However, most Earth system models still lack realistic representations of biological processes and their potential feedbacks to climate change, which is in part due to the paucity of experimental data," she added. "Our results thus provide one more step towards a better understanding of the biotic response to a changing climate and will help to improve future climate projections." For a global assessment of the effects and feedbacks, data are also needed from areas such as polar oceans, which are expected to be particularly sensitive to climate change.
Now the team plans to look at the interaction of the direct effects of ocean warming, as investigated in this study, with other climate change-induced alterations, such as enhanced vertical stratification of the upper water column, and ocean acidification. "We need to learn more about the response of marine organisms and biological processes to these interacting factors, to improve our ability to predict future climate conditions," said Wohlers.
The researchers reported their work in PNAS.