How does rising atmospheric CO2 affect marine organisms?

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Impacts of Ocean Acidification on a Marine Food Web

Paper Reviewed
Taucher, J., Haunost, M., Boxhammer, T., Bach, L.T., Alguerˇ-Mu˝iz, M. and Riebesell, U. 2017. Influence of ocean acidification on plankton community structure during a winter-to-summer succession: An imaging approach indicated that copepods can benefit from elevated CO2 via indirect food web effects. PLoS One 12: e0169737, doi:10.1371/journal.pone.0169737.

Writing as background for their important new study, Taucher et al. (2017) state that "plankton communities form the base of the pelagic food web and provide many important ecosystem services such as productivity, sustenance of fish stocks, or carbon update." However, they note that it remains "one of the major challenges in biological oceanography to find general rules that explain and predict the trophic structure and biogeochemical functioning of marine ecosystems and how underlying ecological processes are affected by environmental drivers, particularly in the context of ongoing climate change and ocean acidification." Thus, it became their objective to investigate the impact of ocean acidification on plankton community structure and biogeochemical cycling during a long-term in situ study.

To accomplish their desires they used an imaging-based approach to obtain size distribution and taxonomic composition data of a natural plankton community housed in ten pelagic mesocosms (50 m3) deployed in the Gullmar Fjord of Sweden under natural or reduced seawater pH (simulate "ocean acidification," corresponding to ~760 µatm pCO2). The experiment ran for 113 days, covering the transition from winter to summer conditions, beginning in January of 2013. And what did their experiment reveal?

Simulated ocean acidification had a stimulatory effect on the biomass and size structure of the entire plankton community, from picoplankton to mesozooplankton. Notably, there were large biomass increases for copepods and diatoms, which increased by 40 and 30 percent, respectively (see figure below). In explaining this observation, it was the belief of the authors that elevated CO2 had an initial direct stimulatory effect on the phytoplankton productivity that indirectly "propagated up the food web and ultimately became visible as elevated biomass of copepods," though it is also quite possible that elevated CO2 directly stimulated the growth of the higher trophic organisms up the food chain as well.

With respect to the implications of their work, Taucher et al. write that "since copepods serve as a major food source for a variety of commercially important fish species, such CO2-driven trophic cascades could have important implications for ecosystem structure and fish stock dynamics in temperate and arctic regions." And based upon the results of their study, we would add that all indications are that those implications are of a highly positive nature.

Figure 1. Box plots of overall biomass of copepods (left panel) and diatoms (Coscinodiscus sp., right panel) in control and high CO2 mesocosms on day 57 of the experiment, corresponding to the peak of a phytoplankton bloom. Adapted from Taucher et al. (2017).

Posted 26 September 2017