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Seagrass Buffering of Ocean Acidification Effects in a Coral Reef Mesocosm

Paper Reviewed
Liu, P.J., Ang, S.J., Mayfield, A.B. and Lin, H.J. 2020. Influence of the seagrass Thalassia hemprichii on coral reef mesocosms exposed to ocean acidification and experimentally elevated temperatures. Science of the Total Environment 700: 134464.

Seagrass beds provide a number of ecosystem services, including helping to stabilize sediments and filter seawater, which can improve water clarity and result in higher irradiance conditions that ultimately benefit the growth of surrounding marine organisms such as corals.

Given their high rates of photosynthesis, seagrasses can also impact their environment by removing CO2 from surrounding seawater, thereby reducing its pH level. Such impacts have been documented in waters of East Asia (Lai et al., 2013), the Pacific Ocean (Anthony et al., 2013), Mediterranean (Hendriks et al., 2014) and the Caribbean (Manzello et al., 2012). The significance of this phenomenon holds great implications for the future of calcifying marine organisms within or near seagrass beds. Predicted to experience calcification and growth declines due to seawater pH reductions brought about by rising atmospheric CO2 (i.e., so-called ocean acidification), these key organisms may actually be buffered from such negative effects thanks to enhanced seagrass photosynthesis caused by the rise in CO2.

Supporting evidence of this buffering thesis has most recently emerged in the work of Liu et al. (2020). As their contribution to this topic, the four Taiwanese researchers set up a controlled tank-environment experiment at the National Museum of Marine Biology and Aquarium in Taiwan. There, in the facility's coral reef mesocosm facility, they investigated "whether the effects of the seagrass Thalassia hemprichii can increase the resilience of ocean acidification-challenged coral reef mesocosms whose temperatures were gradually elevated."

Results indicated, in the words of the authors, that "seagrass shoot density, photosynthetic efficiency, and leaf growth rate actually increased with rising temperatures under ocean acidification." They also found that coral calcification rates at a given temperature were higher in mesocosms where corals were co-incubated with seagrass than when they were not. Consequently, based on these and other key observations, they conclude that "seagrass helped to stabilize the system's metabolism in response to projected climate change stressors." And that is good news for those concerned about the potential impacts of ocean acidification on coral reefs.

Anthony, K., Diaz-Pulido, G., Verlinden, N., Tilbrook, B. and Andersson, A. 2013. Benthic buffers and boosters of ocean acidification on coral reefs. Biogeosciences 10: 4897-4909.

Hendriks, I.E., Olsen, Y., Ramajo, L., Basso, L., Steckbauer, A., Moore, T., Howard, J. and Duarte, C. 2014. Photosynthetic activity buffers ocean acidification in seagrass meadows. Biogeosciences 11: 333.

Lai, S., Gillis, L., Mueller, C., Bouma, T., Guest, J., Last, K., Ziegler, A. and Todd, P. 2013. First experimental evidence of corals feeding on seagrass matter. Coral Reefs 32: 1061-1064.

Manzello, D.P., Enochs, I.C., Melo, N., Gledhill, D.K. and Johns, E.M. 2012. Ocean acidification refugia of the Florida reef tract. PLoS ONE 7: e41715.

Posted 25 May 2020