How does rising atmospheric CO2 affect marine organisms?

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Seagrasses Enable Nearby Corals to Withstand Ocean Acidification
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.

The authors state that although many people expect future ocean acidification (OA) due to rising atmospheric CO2 concentrations to reduce the calcification rates of marine organisms, they say we have little understanding of how OA will manifest itself within dynamic, real-world systems, because, as they correctly note, "natural CO2, alkalinity, and salinity gradients can significantly alter local carbonate chemistry, and thereby create a range of susceptibility for different ecosystems to OA."

What was done
"To determine if photosynthetic CO2 uptake associated with seagrass beds has the potential to create OA refugia," as they describe it, Manzello et al. repeatedly measured carbonate chemistry across an inshore-to-offshore gradient in the upper, middle and lower Florida Reef Tract over a two-year period.

What was learned
During times of heightened oceanic vegetative productivity, the five U.S. researchers found "there is a net uptake of total CO2 which increases aragonite saturation state (Ωarag) values on inshore patch reefs of the upper Florida Reef Tract," and they say that "these waters can exhibit greater Ωarag than what has been modeled for the tropical surface ocean during preindustrial times, with mean Ωarag values in spring equaling 4.69 ± 0.10." At the same time, however, they report that Ωarag values on offshore reefs "generally represent oceanic carbonate chemistries consistent with present day tropical surface ocean conditions."

What it means
Manzello et al. hypothesize that the pattern described above "is caused by the photosynthetic uptake of total CO2 mainly by seagrasses and, to a lesser extent, macroalgae in the inshore waters of the Florida Reef Tract." And they therefore conclude that these inshore reef habitats are "potential acidification refugia that are defined not only in a spatial sense, but also in time, coinciding with seasonal productivity dynamics," which further implies that "coral reefs located within or immediately downstream of seagrass beds may find refuge from ocean acidification." And in further support of this conclusion, they cite the work of Palacios and Zimmerman (2007), which they describe as indicating that "seagrasses exposed to high-CO2 conditions for one year had increased reproduction, rhizome biomass, and vegetative growth of new shoots, which could represent a potential positive feedback to their ability to serve as ocean acidification refugia."

Palacios, S. and Zimmerman, R.C. 2007. Response of eelgrass (Zostera marina L.) to CO2 enrichment: Possible impacts of climate change and potential for remediation of coastal habitats. Marine Ecology Progress Series 344: 1-13.

Reviewed 6 March 2013