MEPS 416:79-91 (2010)  -  DOI: https://doi.org/10.3354/meps08780

ABSTRACT: Continued anthropogenic carbon emissions are expected to result in an increase in atmospheric CO₂ concentration to 700 ppm by the end of this century. This will cause a corresponding drop in the global average surface water pH of the oceans by ~0.4 units to ~7.8 and an increase in the CO₂ concentration of seawater. Ocean acidification may potentially both stimulate and reduce primary production by marine phytoplankton. Data are scarce on the response of marine phytoplankton growth rates to lowered pH/increased CO₂. Using the acid addition method to lower the seawater pH and manipulate the carbonate system, we determined in detail the lower pH limit for growth rates of 2 model species of common marine phytoplankton. We also tested whether growth and production rates of 6 other common species of phytoplankton were affected by ocean acidification (lowered to pH 7.0). The lower pH limits for growth of the dinoflagellate Heterocapsa triquetra and the cryptophyte Teleaulax amphioxeia were pH ~6.0 and 6.3, respectively. The growth rates of these 2 species were significantly reduced in the range of pH 6.4 to 6.5. Cell volume, growth, and production rates of the 6 other phytoplankton species were statistically similar in the pH range of ~7.0 to 8.5. Our results and literature reports on growth at lowered pH indicate that marine phytoplankton in general are resistant to climate change in terms of ocean acidification, and do not increase or decrease their growth rates according to ecological relevant ranges of pH and free CO₂. We speculate about whether common natural pH fluctuations in time and space from 7.0 to 9.0 make phytoplankton capable of tolerating near-future ocean acidification. However, due to the less fluctuating pH environment of oceanic regions compared to coastal regions, truly oceanic species may be more sensitive to lowered pH than coastal species.

KEY WORDS: Marine phytoplankton ^. Lowered pH ^. Growth rates ^. Primary production ^. Ocean acidification