Background
The authors write that "the atmospheric CO2 concentration is rising, and models predict that by the end of the century it will have increased to twice the amount seen at any given time during the last 15 million years," stating that "this will cause a decrease in average surface water pH of 0.4," while noting that planktonic protists will be among the organisms to be affected first by this change."

What was done
To further explore this important subject, Nielsen et al. "tested whether reduced pH would affect plankton communities over an incubation period of 14 days." This was done, as they describe it, "in a laboratory microcosm setup using a natural plankton community from the Derwent River estuary, Australia," wherein "two treatments with reduced pH (8.0 and 7.7) were compared to an unaltered control of pH 8.3," during which exercise "measured parameters included community photosynthesis, nutrient uptake and biomass build-up, as well as enumeration of 25 protist taxa and quantitative HPLC of phytoplankton pigments."

What was learned
The four researchers determined that nutrient uptake and photosynthetic parameters "were all unaffected by pH treatments 8.3-7.7," treatments that they say "match the predicted 21st century changes in CO2 and pH." In addition, they found that "cellular carbon and total particulate organic carbon were both completely unaffected by pH treatment within this range," and that "the same was true for the succession of all 25 enumerated protist species." In addition, they report that "phytoplankton pigment analysis did not show effects of pH either," and they say that "the investigated plankton community was thus, in all ways, resilient to pH changes between 8.3 and 7.7," noting once again that these changes are equivalent to the predicted changes for the next century.

What it means
In discussing their findings, Nielsen et al. write that "others have also found no or very limited changes in phytoplankton communities in response to 21st century predicted changes in pH and CO2," citing Kim et al. (2006), Riebesell et al. (2007) and Suffrian et al. (2008); and they also note, in this regard, that "many coastal plankton communities are impervious to such changes," additionally citing the work of Nielson et al. (2010). One potential reason for this "broad level of pH-tolerance," as they describe it, is that "pH in coastal waters often fluctuates as a result of respiratory and photosynthetic processes," as well as "hydrographical events," with the result that "seasonal, and even diurnal, fluctuations in coastal seawater pH have been shown to encompass 7.5 to 9.6 (Macedo et al., 2001; Hansen, 2002)." And thus they conclude that "it is unlikely that the investigated plankton community would be significantly affected by a pH and CO2 change as predicted for the 21st century."

References
Hansen, P.J. 2002. Effect of high pH on the growth and survival of marine phytoplankton: implications for species succession. Aquatic Microbial Ecology 28: 279-288.

Kim, J.M., Lee, K., Shin, K., Kang, J.H., Lee, H.-W., Kim, M., Jang, P.-G. and Jang M.-C. 2006. The effect of seawater CO2 concentration on growth of a natural phytoplankton assemblage in a controlled mesocosm experiment. Limnology and Oceanography 51: 1629-1636.

Macedo, M.F., Duarte, P., Mendes, P. and Ferreira, J.G. 2001. Annual variation of environmental variables, phytoplankton species composition and photosynthetic parameters in a coastal lagoon. Journal of Plankton Research 23: 719-732.

Nielsen, L.T., Jakobsen, H.H. and Hansen, P.J. 2010. High resilience of two coastal plankton communities to twenty-first century seawater acidification: evidence from microcosm studies. Marine Biology Research 6: 542-555.

Riebesell, U., Schulz, K., Bellerby, R., Botros, M., Fritsche, P., Meyerhofer, M., Neill, C., Nondal, G., Oschlies, A., Wohlers, J. and Zollner, E. 2007. Enhanced biological carbon consumption in a high CO2 ocean. Nature 450: 545-548.

Suffrian, K., Simonelli, P., Nejstgaard, J.C., Putzeys, S., Carotenuto, Y. and Antia, A. N. 2008. Microzooplankton grazing and phytoplankton growth in marine mesocosms with increased CO2 levels. Biosciences 5: 1145-1156.