How does rising atmospheric CO2 affect marine organisms?

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Siliceous Phytoplankton of a Middle Eocene Warming Event
Renaudie, J., Danelian, T. Saint Martin, S., Le Callonnec, L. and Tribovillard, N. 2010. Siliceous phytoplankton response to a Middle Eocene warming event recorded in the tropical Atlantic (Demerara Rise, ODP Site 1260A). Palaeogeography, Palaeoclimatology, Palaeoecology 286: 121-134.

The Eocene was a period of time that lasted from about 56 to 34 million years ago (Ma), which was characterized by a progressive drop in mean global temperature. However, this long-term cooling trend was interrupted by several short-lived warming reversals, one of which occurred between approximately 40.0 and 40.2 Ma; and this 200,000-year period is the one that was the focus of the research of Renaudie et al.

What was done
The authors conducted a quantitative micropalaeontological analysis of siliceous phytoplankton remains found in a sediment core extracted from the seabed at an ocean depth of 2,549 meters at ODP Site 1260 (~9.2N, 54.65W) on the Demerara Rise, which is a continental shelf located off the coast of Surinam.

What was learned
The five French scientists report that "the pre-warming flora, dominated by cosmopolitan species of the diatom genus Triceratium, was replaced during the warming interval by a new and more diverse assemblage, dominated by Paralia sulcata (an indicator of high productivity) and two endemic tropical species of the genus Hemiaulus." In addition, they state that "the critical warming interval was characterized by a steady increase [italics added] in tropical species of the genus Hemiaulus." In addition, they state that "the microflora preserved above the critical interval was once again of low diversity and dominated by various species of the diatom genus Hemiaulus."

What it means
On the basis of Renaudie et al.'s findings, we can conclude (as nearly always seems to be the case) that warmer is better, which maxim is in the current case exemplified by (1) the greater productivity of the tropical ocean during the warmer period, and (2) the ocean's continuous upward trend in the diversity of phytoplanktonic species throughout the period of warming.

Reviewed 7 July 2010