How does rising atmospheric CO2 affect marine organisms?

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Lake Mina, Minnesota, USA
St. Jacques, J.-M., Cumming, B.F. and Smol, J.P. 2008. A 900-year pollen-inferred temperature and effective moisture record from varved Lake Mina, west-central Minnesota, USA. Quaternary Science Reviews 27: 781-796.

Working with sediment cores retrieved from Lake Mina (4553.40'N, 9528.68'W) in west-central Minnesota, USA, the authors derived mean February temperatures over the period AD 1116 to 2002 at a four-year resolution using "a pre-settlement pollen-climate calibration set." This work revealed, in their words, that "there was a cold Little Ice Age (AD 1500-1870) preceded by a warmer Medieval Climate Anomaly (AD 1100-1500) on the northeastern border of the Great Plains." From the peak temperature of the warmer of these two periods, which we call the Medieval Warm Period, to the coldest temperature of the Little Ice Age, their data indicate a temperature decline of 1.8C, which is followed by a complete recovery by the year 1900, where their data terminate. Hence, since we know the planet warmed a good deal over the 20th century, we can conclude that the peak warmth of the MWP was less than that of the CWP for this study.