How does rising atmospheric CO2 affect marine organisms?

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Lake Stora Višarvatn, Northeast Iceland
Axford, Y., Geirsdottir, A., Miller, G.H. and Langdon, P.G. 2009. Climate of the Little Ice Age and the past 2000 years in northeast Iceland inferred from chironomids and other lake sediment proxies. Journal of Paleolimnology 41: 7-24.

The authors developed a regional climatic record from a sediment core retrieved from lake Stora Višarvatn in northeast Iceland (66°14.232'N, 15°50.083'W) in the summer of 2005, based on chironomid assemblage data that were well correlated with nearby measured temperatures over the 170-year period of the instrumental record. With respect to the MWP, the four researchers report that their data indicated "warm temperatures in the tenth and eleventh centuries, with one data point suggesting temperatures slightly warmer than present," which -- as best we can determine from the graph of their results, reproduced below, -- yields a peak MWP temperature 0.4°C greater than the peak CWP temperature.