How does rising atmospheric CO2 affect marine organisms?

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Gdansk, Northern Poland
Swieta-Musznicka, J., Latalowa, M., Szmeja, J. and Badura, M. 2011. Salvinia natans in medieval wetland deposits in Gdańsk, northern Poland: evidence for the early medieval climate warming. Journal of Paleolimnology 45: 369-383.

Working with pollen and macrofossils taken from trench walls exposed during archaeological excavations in Gdansk (54°22'N, 18°40'E), northern Poland, as well as with similar materials contained within cores retrieved from sediments lying beneath the trenches, Swieta-Musznicka et al. discovered evidence for a population expansion of Salvinia natans (an aquatic fern) around the 7th-8th century AD, which was similar to a climate-driven population expansion during the last decade. In this regard, they further state that "the co-occurrence of S. natans with other aquatic plant species was similar in both the medieval and present-day vegetation," and they report that "the high density of S. natans in the medieval population caused impoverishment of the local ecosystems in a way that has been observed in recent water bodies affected by invasive pleustophytes (free-floating plants)." Thus, they conclude that "the S. natans 'blooms' in the Early Middle Ages may be regarded as an extraordinary occurrence that has an analogue in the climate-driven population of this species during the last decade."

More specifically, in terms of relative warmth, the four researchers describe the early period of S. natans population expansion as being due to "climate warming similar to the present time." Yet in another place they say that "in the early medieval period, the population density of S. natans was similar to or higher than that observed today in shallow waters invaded by this species in the Gdansk region [italics added]." Therefore, the medieval warmth of this part of the world could well have been greater than that of the present. However, we will conservatively settle for the warmth of the MWP (AD 600-700) being approximately equal to that of the CWP.