How does rising atmospheric CO2 affect marine organisms?

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Kattegat Region of North Sea
Fjellsa, A. and Nordberg, K. 1996. Toxic dinoflagellate "blooms" in the Kattegat, North Sea, during the Holocene. Palaeogeography, Palaeoclimatology, Palaeoecology 124: 87-105.

Working with an ocean sediment core extracted from the Kattegat region of the North Sea between Sweden and Denmark (56°32'48" N, 12°11'15" E), the authors derived a temporal history of the Holocene distribution of the dinoflagellate Gymnodinium catenatum. This work revealed an early abundance of the species at about 4300-4500 yr BP. Then, in connection with what they called a "climatic deterioration, the species decreased abruptly and subsequently disappeared." It reestablished its presence some time later, however, and, as they described it, "occurred in massive 'blooms' during the so-called mediaeval warm epoch round about 700-800 yr BP." But they went on to add that "at the time of the so-called Little Ice Age, approximately 300 yr BP, G. catenatum again became extinct in the Kattegat area."

In light of these findings, Fjellsa and Nordberg reported "there appears to be a close relationship between climatic fluctuations and the presence and abundances of G. catenatum, which from its present ecology is considered a warmer water species." Therefore, they concluded that the two bloom periods were "characterized as warmer periods with climatic optima correlated to the peak phases," noting that "the most massive blooms took place during the so-called 'Medieval warm epoch'."

With respect to the more recent past, the two researchers report that "cysts from G. catenatum have been found in the surface sediments from the Danish coast bordering the Kattegat (Ellegaard et al., 1993)," but they say they "do not know if G. catenatum has lived as a small part of the plankton since the 'Little Ice Age,' or if the species has been re-introduced by the current system or via ships' ballast tanks." And they note that "there is also a possibility that the species has become re-established in conjunction with global warming during the past 80 years." In any event, it is clear that the AD 1996 abundance status of the key dinoflagellate was nowhere near that of the "massive blooms" of the Medieval Warm Period. Hence, we relegate the MWP in the Kattegat region of the North Sea to the period AD 1200-1400, concluding that the peak warmth of the MWP was greater than that of the CWP.

Additional Reference
Ellegaard, M., Christensen, N.F. and Moestrup, O. 1993. Temperature and salinity effects on growth of a non-chain-forming strain of Gymnodinium catenatum (Dinophyceae) established from a cyst from recent sediments in the Sound (Oresund), Denmark. Journal of Phycology 29: 418-426.