How does rising atmospheric CO2 affect marine organisms?

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Smreczynski Straw Lake, Tatra Mountains, Southern Poland
Gasiorowski, M. and Sienkiewicz, E. 2010. The Little Ice Age recorded in sediments of a small dystrophic mountain lake in southern Poland. Journal of Paleolimnology 43: 475-487.

The authors inferred the thermal status of Smreczynski Staw Lake (4912'N, 1951'E) in the Tatra Mountains of southern Poland via analyses of the distributions of various cladocera, chironomid and diatom species they identified and quantified in a sediment core they had extracted from the center of the lake in the spring of 2003, which contained sediments that had accumulated there over the prior 1500 years. This work revealed the presence of "a diverse ecosystem at the beginning of [the] record, ca. AD 360-570," which time interval is typically assigned to the Dark Ages Cold Period. Thereafter, however, they found that from AD 570 to 1220 "environmental conditions were better," and that various cold-water taxa were "totally absent." In addition, they discovered that the younger section of this portion of the core -- approximately its upper third (AD 850-1150), which contained the highest concentration of warm-water Chironomus species -- "can be correlated with the Medieval Warm Period (Moberg et al., 2005)." Then came the Little Ice Age, extending all the way to the start of the 20th century, after which relative warmth once again returned, persisting to the present in the form of the Current Warm Period; and based on the peak Chironomus concentrations of this latter portion of the record, their data suggest that the peak warmth of the CWP and the MWP were about the same, which is also what the temperature reconstruction of Moberg et al. suggests.