How does rising atmospheric CO2 affect marine organisms?

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Response of a Common Isoetid to Ultra-CO2-Enriched Water
Andersen, T., Andersen, F.O. and Pedersen, O. 2006. Increased CO2 in the water around Littorella uniflora raises the sediment O2 concentration. Aquatic Botany 84: 294-300.

Isoetids are small slow-growing evergreen perennials that live submerged along the shores of numerous freshwater lakes. According to the authors, they "are known to rely primarily on sediment-derived CO2 for their photosynthesis (e.g. up to 100% for Lobelia dortmanna, Sand-Jensen and Sondergaard, 1979)."

What was done
Anderson et al. grew specimens of the isoetid Littorella uniflora (L.) in sediment cores removed from Lake Hampen (Denmark) in 75-liter tanks with an overburden of 10 cm of filtered lake water for a period of 53 days, while measuring a number of plant, water and sediment properties, after which they harvested the plants and measured their biomass. Throughout the experiment, one set of plants had normal ambient air bubbled through the water covering the sediments, while an equivalent set of plants had a mixture of ambient air and CO2 (sufficient to lower the water's pH by one unit) bubbled through its water to simulate a 10-fold increase in atmospheric CO2 concentration.

What was learned
The ultra-CO2-enriched water led to an approximate 30% increase in plant biomass, as well, in the words of the authors, as "higher O2 release to the sediment which is important for the cycling and retention of nutrients in sediments of oligotrophic softwater lakes."

What it means
Even an order of magnitude increase in the air's CO2 concentration would likely not be a problem for the world's isoetids. In fact, it would probably benefit them.

Sand-Jensen, K. and Sondergaard, M. 1979. Distribution and quantitative development of aquatic macrophytes in relation to sediment characteristics in oligotrophic Lake Kalgaard, Denmark. Freshwater Biology 9: 1-11.

Reviewed 26 July 2006