First published in IGBP's Global Change Newsletter Issue 73, April 2009

Low winter CaCO3 saturation in the Baltic Sea: consequences for calcifiers

Features |
The Ocean in a High-CO2 World: Science highlights from the symposium
Toby Tyrrell
National Oceanography Centre  
Southampton University
Southampton, UK

Bernd Schneider
Institut für OstseeforschungWarnemünde
Rostock, Germany

Ocean acidification lowers the calcium carbonate (CaCO3) saturation state (Ω) of seawater and thus the ability of calcifying organisms to form shells or skeletons. All surface oceans are presently supersaturated with respect to CaCO3 (Ω>1) but under continued emissions of CO2 this will change. The first oceans to experience surface undersaturation  (Ω<1) will be the Arctic and Southern Oceans, and it will most likely occur in wintertime, the time of the year at which Ω is typically lowest. Measurements in the Baltic Sea show that a similar situation already pertains there: the central Baltic becomes undersaturated (or nearly so) in winter, with respect to both aragonite and calcite mineral forms of CaCO3 (Figure). Undersaturation appears even more severe in the most northerly part of the Baltic Sea, the Bothnian Bay. Low wintertime Ω is matched by unusual patterns of chemical etching (dissolution) of CaCO3 shell fragments in sediments. We are taking advantage of this natural analogue to better understand future impacts of ocean acidification, by comparing biogeographical distributions of calcifying organisms in the Baltic with carbon chemistry (mindful that there are also strong gradients in salinity and other parameters). For instance, while many calcifiers are scarce in the Baltic, the blue mussel Mytilus edulis occurs even in the low Ω Bothnian Bay.
Calcium carbonate saturation state of Baltic seawater over the seasons.
Ω<1 indicates undersaturated waters where carbonate dissolution occurs.
Map: Norman Einstein