Climate Fact: Diatoms and Dinoflagellates
During the warm seasons (spring through fall), the water in the Baltic Sea is stable and stratified. This means that the warmest and least dense water is on the surface, and as you dive deeper and deeper, layers of progressively colder, saltier, and denser water are encountered. During the decades of the 1970′s and 1980′s, the North Atlantic Oscillation (NAO) was in a negative phase, and winters in the Baltic Sea were generally cold and dry. As a result, during the winter, the water on the surface would cool to the point where it was colder than the water below it, and mixing between the layers would occur until a new stable state (one where the cold water is on the surface) was reached. This annual mixing was important for the survival of the sea’s diatoms, which depended on the mixing that would take place during the spring as the sea reverted from the winter to summer water column formation. Diatoms are photosynthetic, single-celled organisms, which form a large part of the marine food pyramid’s base. A positive state of the NAO since the late 1980′s has corresponded to milder winters, and the deep mixing that would occur as the seasons changed no longer occurs. As a result, the diatoms that depend on the mixing have been largely replaced by another type of single-celled organism known as dinoflagellates, which may be best known for their whip-like tails that propel them through the water. This change has also corresponded to other changes in the sea’s species’ composition.
Seasons: Winter, Spring
Sources: Alheit, J. et al. “Synchronous ecological regime shifts in the central Baltic and the North Sea in the late 1980′s.” ICES Journal of Marine Science 62 (2005): 1205-1215 and Hinrichsen, HH et al. “Correlation analyses of Baltic Sea winter water mass formation and its impact on secondary and tertiary production.” Oceanologia 49 (2007): 381-395.
