Biogeographic distribution of diversity and size-structure of organic-walled dinoflagellate cysts

Global biogeographic patterns and environmental correlates of diversity and size structure of extant marine organic dinoflagellate cysts were determined. Dinoflagellate cyst diversity, like that of many other terrestrial and marine groups, is lowest at the poles and higher at lower latitudes. Temperature is responsible for much of the positive correlation between dinoflagellate cyst diversity and latitude. In contrast, the most obvious correlate with the median size of dinoflagellate cysts is the depth of the water column, especially in warm-water regions, perhaps due to changes in mixing regime and the advantages associated with the lower sinking rates and lower nutrient requirements of smaller species in offshore waters. The unique biogeographic pattern in dinoflagellate cyst diversity may reflect the unique physiological features of dinoflagellates: preference for warm, stable water columns, slower inherent growth rates, and their ability to act as mixotrophs or heterotrophs. Dinoflagellates tend to be 'gleaners', slow-growers adapted to oligotrophic conditions and less sensitive to inorganic nutrient supply, as compared to inorganic resource 'opportunists' with high growth rates, such as diatoms. These ecophysiological differences between dinoflagellates and other functional groups, such as diatoms and coccolithophores, may account for the major differences in the biogeographic and latitudinal diversity gradients between these groups.