8th International Nannoplankton Association Conference
Karl-Heinz Baumann, Jeremy R. Young, Mário Cachão, Patrizia Ziveri:
Coccolithus pelagicus (Wallich) Schiller is well known as a dominant component of sub-Arctic coccolith assemblages. Its biogeography indicates a preference for relatively cold water temperatures (below 10°C), but also includes upwelling regions in lower latitudes with a quite different environment and surface-water temperatures upto 18°C.
Biometric data on C. pelagicus coccoliths suggest significant size variation. Overall, the distal shield is from 6.0 to17.0µm long and between 5.0 and 14.0µm wide. Element counts for the distal shield are 30 to 65. Placolith length, width, as well as the number of elements are extremely well correlated to each other. Moreover, the measurements show a clear bimodal distribution pattern, strongly suggesting that there are two subspecies, a smaller sub-Arctic and a larger temperate form, with discrete morphologies and, probably, ecological preferences. The individual-rich sub-Arctic population is characterised by medium-sized coccoliths of usually 6-11µm, whereas populations of the temperate and upwelling regions had significantly larger coccoliths, 10-16µm long. In addition, samples from intermediate and temperate latitudes show bimodal populations. These results suggest major genotypic variation within the population, a hypothesis that is supported by culture experiments on isolates of both subspecies.
Probably this finding will provide the key to understanding the somewhat enigmatic palaeobiogeography of this species, among others, during relatively warm Pliocene times. Strong temporal size variability has been observed, even within the Plio-Pleistocene. In addition, its role as a major cold-water species (= sub-Arctic type) must be rechecked in mid-latitude sediment cores to get a meaningful interpretation of C. pelagicus abundance fluctuations.
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