Seasonal variations in the morphology of the coccolithophore Calcidiscus leptoporus off Bermuda (N. Atlantic)

Calcidiscus leptoporus is a cosmopolitan coccolithophore specieswith one of the longest fossil records. A previous morphometricstudy on coccoliths of C. leptoporus, carried out on a globalset of Holocene sediment samples, indicated that temperaturemight be a significant factor influencing the size distributionof C. leptoporus. In the present study, C. leptoporus morphologicalvariations and dynamics are studied in samples from HydrostationS time series, near Bermuda, over a 3 year interval (1991 to1994) with monthly sampling. Calcidiscus leptoporus was presentall year round in very low abundances but displayed strong variationsthrough the time-series, both in absolute abundances (maximum551 cells/l and minimum <10 cells/l) and in morphology. Nosignificant correlation between environmental parameters andC. leptoporus abundances was found. The seasonal dynamics ofC. leptoporus, therefore, appears to be the consequence of planktoncommunity interactions and, possibly, of life-cyle. The morphologicalstudy of C. leptoporus shows a sharp decrease in size and variabilityoccurring during the spring–summer transition at the onsetof water stratification. Our results give no conclusive evidenceas to whether C. leptoporus at Hydrostation S consists of anassemblage of different species or ecophenotypes. However, weprovide evidence for significant morphological plasticity linkedto seasonality and possibly temperature. More importantly, ourresults show a trend between temperature and morphology oppositeto the study of Knappertsbusch et al. (Knappertsbusch et al.,1997) based on Holocene sediments. These results emphasize thenecessity of plankton studies in order to interpret patternsobserved in sediments.