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

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Abstract
Calcidiscus leptoporus is a cosmopolitan coccolithophore species with one of the longest fossil records. A previous morphometric study on coccoliths of C. leptoporus, carried out on a global set of Holocene sediment samples, indicated that temperature might be a significant factor influencing the size distribution of C. leptoporus. In the present study, C. leptoporus morphological variations and dynamics are studied in samples from Hydrostation S time series, near Bermuda, over a 3 year interval (1991 to 1994) with monthly sampling. Calcidiscus leptoporus was present all year round in very low abundances but displayed strong variations through the time-series, both in absolute abundances (maximum 551 cells/l and minimum <10 cells/l) and in morphology. No significant correlation between environmental parameters and C. leptoporus abundances was found. The seasonal dynamics of C. leptoporus, therefore, appears to be the consequence of plankton community interactions and, possibly, of life-cyle. The morphological study of C. leptoporus shows a sharp decrease in size and variability occurring during the spring–summer transition at the onset of water stratification. Our results give no conclusive evidence as to whether C. leptoporus at Hydrostation S consists of an assemblage of different species or ecophenotypes. However, we provide evidence for significant morphological plasticity linked to seasonality and possibly temperature. More importantly, our results show a trend between temperature and morphology opposite to the study of Knappertsbusch et al. (Knappertsbusch et al., 1997) based on Holocene sediments. These results emphasize the necessity of plankton studies in order to interpret patterns observed in sediments.

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