Coccolithophore dynamics off Bermuda (N. Atlantic)

Standing stocks of coccolithophores in the photic zone and their taxonomic composition were investigated at Bermuda Hydrostation "S" from January 1991 to January 1994 (217 samples from 32 vertical profiles, 0-200 m water dept h). Their variability is compared with environmental parameters measured by the JGOFS Group at nearby BATS station during the same time period. Coccol ithophores showed a pronounced seasonal variability, with the highest stand ing stocks (106 x 10(3) cells 1(-1)) during late winter to Spring and lowes t during summer. Average cell density was 20 x 10(3) cells 1(-1). Emiliania huxleyi dominated in spring (up to 93 x 10(3) cells 1(-1)) and Florisphaer a profunda accounted for the regional subsurface cell density peak in fall (up to 68 x 10(3) cells1(-1)). In summer and fall, during maximum stratific ation, Umbellosphaera irregularis dominated near the surface and U. tenuis at 50-75 m water depth. Both species had much lower cell densities than E. huxleyi or F. profunda. The interannual variability was large, with average cell densities in 1993 only about half those of 1992. Cell densities decre ased rapidly below 100 m water depth. Seasonal relative abundance changes w ere rather stable from year to year. Total coccolithophore (mostly E. huxle yi) cell densities were highest where light was available (upper 50 m) and increased within a few weeks after the seasonal advection of nitrate (at te mperatures of 18-23 degreesC). They remained relatively low when only phosp hate increased in the upper photic zone. F. profunda peaks occurred at slig htly elevated nitrate and phosphate concentrations and at moderate light in tensities. U. irregularis and U. tenuis developed in nitrate-depleted water s. The optimum temperature and light ranges of U. irregularis were higher t han those of U. tenuis. Among the minor taxa, Gephyrocapsa oceanica, small coccospheres, Calciosolenia spp., Ophiaster hydroideus and Syracosphaera mo lischii appeared to have ecological requirements similar to those of E. hux leyi. Holococcolithophores, Calcidiscus leptoporus, Ceratolithus cristatus and Helicosphaera spp. increased at lower nitrate concentrations and higher temperatures and light intensities than E. huxleyi. Population changes of Thorosphaera flabellata were similar to those of F. profunda. Discosphaera tubifera and Rhabdosphaera clavigera increased in warm waters depleted in n itrate and showed a preference for very high light intensities. Most specie s seem to survive at a wide range of local environmental conditions but usu ally at very low cell densities. Individual taxa flourished under a relativ ely restricted range of favourable environmental conditions. Correlations b etween coccolithophores and abiotic parameter variability suggest that up t o 50% of the dynamics of the most abundant coccolithophore species may be d riven by variability in the physical parameters, i.e. by "bottom-up" contro ls. (C) 2001 Elsevier Science Ltd. All rights reserved.