Coccolithophore dynamics off Bermuda (N. Atlantic)

Abstract

Abstract 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 depth). Their variability is compared with environmental parameters measured by the JGOFS Group at nearby BATS station during the same time period. Coccolithophores showed a pronounced seasonal variability, with the highest standing stocks (106×103 cells l−1) during late winter to spring and lowest during summer. Average cell density was 20×103 cells l−1. Emiliania huxleyi dominated in spring (up to 93×103 cells l−1) and Florisphaera profunda accounted for the regional subsurface cell density peak in fall (up to 68×103 cells l−1). In summer and fall, during maximum stratification, 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 decreased rapidly below 100 m water depth. Seasonal relative abundance changes were rather stable from year to year. Total coccolithophore (mostly E. huxleyi) cell densities were highest where light was available (upper 50 m) and increased within a few weeks after the seasonal advection of nitrate (at temperatures of 18–23°C). They remained relatively low when only phosphate increased in the upper photic zone. F. profunda peaks occurred at slightly elevated nitrate and phosphate concentrations and at moderate light intensities. U. irregularis and U. tenuis developed in nitrate-depleted waters. The optimum temperature and light ranges of U. irregularis were higher than those of U. tenuis. Among the minor taxa, Gephyrocapsa oceanica, small coccospheres, Calciosolenia spp., Ophiaster hydroideus and Syracosphaera molischii appeared to have ecological requirements similar to those of E. huxleyi. 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 nitrate and showed a preference for very high light intensities. Most species seem to survive at a wide range of local environmental conditions but usually at very low cell densities. Individual taxa flourished under a relatively restricted range of favourable environmental conditions. Correlations between coccolithophores and abiotic parameter variability suggest that up to 50% of the dynamics of the most abundant coccolithophore species may be driven by variability in the physical parameters, i.e. by “bottom-up” controls.