Coccolithophores in the Gulf of Mexico (M67/2)
Zusammenfassung der Projektergebnisse
In the proposed study we have investigated the spatial distribution of the living coccolithophores in 52 water samples from the southeastern Gulf of Mexico in order to clarify the relationships between the environment and the nannoflora in this region. Total coccolithophore numbers range from zero in deep water samples (200m) to about 23,000 to 46,500 coccolithophore cells per litre (cells/l) at intermediate water depths (50-75m). From 39 identified taxa, only nine species contribute significant cell numbers of more than 1000 cells/l and comprise more than 5% of the communities in at least one sample. Emiliania huxleyi was the most abundant species throughout the stations with concentrations of up to 22,700 cells/l. At all stations, a typical vertical succession of coccolithophore species was found. Umbellosphaera tenuis (type IV), Discosphaera tubifera, Rhabdosphaera clavigera, and, to a minor content, species of the genus Syracosphaera, were mainly restricted to surface waters (≤50m), all adapted to lower nutrient levels. The assemblage in the deep photic zone (>50m) was mainly composed of abundant Florisphaera profunda, Gladiolithus flabellatus, and Algirosphaera robusta indicating a tolerance of lower light availability. When comparing the presently observed coccolithophore assemblage with assemblages sampled in October 1990 the main pattern may be similar, but discrepancies are obvious. The overall coccolithophore cell concentrations did not vary much, but proportions of some species changed a lot. However, even more striking are the discrepancies when comparing the present findings with coccolithophore assemblages found in March 1991. The latter were completely different to those in the present samples. No species was very numerous compared to E. huxleyi, which almost exclusively occurred throughout the water column. Thus, the vertical distribution of all coccolithophorid taxa, except the placolith-bearing species, in particular E. huxleyi, was probably controlled by upper photic-zone water temperature and stratification of the water-column. The coccolithophore communities therefore mainly reflected the local oceanographic situation and seemed to be more dependent on mixed layer depth and nutrient availability than on temperature and salinity changes.
