Seasonal and inter-annual dynamics of coccolithophore fluxes from the upwelling region off Cape Blanc, NW Africa

Abstract

Coccolithophore fluxes were determined at a mooring site off Cape Blanc (21° 08.79 N, 20° 41.29 W) from March 1989 to November 1991 to reveal inter-annual and seasonal changes in species composition. In total, 74 coccolithophore species were recorded, with Emiliania huxleyi, Florisphaera profunda and Gephyrocapsa spp. dominating the assemblage. Two major conditions were identified, leading to three different intervals with characteristic species composition and associated changing environmental factors: a ‘normal’ condition characterized by fluctuating fluxes with rather constant species assemblage and a ‘pulse-like event’ that reveals a significant increase in coccolithophore fluxes and change in species composition. From March 1988 to October/November 1990 and from May 1991 the coccolithophore assemblage was dominated by E. huxleyi, F. profunda, G. flabellatus, G. oceanic , and G. ericsonii . Variability within this assemblage points to the occurrence of different surface waters, such as offshore meandering filaments. In winter 1990/1991 a ‘pulse-like event’ followed that was characterized by a significant increase in coccolithophore fluxes and in the contribution of species of the genera Ophiaster, Acanthoica, Calciosolenia, Calciopappus and Syracosphaera . It is assumed that a fast settling took place after enhanced production occurred. The third interval from May to November 1991 displays reduced fluxes of all coccolithophore species. However, the species composition is similar to the first interval, thus seemingly the assemblage converges back to ‘normal’ conditions. Statistical analysis revealed small correlations of the tested environmental parameters, such as sea surface temperature, nutrient availability and lithogenic fluxes. Nevertheless a certain seasonality of the assemblages was obvious in terms of different correlation of these environmental factors. In autumn the variability of the coccolithophorid assemblage is correlated more to sea surface temperature, whereas the spring assemblage of coccolithophores is related more to nutrient availability. In contrast, the summer assemblage is, to some extent, influenced by all tested environmental factors, suggesting that further environmental parameters are more important. This may also explain that although the observed mean coccolith fluxes off Cape Blanc are well within the range of those in the SW African upwelling, as well as in more oligotrophic sites of the temperate North Atlantic and the Canary Islands, the assemblage composition is conspicuously different. The region off Cape Blanc stands out in terms of a lesser content of E. huxleyi but an enhanced proportion of a number of small-sized coccolithophore species.