Modern coccolithophore and carbonate sedimentation along a productivity gradient in the Canary Islands region: Seasonal export production and surface accumulation rates

Coccolithophores, one of the main primary and carbonate producers in the surface waters of the Canary Islands region, were investigated to quantify coccolith and coccolith-carbonate export fluxes and to determine possible trends in species composition related to different productivity conditions. Present-day fluxes were compared to the long-term records of coccolith and carbonate accumulation in Holocene sediments underneath the mooring locations. Coccolithophore flux variations in sediment traps at three stations along an E-W transect at approximately 29°N from the upwelling influenced north-west African shelf region to the open ocean were recorded from January to September 1997. Total coccolith fluxes along the studied transect were highly seasonal, with pronounced sedimentation events occurring in February (upper traps) and March (deep traps), related to the winter phytoplankton bloom in surface waters. While total coccolith flux rates and fluxes of different species at the two oceanic sites LP and ESTOC were comparable (mean: 6.4 and 5.4 × 10⁸ coccoliths m^-2 d^-1), fluxes increased significantly towards the shallower site EBC (mean: 13.7 × 10⁸ coccoliths m^-2 d^-1). This gradient points to increased productivity in surface waters at the nearshore station, probably associated with nutrient-rich upwelling filaments detaching from the coast and/or the easternmost islands. Additionally, a strong influence of laterally advected, coccolith-rich material was demonstrated for the deeper sediment traps at the two offshore sites, resulting in an increase of mean daily coccolith fluxes by a factor 3 (LP) and by a factor 2 (ESTOC), respectively. Estimated coccolith-carbonate fluxes along the transect were 2.8 gm^-2 (EBC) and 1.2-1.3 g m^-2 (ESTOC, LP) in the upper traps, representing 31-33% of the total annual carbonate flux. The coccolith-carbonate supply was higher in the deeper traps, with 63% at LP and 44% at ESTOC, respectively. Average Holocene coccolith and (coccolith-) carbonate accumulation rates derived from surface samples at the mooring locations generally confirm the sediment trap results. Discrepancies mainly result from near-bottom and down-slope particle transfer processes, which especially influence the site closer to the shelf. Dissolution processes are likely to affect primarily the deep sediments at the oligotrophic site LP. The diverse subtropical coccolithophore assemblages were quite similar in all sediment-trap samples from the three mooring sites, with three species (Emiliania huxleyi, Florisphaera profunda, Gephyrocapsa ericsonii) contributing significantly to the assemblage. Despite the good correspondence between the assemblage compositions along the transect, some trends towards the upwelling influenced site EBC could be observed. Whereas all Gephyrocapsa species (mainly G. ericsonii) respond positively to higher productivity conditions at the near-shore station, Calcidiscus leptoporus and the deep-photic zone species F. profunda increased in mean relative abundances towards the two offshore sites, pointing to a preference for the more stable, oligotrophic conditions of the subtropical gyre.