Coccolithophore export production in response to monsoonal upwelling off Somalia (northwestern Indian Ocean)

Coccolithophores, collected in a sediment trap at 1032 m depth off Somalia from June 1992 to February 1993, responded rapidly to the SW and NE Monsoon s with changes in productivity, species composition, coccolith:coccosphere ratio, and CaCO3 production. Statistically identified species clusters and sample clusters consistently matched hydrographic periods distinguished by sea-surface temperature, wind velocities and nutrient concentrations, as we ll as delta(15)N values in the sediment-trap samples. The deep-photic zone species Florisphaera profunda and Gladiolithus flabellatus were dominant in the early SW Monsoon when coastally upwelled water carried by a large gyre , passed over the station. The opportunistic species Emiliania huxleyi and Gephyrocapsa oceanica dominated during the height of the SW Monsoon and exh ibited maximum fluxes during the beginning of upwelling relaxation in Septe mber. Most other coccolithophore species showed maximum relative abundances in a later stage of the upwelling relaxation when coccolithophore fluxes s tarted to decrease. In the inter-Monsoon period stratificd, nutrient-deplet ed, surface waters coincided with low coccolithophore fluxes. Several less abundant, oligotrophic species exhibited maximum percentages in this period . During the NE Monsoon both deep and shallow living species displayed a ra pid increase in fluxes in response to a moderate wind-induced entrainment o f nutrients. During this period maximum fluxes were recorded, with 1.1 x 10 (9) coccoliths m(-2) d(-1) and 4.8 x 10(6) coccospheres m(-2) d(-1). Average, time-weighted, total fluxes were 4.7 x 10(8) coccoliths m(-2) d(-1 ) and 12.2 x 10(5) coccosphcrcs m(-2) d(-1), corresponding to a (computed) coccolithophore-CaCO3 flux of 8.1 mg m(-2) d(-1) ( = 2.9 g m(-2) yr(-1)), w hich is 13.2% (13.5% including Thoracosphaera spp.) of the yearly total CaC O3 flux. The average CaCO3 flux in the fine fraction (< 32 mu m) was 21.4 m g m(-2) d(-1) ( = 7.8 g m(-2) yr(-1)), contributing 34.3% to the total CaCO 3 flux on a yearly basis. (C) 2000 Elsevier Science Ltd. All rights reserve d.