Abstract
A systematic investigation of the extant coccolithophore community around Cabo Verde archipelago was performed during the cruise MSM49 of RV Maria S. Merian, which took place in the late fall of 2015. The description of the spatial and vertical distributions of coccolithophores was based on a survey performed to the north, east and south of Cabo Verde archipelago, between the surface and 150 m water depth. The total cell densities obtained for the studied region were relatively low, reaching to a maximum of 30 × 103 cell L−1 in the upper 50 m over the southeastern slope of the Senghor seamount. Emiliania huxleyi and Gephyrocapsa oceanica were the dominant species, followed by Florisphaera profunda. The coccolithophore distribution off Cabo Verde was essentially explained by relatively warm and nutrient-depleted waters in the region during the surveyed interval, in result of the weaker NE trade winds and the northward migration of the Intertropical Convergence Zone. In these conditions, a notable zonation of coccolithophores along depth was depicted, in consequence of the inferred general well-stratified water column. Four typical depth-related groups were identified: (i) a Shallow oligotrophic (10–30 m), represented by Discosphaera tubifera and Umbellosphaera spp.; (ii) an Intermediate (40–50 m), formed by the three placolith-bearing species E. huxleyi, G. ericsonii and G. oceanica, and by Algirosphaera robusta, Helicosphaera spp., Michaelsarsia spp., Syracosphaera spp. and Umbilicosphaera spp.; (iii) a Deep (60–75 m) with F. profunda, Ophiaster spp., Oolithotus spp. and Reticulofenestra sessilis as typical members; (iv) and The Deepest (>80 m), composed by Gladiolithus flabellatus and Syracosphaera lamina. In addition, high abundances of G. oceanica related with the Eddy station were attributed to the transport and thriving of the coastal coccolithophore community, dominated by this species, from the African coast towards Cabo Verde.
Highlights
The eastern subtropical North Atlantic is an oligotrophic to meso trophic region, bordered to the west and north by the oligotrophic subtropical gyre, to the east by the NW African coast, with frequent formation of eddies and filaments transporting upwelled nutrient-rich water from the coast offshore, and to the south by the equatorial convergence often transporting upwelled waters towards Cabo Verde
The minimum temperature of the studied surface layer was observed at 150 m depth, varying be tween 12.9 ◦C at the southernmost station S2 and 14.9 ◦C at station N2
Between 35.7 (S1) and 36.4 (CVO), were recorded at the thermocline for the several sites. This depth a general salinity decrease was observed until the lowest depth recorded
Summary
The eastern subtropical North Atlantic is an oligotrophic to meso trophic region, bordered to the west and north by the oligotrophic subtropical gyre, to the east by the NW African coast, with frequent formation of eddies and filaments transporting upwelled nutrient-rich water from the coast offshore, and to the south by the equatorial convergence often transporting upwelled waters towards Cabo Verde. An increase in nutrients in the proximity of an island, known as Island Mass Effect (IME), may rise the nearshore standing stock of phytoplankton biomass by more than 80% over background oceanic productivity (Gove et al, 2016). Acting as natural barriers in the main current pathways, seamounts lead to similar geophysical and biological effects when compared to the surrounding open ocean (Heywood et al, 1990; Hasegawa et al, 2004). The movement of water masses around these prominent features of seabed, usually associated with the upwelling of nutrient-rich deep sea water, lead to a higher primary production and the subsequent increase of fauna and flora around the peaks (Abecasis et al, 2009), considered hotspots of biodiversity (Morato et al, 2010).
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