Abstract. A systematic investigation of the spatial and temporal variability in
coccolithophore abundance and distribution through the water column of the NW
Iberian coastal upwelling system was performed. From July 2011 to June 2012,
monthly sampling at various water depths was conducted at two parallel
stations located at 42∘ N. Total coccosphere abundance was higher at the outer-shelf station, where
warmer, nutrient-depleted waters favoured coccolithophore rather than
phytoplanktonic diatom blooms, which are known to dominate the inner-shelf
location. In seasonal terms, higher coccosphere and coccolith abundances were
registered at both stations during upwelling seasons, coinciding with high
irradiance levels. This was typically in conjunction with stratified,
nutrient-poor conditions (i.e. relaxing upwelling conditions). However, it also
occurred during some upwelling events of colder, nutrient-rich subsurface
waters onto the continental shelf. Minimum abundances were generally found
during downwelling periods, with unexpectedly high coccolith abundance
registered in subsurface waters at the inner-shelf station. This finding can
only be explained if strong storms during these downwelling periods favoured
resuspension processes, thus remobilizing deposited coccoliths from surface
sediments, and hence hampering the identification of autochthonous
coccolithophore community structure. At both locations, the major coccolithophore assemblages were dominated by
Emiliania huxleyi, small Gephyrocapsa group,
Gephyrocapsa oceanica, Florisphaera profunda,
Syracosphaera spp., Coronosphaera mediterranea,
and Calcidiscus leptoporus. Ecological preferences of the different
taxa were assessed by exploring the relationships between environmental
conditions and temporal and vertical variability in coccosphere abundance.
These findings provide relevant information for the use of fossil coccolith
assemblages in marine sediment records, in order to infer past environmental
conditions, of particular importance for Paleoceanography. Both E. huxleyi and the
small Gephyrocapsa group are proposed as proxies for the upwelling regime with a
distinct affinity for different stages of the upwelling event: E. huxleyi was
associated with warmer, nutrient-poor and more stable water column (i.e.
upwelling relaxation stage) while the small Gephyrocapsa group was linked to colder
waters and higher nutrient availability (i.e. early stages of the upwelling
event), similarly to G. oceanica. Conversely, F. profunda is suggested as a proxy for the
downwelling regime and low-productivity conditions. The assemblage composed
by Syracosphaera pulchra, Coronosphaera mediterranea, and Rhabdosphaera clavigera
may be a useful indicator of the presence of subtropical waters
conveyed northward by the Iberian Poleward Current. Finally, C. leptoporus is proposed as
an indicator of warmer, saltier, and oligotrophic waters during the
downwelling/winter regime.
Low nutrient and high chlorophyll a coastal upwelling system – A case study in the southern Taiwan Strait
