Plankton distribution across a slope current-induced front in the southern Bay of Biscay

Relatively warm (12.50–12.75°C) and high-salinity [4.5 mg m−3) were found at the outer edge of the frontal boundary. Small autotrophic flagellates dominated the phytoplankton community. Primary production peaked at the boundary region. Estimated phytoplankton growth rates indicated that active growth was taking place, with lower turnover times integrated over the water column at the frontal station (2.5–5 days) than at coastal (1.5–2.8 days) or oceanic (1.5–3.5 days) stations. The lowest doubling times (1–2 days) were calculated for surface frontal populations. Accumulation of zooplankton was also observed associated with the convergent physical structure, although this relationship was less marked than for phytoplankton. Copepods, mainly Paracalanus parvus, Acartia clausi and Oithona helgolandica, formed the bulk of the mesozooplankton biomass. Compatibility between the size of phytoplankton cells and copepod size spectra indicate high food availability for these animals, particularly in the vicinity of the front. The distribution of fish eggs and fish larvae was also coupled with the slope current-induced front. Sardine larvae were more abundant at the coastal side of the front, whereas larval stages of blue whiting reached the highest densities at off-shelf stations. Larvae of lamellibranch molluscs and bryozoa were restricted to nearshore waters, as the frontal boundary prevented larval dispersion to the open ocean. The results presented in this paper suggest that the Iberian slope current and its associated shelf-break frontal structure were crucial in controlling phytoplankton primary production, activity of grazers, distribution of larvae of fishes and benthic invertebrates, and ultimately in determining the structure of the pelagic food web in the southern Bay of Biscay during the seasonal period of vertical mixing.