PLANKTON DISTRIBUTION ACROSS A SLOPE CURRENT-INDUCED FRONT IN THE SOUTHERN BAY OF BISCAY

Relatively warm (12.50-12.75-degrees-C) and high-salinity [<35.640 pra ctical salinity units (PSU)] water flowing eastward was detected at th e shelf-break during a cruise carried out in the southern Bay of Bisca y in Spring 1987. The slope current induced the formation of a converg ent front separating well-mixed oceanic waters from haline-stratified coastal waters. Very high concentrations of dissolved oxygen (295 mumo l kg-1) and chlorophyll a (>4.5 mg m-3) were found at the outer edge o f the frontal boundary. Small autotrophic flagellates dominated the ph ytoplankton community. Primary production peaked at the boundary regio n. Estimated phytoplankton growth rates indicated that active growth w as taking place, with lower turnover times integrated over the water c olumn at the frontal station (2.5-5 days) than at coastal (1.5-2.8 day s) 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 phytop lankton. Copepods, mainly Paracalanus parvus, Acartia clausi and Oitho na helgolandica, formed the bulk of the mesozooplankton biomass. Compa tibility between the size of phytoplankton cells and copepod size spec tra indicate high food availability for these animals, particularly in the vicinity of the front. The distribution of fish eggs and fish lar vae was also coupled with the slope current-induced front. Sardine lar vae were more abundant at the coastal side of the front, whereas larva l stages of blue whiting reached the highest densities at off-shelf st ations. Larvae of lamellibranch molluscs and bryozoa were restricted t o nearshore waters, as the frontal boundary prevented larval dispersio n to the open ocean. The results presented in this paper suggest that the Iberian slope current and its associated shelf-break frontal struc ture were crucial in controlling phytoplankton primary production, act ivity of grazers, distribution of larvae of fishes and benthic inverte brates, and ultimately in determining the structure of the pelagic foo d web in the southern Bay of Biscay during the seasonal period of vert ical mixing.