Small-scale effects of a river plume front on the distribution of salps and doliolids

River plumes are important features in coastal ecosystems and more studies are necessary to improve our knowledge about their influence on the distribution and dispersion of plankton, especially at small scales. Four surveys were conducted in the Nalón River mouth (Spain, southern Bay of Biscay) to study the effect of the river plume front on the distribution of doliolids (Doliolum nationalis) and salps (Thalia democratica). During each survey, surface currents, physicochemical variables and plankton distribution were measured by means of several GPS-tracked drifters, a CTD probe and a surface floating bongo net. Thaliacean abundance was related mainly to the intensity of the offshore convergent flow at the plume front, and less to physicochemical or biological parameters. Surface thaliacean abundances were lower at the plume and higher on the oceanic side of the front, and peaked closer to the foam line in fronts with stronger convergent flow. the distance between the point of maximum thaliacean abundance and the front was significantly correlated with a ratio of individual swimming speed to convergent current velocity. We interpret this result in terms of their capacity to oppose convergent flow.