We studied a finescale frontal structure in order to highlight its influence on the dynamics and distribution of phytoplankton communities. We computed the growth rates of several phytoplankton groups identified by flow cytometry in two water masses separated by the front. We found contrasted phytoplankton dynamics on the two sides of the front, consistent with the distribution of their abundances. Our study gives new insights into the physical and biological coupling on a finescale front.