The contrasted phytoplankton dynamics across a frontal system in the southwestern Mediterranean Sea

Numerical simulations have shown that finescale structures such as fronts are often suitable places for the generation of vertical velocities,transporting subsurface nutrients to the euphotic zone and thus modulating phytoplankton abundance and community structure. In these structures,direct in situ estimations of the phytoplankton growth rates are rare; although difficult to obtain, they provide precious information on theecosystem functioning. Here, we consider the case of a front separating two water masses characterized by several phytoplankton groups withdifferent abundances in the southwestern Mediterranean Sea. In order to estimate possible differences in growth rates, we measured thephytoplankton diurnal cycle in these two water masses as identified by an adaptive and Lagrangian sampling strategy. A size-structured populationmodel was then applied to these data to estimate the growth and loss rates for each phytoplankton group identified by flow cytometry, showing thatthese two population parameters are significantly different on the two sides of the front and consistent with the relative abundances. Our resultsintroduce a general method for estimating growth rates at frontal systems, paving the way for in situ exploration of finescale biophysicalinteractions.