engleski

Effect of mixing on microbial communities in the Rhone River plume

The biological processes involved during mixing of a river plume with the marine underlying water were studied off the Rhone River outlet. Samples of suspended and dissolved matter were collected while tracking a drifting buoy. Three trajectories were performed, at 2-day intervals, under different hydrological and meteorological situations. A biological uptake was evidenced from ammonium (NH4) and phosphate (PO4) shortage, indicating an early "NH4-dependent" functioning occurring before the well-known "NO3-based" cycle. The different ratios between NH4, NO3 and PO4, as a function of salinity, were discussed to detail the preferential use in PO3 and NH4. Salinity zones with enhanced bacterial production, high chlorophyll a concentration, as well as DOG, NH4 and PO4 consumption were evidenced from 20 to 35 in salinity. It was shown that the successive abundance of bacteria and phytoplankton during transfer reflected the competition for PO3 of both communities. On the Rhone River plume, the role played by temperature, light conditions and suspended matter upon biological activity seems relatively minor compared to salinity distribution and its related parameter: nutrient availability. It can be concluded that biological uptake in the Rhone River plume was closely related to the dilution mechanism, controlled itself by the dynamics of the plume. In windless conditions and close to the river mouth, the density gradient between marine and river water induced limited exchanges between the nutrient-rich freshwater and the potential consumers in the underlying marine water. Consequently, Little biological activity is observed close to the river mouth. Offshore, mixing is enhanced and a balance is reached between salinity tolerance and nutrient availability to form a favourable zone for marine phytoplankton development. This can be quite far from the river mouth in case of a widely spread plume, corresponding to high river discharge. Under windy and wavy conditions, the plume freshwater is early and rapidly mixed, so that the extension of the "enhanced production zone" is drastically reduced and even bacteria could not benefit from the fast mixing regime induced.

coastal processes; plume dynamics; dissolved organic carbon; bacterial production; chlorophyll a

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