Effect of mixing on microbial communities in the Rhone River plume

The biological processes involved during mixing of a river plume with the m arine underlying water were studied off the Rhone River outlet. Samples of suspended and dissolved matter were collected while tracking a drifting buo y. Three trajectories were performed, at 2-day intervals, under different h ydrological and meteorological situations. A biological uptake was evidence d from ammonium (NH4) and phosphate (PO4) shortage, indicating an early "NH 4-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 zone s with enhanced bacterial production, high chlorophyll a concentration, as well as DOG, NH4 and PO4 consumption were evidenced from 20 to 35 in salini ty. It was shown that the successive abundance of bacteria and phytoplankto n during transfer reflected the competition for PO3 of both communities. On the Rhone River plume, the role played by temperature, light conditions an d 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 cl osely related to the dilution mechanism, controlled itself by the dynamics of the plume. In windless conditions and close to the river mouth, the dens ity gradient between marine and river water induced limited exchanges betwe en the nutrient-rich freshwater and the potential consumers in the underlyi ng 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 z one for marine phytoplankton development. This can be quite far from the ri ver mouth in case of a widely spread plume, corresponding to high river dis charge. Under windy and wavy conditions, the plume freshwater is early and rapidly mixed, so that the extension of the "enhanced production zone" is d rastically reduced and even bacteria could not benefit from the fast mixing regime induced. (C) 2001 Elsevier Science B.V. All rights reserved.