engleski

The impact of hydrographic conditions on the distribution of photosynthetic pigments in the plume of the Rhone River

The Rhone River with an average discharge of 1715 m3/s strongly influences the biogeochemistry and productivity of the Gulf of Lion but the role of the individual physicochemical and biological processes is still not fully understood. A series of Lagrangian drifting experiments was performed in the plume of the Rhone River in order to study the impact of hydrological and meteorological conditions on the distribution of phytoplankton biomass as reflected by photosynthetic pigments and their breakdown products. The experiments were conducted in November 1993 and November 1994 and encompassed two hydrologically contrasting situations. The sampling strategy comprised the collection of seawater samples in the plume and underlying marine water at five different depths along one-day trajectories of a drifter that was launched from the Roustan buoy situated 1 mile from the river mouth. The pigment analyses were performed using a reversed-phase HPLC technique, including serially coupled spectrophotometric and spectrofluorimetric detection. Hydrographic parameters, nutrient distribution and wind conditions were recorded during the experiment using standard oceanographic techniques. The results indicated that both hydrological and wind conditions and the resulting salinity and nutrient gradients exhibit a strong impact on the distribution and composition of phytoplankton biomass resulting in a high variability of the photosynthetic pigments. A significant accumulation of phytoplankton was observed along the nitrocline, which was characterised with salinity values of 30-35 PSU, while the concentration in the nutrient-rich uppermost layer was often much lower due to the low salinity (<20 PSU), which prevented development of marine phytoplankton. The stability of the stratified plume was shown to be very sensitive to wind stress, and consequently an accumulation of phytoplankton biomass can be observed only during period of calm weather conditions. The composition of chlorophyll and carotenoid chemotaxonomic biomarker pigments revealed a pronounced predominance of diatoms (fucoxanthin), especially in the in the low-salinity layer and the halocline. The concentration of prymnesiophytes (19'-hexanoyloxyfucoxanthin) and green algae (chlorophyll b) was relatively low. Moreover, comparatively low concentrations of chlorophyll a breakdown products indicated that the present biomass was freshly formed in the river plume and was not significantly altered by grazing.

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