Suspended matters mean distribution and seasonal cycle in the Río de la Plata estuary and the adjacent shelf from MODIS and in-situ observations

SIMIONATO, C. G.; MOREIRA, D.; GOHIN, F.; CAYOCCA, F.; LUZ CLARA TEJEDOR, M.

Buenos Aires

Workshop; Workshop on satellite ocean color applications to the Río de la Plata estuary.; 2012

Instituto Argentino de Física del Espacio

Draining the second largest basin of South America, the Río de la Plata is one of the largest and most turbid estuaries of the world. Its fresh water plume impacts the properties of the shelf for more than 500 km. Even though turbidity in this estuary has been the object of several papers, inorganic suspended matter has deserved less attention because of the lack of observations. In this sense, daily 1 km resolution MODIS-Aqua data were processed for surface inorganic suspended matter in the estuary and the adjacent shelf using the IFREMER algorithm for Type 2 waters, providing 10 years of observations. The aim of this work is to analyze those data to characterize mean values and variability on seasonal time scales, so as to provide clues about the involved physical mechanisms. The analysis is complemented with in situ observations collected during six cruises to the area and from fixed instruments deployed during the FREPLATA/FFEM Experiment 2009-2010. The comparison between direct and remote sensed observations is very satisfactory, showing a good agreement in the magnitude and spatial distribution of the mean suspended sediments concentration, its standard deviation, so as the seasonal variability. Results suggest that the surface suspended matters’ concentration in the Río de la Plata is strongly associated with the hydrodynamics and the scales of variability of the tributaries and the winds. All along the year the concentration of surface suspended matter maximizes along the southern (Argentinean) coast of the upper and intermediate estuary and at the tips of Samborombón Bay. This fact seems to be linked in part with the higher solid discharge of the Paraná River -flowing along the southern coast of the estuary- compared to the Uruguay River -which flows following the northern coast-. The former receives most of the sediments load to the Río de la Plata from the Bermejo River located about 1000 km upstream the estuary’s head. The observed mean pattern also seems to be related to the stronger tidal currents along the southern coast of the estuary and at the tips of Samborombón Bay, which would act resuspending sediments near the bottom. Then wind waves during storms would enhance vertical mixing, consequently increasing surface concentration. The concentration of suspended sediments rapidly falls seawards the Barra del Indio shoal, in the area of the salt wedge. Probably this is, at least in part, a consequence of flocculation. In the outer estuary, suspended matters concentration seems to be also strongly associated to the wind-forced motion of the fresh water plume. Suspended matter concentration exhibits a maximum in winter and a minimum in summer, that cannot be fully explained in terms of the cycle of the solid discharge of the tributaries, but seems to be related to a raise in the frequency of the storms in winter, increasing the frequency of strong wind events in the region and, therefore, a growth of