SUSPENDED MATTER MEAN DISTRIBUTION AND SEASONAL CYCLE IN THE RÍO DE LA PLATA ESTUARY AND THE ADJACENT SHELF FROM OCEAN COLOR SATELLITE (MODIS) AND IN-SITU OBSERVATIONS

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

Puerto Madryn

Conferencia; 9th International Conference on Tidal Sedimentology ? Tidalites 2015.; 2015

Centro Nacional Pagatonico y Asociacion Argentina de Sedimentologia

The Río de la Plata (RDP) is one of the largest and most turbid estuaries of the world, carrying a total of 160 million tons y-1 of suspended sediments. The knowledge of their spatial distribution and their scales of variability is fundamental for management and scientific reasons, but has been limited by the scarcity of observations. During 2009 and 2010, in-situ data (CTD and turbidity profiles, and water and bottom sediment samples) were collected at 26 sites during six repeated cruises and from three fixed instruments deployed in the frame of the FREPLATA/FFEM Experiment (Figure 1). Oceanographic cruises were performed every two months approximately, and provided quasi-synoptic fields and profiles of the estuary. Water samples were analyzed for SM concentration and turbidity. Additionally, we used time series gathered at three fixed points of the RDP, providing high temporal resolution observations of waves, winds and turbidity, so as tidal observations. We complement the study with ten years of 1 km-resolution MODIS-Aqua ocean color observations in the RDP and the adjacent shelf, processed for surface SM concentration with the IFREMER algorithm for coastal turbid waters. The aim of this work is to provide a comprehensive characterization of the annual mean suspended matter concentration distribution, to study its variability on seasonal time scale and to identify the involved physical mechanisms (Figure 2).The simultaneous use of in-situ and remote observations has allowed, for the first time, a robust characterization of the mean distribution pattern of the SM concentration and turbidity in the Río de la Plata and its variability on seasonal time scales, so as the identification of the most likely driving processes. The mean surface suspended matter concentration inferred from the color remote observations using the IFREMER algorithm and their variability compare well with those derived from the in-situ observations. There is a good correspondence in the magnitude and the spatial distribution of the maxima, minima and gradients. A marked resemblance is observed between the spatial patterns of the surface SM concentration and turbidity. A linear link between both variables is observed over the whole estuary, without apparent spatial differences.Our principal results can be summarized as follows:?In the upper and intermediate estuary the spatial pattern of the mean surface SM concentration is mainly controlled by both the solid discharge and the re-suspension by tides. As both, the tributaries? solid discharge and tidal currents are larger to the south, the mean surface SM concentration maximizes along that coast all along the year. Maximum concentrations are observed close to Punta Piedras and seem to be highly correlated with the one of the areas of maximum tidal currents and tidal dissipation by bottom friction of the estuary shown by Simionato et al. (2004a). Our results suggest that tidal currents act re-suspending the sediments near the bottom, but wind waves vertically mix the water column making the sediments reach the surface.?In the outer RDP the mean surface SM concentration decays seawards, but displays maximum temporal variability and horizontal gradients. This area coincides with the tip of the bottom salinity front and the region were both the horizontal and vertical salinity gradients are maxima. Here the leading processes would be flocculation and decantation and the wind-forced fresh water plume dynamics. Tidal currents and waves act re-suspending sediments and mixing, but waves play a secondary role, because depth here is larger.?There is a secondary maximum of the mean surface SM concentration in the outer estuary, close to Punta Rasa, which seems to be connected to another area of maximum tidal currents and tidal dissipation by bottom friction. In this area wave action becomes more important, as the depth decreases.?Surface SM concentration displays a marked seasonal cycle (Figure 2) with maximum values in the austral fall-winter and minimum values in summer in most of the estuary, excepting the Uruguayan coast around Montevideo. The rapid increase observed in the SM concentration during March and April seems to be related with the seasonal cycle of the main solid discharge tributary to the estuary, the Bermejo River, which occurs in February and March. Nevertheless, SM concentration maintains high in the entire estuary up to September and maximizes in August (end of the austral winter). This maximum would be forced by the higher frequency of storms, inducing stronger southwesterly winds, during that season. They would produce an increase in the frequency of higher waves, which could cause further re-suspension and/or vertical mixing.?Along the Uruguayan coast and the exterior RDP, close to Montevideo, a weaker seasonal signal with a relative minimum in summer and a maximum in winter is observed. This cycle would be linked to the seasonal displacement of the RDP fresh water plume, which more often advects fresh water (and presumably sediments and nutrients) to the northeast, or the Uruguayan coast, in winter and to the southwest, or Argentinean coast, in summer.Our conclusions do not only contribute to a better understanding of the sedimentological processes in this important estuary, but will also help on the construction and validation of numerical models, which are needed for management and scientific purposes. Those studies, including future biogeochemical modeling are envisaged as a continuation of this research.It is important to note that further development of the algorithms used to calibrate the MODIS-Aqua color observations is needed for the RDP, particularly over its most turbid areas. In this sense, the collection of radiometric samples (not available yet) together with in-situ observation of turbidity, SM and Chl-a and bio-optical properties are fundamental. Efforts to progress in this direction are being made by the working group.