THE POTENTIAL ROLE OF WIND VARIABILITY ON PLANKTON RETENTION IN THE RÍO DE LA PLATA ESTUARY: A NUMERICAL STUDY.

SIMIONATO, C. G.; BERASATEGUI, A.; MECCIA, V. L.; ACHA, M.; MIANZAN, H.

Foz do Iguazú, Brasil

Conferencia; 8th International Conference on Southern Hemisphere Meteorology and Oceanography; 2006

AMS

The Río de la Plata is one of the most important estuarine systems of the world. It drains the waters of the Paraná and Uruguay rivers, which constitute the second largest basin of South America. As a result, it has a huge discharge with a mean of around 25,000 m3s-1, and maximum values as high as 50,000 m3s-1 under extreme conditions. Water stratification is controlled by the confluence of high buoyancy continental discharge advecting offshore, lying on denser shelf waters that intrude into the estuary as a topographically controlled wedge, typically between 100 and 250 km long. The upstream reach of the wedge defines a bottom salinity front, located over a submersed bar named Barra del Indio shoal following the 10 m isobath. Several authors have pointed out the importance of this bottom salinity front in structuring plankton communities as well as in the development of spawning grounds for several coastal fishes. It has been suggested that observed patterns result from retention processes associated to the bottom salinity front. However neither the occurrence of this retention nor the physical and/or behavioral mechanisms involved have been well studied. It has been classically thought that retention would be a natural consequence of the theoretical circulation associated to a salt wedge: salty water incoming from the bottom would push plankton upstream to the bottom salinity front. Nevertheless, several recent papers based on numerical simulations and ADCP current observations have shown that estuarine circulation does not necessarily follow this pattern and, moreover, that it is highly variable and essentially wind dominated. These papers indicate that estuarine scales of variability replicate atmospheric ones, and that currents response to changes in the wind field is very fast, occurring in a lapse of around 6 hours. They also demonstrated that estuarine response to winds can be explained in terms of two modes, both implying currents with a phase lag with respect to winds that depends on the location in the estuary, but that in any case, produce a response in the bottom layer. As a result of the described features, in the time scales relevant to biota, the Río de la Plata would display weather and climate, as the atmosphere does. Atmospheric circulation in the region is characterized in synoptic to intra-seasonal scales by a high variability. As winds in the region rarely blow from the same direction for more than a few days, currents present the same feature. A striking question is, then, whether this highly variable system can favor retention and if this is the fact, what are the involved mechanisms. The aim of this paper is to explore this matter. A set of process oriented numerical experiments in which neutral particles are released along the frontal zone of the Río de la Plata and its vicinity and tracked for different wind conditions in short time scales is conducted.  Results of numerical experiments are complemented with an analysis of local wind statistics. As a result a possible mechanism for plankton retention is suggested.