Oceanography of the Argentina continental shelf between 33 and 55ºS.

PERILLO, G.M.E.; PICCOLO, M.C.; MARCOVECCHIO, J.O.

Paris

Simposio; COASTS 2001; 2001

UNESCO

The continental shelf along the western border of the South Atlantic Ocean (SWAO) between 33 and 55º S represents an unique example of a wide, fully open shelf highly affected by to major western boundary currents (Malvinas and Brazil) and remarkable strong wind systems blowing from the continent. It is also an excellent example of a passive or Atlantic continental margin having all basic elements included in this classification. The area covered ppp by the present article corresponds to the complete Argentina and Uruguay shelves. The continental margin includes the following elements that will be analyzed: coast, continental shelf, continental slope and continental rise. The surface covered by the continental margin within the study area is over 2.4 x 106 km2 and the width varies significantly at various latitudes.  Between Mar del Plata and the Rio de la Plata it has a width of the order of 500 km, but the minimum extension is on the southern end of Tierra del Fuego where is only 100 km. However, the maximum widths are what makes this area really exceptional. Across the Malvinas Plateau the margin is over 2000 km wide. Wide variety of environments occur along the coast of the region. Extensive mostly dissipative beaches backed by wide coastal dune systems are interspersed by major estuaries such as Rio de la Plata and Bahía Blanca Estuary, and large low, muddy coasts as Samborombón and Anegada bays . However, from the Negro river to the south, the coast is dominated by Tertiary cliffs that reach their maximum height of about 150 m along the coast of Santa Cruz and Tierra del Fuego. Due to their particular shape, San Matías, San José and Nuevo gulfs are prominent features that border the Valdés Peninsula. Other major, open gulfs are San Jorge and Bahía Grande. Major sources of freshwater into the shelf are the Río de la Plata (mean discharge 15,000 m3 s-1), Patos Lagoon (1,000  m3 s-1 just to the north o fthe study area) and Negro River (1,000  m3 s-1). Because the former enter at the narrower portions of the shelf, both have a strong influence over the shelf. Other inputs are significantly lower and their influence on the shelf circulation is mostly restricted to the inner shelf as their plumes are generally deflected to the north by Coriolis. Two other features are dominant on the southern portion of the study area: Magellan Strait and the Drake Passage. Both highly affect the circulation of the shelf (the former) and the slope (the latter). Considering only the continental shelf, its surface within the study area is about 1.5 x 106 km2 and its width varies accordingly to the continental margin. The shelf break occurs at depths between 115 an 240 m. Although the shelf is of very low relief, four well-defined terrace are located from the coast seaward at 20-30 m, 85-100 m, 110-120 m and 130-150 m. Even though there are no definitive evidences of their origin, they have been related to sea-level still stands during the last transgression. Other significant geomorphologic features of the shelf is the presence of shoreface-connected sand ridges and gigantic dunes in San Matías Gulf. The ridges are found from Uruguay to the south of Bahía Blanca Estuary. The are open between 25 and 45º with the coast and are the specular reflexion of similar ridges found along the North American shelf. Whereas the dunes have heights up to 20 m and are found along the sill that closes the gulf between 75 and 110 m depth. The continental slope varies in width between 50 and 300 km with a mean slope of 1:50. The estimated surface is about 700,000 km2. There only few submarine canyons, none of which have been observed to continue into the shelf. The only one probably related to a river is the Río de la Plata canyon. Other canyons are: Mar del Plata, Bahía Blanca, Ameghino and Almirante Brown. The last two have the unique particularity that they are parallel to slope rather than crossing it. The slope ends at about the 3200-m isobath where the continental rise starts. The latter has a very smooth slope with a mean width of 250 km and a surface of 300,000 km2.The water masses of the Argentinian shelf are of subantarctic origin but modified by the continental discharge and heat and mass transfer with the atmosphere. This water mass enter the shelf between Malvinas and Tierra del Fuego islands (from the Drake Passage) and significantly diluted by the Magellan Strait waters. To the south of 42º S, the surficial flow is seaward and evaporation is higher than precipitation. Also, the strong westerly winds dominant in the region during most of the year, helps in the seaward drift. However, to the north of 42º S, the atmospheric forcing diminishes and the circulation presents seasonal patterns. There is a marked increase in continental discharge and also the relation precipitation-evaporation is positive. Although most data suggest that the general circulation is towards the NNE, there are physical and biological evidences that a countercurrent to the SSW during the summer. Although the Malvinas and Brazil currents have a dominant effect on the slope region, they have little influence over the shelf area, except for some minor fingers that surge along the outer shelf. The formation of strong fronts is an effective barrier that prevents the invasion of these waters onto the shelf. However, the degree of influence is larger on the northern portion of the study area as the shelf is much narrower. This is detected by the presence of planktonic species typical of subtropical waters that are found all the year. SWAO physicochemical characteristics are mainly regulated by both the own water properties as well as interactive processes occurring within the region. Considering their behavior, two domains can be mentioned: i) shelf waters ; and  ii) coastal waters. Physicochemical properties of continental shelf waters are fully characterized by three subantarctic originated water masses, all of them derived from the Malvinas current: a) internal shelf; b) middle or central shelf, and c) external shelf or slope. These three water types are nutrient enriched (both N and P); their input from the SWAO fertilize the Argentine continental shelf. Furthermore, and due to the increase of salinity, temperature and/or density horizontal gradients within the shelf, several frontal areas are developed, which occur in two basic types: tidal fronts and slope fronts. These systems usually act as retention or concentration zones, accumulating not only nutrients but also other dissolved compounds and particulate matter (i.e., detritus, plankton, eggs, larvae) that generate highly productive areas. In the northern area of the shelf the Subtropical Confluence occurs, due to the meeting of the Malvinas and Brazil currents. This confluence generates many frontal systems from the internal shelf to the slope. Shelf waters also show a clear seasonal stratification, which occurs during spring and summer. From early autumn and during winter, stratification is broken because of both mechanical mixing and convective processes, thus homogenizing nutrient distribution along the whole water column. This process completes the system´s fertilization. The areas with highest productivity levels are linked to slope frontal zones (38° to 42°S.) as well as to tidal frontal areas close to Peninsula Valdés (42°S.) and Buenos Aires province shelf (38° to 40°S.). On the other hand, coastal waters are usually homogeneous (or with minor stratification), showing a strong interaction with inflowing freshwater. Their nutrient loads also present an annual cycle, but with highly modified concentrations due to inputs from land sources. Thus, concentration areas of nutritive compounds are generated, like tidal fronts, estuaries or coastal systems with restricted circulation, where biological production reaches the highest levels as recorded within the whole region. In case of coastal waters, the highest production levels are measured in littoral systems of Buenos Aires province (i.e., Bahía Blanca Estuary, Mar Chiquita coastal lagoon), in frontal systems linked to large rivers outlet (i.e.,Río de la Plata, Negro River), or in systems with reduced water exchange with the shelf area (i.e., Patagonian gulfs). Moreover, coastal waters can be locally affected by urban or industrial discharges, even though in all cases the dilution effect as well as the general circulation of the system seems to dominate over these anthropic impacts. In this sense, several areas within the littoral have been identified as locally impacted by different pollutants: i) Río de la Plata Estuary (hydrocarbons, heavy metals, pesticides, organic matter ; litter) ; (ii) Bahía Blanca Estuary (heavy metals, organic matter); iii) San Antonio Bay (heavy metals, organic matter); iv) Nuevo Gulf (organic matter); v) San Jorge Gulf (hydrocarbons, organic matter); vi) Tierra del Fuego island (organic matter, heavy metals, hydrocarbons) The high environmental variability of the SWAO allowed the delimitation of four different complexes (previously designated as regions): Antarctic, Subantarctic, Subtropical and Tropical. Each one of these complexes is intimately linked with water mass dynamics, and may be divided in several biogeographical provinces (Magellanic, Argentinian and Brazilian Provinces). Each one show differences in their biodiversity and biomass production levels and strong biological/ecological interactions between continental and insular areas, but also some biological/ecological links with Antarctica and the South American Pacific coast. Several schemes may be proposed to the analysis of the biodiversity of the region. The existing biogeographical provinces may be traditionally studied or the subantarctic-subtropical interactions deeply analyzed; a more dynamic point of view arises when comparisons between the inner and outer continental shelf are drawn. Either scheme may be useful for the identification of the biodiversity characteristics and problems of the region. In the present article emphasis is placed on studies of benthic and planktonic communities, but also with a strong reference to fishes, birds and marine mammals. A gross environmental characterization of the SWAO show sectors of high sensitivity due to the presence of breeding grounds of both commercial resources (fish and squid) or protected resources (birds and mammals). Also important wetland areas can be identified along the coast (Samborombón and San Antonio bays, Tierra del Fuego, etc.) where many migratory birds use these sites as a stop-over area on their annual migration between southern wintering and northern breeding grounds.