Rio de La Plata

• Category: Marine and Oceanic Biomes.
• Geographic Location: South America.
• Summary: This river is the second-largest estuary on the continent and represents an important area for the coastal fisheries of Argentina and Uruguay.

Rio de la Plata is an immense, funnel-shaped river estuary that runs from the juncture of two major rivers—the Uruguay and Paraná—to the South Atlantic Ocean. With Argentina on its south shore and Uruguay to the north, Rio de la Plata extends 186 miles (300 kilometers), broadening from 25 miles (40 kilometers) wide at the juncture to 124 miles (200 kilometers) wide near the sea. The immediate drainage basin covers 11,583 square miles (30,000 square kilometers). The combined hydrographic system of all three rivers extends from the subequatorial zone through the tropics, and includes part of Bolivia, Paraguay, Uruguay, Brazil, and Argentina.

As with any estuary, seawater meets freshwater to form an overall brackish mix. However, here a submerged shoal, the Barra del Indio, represents a geomorphological barrier in the Rio de la Plata, and defines a salinity front characterized by strong vertical salinity stratification. Marine waters (saltier and denser) penetrate farther into the estuary along the bottom from this point, while freshwater advances oceanward on the surface, forming a salt wedge.

Hydrology

The high turbidity front in the inner part of the estuary constrains photosynthesis, and food chains are probably detritus-based more than founded upon plankton or algae. Immediately offshore from the turbidity front, however, water is less turbid, and phytoplankton increases. All the valuable species for the coastal fisheries concentrate near this area.

The physical characteristics of this estuary play a key role in the biodiversity of the benthos or bottom-dwellers, the nekton or free-swimmers, and the marine mammals. The reproductive processes of fish species occur in the outer part of the estuary.

The riverine zone of the Rio de la Plata, located in the inner part of the estuary, is characterized by shallowness with an average depth of less than 23 feet (7 meters) and a vertically homogeneous water column occupied by freshwater (very low average salinity of 0.225 parts per thousand). Water temperature averages 73 degrees F (23 degrees C) at the surface and 74 degrees F (23 degrees C) at the bottom.

In the estuarine section of the river, the average depth increases up to 23–82 feet (7–25 meters). Water temperatures here average 72 degrees F (22 degrees C) at the surface and 68 degrees F (20 degrees C) near the bottom. Beyond the estuary, in the marine zone, the water column is vertically homogeneous and characterized by high salinity (more than 30 parts per thousand). The temperature mean value in this offshore area is 68 degrees F (20 degrees C) along the entire column.

The shallow, highly turbid tidal river and the outer estuary open to the shelf are separated by a turbidity front, closely related to the salinity one. Its extent and location are highly variable, however, depending on the river discharge and wind forcing. Winds are extreme in the Río de la Plata region, and water column stratification is disrupted and the salt wedge becomes well mixed after several hours of strong onshore winds. Upstream of the turbidity front, within the tidal river, primary production is strongly light-limited. Downstream of this maximum, the concentrations of dissolved inorganic nutrients generally decrease rapidly as phytoplankton biomass increases along the salinity gradient. Upstream and downstream of the turbidity-salinity-temperature front, the species composition of fishes is completely different.

Biodiversity

There are several hundred species of fish in the Rio de la Plata biome, with around 100 of them ranging in both the estuary and in upstream habitats of the Uruguay and Paraná Rivers. Among the most abundant in the estuary proper, particularly toward the marine end, are whitemouth croaker (Micropogonias furnieri), stripped weakfish (Cynoscion guatucupa), king weakfish (Macrodon ancylodon), black drum (Pogonias cromis), Patagonian smoothhound (Mustelus schmitti), flounder (Paralichthys orbignyanus), and eagle ray (Myliobatis goodei). Hake (Merluccius hubbsi) is a migratory species that appears along the ocean front from May to October.

Additional fish species recorded here include red porgy (Sparus pagrus), hawkfish (Cheilodactylus bergi), and Parona leatherjack (Parona signata). Many tropical fish, such as the Buenos Aires tetra (Hyphessobrycon anisitsi), inhabit the Rio de la Plata; they are prey for the larger fauna, as well as being prized for the aquarium industry. There is also an abundance here of Argentine squid (Illex argentinus), quite valuable in the food web and as a commercial fishery take.

Many types of ray and shark patrol the estuary, and among their preferred prey is La Plata dolphin (Pontoporia blainvillei), a hallmark marine mammal species here. The La Plata dolphin is one of the smallest cetaceans, and among the river dolphins of the world, it is probably the one species that spends most of its life in saltwater and brackish environments. Green, leatherback, and loggerhead sea turtles depend on the marine and beach habitats of the Rio de la Plata at different points of their life cycles.

Along the beaches, lagoons, and marshes that line the estuary, and in some of the quite-dense forest that has crept up to the very edge of the Rio de la Plata, many resident and migratory bird populations are in evidence. Plovers, albatross, herons, and southern lapwings—occasionally joined by the flightless greater rhea—feast upon mollusks, small fish, seaweeds, and crustaceans. Hummingbirds—at least nine species—buzz around for nectar. A plethora of parrots haunt the forest, along with species like green-barred woodpecker, red-rumped warbling finch, and scarlet-headed blackbird.

Human Interaction

Human effect on the estuary through agriculture, cattle raising, and industrial and port activities have seriously damaged the environment and threaten the sustainability of various habitats around the Rio de la Plata. On the two shores of the estuary lie the capitals of each country: Buenos Aires, Argentina, and Montevideo, Uruguay—together with more than 13 million inhabitants. Ports, dredging, and commercial fisheries cause different ecological pressures. Upstream, several cities and other industrial centers located near the banks of the main tributary rivers also discharge wastes into the waters. The Rio de la Plata system is highly sensitive to changes in nutrient loading and freshwater input, which may modify the ecosystem structure by the development of harmful algal blooms and consequent eutrophication (nutrient-overload and oxygen depletion).

Land-use patterns, especially logging, agricultural clearance for food crops and biofuels, and general human infrastructure construction, all combine to accelerate forest loss and resultant erosion, and greatly increase sediment supply to the estuary. Drastic changes in the quantity and quality of organic inflows also exert a strong influence on the distribution, composition, and metabolism of the aquatic communities here.

Two important and relatively novel consequences of human impact should be considered in this environment: climate change and biological invasions. Changes in the hydrological cycle have resulted in more instances of flooding and drought within the area. Annual mean temperature and rainfall significantly increased over the last 100 years—as has the mean sea level. These climatic tendencies, coupled with the direct effects of the increase in the human population, have generated an increase in algal bloom frequencies in the estuary. Such changes often favor invasion by exotic species; two important cases of this were Limnoperna fortunei and Rapana venosa, two mollusk species that first were recorded here in the 1990s. Both had extensive economic and ecological impact.

There are some efforts to protect native species and habitats. Near the confluence with the Paraná River, for instance, is the Reserva Ribera Norte, a sanctuary that hosts a wonderful range of habitats—willow forest, aliso forest, ceibo forest, riverine brushland, freshwater marsh, and others—and gives respite to a very broad range of flora and fauna. Since human population will tend to increase in the region, and climate change seems certain to intensify, such conservation initiatives in and around this important and unique biome are highly desirable.

Bibliography

Acha, M. E., et al. “An Overview of Physical and Ecological Processes in the Rio de la Plata Estuary.” Continental Shelf Research 28 (2008).

“Climate Actions at the La Plata River Basin.” UNECE, unece.org/sites/default/files/2022-04/5.%20Session1‗Juan%20Carlos%20Alurralde‗CICPlata‗0.pdf. Accessed 31 Dec. 2024.

Mianzan, H. W., et al. “The Río de la Plata Estuary, Argentina-Uruguay.” In U. Seeliger, ed., Ecological Studies: Coastal Marine Ecosystems of Latin America. Berlin, Germany: Springer-Verlag, 2001.

Piecuch, Christopher. "River Effects of Sea-Level Rise in the Rio de la Plata During the Past Century." EGUsphere, 27 July 2022, egusphere.copernicus.org/preprints/2022/egusphere-2022-700/. Accessed 31 Dec. 2024.

Quiros, R., J. A. Bechara, and E. K. de Resende. “Fish Diversity and Ecology, Habitats and Fisheries for the Un-Dammed Riverine Axis Paraguay-Parana-Rio de la Plata (Southern South America).” Aquatic Ecosystem Health & Management 10, no. 2 (2007).