An overview of physical and ecological processes in the Rio de la Plata Estuary

The Rio de la Plata is a large-scale estuary located at 35 degrees S on the Atlantic coast of South America. This system is one of the most important estuarine environments in the continent, being a highly productive area that sustains valuable artisanal and coastal fisheries in Uruguay and Argentina. The main goals of this paper are to summarize recent knowledge on this estuary, integrating physical, chemical and biological studies, and to explore the sources and ecological meaning of estuarine variability associated to the stratification/mixing alternateness in the estuary. We summarized unpublished data and information from several bibliographic sources. From study cases representing different stratification conditions, we draw a holistic view of physical patterns and ecological Processes of the stratification/mixing alternateness. This estuary is characterized by strong vertical salinity stratification most of the time (the salt-wedge condition). The head of the estuary is characterized by a well-developed turbidity front. High turbidity constrains their photosynthesis. Immediately offshore the turbidity front, water becomes less turbid and phytoplankton peaks. As a consequence, trophic web in the estuary could be based on two sources of organic matter: phytoplankton and plant detritus. Dense plankton aggregations occur below the halocline and at the tip of the salt wedge. The mysid Neomysis americana, a key prey for juvenile fishes, occurs all along the turbidity front. A similar spatial pattern is shown by one of the most abundant benthic species, the clam Mactra isabelleana. These species could be taken advantage of the particulate organic matter and/or phytoplankton concentrated near the front. Nekton is represented by a rich fish community, with several fishes breeding inside the estuary. The most important species in terms of biomass is Micropogonias furnieri, the main target for the coastal fisheries of Argentina and Uruguay. Two processes have been identified as producing partially stratified conditions: persistent moderate winds (synoptic scale), or low freshwater runoff (interannual scale). Less frequently, total mixing of the salt wedge occurs after several hours of strong winds. The co-dominance of diatoms (which proliferate in highly turbulent environments) and red tides dinoflagellates and other bloom taxa (better adapted to stratified conditions), would indicate great variability in the turbulence strength, probably manifested as pulses. Microplankton and ichthyoplankton assemblages defined for the stratified condition are still recognized during the partially mixed condition, but in this case they occupy the entire water column: vertical structure of the plankton featuring the stratified condition become lost. Bottom fish assemblages, on the contrary, shows persistence under the different stratification conditions, though the dominant species of the groups show some variations, Summarizing, the Rio de la Plata Estuary is a highly variable environment, Strongly stratified most of the time but that can be mixed in some few hours by strong wind events that occur in an unpredictable manner, generating stratification/partially mixed (less frequently totally mixed) pulses all along the year. At larger temporal scales, the system is under the effects of river discharge variations associated to the ENSO cycle, but their ecological consequences are not fully studied. (C) 2008 Elsevier Ltd. All rights reserved.