Oceanographic Processes on Freshwater-Dominated Continental Shelves I
Presiding: E J Campos, Insituto Oceanografico da Universidade de Sao Paulo; Y Ro, Department of Oceanography, Chungnam National University
OS21A-01 INVITED 08:30h
An Overview of Ecological Processes in the Rio de la Plata Estuary
The Rio de la Plata, one of the most important estuarine environments in South America, is characterized by a salt-wedge regime. Large extension and shallow water depth make the estuary highly susceptible to atmospheric forcing. The estuary is a highly productive area, which sustain important artisanal and coastal fisheries in Uruguay and Argentina, mainly based on the whitemouth croaker, Micropogonias furnieri. The main goal of this paper is to summarize recent knowledge on this system, integrating physical, chemical and biological studies. This estuary is characterized by strong vertical salinity stratification, with marine waters (saltier and denser) penetrating deeper into the estuary along the bottom, while fresh waters advance ocean-ward on the surface, forming a salt wedge. The upstream reach of the salt wedge defines a bottom salinity fronts, whose location is controlled by the topography, a submerged shoal called Barra del Indio and at the opposite area, the convergence between the estuarine and marine waters define a surface salinity front. The convergence of water masses and the strong picnoclines at the head of the salt wedge produce the accumulation and retention of plankton, including the eggs of those fishes that concentrate here to spawn (e.g. Micropogonias furnieri and Brevoortia aurea) and even debris. High turbidity constrains here photosynthetic production and food chains are probably detritus based, supporting high densities of Acartia tonsa (Copepoda) and Neomysis americana (Mysid), both omnivorous species that complete its entire life-cycle within the estuary. In agreement, heterotophic microzooplankton is abundant. Moreover, high deposition of suspended matter support dense beds of the deposit feeding clams Mactra isabelleana. As soon as the water become less turbid, an extremely high chlorophyll signal is observed. The largest portion of the salt wedge regime (more than 200 km) is characterized by dense plankton aggregations below the picnocline, meanwhile the upper layer is biologically poor. Marine species penetrate the estuary advected or following the saline waters. The surface salinity front is the offshore end of the salt wedge. At this portion of the estuary, mixing of estuarine and marine waters and enhancement of vertical nutrient flux fertilize the frontal area and another maximum chlorophyll-a concentrations is observed. Zooplankton concentrations, mainly gelatinous plankton have been reported here. Reproduction of some fishes seem also associated to this surface front. This front is the more subtle and high dynamic portion of the wedge, showing a seasonal pattern driven by winds. It delineates the boundary between the estuary and the continental shelf waters. The influence of this large river in the ecological processes of the continental shelf is largely unknown. The knowledge on the ecology of the Rio de la Plata estuary has been remarkably enlarged during the last ten years, and some ongoing researches such as the study of different sources of primary production and the complexity of the trophic pathways, looks promissory and surely will contribute to the understanding of the regional importance of the estuary.
OS21A-02 09:00h
The Subtropical Shelf Front off Eastern South America: Winter 2003 and Summer 2004
In Austral winter 2003 and summer 2004 we conducted two large-scale hydrographic surveys on the continental shelf and slope off eastern South America. Among other goals, the field work was designed to determine the relation between the low salinity waters derived from the Rio de la Plata, and the subsurface front found between Subantarctic Shelf Waters (SASW), derived from the Patagonia continental shelf, from Subtropical Shelf Waters (STSW), primarily influenced by the Brazil Current. In August 2003 the Plata plume extended beyond 26S creating a region of strong vertical stratification in the offshore edge of the plume, near the 40 m isobath. The cross-shore salinity structure at Albardao (33S) reveals two distinct salinity fronts: the inner shelf front at the offshore edge of the Plata plume and the STSF located in the mid-shelf. The plume front is isothermal and associated with a sharp density front while the STSF presents nearly density-compensating salinity and temperature gradients. Though the density gradients across the STSF are moderate, isopycnals intersect the sea surface and the bottom, inhibiting isopycnal mixing between SASW and STSW. The strong northeasterlies in February 2004 retracted the Plata plume to 32S and expanded it to the shelf break. Consequently, the summer halocline extends over the entire shelf width causing the plume front to merge with the STSF in the upper layer. Off Albardao there is strong isopycnal mixing between mid-shelf and Brazil Current waters. The data reveal how seasonal changes in the upper layer associated to wind variations induce significant modifications of the lower layer water masses and completely rearrange the frontal structure. These thermohaline alterations are likely to have a profound impact on the distribution of species.
OS21A-03 09:15h
Oceanography of the Subtropical Shelf Front Zone in the SW-Atlantic Continental Shelf
Only physical aspects of the Subtropical Shelf Front (STSF) have been described for the Southwest Atlantic Continental Shelf. The main goal of this paper is to present results of an integrated physical, chemical and biological study at the STSF conducted during the winter of 2003 and summer of 2004. A cross section was established at the historical determined location of the STSF. Nine stations were sampled during the winter cruise and 7 stations during summer. Each section included a series of conductivity-temperature-depth (CTD) stations fitted with dissolved oxygen and turbidity sensors. Selected water samples were filtered and frozen at -20oC for nutrient determination. Samples for chlorophyll were concentrated on filters and these were stored frozen for later processing. Plankton net tows were carried out above and below pycnocline. Surface benthic foraminifera were collected with a bottom snapper. Results revealed that winter was marked by an inner shelf salinity front and the STSF located in the mid-shelf. Inner salinity showed the strong influence of freshwater, with high silicate (71.98 ΜM), phosphate (2.70 ΜM), nitrate (1.01 ΜM), Total Dissolved Nitrogen (22.98 ΜM) and suspended matter (44.80 mg/L). With distance from the coast and reduction of terrestrial input, subsurface high values of nutrients were associated with SACW upwelling. As a result, chlorophyll a concentration decreased from coastal well-mixed waters, where values up to 3.0 mg.m3 are registered, to offshore waters. Zooplankton abundance and biomass, and ichthyoplankton abundance follows the same trend. Zoo and ichthyoplankton abundance revealed the presence of 3 groups associated to the inner, mid and offshore shelf region. Benthic foraminifera composition suggested that shallow stations are dominated by few large freshwater species, while offshore stations presented smaller forms and higher species diversity. During summer, the halocline extended over the shelf and joined the STSF in the upper layer. The concentration of inorganic nutrients is reduced in the shallow waters in relation to the winter period, but high values are observed between 40 and 60 m depth and offshore deep waters. Zooplankton biomass was not as high as during winter, and largest values are observed around the STSF. Zoo and ichthyoplankton presented 3 groups, separated by the presence of the STSF. Despite less influence of freshwater during this season, benthic foraminifera were still marked by the presence of freshwater species. The results from this study suggest that during winter, freshwater influence is strong and physical-chemical-biological interactions develop in the front along its plume. However, during summer, when the presence of freshwater is less intense, these interactions seem to take place at the STSF.
OS21A-04 09:30h
Effects of the Seasonal Wind Variations on the Shelf Waters off Southeastern South America
The impact of Rio de la Plata derived waters along the Uruguayan and southern Brazilian shelves has been recognized since the first oceanographic surveys were carried out in this region. These waters form a low salinity tongue that affects the circulation and stratification as well as the distribution of nutrients and biological species. The coastal low salinity plume presents a seasonal meridional displacement reaching lower latitudes (28o S) during winter and 32 o S during summer months. Historical data suggests that the seasonal plume fluctuations are a response to the variations in the along-shore wind stress. To provide a synoptic view of these processes and to establish the connection between wind and outflow variations on the distribution of the coastal waters, two oceanographic cruises were carried out in late August 2003 and February 2004. The surveys covered the continental shelf and slope of southeastern South America, between Mar del Plata, Argentina and the northern coast of Santa Catarina, Brazil. On each cruise about 90 stations were occupied, distributed in 11 cross-shelf profiles using a Seabird 911 CTD. The observations show how the Plata plume alters the circulation and property distributions causing the dominant environmental variations on the continental. The seasonal wind variations are the most important factor for the plume dynamics in the sense that: A) SW winds prevailing in winter force the northward spreading of the plume to low latitudes even during low river discharge periods. In the winter of 2003 mixtures of about 5% of Plata river waters were found at the northernmost section, located 1300 km from the Plata estuary; B) NE winds retain the plume in the southern area and spread it over the entire width of the continental shelf east of the estuary. Another effect of the summer NE winds that inhibit the northward plume penetrations is the development of coastal upwelling near Cabo de Santa Marta. Wind action also influences the dynamics and water masses variations along the continental shelf area. In this case, in areas influenced by the river discharge, Subtropical Shelf waters are kept away from the coastal region. When the influence of freshwater decreases, NE winds induce coastal upwelling cells such as observed near Cabo de Santa Marta.
OS21A-05 09:45h
Patos Lagoon Outflow Within the Rio de la Plata Plume Using an Airborne Salinity Mapper
Major river systems discharging into continental shelf waters frequently form buoyant coastal currents that propagate along the continental shelf in a direction corresponding to that of coastal trapped wave propagation (with the coast on the right/left, in the northern/southern hemisphere). The combined flow of the Uruguay and Parana Rivers, which discharges freshwater into the Rio de La Plata estuary (Latitude ~36 S), frequently gives rise to a buoyant coastal current (the 'La Plata plume') that extends northward along the continental shelf off Uruguay and Southern Brazil. Depending upon the prevailing rainfall, wind and tidal conditions, the Patos/Mirim Lagoon complex (Latitude ~ 32 S) also produces a freshwater outflow plume that expands across the inner continental shelf. Under these circumstances the Patos outflow plume may be embedded in temperature, salinity and current fields that are strongly influenced by the larger Plata plume. The purpose of this paper is to present observations of such an embedded plume structure and to determine the implications for the dynamics of the smaller Patos plume. We describe the results of an airborne remote sensing and shipboard in situ study of the salinity distribution and extent of the La Plata and Patos/Mirim Lagoon plumes conducted under contrasting winter (2003) and summer (2004) conditions. The survey was conducted using an aircraft carrying NRL's Salinity, Temperature and Roughness Remote Scanner (STARRS). A series of broad-scale flights was conducted over the continental shelf off Argentina, Uruguay and Brazil, and a detailed mapping flight was undertaken over the Patos/Mirim outflow region. Their purpose was to determine the distribution and behavior of the Plata and Patos Lagoon plumes on the continental shelf under representative winter and summer conditions. The resulting airborne and shipboard hydrographic data are compared with dynamical model parameter estimates to address the following questions: What is the influence of a large-scale plume on the behavior of a smaller scale outflow embedded within it? How might the observed Patos plume be classified in comparison with other plumes, and would this classification likely change when it is embedded in the larger Plata plume? The situation of the Patos plume embedded within the Plata Plume during winter-time may be contrasted with the simpler situation in which either one of the plumes is absent. The case of a weak Plata plume being present during summer (in the absent of the Patos plume), serves as an approximation to the ambient conditions, or background state, pertaining to the embedded case.