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

ARTICLE IN PRESS Continental Shelf Research 28 (2008) 1579– 1588

Contents lists available at ScienceDirect

Continental Shelf Research journal homepage: www.elsevier.com/locate/csr

An overview of physical and ecological processes in the Rio de la Plata Estuary E. Marcelo Acha a,b,c,, Hermes Mianzan a,c, Rau´l Guerrero a,b, Jose´ Carreto a,b, Diego Giberto a,c, Norma Montoya a, Mario Carignan a a b c

´n y Desarollo Pesquero, Paseo Victoria Ocampo No. 1, 7600 Mar del Plata, Argentina INIDEP—Instituto Nacional de Investigacio Universidad Nacional de Mar del Plata, Argentina CONICET—Consejo Nacional de Investigaciones Cientı´ficas y Te´cnicas, Argentina

a r t i c l e in fo

abstract

Article history: Received 22 May 2006 Received in revised form 9 January 2007 Accepted 10 January 2007 Available online 12 March 2008

The Rio de la Plata is a large-scale estuary located at 351S 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 We summarized unpublished data and information from several bibliographic sources.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 Rı´o 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. & 2008 Elsevier Ltd. All rights reserved.

Keywords: Brackishwater ecology Salt-wedge estuaries Vertical mixing Review South America Argentina 34–371S latitude 54–581W longitude

1. Introduction  Corresponding author at: INIDEP—Instituto Nacional de Investigacio´n y

Desarollo Pesquero, Paseo Victoria Ocampo No. 1, 7600 Mar del Plata, Argentina. Tel.: +54 223 4862586; fax: +54 223 4861830. E-mail address: [email protected] (E. Marcelo Acha). 0278-4343/$ - see front matter & 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.csr.2007.01.031

The Rio de la Plata, located at 351S on the Atlantic coast of South America, drains the second largest basin of this continent, following that of the Amazon. Its drainage area covers ca. 3.1 106 km2, which represents about 20% of the South American

ARTICLE IN PRESS 1580

E. Marcelo Acha et al. / Continental Shelf Research 28 (2008) 1579–1588

Fig. 1. The Rı´o de la Plata Estuary. Mean surface salinity for spring (from Guerrero et al., 1997b). Grayed zone (isohalines each 2.5 unit) represents the mixing zone. Black region shows the tidal river. White star shows Ponto´n Recalada weather station. Symbols show stations locations of the sections in Fig. 2 (+, 2A; K, 2B; n, 2C; %, 2D). Inset: Rı´o de la Plata basin.

continental area (Fig. 1). This river forms 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, mainly based on the whitemouth croaker (Micropogonias furnieri). The estuary is characterized by a salt-wedge regime; low seasonality in the river discharge; low tidal amplitude (o1 m); a broad and permanent connection to the sea; and high susceptibility to atmospheric forcing, due to its large extension and shallow water depth (Guerrero et al., 1997a, b; Mianzan et al., 2001; Simionato et al., 2004). This estuary has received considerable attention over the past years, principally because of its recognized importance to coastal communities (e.g., Boschi, 1988; Mianzan et al., 2001). However, studies in the Rı´o de la Plata are technically and financially difficult due to its large spatial scale. A large number of observations are needed to obtain a synoptic description and to adequately resolve the spatial and temporal variability of the ˜ an et al., 1999). Much of the previous work has system (Framin been focused on the mean distribution of water properties, showing that the estuary is a stratified system most of the time ˜ an and Brown, 1996; Guerrero et al., 1997a, b). Biological (Framin studies on fish reproduction (Macchi et al., 1996; Acha et al., 1999; Acha and Macchi, 2000; Macchi et al, 2002, 2003; Militelli and Macchi, 2004), fauna assemblages (Jaureguizar et al., 2003, 2004; Berasategui et al., 2004, 2006; Giberto et al., 2004), plankton and benthos distribution (Madirolas et al., 1997; Mianzan et al., 2001b; Alvarez Colombo et al., 2003; Schiariti et al., 2006), and habitats of the salt wedge (Boschi, 1988; Mianzan et al., 2001) focused also on mean patterns. Description of mean conditions represents the initial stages of our knowledge of the system, but it is accepted that the estuary shows high variability. For example, ˜ an et al. (1999) described spatial variability of the vertical Framin structure showing that in some conditions, strongly stratified and totally mixed areas can occur at the same time. Moreover, stratification in the estuary varies over a range of time scales,

involving several physical forcing. This varying strength of turbulence and stratification would have different ecological consequences, controlling for instance the light environment experienced by primary producers in the turbid estuarine environment, or determining the retentive properties of the estuary for planktonic organisms. The main goals of this paper are to summarize recent knowledge on this system, 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.

2. Data sources and methodology We employed unpublished data and information coming from different bibliographic sources, several of them rather cryptic (‘‘gray literature’’), normally not available to the international scientific community. We summarized the physical oceanographic information and we compiled and condensed the information about main ecological processes studied for the region. From study cases representing different stratification conditions, we draw a holistic view of physical patterns and ecological processes of the stratification/mixing alternateness in the estuary. Four salinity sections, representing different stratification conditions, were constructed on the basis of CTD data (pressure, temperature, and salinity). Wind condition from 7 days previous and throughout the observation period was also analyzed using Ponto´n Recalada weather station data (Fig. 1). In order to measure stratification in the estuary, we employed the Simpson parameter of stability (f) (Simpson, 1981). This parameter quantifies the work or energy needed to break down the density gradient to totally mixed condition in a stratified water column. Simpson parameter is calculated as Z f ¼ gh ðr  r0 Þ  z dz

ARTICLE IN PRESS E. Marcelo Acha et al. / Continental Shelf Research 28 (2008) 1579–1588

where g is the gravity acceleration, h the total depth, r the in situ density, r0 the water column mean density, and z is the depth. Due to the fact that our information comes mostly from fish stock assessment cruises, great part of data refers to the fish community. We analyzed then bottom fish assemblages under different stratification conditions. A classification analysis (group average sorting of the Bray–Curtis similarity measures as log x+1 transformed biomass fish data) based on community structure was performed to define fish grouping of sampling sites (Clarke and Warwick, 2001). The samples that showed a marked tendency to cluster (similarity X20% in the biomass classification analysis) were used to perform a SIMPER analysis (‘‘similarity percentages’’) in order to identify those species which contributed most to similarities or differences within groups (‘‘typical’’ and ‘‘discriminating’’ species, respectively, Clarke, 1993).

1581

3. Analysis of estuarine stratification variability 3.1. Physics and ecology under the stratified condition This estuary is characterized by strong vertical salinity stratification most of the time, with marine waters (saltier and denser) penetrating deeper into the estuary along the bottom, while fresh waters advance oceanward on the surface, forming a salt wedge (Guerrero et al., 1997a, b) (Fig. 2A). The upstream reach of the salt wedge defines a bottom salinity front, whose location is controlled by the topography. A submersed bar, called Barra del Indio shoal, divides the system into an inner tidal river and an outer mixing zone (the estuary proper) (Fig. 1). The tidal river has depths lesser than 5 m and covers an area of about 13,000 km2. The estuarine region is larger, extending over an area of ca. 38,000 km2. The bottom of the estuary shows a gentle slope and

Fig. 2. Salinity sections of the Rı´o de la Plata Estuary: (A) the stratified (salt wedge) condition, (B) partially mixed condition forced by wind, (C), partially mixed condition induced by extreme low river discharge, and (D) totally mixed condition due to strong winds. Numbers at the top axis show values of the Simpson stratification parameter. Double headed arrows indicate the transition zones between stable (stratified) and unstable (mixed) regions of the estuary.

ARTICLE IN PRESS 1582

E. Marcelo Acha et al. / Continental Shelf Research 28 (2008) 1579–1588

depths are lesser than 25 m (10 m mean depth). In the offshore boundary of the estuary, convergence between estuarine and marine waters defines a surface salinity front (Fig. 2A). In our example in Fig. 2A, the vertical section exhibits two distinctive layers: a low salinity surface layer (5–7 m thick) overlying a wedge of salty shelf water. A stable (stratified) region, with f values above 200 J m2, extends more than 100 km seaward. This region is bounded by unstable (mixed) areas, both at the head and near the mouth of the estuary. The wind condition during the previous week exhibited mild to moderate winds (2–6 m s1). Horizontal salinity gradients of the surface front are weaker than those of the bottom front, and its form and location are more variable. Physical forcing in the estuary are characterized by low seasonality in the river run off (mean river discharge estimated is 24,000 m3 s1), seasonality in winds pattern (spring–summer dominated by onshore winds (NE, E, SE, and S), and fall–winter characterized by a balance between onshore and offshore winds (N, NW, W, and SW)), and low tidal amplitude (0.3–1 m tidal range) (e.g., Mianzan et al., 2001). Mean distribution of bottom salinity is mainly controlled by bathymetry. The high surface to volume ratio makes the system highly sensitive to the atmospheric forcing, and in this way distribution of surface salinity is mainly driven by winds generating a seasonal pattern (towards NE during spring–summer and to the SW during fall–winter) (Guerrero et al., 1997a). Numerical simulations (Simionato et al., 2004) explain the observed bimodal pattern of discharge in terms of two variability modes: a first one controlled by winds with a dominant component perpendicular to the estuary axis (NE/SW), and a second mode dominated by winds parallel to the estuary axis (SE/NW). These modes result from the fact that the estuary is a semi-enclosed basin, where bathymetry and coastline determine the current direction at every point in the domain. Dynamic

conditions of the estuary understood as characteristic of ‘summer’ or ‘winter’ are likely to occur during any season with synoptic to intra-seasonal scale, and in response to winds varying at the same frequency. As a result, variability of the estuarine circulation is coupled to the atmospheric circulation, presenting short-term weather conditions (the instantaneous state and its short-term variation) and climate features (the average condition over a long period) similar to those of the atmosphere (Simionato et al., 2006). A well-developed turbidity front characterizes the innermost ˜ an and Brown, 1996) (Fig. 3B). This part of the estuary (Framin turbidity maximum is due to the suspended matter flocculation at the tip of the salt wedge, and re-suspension of sediment due to tidal stirring. The turbidity front shows a stable position following the geometry of Samborombo´n Bay, and a more variable location ˜ an and Brown, on the deeper and open Uruguayan coast (Framin 1996). High dissolved inorganic nitrogen concentrations (between 18 and 28 mM) are observed at the head of the estuary (Nagy et al., 1997, 2002) but turbidity constrains photosynthesis. The small euphotic to mixing depth ratio (Fig. 4A) should not allow a positive net primary productivity. Thus, in the innermost part of the estuary phytoplankton biomass is relatively poor and nutrients assimilation is light-limited. Food chains at the inner estuary are probably detritus based, supporting high densities of the copepod Acartia tonsa (up to 8000 ind. m3) and the mysid Neomysis americana (up to 2520 ind. m3) (Mianzan et al., 2001; Schiariti et al., 2006) (Fig. 3C). Both species are omnivorous and could take advantage of the high abundance of organic matter concentrated by the front. Similar processes seem to occur near the turbidity maximum of the Chesapeake Bay, where Roman et al. (2001) have shown that the most abundant copepod in that estuary, Eurytemora affinis, also concentrates near the turbidity

Fig. 3. Ecological processes at the Rı´o de la Plata Estuary. (A) Distribution of whitemouth croaker gravid females, and bottom salinity field (symbol size proportional to ˜ an and Brown, 1996). (C) Distribution of females abundance, redrawn from Macchi et al., 1996). (B) Modal position of the turbidity front in the estuary (redrawn from Framin the mysid Neomysis americana (symbol size proportional to abundances, redrawn from Schiariti et al., 2006). (D) Young-of-the-year whitemouth croaker distribution (redrawn from Lagos, 2002).

ARTICLE IN PRESS E. Marcelo Acha et al. / Continental Shelf Research 28 (2008) 1579–1588

Fig. 4. Light and chlorophyll along the Rı´o de la Plata Estuary: (A) light penetration distribution as percentage of the incident radiation and (B) distribution of chlorophyll a (mg m3) as determined by HPLC.

maximum. It is believed that abundant food, in the form of detritus, protozoa and phytoplankton, in addition to the convergence associated with estuarine circulation, results in the high zooplankton concentrations recorded (Roman et al., 2001). Detritus is a low quality food but extremely abundant and with a constant supply in the Rı´o de la Plata Estuary (e.g., some pheopigments measurements were in the range 1.0–25.95 mg m3; CARP, 1989). Moreover, heterotrophic microzooplankton would constitute another important food item for those copepods and mysids, and is very abundant in the inner, highly turbid portion of the estuary (Kogan, 2005). Those processes have influence all along the trophic chains of the estuary. N. americana represents between 75% and 80% of the diet for juvenile croakers smaller than 12 cm (Sa´nchez et al., 1991), and the spatial distribution of croakers young-of-the-year (compare Fig. 3B, C and D) clearly matches that of the turbidity front (Lagos, 2002). High deposition of suspended matter supports also dense beds of the deposit feeding clams Mactra isabelleana, which reaches densities of about 600 ind. m2 (Giberto, unpublished data). This clam is one of the most important items in the diet of adult croakers (Giberto et al., 2004). Immediately offshore the turbidity front, and as soon as waters become less turbid, a high chlorophyll signal is observed with maximum values reaching up to 15.5 mg m3 (Fig. 4B) (Carreto et al., 2003, 2008; Huret et al., 2005). In this way, trophic web in the estuary could be based on two sources of organic matter: phytoplankton and plant detritus. The estuarine dynamics and the strong pycnoclines at the head of the salt wedge favor the accumulation and retention of plankton, like the most abundant copepod, A. tonsa, which is a broadcast spawner releasing free eggs that remain in the estuary. Retention in other estuaries seems to be less efficient: Roman et al. (2001) attribute the success of E. affinis in the Chesapeake Bay in part to the ability of females to carry the eggs until they are ready to hatch, avoiding exportation from the estuary. Retentive properties of the Rı´o de la Plata allow also an intense reproductive activity of fishes. At least 64% (n ¼ 21) of the teleosts that make use of this estuary are also estuarine spawners (Berasategui et al., 2004). This is a high percentage compared to other estuarine systems of the world (e.g., Dando, 1984; Haedrich, 1992; Mann,

1583

2000). Most abundant species concentrate near the tip of the saltwedge to spawn, like the Brazilian menhaden Brevoortia aurea (abundances up to 2100 eggs m3) and the whitemouth croaker M. furnieri (abundances up to 297 eggs m3) (Acha et al., 1999; Acha and Macchi, 2000) (Fig. 3A). This croaker is the most valuable species for the coastal fisheries of Argentina and Uruguay, with mean total landings of about 60,000 ton yr1 during the 1990–1999 decade (Carozza et al., 2004). The sheltered waters of Samborombo´n Bay (Fig. 1) are the main nursery ground for small croakers and most of juvenile fishes in the estuary (Mianzan et al., 2001). The largest portion of the salt wedge is characterized by dense aggregations of mesozooplankton below the picnocline. Marine species penetrate the estuary advected by or following the saline waters of the bottom layer (Madirolas et al., 1997; Mianzan et al., 2001; Berasategui et al., 2004; Cabreira et al., 2006). Mean values for larval fishes abundances below the halocline, ranged between 0.4 and 11.5 individuals per 100 m3 (Berasategui et al., 2004). The upper layer is biologically poor excepting for microzooplankton, specially heterotrophic dinoflagellates and naked ciliates (Kogan, 2005). Estuarine dynamics should have influence during the larval phase of benthic invertebrates, but the potential of water movements in setting adult banks has never been evaluated for this estuary. After settlement, and due to its sessile nature, spatial patterns of benthic community are supposed to show scarce (if any) variations related to relatively short-term processes like the stratification/mixing alternateness. In this estuary, benthic community is less diverse than those of the adjacent marine areas, but has highest densities and biomass (Giberto et al., 2004). Diversity of trophic modes increased also with salinity, meaning that a more even distribution of trophic structure is found at the adjacent sea. Higher biomass values of deposit feeders, like M. isabelleana, and carnivore gastropods like Buccinanops duartei characterized the estuary. In contrast, filter feeders; carnivores; herbivores or suspension feeders are well represented in the marine area. Artemesia longinaris, an omnivorous decapod, is also widely distributed along the study area. Contrary to other similar tropical and subtropical ecosystems, no mangroves, sea grasses nor benthic macroalgae are present in the Rı´o de la Plata Estuary (Boschi, 1988). Therefore, possible food supplies for the high benthic biomasses found at the estuary are organic matter from detritus produced by terrestrial plants and salt-marsh grasses, and from high aggregations of zooplankton found below the halocline at the head of the salt wedge (Mianzan et al., 2001). The spatial distribution of the dominant species, the clam M. isabelleana, coincides with the location of the maximum turbidity zone, reaching highest densities and biomass near the ˜ an region with highest probability of frontal occurrence (Framin and Brown, 1996). This bivalve probably takes advantage of the high-suspended matter deposition at the turbidity front (Giberto et al., 2004). The surface salinity front is the offshore limit of the wedge (Fig. 2A). Phytoplankton biomass shows their lower values since most nitrogen, the main nutrient limiting primary production in coastal waters, was consumed inside the estuary. Surface chlorophyll a during spring may reach up to 7 mg m3 (Carreto et al., 1986, 2007). Reports for this area showed that several chaindiatoms species were dominant in the phytoplankton assemblage (e.g., Thalassionema nitzschioides, Thalassiosira spp., Chaetoceros spp.) of the high silicate estuarine water. However, some bloom forming dinoflagellate species (e.g., Prorocentrum minimum) and the ciliate Myrionecta rubra ( ¼ Mesodinium rubrum) were in occasions highly abundant, co-dominating in some cases with the diatom flora. Some euglenophyceans, prasinophyceans (e.g., Pyramimonas sp.) and cryptophytes (e.g., Cryptomonas sp.) were

ARTICLE IN PRESS 1584

E. Marcelo Acha et al. / Continental Shelf Research 28 (2008) 1579–1588

also frequent; the latter co-dominating in some cases with the diatom flora (Carreto et al., 2008). High abundances of zooplankton (mainly gelatinous plankton) have been reported at the surface salinity front (Mianzan and Guerrero, 2000; Mianzan et al., 2001b; Alvarez Colombo et al., 2003). In many estuaries, ctenophores become very abundant in summer, such as Mnemiopsis leidyi in Narrangansett Bay, Rhode Island (Mann, 2000). At the Rı´o de la Plata surface salinity front, ctenophores (Mnemiopsis maccradyi and Pleurobrachia pileus) comprised 81% of the total organic carbon of the planktonic community (Mianzan and Guerrero, 2000). Also copepods were abundant, with A. tonsa reaching up to 550–17,600 ind. m3 ˜ as et al., 2002, respectively). Reproduc(Marrari et al., 2004; Vin tion of some fishes seem also associated to the surface front (Berasategui et al., 2004). Fig. 5 summarizes in a conceptual way some of the main trophic relations in the estuary. The surface salinity front is the more subtle and highly dynamic portion of the wedge, showing a seasonal pattern driven by winds. For practical purposes, we consider it as the boundary between the estuary and the continental shelf waters, though the river plume can be traced several kilometers away (Piola et al., 2005). Recent studies deal with the influence of this large river on the ecological processes of the continental shelf ecosystem (Muelbert et al., 2008). All along the salt wedge, the structure of bottom fishes community seems related to the highest horizontal gradients of salinity. Jaureguizar et al. (2003) described fish assemblages during the stratified condition and identified three fish assemblage areas along the main axis of the estuary. The innermost was characterized by the presence of freshwater species such as Parapimelodus valenciennes and the anadromous Netuma barbus. The estuarine group was dominated by M. furnieri, B. aurea, Macrodon ancylodon, Paralonchurus brasiliensis and Paralichthys orbignyanus, and the marine assemblage was a more diverse group featured by Cynoscion guatucupa, Conger orbignyanus, Discopyge tschudii, Paralichthys patagonicus, Percophis brasiliensis, Atlantoraja castelnaui, Mustelus schmitti, Sympterigia bonapartei, Stromateus brasiliensis, Squatina guggenheim, Myliobatis goodei and Prionotus punctatus. At the seasonal scale, the assemblages are persistent in specific composition and their link to particular salinity habitats. Geographic variations of the boundaries, indicating contraction or

expansion of fish assemblage areas, were associated with water masses dynamics.

3.2. Physics and ecology under the partially stratified condition The partially stratified condition can be forced by persistent moderate winds (time scale of days or synoptic scale), or low freshwater runoff (interannual scale). Both processes act weakening the stratification, however wind forced mixing events have higher frequency (many times a year) than the diminishing river discharge ones, which are related to the ENSO cycle (Depetris et al., 1996; Piola et al., 2005), and which last for several months. Fig. 2B illustrates a condition in that a strong wind event, lasting for 24 h, occurred while performing the profiles at the center of the section. The section in Fig. 2C represents an event characterized by a great diminishing in river discharge, which represented about 27.5% respect to the historical values for summer (Berasategui et al., 2004), coupled with a southward-dominated alongshore component of the wind stress forcing (Piola et al., 2005) give a description of the historical pattern of both forcing). In both sections, the head of the estuary presents an unstable condition (vertically mixed water columns). The outer estuary shows intermediate f values indicating a partially mixed condition due to the strong winds lasting for 24 h (Fig. 2B). Stratification parameter values allow defining two transition zones, bounding an outer stratified regime that extends 50 km offshore. The outermost station represents the beginning of the relative mixed shelf system. During extreme low river discharge condition, + values for the outer estuary show a relatively stable water column (weakly stratified) (Fig. 2C). Winds were moderate to strong 5 days previous to the cruise. The transition zone, defined by the 100o+o200 J m2 interval, separates an outer fresher lens from the relatively salty and well-mixed water column at the head. Spatial pattern of microzooplankton assemblages is affected by the water masses vertical structure. During high stratification conditions, three microzooplankton assemblages are observed, each one limited by areas of maximum salinity gradients. Upriver the bottom salinity front, the freshwater assemblage occurs, dominated by small organisms like tintinnids. In the estuary, the halocline separates the salt-wedge assemblage from the upper

Fig. 5. Conceptual diagram of some of the main trophic relations in the Rı´o de la Plata Estuary.

ARTICLE IN PRESS E. Marcelo Acha et al. / Continental Shelf Research 28 (2008) 1579–1588

layer group, both dominated by naked ciliates and heterotrophic dinoflagellates. Under partially stratified conditions, when the halocline is weakened by wind action, the three assemblages are still recognized along the horizontal dimension, but occupying each assemblage the entire water column (Kogan, 2005). The same pattern was observed for the radiolarian Lophophaena rioplatensis found in the Rı´o de la Plata Estuary. Radiolarians are oceanic organisms, but in this estuary L. rioplatensis occurs at salinities as low as 15.4 and with abundances (up to 240 cells l1) exceeding in one or two orders those reported for oceanic or upwelling environments. Abundant food resources (particulate matter, including phytoplankton) and skeleton building material, as well as low grazing pressure, could be responsible for the unusually high standing stock. The presence of cytoplasm in most of the observed shells suggest an actively growing in situ population, rather than expatriated specimens (Boltovskoy et al., 2003). When the water column is strongly stratified, the organisms are found below the halocline, with the highest abundances occurring at the middle and outer sectors of the estuary. On the other hand, when vertical stratification became weaker due to wind action, radiolarian densities are evenly distributed from surface to bottom (Boltovskoy et al., 2003). Berasategui et al. (2004) studied larval fish assemblages during the stratified and partially mixed states. The first case showed the typical salt-wedge condition. The second one occurred during the low river discharge event above described. Four assemblages were identified related to the freshwater environment; the bottom salinity front; the mixohaline zone; and the outer portion (surface salinity front) of the estuary. The freshwater group was composed of ‘catfishes’ (Order Siluriforme) larvae, while M. furnieri and B. aurea characterized the bottom salinity front assemblage. The closely related assemblage of the upper/middle estuary was composed by Gobiosoma parri and P. brasiliensis, which occupied the region immediately offshore the bottom front. The middle/ lower estuary assemblage occurred at the outermost region. Several species of this group are coastal or shelf spawners, with the estuary representing a marginal reproductive area. When waters were well stratified, these species occupied the bottom layer below the halocline and this assemblage probably extends offshore outside the estuary. During the partially mixed situation, despite the remarkable reduction in freshwater input, the oceanographic structures (i.e., bottom salinity front; two layers mixohaline region; surface salinity front) were still present, and the larval assemblages were even now recognized. These results showed the constancy of the retentive properties of the estuary

1585

which maintained ichthyoplankton inside the system, and the relative steadiness of the larval groups. The microplankton and ichthyoplankton assemblages (in spite of the different size scales of organisms in these communities) show similar performance regarding stratification/mixing alternateness. Assemblages track the main salinity gradient, that in the partially mixed state develop in the horizontal dimension, as a result the vertical structure of the biological communities is lost, though a weak halocline can be measured. Regarding nektonic community, we analyzed bottom fish assemblages under different stratification conditions. Two assemblages (estuarine and marine) were found in any case. The estuarine assemblage occupied the region of highest bottom salinity gradients while the marine one occupied the outermost part of the system (Table 1; Fig. 6). However, during the totally mixed and low river discharge conditions, bottom salinity gradients were more relaxed and the portion occupied by the estuarine assemblage seems to be smaller. In spite of the persistence of the same assemblages during all the conditions, dominant species showed some variations (Table 1) but M. furnieri is always the dominant species in the estuarine assemblage during any condition. 3.3. Physics and ecology under the mixed condition A disruption of water column stratification and mixing of the salt wedge occurs after several hours of strong winds (Fig. 2D), though salt wedge at the head of the estuary can be reestablished within 48–72 h (Guerrero et al., 1997a). In our example in Fig. 2D, the horizontal distribution of the Simpson parameter of stability presents a totally mixed (unstable) condition. Meteorological records show storm wind (14–16 m s1) from the southeast, 72–24 h before the observations and strong wind (8–11 m s1) while performing the profiles. The vertical section, except a weak stability area at the center of the section (+ 100–170 J m2), displays a totally mixed (unstable) vertical structure with + lower than 70. Extreme winds at the Rı´o de la Plata region are evenly distributed along the year, having no defined temporal pattern (mean frequency about 30–40 days). These events persist for 2–3 days, exceptionally 5 days (Guerrero et al., 1997a). Totally mixed condition occurs in an unpredictable manner, lasting for short periods (usually lesser than 3 days) during which field sampling becomes extremely difficult. As a consequence, biological information during this situation is very scarce. We draw some inferences on the ecological effects of this condition.

Table 1 SIMPER (‘‘similarity percentages’’ analysis) results for comparison between the estuarine and marine groups for the three stratification conditions Condition

Estuarine

Marine

Estuarine vs. marine

Stratified

Micropogonias furnieri (46.47%) Macrodon ancylodon (22.23%) Brevoortia aurea (11.25%) Paralonchurus brasiliensis (9.08%)

Cynoscion guatucupa (34.91%) Sympterygia bonapartei (15.71%) Stromateus brasiliensis (14.41%) Mustelus schmitti (9.40%)

Cynoscion guatucupa (24.07%) Micropogonias furnieri (15.25%) Brevoortia aurea (14.07%) Sympterygia bonapartei (7.22%)

Totally mixed

Micropogonias furnieri (35.64%) Netuma barbus (29.86%) Macrodon ancylodon (18.80%) Parapimelodus valensiennesi (6.50%)

Micropogonias furnieri (26.15%) Macrodon ancylodon (14.47%) Xystreurys rasile (13.71%) Trichiurus lepturus (11.25%)

Xystreurys rasile (9.14%) Netuma barbus (7.28%) Trichiurus lepturus (6.84%) Cynoscion guatucupa (6.49%)

Partially mixed low river discharge

Micropogonias furnieri (43.98%) Trichiurus lepturus (12.13%) Mugil platanus (9.48%) Anchoa marinii (8.58%)

Trichiurus lepturus (21.78%) Prionotus punctatus (16.81%) Anchoa marinii (14.75%) Cynoscion guatucupa (12.89%)

Micropogonias furnieri (13.43%) Cynoscion guatucupa (11.17%) Trichiurus lepturus (8.68%) Anchoa marinii (8.52%)

Typical species (those contributing more than 90% to the average similarity for each group ‘‘estuarine’’ and ‘‘marine’’) and discriminating species (those contributing more than 90% to the average dissimilarity between groups ‘‘estuarine’’ and ‘‘marine’’) are listed (% contribution in parenthesis).

ARTICLE IN PRESS 1586

E. Marcelo Acha et al. / Continental Shelf Research 28 (2008) 1579–1588

Fig. 6. Bottom fish assemblages and bottom salinity distribution for three different stratification conditions: (A) and (B) stratified (salt wedge); (C) and (D) totally mixed condition due to strong winds; (E) and (F) partially mixed condition induced by extreme low river discharge.

In the classical conceptual model of Margalef (1978), the selection of diatoms or dinoflagellates is viewed as being abiotically regulated by interactions between turbulence and nutrients. In nutrient rich waters, stratification is the essential physical condition that dinoflagellate require to bloom because of their relative inability, unlike diatoms, to tolerate the elevated shear-stress associated with water column mixing and turbulence. Although there are not direct observations of the phytoplankton assemblages during partially or totally mixed conditions in this estuary, the observed co-dominance of diatoms, red tides dinoflagellates, and other bloom taxa, indicate a great variability in the turbulence strength, probably manifested as pulses over several spatial and temporal scales. Though wind stress or runoff diminishing both produce partially mixed conditions as above discussed, it is expectable a different degree of turbulence, having consequently different effects on phytoplankton community. Monbet (1992) has discussed the influence of stability of the water column on the phytoplankton of estuaries, stressing the importance of tidal mixing on phytoplankton response to nutrient inputs for micro- and macrotidal estuaries. He showed that increased mixing reduces chlorophyll concentrations because a reduction of the residence time of algae in the photic zone. Low tides at the Rı´o de la Plata have small effect on mixing, which is wind dominated due to the large surface and shallowness. Consequently, a decreased photosynthetic activity during strong wind events and a higher importance of the detritus-based food chain are expected.

Due to the high difference in salinity between the upper and lower layers (wedge condition), turbulent mixing could suddenly expose planktonic organisms to an osmotic shock, taking into account that these organisms mostly concentrate immediately below the halocline and at the bottom salty layer. Dead copepods (A. tonsa) reaching up to 44% of the total copepod abundances were detected concentrated at the surface salinity front, probably as a result of an extreme winds event (Mianzan et al., 2001; Ramirez1, personal communication). Dead specimens were recognized by the presence of tissue vacuolization on adult specimens, proving in this way not to be exuviae. A. tonsa is a cosmopolitan species well adapted to estuarine environments, but in spite of the species salinity tolerance, laboratory experiments showed that mortality increases by reduced time of acclimation caused by the changes rate (Cervetto et al., 1999). We expect that plankton community be severely affected by destruction of the water mass stratification and rapid exposure to different salinities. N. americana, which seems to be a key linkage in the estuarine food web (see Section 3.1), occurs along the bottom salinity front, which is the area of steepest salinity gradients of the estuary (Schiariti et al., 2006). Moreover, shallowness (depth o10 m) make this region highly sensitive to wind stress and consequently to disruptions of the salinity stratification (Guerrero et al., 1997a, b), exposing in this way the mysids to abrupt changes in salinity. High energy demands, related to increased osmoregulatory activities, would be satisfied by the large food amounts concentrated by the bottom salinity front (Schiariti et al., 2006). Although bottom fish assemblages seem not to be affected by the totally mixed condition (Fig. 6), mixing events could have influence on the success of fish reproduction. Most of the fishes that reproduce inside the Rı´o de la Plata Estuary are pelagic eggs spawners (Berasategui et al., 2004). Retentive properties of the estuarine dynamics seem to play a paramount role in allowing this reproductive style. However, when the salt wedge is broken by wind-induced vertical mixing, altering also the dynamics of the estuarine waters, eggs and larvae are probably advected from the system towards the ocean with the mixed waters, and lost for the populations of the estuary. However, the protracted breeding season of the Rı´o de la Plata fishes, their batch-spawning mode of reproduction, and their high spawning frequency, conform a spread risk strategy that could ensure enough offspring survival to maintain the species in this variable and unpredictable environment (Acha et al., 1999; Acha and Macchi, 2000).

4. Conclusions Estuarine variability is a complex issue. It is widely recognized that estuaries are highly variable, and for most of the cases, unpredictable ecosystems (e.g., Day et al., 1989). Different forcing, interacting with geomorphologic configuration, depth range, size of the estuaries, and features of the adjacent ocean, produce a rich spectrum of variability. Moreover, such variability manifest over a range of temporal scales, each one having its own key forcing. The Rı´o de la Plata is a large-scale estuary, with a high freshwater discharge and a broad and permanent connection to the sea. The estuary is characterized by low tidal amplitude and weak seasonality in the river input. The high surface to volume ratio makes this estuary highly sensitive to the atmospheric forcing, specially to wind-induced turbulence. At the seasonal scale, mean winds pattern controls the surface salinity distribution. At shorter time scales, strong winds induce total or partially mixed 1 Dr. Fernando Ramı´rez, Laboratory of Zooplancton, INIDEP, Paseo Victoria Ocampo No. 1, 7600 Mar del Plata, Argentina.

ARTICLE IN PRESS E. Marcelo Acha et al. / Continental Shelf Research 28 (2008) 1579–1588

conditions of the water column. This is the scenario where life must endure in the Rı´o de la Plata Estuary: 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 ˜ o-La Nin ˜ a phenomenon, but variations associated to the El Nin their ecological consequences remains as a poorly studied topic of the estuary.

Acknowledgments This research was supported by Agencia PICT 2000 no. 0708424 and PICT 2003 no. 07-13659; CONICET PIP 5009, EXA 355/ 06 and by Fundacio´n Antorchas no. 13900-13. It is also part of Project PLATA, supported by the Inter-American Institute for Global Change Research (IAI), through Project SACC (CRN-061), by the Sao Paulo State Funding Foundation (FAPESP; Grant 2004/ 01950-3), and by the NICOP Program of the US Office of Naval Research (Grant N00014-02-1-0295). This is INIDEP contribution no. 1434. References Acha, E.M., Macchi, G.J., 2000. Spawning of Brazilian menhaden, Brevoortia aurea, in the Rı´o de la Plata estuary off Argentina and Uruguay. Fishery Bulletin (NOAA) 98 (2), 227–235. Acha, E.M., Mianzan, H.W., Lasta, C.A., Guerero, R.A., 1999. Estuarine spawning of the witemouth croaker Micropogonias furnieri (Pisces: Sciaenidae), in the Rı´o de la Plata, Argentina. Marine and Freshwater Research 50, 57–65. Alvarez Colombo, G., Mianzan, H.W., Madirolas, A., 2003. Acoustic characterization of gelatinous plankton aggregations: four study cases from the argentine continental shelf. ICES Journal of Marine Science 60, 650–657. Berasategui, A.D., Acha, E.M., Ferna´ndez Araoz, N.C., 2004. Spatial patterns of ichthyoplankton assemblages in the Rio de la Plata estuary (Argentina, Uruguay). Estuarine, Coastal and Shelf Science 60, 599–610. Berasategui, A.D., Menu Marque, S., Gomez Erache, M., Ramı´rez, F.C., Mianzan, H.W., Acha, E.M., 2006. Copepod assemblages in a highly complex hydrographic region. Estuarine, Coastal and Shelf Science 66, 483–492. Boltovskoy, D., Kogan, M., Alder, V., Mianzan, H., 2003. First record of a brackish radiolarian (Polycystina), Lophophaena rioplatensis n. sp. in the Rio de la Plata estuary. Journal of Plankton Research 25 (12), 1551–1559. Boschi, E.E., 1988. El ecosistema estuarial del Rı´o de la Plata (Argentina y Uruguay). Anales del Instituto de Ciencias del Mar y Limnologı´a, Universidad Nacional Auto´noma de Me´xico 15, pp. 159–182. Cabreira, A., Madirolas, A., Alvarez Colombo, G., Acha, E.M., Mianzan, H., 2006. Acoustic study of the Rı´o de la Plata estuarine front. ICES Journal of Marine Science 63, 1718–1725. CARP, 1989. Estudio para la evaluacio´n de la contaminacio´n del Rı´o de la Plata. Comisio´n Administradora del Rı´o de la Plata, Montevideo-Buenos Aires. Carozza, C.R., Lasta, C.A., Ruarte, C., Cotrina, C.P., Mianzan, H., Acha, E.M., 2004. Corvina rubia (Micropogonias furnieri). In: Sa´nchez, R.P., Bezzi, S.I. (Eds.), Los peces marinos de intere´s pesquero. Caracterizacio´n biolo´gica y evaluacio´n del estado de explotacio´n, vol. 4. Instituto Nacional de Investigacio´n y Desarrollo Pesquero, Mar del Plata, Argentina, pp. 255–270. Carreto, J.I., Negri, R.M., Benavides, H.R., 1986. Algunas caracterı´sticas del florecimiento del fitoplancton en el Frente del Rı´o de la Plata. Parte 1: Los sistemas nutritivos. Revista De Investigacio´n y Desarrollo Pesquero 5, 7–29. Carreto, J.I., Montoya, N.G., Benavides, H.R., Guerrero, R.A., Carignan, M.O., 2003. Characterization of spring phytoplankton communities in the Rı´o de la Plata maritime front using pigment signatures and cell microscopy. Marine Biology 143, 1013–1027. Carreto, J.I., Carignan, M.O., Montoya, N.G., Cucchi Colleoni, D.A., 2007. Ecologia del fitoplancton en los sistemas frontales del mar argentino. In: Carreto, J.I., Bremec, C. (Eds.), El Mar Argentino y sus recursos Pesqueros. Tomo V. El ambiente Marino, INIDEP, Mar del Plata, Argentina, pp. 11–31. Carreto, J.I., Montoya, N.G., Akselman, R., Carignan, M.O., Silva, R.I., Cucchi Colleoni, A.D., 2008. Algal pigment patterns and phytoplankton assemblages in different water masses of the Rio de la Plata maritime front. Continental Shelf Research 28 (12), this issue, doi:10.1016/j.csr.2007.02.012. Cervetto, G., Gaudy, R., Pagano, M., 1999. Influence of salinity on the distribution of Acartia tonsa (Copepoda, Calanoida). Journal of Experimental Marine Biology and Ecology 239, 33–45. Clarke, K.R., 1993. Non-parametric multivariate analysis of changes in community structure. Australian Journal of Ecology 18, 117–143.

1587

Clarke, K.R., Warwick, R.M., 2001. Change in Marine Communities: An Approach to Statistical Analysis and Interpretation, second ed. PRIMER-E, Plymouth. Dando, P.R., 1984. Reproduction in estuarine fish. In: Potts, G.W., Wootton, R.J. (Eds.), Fish Reproduction: Strategies and Tactics. Academic Press, London, pp. 155–170. ˜ ez-Arancibia, A., 1989. Estuarine Ecology. Day Jr., J.W., Hall, C.A.S., Kemp, W.M., Ya´n Wiley, New York. Depetris, P.J., Kempe, S., Latif, M., Mook, W.G., 1996. ENSO-controlled flooding in the Parana´ River (1904–1991). Naturwissenschaften 83, 127–129. ˜ an, M.B., Brown, O.B., 1996. Study of the Rı´o de La Plata turbidity front. Part Framin I: Spatial and temporal distribution. Continental Shelf Research 16, 1259–1282. ˜ an, M.B., Etala, M.P., Acha, E.M., Guerrero, R.A., Lasta, C.A., Brown, O.B., 1999. Framin Physical characteristics and processes of the Rı´o de la Plata estuary. In: Perillo, G.M., Piccolo, M.C., Pino, M. (Eds.), Estuaries of South America—Their Geomorphology and Dynamics. Springer, Berlin, pp. 161–194. Giberto, D.A., Bremec, C.S., Acha, E.M., Mianzan, H., 2004. Large-scale spatial patterns of benthic assemblages in the SW Atlantic: the Rı´o de la Plata estuary and adjacent shelf waters. Estuarine, Coastal and Shelf Science 61 (1), 1–13. ˜a´n, M.B., Lasta, C.A., 1997a. Physical oceanography Guerrero, R.A., Acha, E.M., Framin of the Rı´o de la Plata estuary, Argentina. Continental Shelf Research 17, 727–742. ˜ an, M., 1997b. HydroGuerrero, R.A., Lasta, C., Acha, E.M., Mianzan, H., Framin graphic Atlas of the Rı´o de la Plata. CARP-INIDEP, Buenos Aires, Montevideo. Haedrich, R.L., 1992. Estuarine fishes. Ecosystems of the World. Estuaries and Enclosed Seas 26, 185–207. Huret, M., Dadou, I., Dumas, F., Lazure, P., Garc- on, V., 2005. Coupling physical and biogeochemical processes in the Rı´o de la Plata plume. Continental Shelf Research 25 (5–6), 629–653. Jaureguizar, A., Menni, R.C., Bremec, C., Mianzan, H.W., Lasta, C.A., 2003. Fish assemblages and environmental patterns in the Rı´o de la Plata estuary. Estuarine, Coastal and Shelf Science 56, 921–933. Jaureguizar, A., Menni, R.C., Guerrero, R.A., Lasta, C.A., 2004. Environmental factors structuring fish communities of the Rı´o de la Plata estuary. Fisheries Research 66, 195–211. Kogan, M., 2005. Estudio de la composicio´n especı´fica, abundancia y distribucio´n espacial del microzooplancton (Protozoos y Micrometazoos) en el estuario del Rı´o de la Plata (Argentina-Uruguay). Ph.D. Thesis, Universidad de Buenos Aires, Argentina, unpublished. Lagos, N., 2002. Distribucio´n espacial de los juveniles de corvina rubia (MicroReport, Freplata (PNUD7GEF7RLA799/G31), Montevido, Uruguay. Macchi, G.J., Acha, E.M., Lasta, C.A., 1996. Desove y fecundidad de la corvina rubia (Micropogonias furnieri, Desmarest, 1823) del estuario del Rı´o de la Plata, ˜ ol de Oceanografı´a 12 (2), 99–113. Argentina. Boletı´n del Instituto Espan Macchi, G.J., Acha, E.M., Lasta, C.A., 2002. Reproduction of black drum, Pogonias cromis (Pisces: Sciaenidae), in Samborombo´n Bay, Argentina. Fisheries Research (1–2), 83–92. Macchi, G.J., Acha, E.M., Militelli, M.I., 2003. Seasonal egg production pattern of whitemouth croaker (Micropogonias furnieri) of the Rı´o de la Plata estuary, Argentina-Uruguay. Fishery Bulletin 101, 332–342. Madirolas, A., Acha, E.M., Guerrero, R., Lasta, C., 1997. Sources of acoustic scattering near a halocline in an estuarine frontal system. Scientia Marina 61 (4), 431–438. Mann, K.H., 2000. Ecology of Coastal Waters. Blackwell Science, Malden, MA, USA. Margalef, R., 1978. Life-forms of phytoplankton as survival alternatives in a unstable environment. Oceanologica Acta 1 (4), 493–509. ˜ as, M.D., Martos, P., Herna´ndez, D., 2004. Spatial patterns of Marrari, M., Vin mesozooplankton distribution in the Southwestern Atlantic Ocean (34–411S) during austral spring: relationship with the hydrographic conditions. ICES Journal of Marine Science 61, 667–679. Mianzan, H.W., Guerrero, R.A., 2000. Environmental patterns and biomass distribution of gelatinous macrozooplankton. Three study cases in the southwestern Atlantic Ocean. Scientia Marina 64 (Suppl. 1), 215–224. Mianzan, H.W., Lasta, C., Acha, E.M., Guerrero, R., Machi, G., Bremec, C., 2001. The Rı´o de la Plata Estuary, Argentina, Uruguay. In: Seeliger, U., de Lacerda, LD., Kjerve, B. (Eds.), Ecological Studies: Coastal Marine Ecosistems of Latin America. Springer, Berlin, pp. 185–204. Militelli, M.I., Macchi, G.J., 2004. Spawning and fecundity of king weakfish, Macrodon ancylodon, in the Rı´o de la Plata estuary, Argentina-Uruguay. Journal of the Marine Biological Association of the United Kingdom 84, 443–447. Monbet, Y., 1992. Control of phytoplankton biomass in estuaries: a comparative analysis of microtidal and microtidal estuaries. Estuaries 15 (4), 563–571. Muelbert, J.H., Acha, E.M., Mianzan, H., Reta, R., Guerrero, R., Berasategui, A., Braga, E.S., Eichler, P., Garcia, V.M., Gomez-Erache, M., Ramirez, F., 2008. Biophysical coupling at the Subtropical Shelf Front Zone in the SW-Atlantic Continental Shelf. Continental Shelf Research 28 (12) this issue, doi:10.1016/j.csr.2007.08.011. Nagy, G.J., Martinez, C.M., Caffera, M.R., Pedrosa, G., Forbes, E.A., Perdomo, A.C., Lo´pez Laborde, J., 1997. The hydrological and climatic setting of the Rı´o de la Plata. In: Wells, P., Daborn, G. (Eds.), The Rı´o de la Plata. An Environmental Overview. An EcoPlata Project Background Report. Dalhousie University, Halifax, Nova Scotia (Chapter 2). Nagy, G.J., Go´mez-Erache, M., Lo´pez, C.H., Perdomo, A.C., 2002. Distribution patterns of nutrients and symptoms of eutrophication in the Rı´o de la Plata River estuary system. Hydrobiologia 475–476, 125–139. Piola, A.R., Matano, R.P., Palma, E.D., Mo¨ller Jr., O.O., Campos, E.J.D., 2005. The influence of the Plata River discharge on the western South Atlantic shelf. Geophysical Research Letters 32, 1603–1606. Roman, M.R., Holliday, D.V., Sanford, L.P., 2001. Temporal and spatial patterns of zooplankton in the Chesapeake Bay turbidity maximum. Marine Ecology Progress Series 213, 215–227.

ARTICLE IN PRESS 1588

E. Marcelo Acha et al. / Continental Shelf Research 28 (2008) 1579–1588

Sa´nchez, F., Marı´, N., Lasta, C., Gianglobbe, A., 1991. Alimentacio´n de la corvina rubia (Micropogonias furnieri) en la Bahı´a Samborombo´n. Frente Marı´timo 8, 43–50. Schiariti, A., Berasategui, A.D., Giberto, D.A., Guerrero, R.A., Acha, E.M., Mianzan, H.W., 2006. Living in the front: Neomysis americana (Mysidacea) in the Rı´o de la Plata estuary, Argentina, Uruguay. Marine Biology 149, 483–489. ˜ ez, M., 2004. A numerical study of the Simionato, C.G., Dragani, W., Meccia, V., Nun barotropic circulation of the Rı´o de la Plata estuary: sensitivity to bathymetry, earth’s rotation and low frequency wind variability. Estuarine, Coastal and Shelf Science 61 (2), 261–273.

˜ nez, M., 2006. Rio de Simionato, C.G., Meccia, V.L., Dragani, W.C., Guerrero, R., Nun la Plata estuary response to wind variability in synoptic to intra-seasonal scales. Part 1: Barotropic response. Journal of Geophysical Research 111, C09031, doi:10.1029/2005JC003297. Simpson, J.H., 1981. The shelf sea fronts: implications of their existence and behavior. Philosophical Transactions of the Royal Society of London (London) A 302, 532–546. ˜ as, M.D., Negri, R.M., Ramı´rez, F.C., Herna´ndez, D., 2002. Zooplankton Vin assemblages and hydrography in the spawning area of anchovy (Engraulis anchoita) off Rı´o de la Plata estuary (Argentina, Uruguay). Marine and Freshwater Research 53, 1031–1043.