Bahia Blanca Estuary Research Articles

Effect of dominant Spartina species on salt marsh detritus production in SW Atlantic estuaries

Two cordgrass species of the genus Spartina cohabit in SW Atlantic (southern Brazil 31º48′ S to Argentinean Patagonia, 43º20′ S) salt marshes. Some salt marshes are dominated by the dense-flowered cordgrass Spartina densiflora (which inhabits the upper intertidal level) and others by the smooth cordgrass Spartina alterniflora (which inhabits the lower intertidal level). We investigated how the different species dominance affects the detritus dynamics in the Bahia Blanca estuary (38º47′ S, 62º20′ W Argentina). Field measurements of annual detritus production using destructive methods show that both plants are similar. However, detritus of S. alterniflora shows higher decomposition rates than that of S. densiflora. This difference may be due to a larger N content, lower lignocellulose content and lower C/N ratio of S. alternifora when compared with S. densiflora. Moreover, field sampling shows that S. alterniflora has a larger amount of trapped litter that, according to the litterbag method, has higher decomposition rates. Therefore it is highly likely that S. alterniflora salt marshes contribute towards more profitable detritus for estuarine food webs than marshes dominated by S. densiflora. These results illustrate that the composition of the coastal plant community can determine the quality and profitability of the detritus that support estuarine food webs. They also illustrate that salt marshes belonging to a same biogeographic group and even coexisting in great proximity can have very different ecosystemic roles.

Single-step chemical extraction procedures and chemometrics for assessment of heavy metal behaviour in sediment samples from the Bahía Blanca estuary, Argentina

The objective of this research was to study heavy metal mobility and availability in sediment samples. A rapid diagnosis about metal behaviour was performed using the combination of several single-step extraction procedures and multi-way chemometric tools. Several single-step procedures for metal lixiviation—ammonium acetate, acetic acid, hydrochloric acid, ethylenediaminetetraacetic acid, trans-1,2-diaminocyclohexane-N,N,N′,N′-tetraacetic acid, nitrilotriacetic acid—with increasing extracting power were simultaneously applied to coastal surface sediment samples. Also, three certified reference materials for total metal concentrations were analysed. The metals studied were Cd, Cr, Cu, Pb and Zn because of their hazardous potential and related abundance in the estuary. The concentrations of metals were determined by inductively coupled plasma optical emission spectrometry. Parallel factor analysis (PARAFAC) was used to obtain a better visualization of the experimental data. Total heavy metal content in the sediment samples ranged from 1.19 to 3.01, 18.6 to 35.6, 55.6 to 102, 20.4 to 79.2 and 70.2 to 508 mg kg−1 for Cd, Cr, Cu, Pb and Zn, respectively. PARAFAC models with two factors describe the data sets appropriately (explained variance of 64.1% and core consistency of 95.4%). Cd appears in the surface sediment samples of Bahia Blanca estuary as the most easily interchangeable, pointing to a possible anthropogenic origin. However, the fraction extracted with ammonium acetate under neutral conditions is low. On the other hand, acetic acid seems to be able to efficiently extract Cd, Cu and Zn. The largest proportions of metals that can be mobilized by complexing reagents belong to the sample S2; Cd, Pb and Zn, and Cu to a lesser extent, appear to be efficiently separated by complexation. Cr appears to be the most difficult to extract, indicating a low mobility of coastal sediments. The combination of single-step lixiviation procedures and multivariate techniques provided useful information about the sediment samples and a better understanding about heavy metal mobility, availability and hazardousness. The information obtained was similar to the one recovered by sequential extraction procedures, but the time saved with the proposed methodology is significantly higher.

Biostabilization of sediments by microbial mats in a temperate siliciclastic tidal flat, Bahia Blanca estuary (Argentina)

Extensive microbial mats have been found in the siliciclastic tidal flats of the temperate Bahía Blanca estuary in the Atlantic coast of Buenos Aires province, Argentina. Several microbially induced sedimentary structures (MISS) occur especially in the upper intertidal and lower supratidal flats, among which the most conspicuous are erosional pockets, gas domes, microbial mat chips, and polygonal oscillation cracks. Biostabilization processes by epibenthic and endobenthic mats are also analyzed. Endobenthic mats occur in the upper intertidal area stabilizing ripples that despite the occurrence of severe storms persist in a fixed position for at least 2 months. Epibenthic mats occurring in the lower supratidal area also protect the substrate forming a thick microbial cover through the studied period. This thick cover was only affected by a strong storm that formed areas with erosional pockets and mat pieces. Nevertheless, the loose sediment within the erosional pockets was quickly colonized by microorganisms that developed a thin biofilm layer after a week. Changes in sediment accumulation were also recorded all over the upper tidal flat during a year, showing an important increase due to bioturbation activities of crabs. This situation also affected microbial mat growth, which evolved from a thin microbial biofilm into a thick, stratified microbial mat community in almost 2 years, mainly in the lower supratidal areas. The results of this study not only help us to better characterize the complex interactions between the microorganisms forming microbial mats, the tidal-flat sediment and the physical parameters that control this setting, but also have important implications for the understanding of analogous fossil sedimentary successions.

Digenean Parasites in the White-Backed Stilt Himantopus melanurus Vieillot, 1817 (Recurvirostridae) from the Argentine Coast

Abstract Four species of digeneans were recovered from 1 specimen of Himantopus melanurus (Aves: Recurvirostridae) from Bahia Blanca estuary, Argentina. Digeneans were assigned to Psilochasmus oxyurus (Psilostomidae), Pygidiopsis crassus (Heterophyidae), Ascocotyle (Ascocotyle) felippei (Heterophyidae), and Levinseniella cruzi (Microphallidae). Psilochasmus oxyurus and A. (A.) felippei have been reported from other species of hosts, mostly from South America, but this is the southernmost report for these species. Levinseniella cruzi has been reported from H. melanurus from Mar Chiquita lagoon, to the north of Bahia Blanca estuary. This is the first report of P. crassus parasitizing a natural host.

Organotin compounds in sediments of three coastal environments from the Patagonian shore, Argentina

One of the most toxic compounds that have been introduced into the marine environment is Tributyltin (TBT) used as a biocide in antifouling paints. The aim of this study was to obtain the first information of TBT pollution in sediments of three environments of the Argentinean coastline. Sediments from the Bahia Blanca estuary, San Antonio Bay and Ushuaia Bay were sampled to determine TBT and dibutyltin (DBT) concentration. The butyltin compounds were identified and quantified by gas chromatography and mass spectrometry (GC?MS). The different amount of organotin compounds detected shows dissimilar pollution levels and degradation processes along the Argentinean coasts.

Isolation, identification and antimicrobial activity of lactic acid bacteria from the Bahía Blanca Estuary

This study analyzed the biodiversity of lactic acid bacteria present in the Bahia Blanca Estuary and their antimicrobial activity against pathogens associate...

Long-term changes in phytoplankton phenology and community structure in the Bahía Blanca Estuary, Argentina

The phytoplankton of the Bahia Blanca Estuary, Argentina, has been surveyed since 1978. Chlorophyll a, phytoplankton abundance, species composition and physico-chemical variables have been fortnightly recorded. From 1978 to 2002, a single winter–early spring diatom bloom has dominated the main pattern of phytoplankton interannual variability. Such pattern showed noticeable changes since 2006: the absence of the typical winter bloom and changes in phenology, together with the replacement of the dominant blooming species, i.e. Thalassiosira curviseriata, and the appearance of different blooming species, i.e. Cyclotella sp. and Thalassiosira minima. The new pattern showed relatively short-lived diatom blooms that spread throughout the year. In addition, shifts in the phytoplankton size structure toward small-sized diatoms, including the replacement of relatively large Thalassiosira spp. by small Cyclotella species and Chaetoceros species have been noticed. The changes in the phenology and composition of the phytoplankton are mainly attributed to warmer winters and the extremely dry weather conditions evidenced in recent years in the Bahia Blanca area. Changing climate has modified the hydrological features in the inner part of the estuary (i.e. higher temperatures and salinities) and potentially triggered the reorganization of the phytoplankton community. This long-term study provides evidence on species-specific and structural changes at the bottom of the pelagic food web likely related to the recent hydroclimatic conditions in a temperature estuary of the southwestern Atlantic.

Breaching biogeographic barriers: the invasion of Haynesina germanica (Foraminifera, Protista) in the Bahía Blanca estuary, Argentina

Large populations of the living benthic foraminifera Haynesina germanica (Ehrenberg 1840) are reported for the first time from marsh and intertidal mud flat sediments of the Bahia Blanca estuary (Argentina). Maximum abundance of living specimens was recorded in shallow intertidal environments. The species was previously recorded from many European and North American shallow-water coastal settings, but has not been documented from Argentina. Comparative faunal assemblage analysis from dated core sample material from within the Bahia Blanca estuary shows that the species has not been present for at least the last 8,200 years. This supports the hypothesis that the species has been accidentally introduced outside its natural range as a probable result of ballast water and/or shipping activities. This study is the first report of a successful invasion of non-indigenous benthic foraminifera to the South Atlantic coast of Argentina. The introduction of nonnative foraminiferal species provides new evidence that human-mediated breaching of biogeographic barriers, will ultimately result in the biotic homogenization of foraminiferal intertidal faunal assemblages.

Multi-year monitoring of estuarine sediments as ultimate sink for DDT, HCH, and other organochlorinated pesticides in Argentina

Forty-nine superficial sediment samples used in this study were collected from eight sites in the Bahia Blanca Estuary, located in the Buenos Aires Province southwest, Argentina, in a period between April 2005 and March 2009, to assess the concentration levels, spatial and seasonal distribution, and putative input sources of 18 selected organochlorine pesticides (OCs), following UNEP-IAEA procedures. Average concentration levels in sediments ranged between 3.54 and 63.79 ng g⁻¹, d.w. (mean = 15.99 ng g⁻¹, d.w.), for ∑OCs, and from nondetectable to 8.03 ng g⁻¹, d.w. (mean = 2.16 ng g⁻¹, d.w.), and from nondetectable to 3.20 ng g⁻¹ (mean = 0.97 ng g⁻¹, d.w.) for hexachlorocyclohexanes (HCHs) and dichlorodiphenyltrichloroethanes (DDTs), respectively. Principal component analysis allowed the classification of sampling sites according to the main OCs inputs over the estuary. In comparison to other worldwide locations and to the world coastal sediment concentrations range, sediment DDTs and HCHs levels were in the low range; on the opposite, ∑OCs average was in the medium range. OCs seasonal distribution was found to be highly correlated with the precipitations seasonality and sowing seasons in the adjacent agriculture fields. In addition, an ecotoxicological risk assessment pointed industrial and agriculture catchment zones with intermediate probability of adverse effects to biota, while the city sewage outfall was classified with none indication of an immediate threat. Data indicated that the estuary sediments were functioning as a sink for persistent OC pesticides (either in use or banned), however, it was found a declining trend on their environmental burden.

Distribution of Several Heavy Metals in Tidal Flats Sediments within Bahía Blanca Estuary (Argentina)

Sediment pollution by metals is of high interest considering that it can affect marine life. The estuaries' quality may be reflected by the environmental intertidal zone condition. Subsurface sediments collected at the nude tidal flats from three sampling stations in the Bahia Blanca Estuary were analyzed for total metals concentrations (Hg, Cd, Pb, and Cr), distribution, and geochemical partitioning. Most of the elements (Hg, Cd, and Cr) have shown highest concentration values in the industrial-influenced area. Maximum value of Pb was obtained where the main freshwater input discharges. Intertidal sediments have presented higher values of Cr than the subtidal ones. Cd and Pb contents near the industrial area were strongly higher in the subtidal zones. The distribution of Cd and Pb demonstrated the occurrence of a diffusion pattern from the land toward the sea, showing a dependence on both the metal itself and/or the source. Not all studied metals have shown the highest content in the fine fraction. The chemical partitioning in the fine fractions offered evidence that the tidal flats were an important source as well as sink of metals to the adjacent coastal area. The studies of intertidal sediments provide an integrative knowledge on the potential effects of different trace metals in the environment and they must be used in the contamination studies within coastal areas.

Geochemical Control of Heavy Metal Concentrations and Distribution Within Bahia Blanca Estuary (Argentina)

Chromium and lead concentrations and distribution have been fully studied within Bahia Blanca estuary inner area, which is strongly influenced by urban and industrial stress. Not only metals dissolved in estuarine water but also those included in sediments and suspended particulate matters (SPM) were measured. In all cases, internationally standardized protocols were applied to metal measurements, including analytical quality check test through analysis of certified reference materials. Total metal contents from surface sediments and SPM were compared with those from a historical database of the area, as well as with values representing the natural geochemical baseline within the system. Results showed that heavy metal pollution is mainly localized in the areas close to both industrial effluents discharge system and urban sewage outfall discharge. Data from sequential extractions indicate that metals from anthropogenic sources (i.e., Pb) are potentially more mobile than those inherited from geological parent material (i.e., Cr). The influences of other potential sources of metals (i.e., streams, runoff) were also considered. SPM was clearly identified as the main carrier of the studied heavy metals within the system, and its significance to metals input into sediments and/or biota was verified. Finally, the normalization of measured metal concentrations against background reference elements (i.e., Al or Fe) has allowed to identify that most of the measured Cr was lithogenic, while a significant percentage of Pb was from anthropogenic origin within Bahia Blanca estuary.

Short-term variability in the phytoplankton and physico-chemical variables in a high-tidal regime, Bahía Blanca Estuary, Argentina

Corresponding autor: vguinder@criba.edu.ar Hydrological processes such as advection, resuspension, estuarine circulation, internal waves, tidal stirring, vertical mixing and stratification are the main physical forces affecting the phytoplankton distribution in estuaries (DENMAN; GARGETT, 1983; CLOERN et al., 1989; CUSHING 1989; DE JONGE; VAN BEUSEKOM, 1992). Vertical and horizontal transport of phytoplankton in coastal areas could be induced by winds, water mass intrusions from the open sea, turbulence, freshwater inflow and tides (CLOERN et al., 1989; LI; SMAYDA, 2001; BRUNET; LIZON, 2003; HAGY III et al., 2005; POPOVICH; MARCOVECCHIO, 2008; POPOVICH et al., 2008b) and these sorts of transport are related to biomass accumulation and dispersion. Many studies have revealed intratidal variations in phytoplankton dynamics (e.g. DEMERS; LEGENDRE, 1979; DEMERS et al., 1986; DUSTAN; PINCKNEY, 1989; BRUNET; LIZON, 2003) and even differences in biomass between micro and macrotidal estuaries (MONBET, 1992). Moreover, phytoplankton patchiness in estuaries has been associated with the physical influence of tides (DUSTAN; PINCKNEY, 1989). In estuaries characterized by high-tidal energy, a steady phytoplankton composition over short time periods might be expected owing to the intense mixing processes continuously occurring in the water column. However, heterogeneous distributions of phytoplankton cells in the water column on different temporal and spatial scales have been observed (e.g. CUSHING, 1989; BRUNET; LIZON, 2003). Many studies have used the biomass (estimated by the chlorophyll a concentration) as the main intrinsic property to describe the phytoplankton community dynamics during tidal cycles (DENMAN, 1977; FRECHETTE; LEGENDRE, 1982, CLOERN et al., 1989; DUSTAN; PINCKNEY, 1989; MONBET 1992), without taking into consideration the species-specific patterns related to these regular short-time events. Hence, studies based only on phytoplankton biomass variability throughout tidal cycles may overlook the short-term temporal changes in the species composition. The Bahia Blanca Estuary constitutes a mesotidal system with a semidiurnal cycle, the tides being the single most important input of energy (PERILLO; PICCOLO, 1991; PERILLO et al., 2004). The temporal and spatial dynamics of the phytoplankton in the estuary have been extensively studied (GAYOSO, 1998, 1999; POPOVICH; MARCOVECCHIO, 2008; POPOVICH et al. 2008a, 2008b) but none of these works has considered the short-term variability associated with the tidal cycle. The main goal of this study was to describe the temporal phytoplankton and physico-chemical dynamics over a short-time period at a fixed station under strong tidal influence. This study presents the first integrated description of daily phytoplankton-community dynamics together with physical and chemical variables in the Bahia Blanca Estuary. Beside its local relevance, it is also a contribution to the understanding of the short-term variability in estuaries characterized by high tidal energy. Bahia Blanca is a coastal plain estuary (38°45´S; 62°22´W) on the Atlantic coast of Argentina (Fig. 1). Tidal currents are reversible with maximum surface velocities of about 1.3 m s

Dynamic issues in the SE South America storm surge modeling

The southeastern coast of South America is located in one of the most cyclogenetic areas in the Southern Hemisphere. Occasionally, those systems may lead to disastrous flooding. Strong and persistent along-shore winds on the shelf may also result in some coastal flooding of smaller magnitude. Nested depth-averaged numerical models are used for the simulation of the storm surge. This study describes the storm surge dynamics in the area and the consequences of different regimes to the storm surge prediction. The large extension and limited depth of the Argentinean Shelf permits a smooth transition of the storm surge to the Rio de la Plata and a relatively long forecast horizon for its innermost zone. On the other hand, it is demonstrated that currents in the navigable channels of the Bahia Blanca estuary are the main cause for a strong tide–surge interaction. There, the relative phase between the meteorological forcing and the tide can shift the peak of the storm surge several hours.

Contribution of planktonic and detritic fractions to the natural diet of mesozooplankton in Bahía Blanca Estuary

The relative importance of phytoplankton and microzooplankton in the natural diet of mesozooplankton was assessed in Bahia Blanca Estuary, Argentina, in December 2005. Grazing experiments were performed using 200–2,000 μm grazers and natural food <100 μm. Individual and community filtration and ingestion rates were estimated for each food fraction after 24 h incubation. Abundance and carbon data of prey and grazers were qualitatively and quantitatively analyzed. Phytoplankton was mainly composed of diatoms and microzooplankton mainly of tintinnids. Both fractions were less abundant than detritus. Most of the grazers belonged to the copepod Acartia tonsa. Mean filtration and ingestion rates on phytoplankton + microzooplankton were 6.44 ml grazer−1 day−1 and 0.03 μg C grazer−1 day−1, respectively. This figure increased to 6.95 ml grazer−1 day−1 and 1.65 μg C grazer−1 day−1 when detritus was included. Mean carbon-specific ingestion rates on phytoplankton and microzooplankton were 0.006 and 0.005 μg C μg C−1 day−1, respectively, whereas after the addition of detritus, the overall rate increased to 0.588 μg C μg C−1 day−1. Highly significant differences were found between grazing rates on detritus and planktonic fractions. Consumers showed higher filtration rates on microzooplankton than on phytoplankton, although 78% of the cells ingested (54.7% μg C) came from the latter. The results point to a higher contribution of detritus to the natural diet of mesozooplankton in late spring. The omnivory of A. tonsa and the high turbidity of Bahia Blanca Estuary may explain the differences observed among food fractions in terms of carbon intake.

First record of the sea anemone Diadumene lineata (Verrill 1871) associated to Spartina alterniflora roots and stems, in marshes at the Bahia Blanca estuary, Argentina

We report the occurrence of the orange-striped green anemone Diadumene lineata (Verrill 1871) (=Haliplanella lineata) in salt marshes at the Bahia Blanca Estuary for the first time in August 2005. We also found this species attached to roots and stems of Spartina alterniflora, an association that has never been registered before. After their determination, sampling was performed during a year to evaluate seasonal abundance of this sea anemone. Results showed that D. lineata was present through the whole year, indicating the existence of a stable population. All individuals sampled were found attached to roots or stems of S. alterniflora, with the higher abundances detected in summer. Further studies are necessary to precise the potential effects of this exotic sea anemone on salt marsh communities.

Dissolved Nutrient Availability during Winter Diatom Bloom in a Turbid and Shallow Estuary (Bahía Blanca, Argentina)

Abstract The inner zone of Bahia Blanca Estuary is a shallow, well-mixed, highly turbid, and nutrient-rich temperate ecosystem. The phytoplankton annual cycle is characterized by a recurrent winter/early-spring diatom bloom. From May to August 2002 a research program aimed to describe the potential relations between the dynamics of phytoplankton and nutrients was carried out. Phytoplankton composition, abundance, chlorophyll a (Chl a), and dissolved inorganic nutrients (nitrate, nitrite, ammonium, phosphate, and silicate) were measured. The species involved were classified in blooming (including Thalassiosira spp. and Chaetoceros spp.) and nonblooming species (including other diatoms and dinoflagellates). The results show: (1) a prebloom period, which includes low biomass values and a strong decrease of NO3−; (2) an initial bloom period characterized by high diatom abundance and Chl a, a marked dominance of Thalassiosira curviseriata, and a strong decrease of NH4+; (3) an intermediate bloom period of high...

Incidental mortality of franciscana dolphin (Pontoporia blainvillei) in Argentina

La Plata River dolphin or franciscana, Pontoporia blainvillei , is an endemic small cetacean of the Southwest Atlantic coast. It is threatened all along its distribution by a sustained high level of incidental mortality in fisheries. Here we assess levels of franciscana bycatch in Argentine waters between 1997 and 2003. We surveyed 18 localities along the coast of the Buenos Aires Province, between General Lavalle (35°06'S, 57°08'W) and Bahia Blanca (Puerto Rosales-Ingeniero White harbour: 38°47'S, 62°16'W). We recorded data on incidental mortality, fishing gears and fishing effort through 209 personal interviews with fishermen. We estimated annual mortality, fishing effort and catch per unit of effort (CPUE) for each locality and period of time. Mortality was caused by gillnets and trawling gears, purse seine nets and shrimper gears. The total mortality estimated for 1997-2000 was 354 dolphins/year (95% CI = 318-392) and 307 dolphins/year (95% CI = 273-343) for 2002-2003. In the entire survey, CPUE of the northern coast of Buenos Aires Province (Bahia Samborombon and Cabo San Antonio) was significantly higher than CPUE for the southern coast (from Mar del Plata to Bahia Blanca estuary). In addition, CPUE of the northern coast decreased significantly throughout the years. This study suggested that even though the gears or fleet behavior changed locally, Buenos Aires Province evidenced an overall mortality relatively constant during the survey. If we consider a minimum of 400 dolphins killed each year in fishing gear and the estimated population values of 15000 individuals for the Argentine coast; mortality represent more than 2% of the Argentine franciscana population, suggesting that it would be subject to decline. Trends in mortality need to be periodically monitored in this area in order to articulate programs of conservation for the species.

Vulnerability to Sea-Level Rise on the Coast of the Buenos Aires Province

Abstract The purpose of this work is to identify and quantify those stretches in the Buenos Aires province that present different degrees of risk in view of the rise of the mean sea level and to determine its nature. The study focused on the Buenos Aires province coast because its variant geomorphology represents the different morphologies along the whole of the Argentinean coastline. The aim of this article is to study the response of two of the coastal vulnerability equations to the environmental diversity and to determine which one is more suitable to be applied to the rest of the country. On verifying the equations, CVI6 was found to be more appropriate for the analysis of coasts with different morphologies. Depending on the physical characteristics of each area, the consequences would include flooding and the loss of low lands in areas such as the Samborombon Bay, Bahia Blanca Estuary, and Anegada Bay, and the eroding of the beaches between Punta Rasa and Bahia Blanca.

Temporal variability of diversity and biomass of tintinnids (Ciliophora) in a southwestern Atlantic temperate estuary

Seasonal variations in diversity and biomass of tintinnids (Ciliophora: Tintinnida) were investigated at two fixed stations in the inner part of the Bahia Blanca Estuary (38°42' S, 61° 50' W) during an annual cycle. The variations were analysed in relation to surface temperature, salinity, transparency, solar radiation and chlorophyll a (Chl a)concentration. Biomass was calculated in terms of biovolume and carbon units. Diversity was estimated as the number of species and the Shannon Index (If, In based). Density of tintinnids ranged from 100 to 7800 individuals L - 1 H ranged from 0 to 1.81. The biomass varied from 0.3 to 127.78 x 10 6 μm 3 L - 1 (0.02-39.4 μg C L - 1 ). Density was significantly related to temperature, solar radiation and Secchi distance (P < 0.01); diversity was significantly related to temperature (P < 0.01) and solar radiation (P < 0.05). Biomass was significantly related only to temperature (P < 0.01) in one of the stations. According to principal components analysis (PCA) tintinnids exhibited a segregation of three groups: winter, spring-summer and autumn for the most internal station and winter, spring and summer-autumn for the most external station. H' values were lower than those observed in other coastal systems found at about the same latitude in the northern hemisphere.

Food and Feeding Habits of Flounder Paralichthys Orbignyanus (Jenyns, 1842) in Bahía Blanca Estuary, Argentina

Specimens of flounder Paralichthys orbignyanus (Jenyns, 1842) were collected in Bahia Blanca estuary between February 1997 and January 1998, and their feeding habits were examined in relation to season and size class. The stomach contents of 823 specimens, ranging from 70 to 875 mm total length, were analysed. Their diet included organisms from 17 taxa. The highest vacuity index values were found during autumn and winter. The stomach fullness index indicated that flounders increased their feeding activity between October and March, reaching a highest point in February and decreasing after February. Fish were the primary prey item in frequency, number and weight, followed by crustaceans, such as shrimps and crabs. A seasonal and size class variation was detected in the diet. During summer all-size flounders consumed mainly fish. In autumn, for all-size classes the main food were fish and crustaceans even though fish were dominant in terms of biomass. In spring, crustaceans (mysids Arthromysis magellanica (Cunningham), shrimps Artemesia longinaris Bate, and prawns Pleoticus muelleri (Bate)) were the dominant prey in terms of number and biomass for flounders ≤ 450 mm TL. Size classes larger than 451 mm TL fed on crustaceans and fish though the most important prey item in terms of biomass was fish. In Bahia Blanca estuary,P. orbignyanus evidenced mainly an ichthyophagou – carcinophagous diet. The results collected from the present study lead to conclude that P. orbignyanus shows, in this region, a clear preference for fish.