Blanca Estuary Research Articles

Mussels as Bioindicators of PAHs Pollution within Argentinean Coastal Environments, South America

The Bahia Blanca Estuary, located at the Atlantic coast of Argentina, includes a governmental nature reserve to protect biodiversity, lands and water; however, as the process of industrialization and urbanization is stepping rapidly at the neighbouring area, the potential increase in contaminant loads is a serious concern. After several contaminants surveys at the estuary, no single study has been yet performed in order to study the native mussels population vs. the chemical environment; then, to test the ability of Brachidontes rodriguezii assemblages as pollution indicators, native populations were chosen at three different sites from the Bahia Blanca Estuary. After quadrants sampling, the populations were assessed for size classification, density, sex rate, surrounding sediment and body burden of Polycyclic Aromatic Hydrocarbons (PAHs) as chemical pollution markers. Soxhlet extracted samples were measured through Capillary Gas Chromatography (Hewlett-Packard P68906C)/Mass Spectrometry (HP 5972). Mussels appeared to be better pollution indicators than sediments and could be used to classify the average pollution level at the area as “moderate” along the studied period. A positive correlational analyses was found between the population assemblage disruption vs. PAH’s tissue burden, supporting the suitability of this specie to be used as biomonitor. In addition, source apportionment procedures could successfully discern between PAHs petrogenic and combustion inputs, leading to tag each site with the major contributing PAHs sources. Finally, screening values analysis concluded a certain health risk to potential consumers; thus, highlighting the urgent need of a systematic monitoring program.

Exceptionally high Cd levels and other trace elements in eggshells of American oystercatcher (Haematopus palliatus) from the Bahía Blanca Estuary, Argentina

Concentrations of six trace metals were determined in the eggshells of American oystercatchers (Haematopus palliatus) from the Bahía Blanca estuary, Argentina. All metals measured in selected samples presented concentrations above the detection limit. Means found for Cu, Pb, Cr, Zn and Ni were as follow: 2.02±0.52, 7.23±2.33, 0.78±0.03, 2.22±1.13 and 6.05±0.89mg/kg dw. The mean concentrations of Cd found were surprisingly high: 13.28±3.38mg/kg dw. Previous studies made on prey items of the American oystercatchers showed low to medium concentrations of the six trace metals. This may indicate a possible transfer of the metals that are available in the environment through food chains. Our study indicates that American oystercatchers sequester heavy metals in their eggshells. Therefore the eggshells may be useful as biomonitors for trace metal contamination in the Bahía Blanca estuary.

Morphology and dynamics of large subtidal dunes in Bahia Blanca estuary, Argentina

The purpose of this paper is to carry out a detailed analysis of subtidal dune morphology and temporal variability and to estimate dune migration rate to improve knowledge of this topic, and so enhance the existing data on different marine environments in the world and especially in South America where this information is limited. Two swath bathymetry surveys were conducted across a dune field in the Bahia Blanca Estuary (Argentina). Morphometric parameters and migration rate according to the dune type, were analyzed. The field is composed of large dunes exhibiting two morphological configurations, which are differentiated into sinuous and barchan dunes. The dunes studied are the largest of the estuary, with heights and wavelengths greater than 5m and 130m, respectively. The crests of the large dunes are arranged with an orientation perpendicular to the axis of the channel. From geometrical analysis of the parameters, the dunes show a weakly positive correlation between dune height and wavelength as too between dune height and water depth. No clear relationship was observed between maximum height and wavelength parameters with water depth. Across the estuary, the bedforms migrate in the ebb direction, with mean rate of 43myear−1. Comparison of our results with previous data shows that during three decades the western boundary of dune field has been displaced 900m towards the outer estuary, however the dune field configuration and distribution of diverse types of bedform appear to be relatively stable.

Distribution, Sources, and Potential Ecotoxicological Risk of Polycyclic Aromatic Hydrocarbons in Surface Sediments from Bahía Blanca Estuary, Argentina.

Thirty-four surface sediment samples were collected from Bahía Blanca Estuary, Argentina, to evaluate polycyclic aromatic hydrocarbon (PAH) contamination and ecotoxicity risk by applying sediment-quality guidelines (SQGs) and toxic equivalent factors (TEQ). Total concentrations of 17 parent PAHs, including the 16 United States Environmental Protection Agency priority PAHs, were measured using gas chromatography-mass spectrometry, and their levels ranged from 19.7 to 30,054.5ng/g dry weight. The greatest values were found near the urban/industrial core and decreasing as the distance from that site increased. Molecular ratios determined mixed sources of PAHs with a slight imposition of pyrolitic over the petrogenic inputs. The ecotoxicological evaluation, based on the SQG model, showed that some of the individual PAHs were in excess of the effects range low (ERL) and the effects range median's threshold; then, predicted occasional (ERL) and frequent adverse effects over the surrounding biota at the area of study were determined. Total PAH levels were expressed as benzo-a-pyrene TEQ and compared with literature data.

Tidal effects on short-term mesozooplankton distribution in small channels of a temperate-turbid estuary, Southwestern Atlantic

Abstract The short-term variability of mesozooplankton distribution and physicochemical variables was examined in two different channels of the Bahía Blanca Estuary, Argentina, during two tidal cycles. All the physicochemical measurements and mesozooplankton sampling were performed at a fixed site during approximately 22-23 h at 3-h intervals. Pumps were used to obtain surface and bottom mesozooplankton samples and the water speed of each stratum was measured with an Acoustic Doppler Current Profiler (ADCP). In all, 23 mesozooplanktonic taxa belonging to four phyla (Arthropoda, Annelida, Echinodermata and Chordata) were identified. The most abundant taxa during the two tidal cycles were Balanus glandula larvae, Eurytemora americana and Acartia tonsa. A discernible variability in the water conditions and vertical mesozooplankton distribution (VMD) different from that known for the estuary's main channel, was found in the other two selected channels. VMD varied during the tidal cycle in both channels in accordance with the channel's geomorphology and water dynamic characteristics of each of them. The variation of the abundance of the different taxa during ebb and flood currents might indicate the existence of a tidal vertical migration of the mesozooplankton as a response to particular dynamic water conditions.

Dissolved and particulate metals dynamics in a human impacted estuary from the SW Atlantic

In order to evaluate metal behavior in urban stressed estuaries, the distribution of major elements (Fe and Mn) and trace elements (Cd and Cu) between suspended particulate matter (SPM) and subsuperficial seawater in the Bahía Blanca Estuary, Argentina, was studied. Four different impacted areas were selected to study the spatial and temporal distribution of these metals in an estuary in continuous industrial development and where an environmental law was implemented to supervise industrial discharges in waters. Sampling was performed within intertidal areas. Physicochemical conditions usually influence the partitioning of metals between the dissolved and particulate fraction thus, salinity, pH, turbidity, temperature and dissolved oxygen were also measured.Dissolved metals were analyzed with atomic absorption spectrophotometry (AAS) and the particulate fraction with inductively coupled plasma optical emission spectrometry (ICP OES). Metals concentration ranges, within the dissolved fraction (μg/L), were from below the method detection limit for all the elements to 4.7 in the case of Cd, 6.0 for Cu and 62 for Fe. Minimum and maximum values in the particulate fraction (μg/g, d.w.) were from below the method detection limit to 11 for Cd; from 24 to 220 for Cu and from 630 to 1500 for Mn. For Fe, concentrations ranged from 2.2 to 9.6 (%). The general order of the dissolved/particulate partition coefficients (Log10Kds) for the studied metals, considering mean values, were: Fe (7.0) > Cu (4.2) > Cd (3.3).The metals values as well as the physicochemical parameters showed temporal variations and many correlations were found among them. Log10Kd Fe values were the highest, highlighting its strong affinity for particles. Metals concentrations were sometimes higher than those from other polluted areas as well as from previous studies from the same estuary, which highlights the potential impact of these elements in the study area.The concentrations of particulate metals achieved in the present work were in some cases of higher ranges than the previous from the same estuary and/or from other polluted estuarine environments. Thus, the environmental law that regulates the discharge of industrial waters appeared not to have a specific impact on the metals concentrations found. Moreover, human activities that surround the estuary and are in continue development should be considered as they might constitute a source of metals for the estuarine system.

Microplankton dynamics under heavy anthropogenic pressure. The case of the Bahía Blanca Estuary, southwestern Atlantic Ocean

Quantifying biotic feedbacks in response to environmental signals is fundamental to assess ecosystem perturbation. We analyzed the joint effects of eutrophication, derived from sewage pollution, and climate at the base of the pelagic food web in the Bahía Blanca Estuary (SW Atlantic Ocean). A two-year survey of environmental conditions and microplankton communities was conducted in two sites affected by contrasting anthropogenic eutrophication conditions. Under severe eutrophication, we found higher phytoplankton abundance consistently dominated by smaller sized, non siliceous species, while microzooplankton abundance remained lower and nutrient stoichiometry showed conspicuous deviations from the Redfield ratio. Phytoplankton growth in such conditions appeared controlled by phosphorous. In turn, microplankton biomass and phytoplankton size ratio (<20μm:>20μm) displayed a saturation relationship with nutrients in the highly eutrophic area, although mean phytoplankton growth was similar in both eutrophic systems. The strength of links within the estuarine network, quantified through path analysis, showed enhanced relationships under larger anthropogenic eutrophication, which fostered the climate influence on microplankton communities. Our results show conspicuous effects of severe sewage pollution on the ecological stoichiometry, i.e., N and P excess with respect to Si, altering nutrient ratios for microplankton communities. This warns on wide consequences on food web dynamics and ultimately in ecosystem assets of coastal pelagic environments.

Variations in macrobenthic community structure in relation to changing environmental conditions in sandy beaches of Argentina

This study describes for the first time the intertidal macrobenthic community of exposed sandy beaches located near the Bahía Blanca Estuary (38°S) and reports the physical characterization of this coastal fringe. The main objective of the study was to link environmental variables to biotic information, analyzing the results in the context of the Swash Exclusion Hypothesis (SEH) and possible estuarine influence. Four beaches were sampled seasonally at different distances from the mouth of the Bahía Blanca Estuary. To characterize the morphodynamic state of the beaches, the Dean parameter (Ω) was calculated. Multivariate analyses were used to assess benthic community structures and their relationships with physical variables. The two beaches located closest to the Bahía Blanca Estuary were classified as intermediate and those located further from the estuary as dissipative. Richness, diversity and biomass of intertidal macrobenthic communities varied with the SEH, increasing towards the dissipative beaches. However, total density was higher on intermediate beaches, possibly because of nutrient-rich silt-clay sediment input from the estuary, enabling them to maintain a higher density of organisms than dissipative beaches. The estuary acts as a moderator of habitat hardness, which together with the morphodynamic state of the beaches is an important factor in the structuring of the macrobenthic community along this coastal fringe.

Microscopic Analysis of Mustelus schmitti´s Uteri During Reproductive Cycle

Aims: It was analysed with light and electron microscopy the different zones of the Mustelus schmitti`suteri along the reproductive cycle. Study Design: Histological routine techniques were used. Place and Duration of Study: Specimens of Mustelus schmitti were collected from the Bahia Blanca estuary, each month, from 2002 to 2006. They were caught by longlines. The fishing was made at the top of the tide during the day. Methologogy: A total of 24 animals were collected and processed; 11 were immature, 6 were maturing and 7 mature females; 4 of this last group were pregnant. Each animal was humanely sacrificed. A longitudinal ventral incision was made in each animal so as to remove the reproductive organs. The pieces were stored in Bouin solution for light microscopy and in 2.5% glutaraldehyde in 0.1 M cacodylate with 12% saccarose for transmission. Results: In immature animals, the simple columnar epithelium shows small and numerous folds, PAS (-) and AB (-). Beneath this epithelium, muscular fibers in longitudinal disposition alternate with connective tissue, finishing in a simple serosa. The uterine epithelium of the anterior portion in mature females, between the isthmus and the cervix, is stratified, PAS (+) and AB (+). Three muscle layers continue and the first one displays a disposition like a net. The second layer has longitudinal fibers divided into two by vascularized connective tissue and the third, is very thin. A typical serosa encloses the organ. In the posterior Original Research Article Elias; ARRB, 6(1): 20-32, 2015; Article no.ARRB.2015.060 21 portion of the uterus in mature females, the epithelium is also stratified but with more secretive cells. The presence of sperm was detected in the cervix and in the posterior portion of the uterus suggesting a special uterine epithelial-sperm interaction. Conclusion: The uteri suffer a cyclic remodelation not only in the epithelia but in the muscle layers.

First molecular identification of Ascocotyle (Phagicola) longa in its first intermediate host the mud snail Heleobia australis.

This is the first study that used species-specific DNA primers to confirm the presence of the heterophyid Ascocotyle (Phagicola) longa Ransom, 1920 in its first intermediate host. The larval stages (rediae and cercariae) of this parasite were morphologically and genetically identified in the gonad of the intertidal mud snail Heleobia australis (d'Orbigny, 1835) (Cochliopidae) in the Bahía Blanca estuary, Argentina. In addition, we asked whether the prevalence in H. australis varied between seasons. Mullets - the second intermediate host of this heterophyid - migrate in estuaries during the warmer seasons and it is expected that piscivorous birds and mammals - the definitive hosts - prey more intensively on this species at those times. Thus, the number of parasite eggs released into the tidal flat within their feces should be higher, thereby increasing the ingestion of the parasite by H. australis.We therefore expected a higher prevalence of A. (P.) longa in H. australis in the Bahía Blanca estuary during spring and summer than autumn and winter. We found that 16 out of 2,744 specimens of H. australis had been infected with A. (P.) longa (total prevalence of 0.58%). Nonetheless, the prevalence showed no significant variation between seasons. Hence, we discuss an alternative scenario where the lack of seasonal changes might be mostly related to the permanent residence of definitive hosts in the estuary and not to the seasonal recruitment of mullets. Finally, we highlight the need for more experimental and comparative approaches in order to understand the diagnosis and geographical distribution of this worldwide heterophyid.

Role of Nutrients in Phytoplankton Development during a Winter Diatom Bloom in a Eutrophic South American Estuary (Bahía Blanca, Argentina)

Spetter, C.A.; Popovich, C.A.; Arias, A.; Asteasuain, R.O.; Freije, R.H., and Marcovecchio, J.E., 2015. Role of nutrients in the phytoplankton development during a winter diatom bloom in a eutrophic South American estuary (Bahía Blanca, Argentina).The Bahía Blanca Estuary is considered highly eutrophic. Long-term studies have shown that the winter–early spring bloom can be considered the development of a diatom assemblage with Thalassiosira curviseriata as the dominant species. Since 2003, several changes have been observed in the annual pattern of nutrients and phytoplankton. To assess the availability of nutrients and their relationship with phytoplankton development, nutrient variability was studied during a winter bloom in 2003 (April 22–September 4). Nitrite, nitrate, ammonium, phosphate, silicates, chlorophyll-a, phytoplankton abundance, particulate organic matter, and physicochemical parameters were measured in surface water at Cuatreros Port. In the Sauce Chico River mouth, we determined the concentration of nutrients to estimate the input of this river. The results of this study were compared with those of a previous usual diatom bloom from 2002. In 2003, the bloom was dominated by the Cyclotella sp. with high chlorophyll-a concentrations (26.5–40.4 μg L−1). Thalassiosira curviseriata was present only at three sampling dates, reaching up to 19% of the total abundance. Mean values of dissolved oxygen, nitrate, oxygen saturation percentage, and salinity were significantly higher in 2003 than in 2002, while N:P ratios were significantly lower. Si was never limiting. A shift in the limiting nutrient between the years (N in 2003 and P in 2002) could have lead to a change in species dominance during the blooms. Results suggest that the nutrients levels in the coastal ecosystems at Cuatreros Port play an important role in the control of phytoplankton dynamics during the productive period and partially explain its interannual variability.

Effect of Spartina alterniflora Loisel, 1807 on growth of the stout razor clam Tagelus plebeius (Lightfoot, 1786) in a SW Atlantic estuary

Aquatic organisms that produce epibenthic structures such as saltmarsh plants play key roles in habitat functioning through their ecosystem engineering effects. Intertidal Spartina species are well known for modifying tidal hydrodynamic and sediment processes, affecting cohabiting infaunal organisms. The stout razor clam Tagelus plebeius is an infaunal filter-feeding species that inhabits a broad range of mudflats (MF) and Spartina saltmarshes (SM) along the American Atlantic coast. Through descriptive and experimental approaches conducted at the Bahía Blanca estuary (Argentina, 38°47′S, 62° 20′W), we evaluated ecosystem engineer effects of Spartina alterniflora (due to changes in sediment and water characteristics) on somatic and shell growth of T. plebeius. Comparison of bulk sediment parameters indicated that the sediment is more easily eroded in the SM than in the MF, but softer in the latest. In addition, food supply for clams (i.e., total particulate organic matter) was higher in the MF although food quality (i.e., percentage of organic matter in the water column) was in general higher in SM. Shell growth was higher but clam condition index (CI) was lower in the MF, and the opposite pattern was found in the SM. These results represent an uncoupling between shell and somatic growth with a positive effect of S. alterniflora on the CI of clams. Nevertheless, results of field experiments manipulating stems and roots of plants separately suggested that the effect of Spartina on clam's CI was not due to the structural changes generated by plant presence itself, although a better food quality in the SM may be related to the larger clams CI.

Biomass, decomposition and nutrient cycling in a SW Atlantic Sarcocornia perennis marsh

Biomass dynamics, decomposition and nutrient cycling were studied in a Sarcocornia perennis salt marsh in the Bahia Blanca estuary (Argentina) to achieve a better understanding of these processes and provide information about a species and a region underrepresented in the literature. Above and belowground biomass stocks and carbon (C), nitrogen (N) and phosphorus (P) concentration in plant tissues were monitored every 2months during a year. The decomposition rate and the concentration of C, N and P during the process were also estimated in above and belowground tissues. Biomass values were low (mean of 363±43 and 242±27gm-2 for aboveground and belowground tissues, respectively), presumably associated with the high salinity of this estuary. The general trend of higher values for aboveground biomass is in agreement with other reports for this species and has an effect on nutrients pools, which are higher for aboveground tissues for C and N. Above and belowground decomposition rates were high (64 and 70% after a year, respectively), meaning this process plays a significant role in the cycling of organic matter. C/N and C/P ratios changed during decomposition, but final ratios were usually higher, suggesting a net release of nutrients. Our results indicate that significant amounts of C, N and P are recycled by S. perennis, highlighting the role of this species and suggesting important consequences of its lost in the study area.

Assessment of the physicochemical conditions sediments in a polluted tidal flat colonized by microbial mats in Bahía Blanca Estuary (Argentina)

The aim of this work is to assess the physicochemical conditions of the supratidal sediments colonized by microbial mats at two sites from Rosales Harbor (Bahía Blanca Estuary, Argentina) close to sewage discharge. Both sites differed in the size grain. No differences in pH, Eh and temperature were observed. Moisture retention and chlorophyll a concentration were significantly different between sites and sediment layers. Heavy metals and organic matter content were significantly higher in SII. No statistical differences were found in porewater nutrients concentration, being higher in SI (except DSi). The presence of Escherichia coli in water and sediment (1000CFU/100mL – uncountable and 35–40CFUg−1dw, respectively) evidenced microbial contamination in the study area. The relationships between the physicochemical parameters evaluated and the influence of the sewage discharge allow defining two different areas in the Rosales Harbor despite the proximity and the presence of microbial mats.

Influence of the winter phytoplankton bloom on the settled material in a temperate shallow estuary

Summary The development of the phytoplankton winter bloom and the accumulation of particulate suspended matter (PSM) inside sediment collectors were assessed in the inner zone of the Bahia Blanca Estuary. The phytoplankton bloom (chlorophyll up to 25 μg l −1 and abundance up to 8 × 10 6 cells l −1 ) was related with high levels of dissolved inorganic nutrients and underwater light availability ( I m up to 355 μE m −2 s −1 ) and was dominated by relatively small diatoms, e.g. Chaetoceros sp.1 (3–8 μm). Conversely, large planktonic diatoms, mostly Thalassiosira spp. 20–60 μm, were found in the accumulated material inside the collectors, together with benthic microalgae and high concentrations of chlorophyll, phaeopigments, particulate organic matter (POM between 18 and 32% of total PSM) and C:N ratios >12. The composition of the settled material indicated vertical exportation of phytoplankton to the benthos, external loads of detritus and bottom resuspension. The present study highlights the close benthic‐pelagic interactions in shallow coastal environments characterized by high productivity.

A new species of Quinquelaophonte (Copepoda: Harpacticoida) from Argentina

Quinquelaophonte aestuarii sp. nov. (Copepoda, Harpacticoida) from Bahía Blanca estuary is the first record of Quinquelaophonte Wells, Hicks & Coull, 1982 in Argentina and its southernmost location in the world. The setal formula (P2: exopod 1.2.3-endopod 1.2.0; P3: exopod 1.2.3-endopod 2.2.1; P4: exopod 1.2.3-endopod 1.2.0) of the last segment of P2-P4, in both the exopod and the endopod, distinguishes this species from all known species in Quinquelaophonte, except for Quinquelaophonte varians Bjornberg, 2010. The new species differs from the latter in the setal formula of the antennule, the mouth parts and maxillipeds, the absence of an inner seta on the second exopod segment of P2, the length of the second exopod segment of P4 and in the shape of baseoendopod setae of P5.

Biogeomorphically driven salt pan formation in Sarcocornia-dominated salt-marshes

Salt-marshes are under increasing threat, particularly from sea-level rise and increased wave action associated with climate change. The development and stability of these valuable habitats largely depend on complex interactions between biotic and abiotic processes operating at different scales. Also, interactions between biotic and abiotic processes drive internal morphological change in salt-marshes. In this paper we used a biogeomorphological approach to assess the impact of biological activities and interactions on salt pan formation in Sarcocornia-dominated salt marshes. Salt pans represent a key physiographic feature of salt-marshes and recent studies hypothesized that biogeomorphic processes could be related to salt pan formation in SW Atlantic salt-marshes. The glasswort Sarcocornia perennis is one of the dominant plants in the salt-marshes of the Bahía Blanca Estuary (Argentina) where they form patches up to 8m in diameter. These salt-marshes are also inhabited in great densities by the burrowing crab Neohelice (Chasmagnathus) granulata whose bioturbation rates are among the highest reported for salt-marshes worldwide. A set of biological interactions between N. granulata and S. perennis appears to be responsible for salt pan development in these areas which has not been described elsewhere. The main objective of this work was to determine the ecological interactions occurring between plants and crabs that lead to salt pan formation by using field-based sampling and manipulative experiments. Our results showed that S. perennis facilitated crab colonization of the salt-marsh by buffering otherwise stressful physical conditions (e.g., temperature, desiccation). Crabs preferred to construct burrows underneath plants and, once they reach high densities (up to 40 burrowsm−2), the sediment reworking caused plant die-off in the central area of patches. At this state, the patches lose elevation and become depressed due to the continuous bioturbation by crabs. Thus, salt pans are generated in this case by a set of biogeomorphic processes that include pure ecological interactions such as plant facilitation of crab settlement and also indirect negative effects of crabs on plant survival. Furthermore, crab bioturbation affects sediment structure due to concentration of burrowing activity under plant canopies promoting elevation loss and leading, after a few years, to salt pan formation in a previously vegetated substrate.

Influence of Experimental Thermal Shifts and Overcrowding on Fecundity in Wild Females of Acartia Tonsa of the Bahía Blanca Estuary

The copepod Acartia tonsa plays an important role in the food web of many estuaries influenced by changes due to anthropic activities, hence it is relevant to know its reproductive plasticity under stressful conditions. We examined the influence of thermal shift (6 ± 1ºC and 18 ± 1ºC) and overcrowding stress on egg production and hatching success in wild females of A. tonsa of the Bahía Blanca Estuary. Egg production rate (EP), hatching success during seven days (E7 %) and time taken to hatch 50 % of eggs (T 50) were registered. The high individual density used in the incubations caused a decrease in EP values at both temperatures. Nevertheless, all the females responded favorably to 18°C. EP and E7 % obtained at both temperatures were significantly different the values at 18ºC, being twice as high as those obtained at 6°C. T 50 values at 18ºC indicated a shorter developmental time in eggs from spring females, whereas the opposite effect was observed in eggs from winter females. Our results indicate that the reproductive performance is affected by short exposures to thermal shift. The pattern of egg production and hatching is also modified in accordance with the maternal history.

Morphological indicators of initial reproductive commitment in Mustelus schmitti (Springer 1939) (Chondrichthyes, Triakidae): folliculogenesis and ovarian structure over the life cycle.

This work provides information about the sexual commitment and the folliculogenesis of the gatuzo, Mustelus schmitti. A total of 112 females of all maturity stages were fished in the Bahía Blanca estuary, between 2009 and 2010. The oogonia were present throughout the life cycle of the animals. The folliculogenesis follows a pattern similar to other elasmobranchs. The granulosa layer keeps monolayered throughout the folliculogenesis, but with two cell types in the vitellogenic follicle. The zona pellucida forms in the primordial follicles. The thecal system shows a connective inner layer and a glandular outer sheath. The microscopic beginning of the sexual commitment, indicated by the vitello hoarding, takes place in follicles from 500 micrometres, while the macroscopic evidence appears in follicles of 2500-3000 micrometres. The results presented in this study suggest that the fishery pressure may affect a susceptible range of sizes of the species, not previously considered and provides a biological framework for the development of fisheries policy.

Seismic Stratigraphy of Pleistocene Deltaic Deposits in Bahía Blanca Estuary, Argentina

The Bahía Blanca estuary (Argentina) has a morphological configuration resulting from hydrological and sedimentary processes related to Late Quaternary sea level changes. This estuarine system occupies a large coastal plain with a dense net of tidal channels, low-altitude islands and large intertidal flats. Little is known about the sedimentary units of the marine subbottom. Therefore, a stratigraphical analysis of the northern coast of Bahía Blanca estuary was carried out using high resolution seismic (3.5 kHz) in order to: i) define Quaternary sequences, ii) describe sedimentary structures, and iii) determine the paleoenvironmental conditions of sedimentation. The seismic stratigraphic data collected and their correlation with drilling lithological data show five seismic sequences (S1, S2, S3, S4 and S5), of which S1-S2 were found to be associated with a continental paleoenvironment of Miocene-Pleistocene age. Sequences S3 and S4, whose lithology and seismic facies (paleochannel structures and prograding reflection configurations), were defined on these materials, to evidence the development of an ancient deltaic environment which was part of a large Pleistocene drainage system. The S5 sequence was formed during the Holocene transgressive-regressive process and complete the seismostratigraphic column defined in the present study.