Rio De La Plata Estuary Research Articles

Time series modeling of coastal fishery landings on the Southwestern Atlantic shelf: Influence of environmental drivers

AbstractTime‐series modeling of fisheries provides insights into stock tendencies and enables short‐term forecasting of landings, aiding decision makers in establishing management priorities. The Rio de la Plata Estuary and its maritime front sustain valuable fisheries for Argentina and Uruguay, with striped weakfish (Cynoscion guatucupa), whitemouth croaker (Micropogonias furnieri), and Argentine hake (Merluccius hubbsi) historically representing highest catches. However, their landings have declined in recent decades. To support resource management, we investigated the effectiveness of Autoregressive Integrated Moving Average (ARIMA) models in capturing fishery landing dynamics and providing reliable short‐term predictions. The best models exhibited a good fit and accurately captured the overall trends of landings. Residual variability unaccounted for by the model was analyzed in relation to time‐lagged environmental conditions. A wavelet coherence analysis was employed to examine the effect of the most significant variables on landings. Results revealed that environmental conditions affect differentially landings of each species as a result of their particular ecological traits. Turbidity and salinity affected mainly M. furnieri, which inhabits the innermost part of the estuary. Additionally, C. guatucupa, inhabiting the outer estuary and coastal region, exhibited a stronger association with river runoff compared to M. hubbsi, which inhabits the continental shelf. This study provides the first evidence of ARIMA models' reliability in representing the temporal evolution of catch in these fisheries, offering valuable tools for short‐term landings forecasting and facilitating sustainable management. Wavelet analysis findings will also contribute to enhancing our comprehension of trends in the correlation between environmental conditions and commercial landings.

Modeling environmental effects on fishery landings: A case study of whitemouth croaker (Micropogonias furnieri) in the Southwestern Atlantic shelf

Estuaries are very productive habitats that serve as spawning and nursery grounds for fisheries species. The Rio de la Plata (RdP) Estuary and its maritime front sustain valuable fisheries for Argentina and Uruguay. Micropogonias furnieri, the coastal fish species historically representing highest catches, has declined landings in recent decades. To enhance our understanding on how natural variability influences this resource and improves its management, we analysed the effect of environmental conditions on M. furnieri’s commercial landings in the long term using generalized linear and additive models (GLMs and GAMs). Explanatory variables, with up to a 10-year lag, included sea surface temperature, turbidity, salinity, wind, river runoff, and climatological indices: El Niño-Southern Oscillation (ENSO), the Southern Annular Mode (SAM) and the Southern Oscillation Index (SOI). Time-lagged analyses for single-covariate models reveal higher deviances explained (DE) at 2, 6, 7, and 8-year lags. Turbidity and salinity, followed by wind and temperature, are the most common variables affecting catches of M. furnieri for GLMs and GAMs. The absence of significant models with only climatological indices or river discharge indicates the impact of large-scale climate fluctuations and river runoff on the RdP Estuary conditions. The best-fitting GLM model includes SAM, turbidity, and wind (DE = 78.8%), whereas the GAM model includes turbidity and wind (DE = 86.4%). Sensitivity of M. furnieri’s catches to climatic signals (ENSO, SAM, SOI) is described for the first time. Our findings provide insights for authorities to implement management measures beyond fishing quotas encompassing environmental shifts in the long-term.

Identification of long-preserved specimens reveals the historical geographic range of the Patagonian lamprey Geotria macrostoma (Burmeister, 1868) in southern South America.

The lamprey genus Geotria Gray, 1851 currently includes only two species: G. australis and G. macrostoma. However, taxonomic relationships within the genus have traditionally been ambiguous and difficult to establish due to the extreme changes in morphology, dentition, and coloration that lampreys undergo during their life cycles, particularly during upstream migration and sexual maturation. Consequently, several lamprey specimens held in museum collections have remained unidentified, especially those from Argentina. In this study, a series of morphometric characters were subjected to discriminant function analysis (DFA) to identify the lamprey species collected during 1867-2004 from the de la Plata River and Patagonia. These specimens are housed at the Museo Argentino de Ciencias Naturales "Bernardino Rivadavia" in Buenos Aires, the Museo de Historia Natural de Montevideo, and the Naturhistoriska riksmuseet in Stockholm. Based on the proportions of the length of the oral disc, prebranchial, and pre-caudal body regions, and the depth of the trunk, DFA provided conclusive evidence that the specimens corresponded to the recently revalidated G. macrostoma (Burmeister, 1868), which was originally incorrectly named as Petromyzon macrostomus Burmeister, 1868, Exomegas macrostomus (Berg, 1899), Geotria chilensis (Berg, 1895), and Geotria macrostoma f. gallegensis Smitt, 1901, as well as other nontype museum individuals of uncertain taxonomic status. The identifications of these long-preserved museum specimens provided key information on the historical geographic range of Argentinian lampreys and suggest that the disappearance of the species reported from northern localities (the Pampean Region) can be attributed to the degradation of their critical habitats, primarily caused by anthropogenic impact and climate change.

Influence of hydrological changes on the composition of phytoplankton and sediment diatoms in the Rio de la Plata estuary

ABSTRACT ENSO phases produce changes in precipitation and river flow in Eastern South America. We aimed to evaluate phytoplankton and surface sediment diatoms' response to ENSO within the Río de la Plata estuary. A 180-km-long transect off the Uruguayan coast was undertaken in May 2009 (La Niña) and May 2010 (El Niño), where physical, chemical and biological variables were measured. During El Niño we registered lower temperatures/salinities throughout the transect, and the riverine zone was specifically more homogeneous in terms of temperature, salinity, turbidity and chlorophyll-a than during La Niña. We observed changes in the species composition of both planktonic and sediment diatoms. Concomitantly, a significant depletion in planktonic diatom abundance and richness in both planktonic and benthic domain during El Niño was observed. In addition, there was a higher proportion of dinoflagellates than during La Niña, which was explained by a higher water column stability. The dominance of the chain-forming and meso-eutrophic diatom Aulacoseira was registered during El Niño in both domains, mainly in the fluvial zone, related to a high river discharge. Our findings are important to anticipate changes in microalgae composition related to oceanographic variability during potential ENSO events.

Status of the invasion of Carpobrotus edulis in Uruguay based on citizen science records

Carpobrotus edulis, a highly invasive plant species repeatedly introduced along the Atlantic coast of South America, poses a significant threat to the ecological integrity of coastal dune ecosystems in Uruguay. This study used 15 years of iNaturalist records to assess the magnitude of Carpobrotus invasion, focusing on its distribution, abundance, and reproductive phenology. Through the analysis of georeferenced and dated data, we determined that Carpobrotus has spread extensively, covering a 10-km-wide coastal area and occupying approximately 220 km along the Atlantic coast and the outer Rio de la Plata estuary. Records have increased in the last three years, with a summer prevalence of 52.9%, due to higher activity on the platform. The species exhibited two flowering peaks in spring and autumn, suggesting an extended reproductive period. The widespread presence of this species in both natural and urbanized areas highlights the urgent need for effective management strategies to mitigate its impact on native biodiversity. Utilizing citizen science platforms like iNaturalist has been instrumental in monitoring and documenting processes of invasion. However, further investigations are necessary, particularly in less sampled regions, to fully understand the extent of Carpobrotus invasion. To protect the country’s coastal ecosystems, it is crucial for local authorities to reassess current practices related to non-native species ornamental planting. By addressing the invasion of C. edulis, the ecological integrity of coastal dune ecosystems can be preserved, ensuring the survival of native flora and fauna along Uruguay’s Atlantic coast.

Antarctic Minke Whale (Balaenoptera bonaerensis) Bio-Duck Call Detection in the Río de la Plata, Uruguay

Passive acoustic monitoring (PAM) has become a popular method for studying both marine mammal behavior and distribution, especially among cetaceans. The Antarctic minke whale (Balaenoptera bonaerensis) occurs in the Southern Hemisphere, and our knowledge concerning its vocal behavior is very limited. Along the coast of Uruguay, minke whales constitute one of the species with the most stranding data collected. Until now, sounds of Antarctic minke whales had never been recorded off the Uruguayan coast. Two different bio-duck call types were recorded along the Atlantic Ocean and the Río de la Plata River coasts by employing PAM stations off the coast of Uruguay in five different positions. One call was recorded off the Atlantic coast, and the four other calls were recorded along the Río de la Plata River, including the classic bio-duck call. In addition to the aforementioned bio-duck call, evidence of a novel call, the bio-duck pulse train, was found. This call was produced in an environment very different from the oceanic environment. The bio-duck pulse train has never been described in previous works, making the findings of the present work all the more novel.

Microbiological hazard identification in river waters used for recreational activities

Pathogenic bacteria, viruses, and parasites can cause waterborne disease outbreaks. The study of coastal water quality contributes to identifying potential risks to human health and to improving water management practices. The Río de la Plata River, a wide estuary in South America, is used for recreational activities, as a water source for consumption and as a site for sewage discharges. In the present study, as the first step of a quantitative microbial risk assessment of the coastal water quality of this river, a descriptive study was performed to identify the microbial pathogens prevalent in its waters and in the sewage discharged into the river. Two sites, representing two different potential risk scenarios, were chosen: a heavily polluted beach and an apparently safe beach. Conductivity and fecal contamination indicators including enterococci, Escherichia coli, F + RNA bacteriophages, and human polyomaviruses showed high levels. Regarding enterococci, differences between sites were significant (p-values <0.001). 93.3% and 56.5% of the apparently safe beach exceeded the recreational water limits for E. coli and enterococci. Regarding pathogens, diarrheagenic E. coli, Salmonella, and noroviruses were detected with different frequencies between sites. The parasites Cryptosporidium spp. and Giardia duodenalis were frequently detected in both sites. The results regarding viral, bacterial, and parasitic pathogens, even without correlation with conventional indicators, showed the importance of monitoring a variety of microorganisms to determine water quality more reliably and accurately, and to facilitate further studies of health risk assessment. The taxonomic description of microbial pathogens in river waters allow identifying the microorganisms that infect the population living on its shores but also pathogens not previously reported by the clinical surveillance system.

Assessment of meso- and microplastics distribution in coastal sediments and waters at the middle estuary of the Rio De La Plata, Argentina (SW Atlantic Ocean)

Estuarine coastal water and sediments collected from multiple locations within the middle Río de la Plata (RDLP) estuary were analyzed in order to identify the presence of microplastics (MPs, <5 mm) and mesoplastics (MePs, 5–25 mm) in one of the most significant estuaries in the Southwestern Atlantic. The present study represents one of the first researches to survey MPs and MePs contamination in key stations at RDLP estuary. Average concentrations of 14.17 ± 5.50 MPs/L and 10.00 MePs/L were detected in water samples, while 547.83 ± 620.06 MPs/kg (dry weight) and 74.23 ± 47.29 MePs/kg d.w. were recorded in sediments. The greatest abundances were observed in the more anthropized areas, near urban settlements. Fibers were the most conspicuous plastic items in water and sediments, followed by fragments. On the other hand, surface sediments, and 50 cm and 100 cm-depth sediments also presented MPs and MePs indicating they could serve as a stratigraphic indicator for recently formed sediments. The main polymer type identified were acrylic fibers, followed by polypropylene (PP) and polyethylene terephthalate (PET). Besides, SEM-EDX detected the presence of Si, Fe, Ti, Al and Cl onto the plastics' surface. These elements may serve as additives to enhance the plastics' properties, such as in the case of Ti, or they could originate from the environment, like biogenic Si or Fe, and Al possibly as a component of the suspended particles or sediments adhered to the micro or meso plastics. Finally, the results of the present study showed that MPs and MePs are commonly found in waters and also tend to be trapped in sediments of the RDLP estuary supporting the assertion that these areas play a substantial role in influencing the transport, dispersion, and buildup of MPs in estuarine regions.

Expansion of Sinotaia quadrata (Mollusca: Gastropoda: Architaenioglossa: Viviparidae) in two major rivers from Argentina.

Invasive non-native freshwater mollusks are a growing concern in South America, with 16 species already recorded in the region. Among them, Sinotaia quadrata has only been documented in Argentina, for the first time in the Punilla Valley, Córdoba (2009) and La Plata, Buenos Aires (since 2015). In this study, we report the presence of S. quadrata in two additional areas, the Río de la Plata River and a stream (unnamed) in the Paraná River basin, two of the most significant rivers in South America, located in the provinces of Buenos Aires and Entre Ríos, respectively. These new records confirm the invasive nature of this species, which has also been identified in Europe, the United States, and Africa in recent years. The findings of this study highlight the need for continued monitoring and management of invasive species in South America's freshwater ecosystems.

The end of an entire biome? World's largest wetland, the Pantanal, is menaced by the Hidrovia project which is uncertain to sustainably support large-scale navigation

The resurgent navigation project known as the Hidrovia Paraguay-Paraná threatens the integrity of the Pantanal, the world's largest wetland, which is considered a biome of its own. Intensive barge navigation is intended to transport crops (soybean, sugar, corn) and cement, iron and manganese from areas of production in Brazil, Paraguay and Bolivia to the oceanic ports of the Plata River. This short communication assembles the information available on the potential impacts of the planned deepening of the natural channel of the Paraguay River in its ~700 km-long upper section. These river channel alterations would disconnect the river from its floodplain, shorten the inundation period, and shrink the wetland area, resulting in severe degradation of the globally outstanding biological and cultural diversity of the Pantanal. The river sediments are mostly sandy and would require perpetual dredging. The reaches needing the most intensive dredging are those of the highest ecological value, protected as a National Park, UNESCO World Heritage and Biosphere Reserves, and various Indigenous reserves and Ramsar sites. Climate change is projected to increase the occurrence of low water periods. Between 2019 and 2021, navigation was impossible even in the already-deepened reaches of the Paraguay River between Corumbá and Asuncion during long periods of the year. Thus, despite considerable financial and technical efforts, the success of the navigation project is doubtful, whereas enormous environmental, cultural, and social impacts can be anticipated. For these reasons, the Brazilian government had already turned down the project in 2000. We suggest alternative, less impactful modes of transport of commodities, e.g., via railway.

Western South Atlantic seasonal variability recorded in a mid-deglacial bivalve from the outer Uruguayan continental shelf

The oceanographic dynamics on the continental shelf off southeastern South America are primarily controlled by the southward-flowing warm Brazil Current, converging with the northward-directed cold Malvinas (Falkland) Current, and interacting with the continental discharge of the Plata River. The seasonally reversing regional wind field together with the seasonal cycle of riverine discharge, determines which of these three components provides the dominant forcing. The Uruguayan shelf is thus located in a transitional zone that extends from the region influenced by the Brazil-Malvinas Confluence (BMC) in the open ocean to the Subtropical Shelf Front (STSF) on the continental shelf. Understanding how the resulting oceanic seasonal variability responded to different climatic boundary conditions may shed light on its future behavior. This study presents the first reconstruction of mid-deglacial seasonal hydrographic variability on the continental shelf off southeastern South America in seasonal resolution based on stable oxygen and carbon isotopes (δ18O, δ13C) from a thick-walled shell of a long-living bivalve. The mid-deglacial (14.3 cal ka BP; Bølling-Allerød interstadial) Retrotapes exalbidus bivalve shows a mean δ18O of 3.27 ± 0.42‰ (2.50 ± 0.42‰ when corrected for changes in global ice volume) and a seasonal δ18O amplitude of 1.69‰ for raw isotopic excursions. Moreover, the δ13C exhibits abrupt negative peaks coincident with more negative δ18O values that indicate seasons of elevated freshwater discharge. Finally, the growth rate of the bivalve suggests that the specimen was closer to the metabolically optimum than modern individual of this species from southern South America. Combining biogeographic and ecologic information with these isotopic data, the results point to colder waters and a slightly lower mid-deglacial seasonal amplitude in temperature compared with modern conditions at this shelf site. Because of the northward-displaced Plata River mouth during deglacial times, negative δ13C peaks are expected to reflect an influence of non-point freshwater sources in the form of small fluvial distributaries along the paleo-coast. Most of this signal may, however, be driven by seasonal metabolic effects associated with low ambient water temperatures related to a shallow-water environment located closer to the respective paleo-coastline due to the low sea level at those times.

Ontogenetic changes in sagitta otoliths of whitemouth croaker Micropogonias furnieri (Acanthuriformes: Sciaenidae) and its implication in acoustic communication

Abstract The otoliths of teleosts are part of the inner ear. They are related to the balance system and are also mechanical components of sound transmission, responsible for hearing in fish. Their morphology is determined by the function they perform and is also influenced by environmental factors. In this context, our goal was to: analyse the possible morphological changes of sagitta otoliths of Micropogonias furnieri (Desmarest, 1823) during its ontogeny, based on six biometric indexes. We analysed 162 sagittae otoliths of Micropogonias furnieri collected in the Rio de la Plata estuary. The variation of the indexes was associated with the different size groups. Rectangularity and aspect ratio [otolith length/total fish length (OL/TL) and sulcus area/otolith area (SA/OA)] were the indexes that contributed the most to the discriminant function. Biometric index differences were related to diverse juvenile environments. The observed shift in the SA/OA relationship could be attributable to a need to respond to a change in the perception of sound (pulse and frequency) experienced by individuals larger than 20 cm that begin to frequent the same areas as spawning adults. In this sense, the ontogenetic morphological change of sagitta otoliths plays an important role in perception and acoustic communication for this species.

Feeding ecology of Brevoortia aurea larvae (Clupeidae, Alosinae) from Río de la Plata estuary off Uruguay

This study analyses the feeding ecology of Brevortia aurea (Brazilian menhaden) larvae during a reproductive season in the Río de la Plata estuary (RPE), and explores its changes along larval development. Data were obtained from stomach content analyses of 205 individuals collected with a Bongo net during spring 2001. Larvae were discriminated into preflexion (PF), flexion (F) and post-flexion (PsF) stages. Feeding was described using complementary metrics like Feeding incidence (FI), the Relative Importance of different prey types consumed (IRI), the Degree of prey digestion (E) and Diet overlap (D), which were estimated and compared between stages. Also, larval morphometric relationships (body length, BL vs mouth width, MW) and between MW and maximum prey width (Wmax) were established for the range of observed larval sizes. The overall FI was 46.3% and increased with the development stage: 40.4% PF, 63.3% F, 78.6% PsF. Feeding occurred mainly during sunlit hours but no clear daily cycle could be established. Copepods and in particular Acartia tonsa were well represented among ingested preys, followed by invertebrate eggs and nauplii. Prey diversity and overlap between stages tended to decrease as development progressed. B. aurea exhibited isometric growth of MW in relation to BL. Average Wmax was 222.5 µm ± 100.9 SD, and increased non-linearly with both BL and MW. Gape size alone did not seem to be the limiting factor for prey choice (size), and we hypothesize that factors involved in the feeding mechanism other than mouth gape and linked with capture performance substantially influence the feeding ecology of this species in the RPE.

Interés público versus interés general: Colonia y el conflicto ambiental en sus costas

The privatization of beaches, with the consequent restrictions to access, a long-standing problem in Uruguay, is currently aggravated by the growing demand for land by foreign capital, real estate, and financial speculation, as well as by the deterioration and vulnerability observed on our coasts.&nbsp; An interesting phenomenon takes place in Departamento de Colonia, on the Río de la Plata River, where local neighbors have taken actions both in the political and media spheres, concerned about the placement of fences, the opening of streets, the approval of subdivisions on public spaces, accesses restrictions to coasts without proper authorizations and considerations of citizens' rights.&nbsp; Delegation, overlapping and conflict of competences between national and local authorities, with their traditional difficulties to coordinate and complement actions, can be added as part of the problem.&nbsp; The article focuses on the abundant regulations aimed both at protecting and preserving coastal spaces as well as settle the ever-present tension between private rights and general interest.

Los invertebrados y la calidad del hábitat como indicadores de la condición biológica de la costa del Río de la Plata, Argentina

Estuaries are productive ecosystems that provide ecosystem goods and services, although human development causes stresses that deteriorate the quality of their waters. The Río de la Plata river presents geographic and socioeconomic relevance, besides areas of importance in the conservation of biodiversity. The objective of this study was to analyze some of the structural and functional characteristics of the meso and macroinvertebrate assemblage along 155 km of the freshwater intertidal zone, applying multivariate analysis and comparing the sampling sites subject to different types of anthropogenic disturbances. Fifty-eight taxa were identified whose total abundance ranged from 570 to 345000 individuals/m2. The highest values of richness, structural and functional diversity, and percentages of sensitive taxa were recorded in recreational areas that have vegetated coasts, a high degree of connectivity and a natural state of riparian habitats. The multivariate analysis organized the sampling sites along a turbidity-conductivity gradient and a eutrophication-contamination gradient. The cluster analysis revealed the following groups: 1) sites with little impact of protected and recreational areas, 2) sites with anthropogenic impact and domestic discharges, 3) sites highly impacted by industrial and sewage discharges, and 4) sites located close to the front of maximum turbidity. These results showed that the characteristics of the habitat and the water determine the assemblages of invertebrates. The structural and functional descriptors analyzed can act reliably in the evaluation of the ecological quality and the biotic integrity of the Argentinian intertidal sector of the river.

Meteotsunamis at the Río de la Plata estuary

A recent numerical study showed that low-amplitude meteotsunamis could be detected at the Río de la Plata (RDP) estuary. In this work, eight sea level series (one year long, 2018) were analyzed: six measured within the RDP estuary and two at the adjacent continental shelf. All meteotsunami events during 2018 were identified and analyzed at the RDP estuary, and the maximum wave height observed was 0.78 m. The atmospheric conditions during the analyzed events were investigated, showing that the presence of atmospheric cold fronts and atmospheric gravity waves seem to happen simultaneously with meteotsunami events. From sea-level observations gathered at the estuary it was inferred that the meteotsunamis could be driven by atmospheric forcings propagating (i) mainly from SE to NW (the meteotsunami would be driven along the estuary from the outer to the upper RDP, and the Proudman resonance mechanism could explain its amplification), (ii) predominantly from SW to NE (the meteotsunami would be simultaneously generated at different locations of the RDP estuary), or (iii) the meteotsunami could be generated in waters of the continental shelf, to get into the RDP estuary, and then freely propagates from the outer to the upper RDP (in this case the Proudman resonance mechanism would not explain the amplification within the estuary). It is important to mention that the results of the present investigation are biased to the high-period band of meteotsunamis because the sampling interval (20 min, for most locations) is rather large to allow the study of the whole meteotsunami frequency band.

Nutrient levels, trophic status and land-use influences on streams, rivers and lakes in a protected floodplain of Uruguay

Land-use intensification impacts freshwater ecosystems in many agricultural landscapes, including the Rio de la Plata grasslands in Southeastern South America. We evaluated water chemistry of rivers, streams and lakes associated with a protected river-floodplain in the Queguay River basin in Uruguay. From 2019–2021, we measured phosphorus (P) and nitrogen (N) forms, dissolved oxygen (DO), turbidity and other chemical parameters; basin-scale land-use via Sentinel satellite imagery; and trophic status based on nutrient concentrations and Principal Components Analysis (PCA). Results highlighted low nutrient concentrations in the Queguay rivers, indicating this watershed as a national reference for evaluating eutrophication in Uruguay. Levels of total nitrogen (TN) in rivers averaged 430–510 µg/L and total phosphorus (TP) averaged 44–51 µg/L, classifying them as oligotrophic-mesotrophic systems. Streams ranged in TP from 51 to 97 µg/L and in TN from 545 to 701 µg/L, suggesting meso- to eutrophic states according to TP levels, and potential anthropogenic eutrophication in smaller basins with crop cover > 25%. Floodplain lakes are a conservation target within the protected area and had TP averaging 74–109 µg/L, suggesting eutrophic-hypereutrophic states based on TP and potential external nutrient inputs. The positive correlation between basin-scale crop cover and mean TP and soluble reactive phosphorus (SRP) concentrations suggested that land-cover plays a role in nutrient levels of fluvial and lotic systems. This study establishes a baseline for water chemistry in lakes and fluvial ecosystems in the Queguay River basin within an extensive agricultural region of the La Plata River Grasslands. By providing the first time series dataset of water chemistry for this area, we fill a major geographical gap regarding the limnological attributes of freshwater ecosystems in northwestern Uruguay. Results highlight the value of the Queguay basin for freshwater habitat conservation, and the vulnerability of these systems to land cover changes and nutrient loading from external sources.

Unravelling the role of local and large-scale factors in structuring sandy beach populations: the wedge clam Donax hanleyanus

Changes in the structure and dynamics of sandy beach macrofaunal populations result from the simultaneous action of local and regional factors acting synergistically. This is particularly noticeable in transitional interfaces between freshwater and marine ecosystems, where large-scale gradients affect local beach morphodynamics and resident fauna. The wedge clam Donax hanleyanus occurs on beaches with contrasting morphodynamics along the salinity gradient generated by the Rio de la Plata estuary. We conducted sampling surveys to determine a fine-grained comprehensive coverage of the species’ distribution in Uruguay to assess the concurrent effects of large-scale variations in salinity and local beach morphodynamics on wedge clam populations along 400 km (16 sandy beaches) over 2 yr. The main factor controlling wedge clam occurrence was salinity, overriding the effects of local habitat features. On beaches where the species was present, total abundance was higher on oceanic dissipative shores, which also harbored a higher abundance and relative representation of recruits. Recruitment was almost lacking in reflective beaches. However, biomass of adults with larger body sizes prevailed on reflective beaches, suggesting a scaling of abundance to body size and potential density-dependent effects. In this metapopulation, estuarine beaches are sinks, whereas oceanic beaches act as source habitats. We concluded that large-scale gradients generated by salinity variations translated into local population patterns (distribution, abundance and population structure) and processes (recruitment), which were secondarily driven by local morphodynamics. The relative importance of pre- and post-settlement processes across the morphodynamic spectrum from reflective to dissipative beaches has yet to be elucidated.

TAFONOMÍA DE UNA CONCENTRACIÓN DE VALVAS DEL BIVALVO INVASOR Corbicula fluminea (MÜLLER, 1774), RÍO DE LA PLATA SUPERIOR, URUGUAY

The bioinvader bivalve Corbicula fluminea (Müller, 1774) is known in the Río de la Plata River since about 35 years ago. This time lapse fills a gap in taphonomical studies, that comprises usually strictly contemporaneous or at best centuries-old phenomena. We sampled shells of this species in one locality in two occasions with the same methodology. Fragmentation, presence of periostracum, corrasion, bioerosion and bioencrustation were studied externally in four sectors of the valve: umbonal, anterior, central, and posterior. We tested the differences in the frequencies of the attributes between the two samples by means of Χ2, resulting only significantly different corrasion in the umbonal sector and fragmentation in the central and posterior areas.The umbonal area was the most affected sector, and it was attributed to a higher exposition to corrasive agents (i.e., moving clasts) compared to other areas of the valve. Neither bioerosion nor bioencrustation was recorded, being this attributed to the less prevalence of these taphonomic features in the low salinity conditions of the study area.

Benthic community responses to organic enrichment during an ENSO event (2009–2010), in the north coast of Rio de la Plata estuary

The 2009–2010 “El Niño”-Southern Oscillation (ENSO) event was characterized by a warm-phase leading to a five-fold increase in the Río de la Plata river flow and an associated change in trophic state because of organic enrichment. We analyzed the biotic response of benthic communities to such an increased trophic state during this ENSO event using community and biotic indices (diversity indices and AMBI). Polychaeta exhibited the highest richness and second the most abundant values while the small gastropod Heleobia australis was the most abundant and frequent species. An increase in richness and diversity of Polychaeta was associated to a concomitant increase of RdlP flow and the organic enrichment of the macrobenthic species. Community indices (richness and diversity) showed variability at both temporal and spatial scale but only AMBI exhibited spatial variability without significant differences between contrasting environmental conditions (ENOS phases). • An ENSO event promoted changes in chemical and biotic sediment variables. • Variability in salinity and protein sediment content were reflected in macrobenthic community indices. • AMBI was useful to infer spatial differences between zones with different environmental quality. • Richness, diversity and AMBI are recommended for the Montevideo Coastal zone environmental monitoring programs.