Large Estuary Research Articles

Nitrous oxide variability along an estuary influenced by agricultural land use (Guadalquivir estuary, SW Spain)

The Guadalquivir Estuary is the largest estuary in the southwest basin of the Iberian Peninsula, which is subject to strong anthropogenic influence such as the damming or the multitude of crop fields on its margins. Nitrous Oxide (N2O) variability is analysed considering the influence of temperature, salinity, water-atmosphere fluxes, benthic fluxes, reactivity and lateral inputs. N2O increases along the salinity gradient, with values ranging from 5.9 to 103.3 nmol L−1. Thus, values of N2O concentration are very close to equilibrium with the atmosphere at the mouth, while in the inner zone the fluxes to the atmosphere are higher, showing the greatest variability of N2O in the estuary (74.26 ± 7.41 μmol m−2 d−1). Sediments act as a source of N2O to the water column, with benthic fluxes presenting a wide range from 2 to 20 μmol m−2 d−1. Denitrification processes in the sediments may be important in the inner part of the estuary, where negative benthic fluxes of nitrate have been observed. Production rates of N2O in the water column are estimated from incubation experiments, resulting in higher production with temperature, and lower with salinity. Lateral inputs are calculated by balance of the different processes characterized and seems to be an important factor influencing N2O variability in the inner zone of the estuary.

Exploring changes in epibenthic food web structure after implementation of a water-sediment regulation scheme

The water-sediment regulation scheme (WSRS) in the Yellow River is a large-scale initiative to artificially regulate the flow of sediment to the sea, thereby increasing the flood-carrying capacity of the riverbed and reservoirs. Currently, systematic studies on ecological impacts of WSRS at ecosystem-level are still insufficient. This limitation hampers the pursuit of a ‘green’, healthy, ecosystem and sustainable fisheries. This study constructed the topological structure of food webs in the Yellow River Estuary (YRE) before, during, and after implementation of the WSRS, analyzing changes in food web complexity and key species based on fishery independent data collected in June, July, and August 2023. The results showed decreases from 59 to 52 in the number of trophic species, and from 539 to 395 in the number of feeding relationships after WSRS implementation. Increased node density, decreased link density, and decreased structural complexity index also indicated a simplification of the YRE food web structure after WSRS implementation. The relatively low value of the characteristic path length indicated that the YRE food web has high connectivity with short path lengths of trophic interaction. Based on the ranking of various topological indices, Japanese seabass (Lateolabrax japonicas) and mantis shrimp (Oratosquilla oratoria) persisted as the key species. Our research revealed limited potential ecological effects that WSRS may have on the YRE food web over a short period. The effects did not persist, and omnivorous key species were identified as being critical in contributing to overall system resilience. These omnivores with high complexity, connectivity and low path lengths allowed the food web to quickly dissipate the exogenous disruption from the WSRS. This provides a theoretical basis for assessing the future ecological health and scientific management of YRE fisheries and similar large estuaries for which sediment transport mitigation is under consideration.

On the role of small estuaries in retaining buoyant particles

Estuaries, as connectors between land and ocean, have complex interactions of river and tidal flows that affect the transport of buoyant materials like floating plastics, oil spills, organic matter, and larvae. This study investigates surface-trapped buoyant particle transport in estuaries by using idealized and realistic numerical simulations along with a theoretical model. While river discharge and estuarine exchange flow are usually expected to export buoyant particles to the ocean over subtidal timescales, this study reveals a ubiquitous physical transport mechanism that causes retention of buoyant particles in estuaries. Tidally varying surface convergence fronts affect the aggregation of buoyant particles, and the coupling between particle aggregation and oscillatory tidal currents leads to landward transport at subtidal timescales. Landward transport and retention of buoyant particles is greater in small estuaries, while large estuaries tend to export buoyant particles to the ocean. A dimensionless width parameter incorporating the tidal radian frequency and lateral velocity distinguishes small and large estuaries at a transitional value of around 1. Additionally, higher river flow tends to shift estuaries toward seaward transport and export of buoyant particles. These findings provide insights into understanding the distribution of buoyant materials in estuaries and predicting their fate in the land-sea exchange processes.

Response of suspended sediment dynamics to human activities in the transitional zone between Changjiang Estuary and Hangzhou Bay

The Changjiang Estuary and Hangzhou Bay system has experienced river damming and estuarine engineering in the last decades. However, few studies focused on the shifts in its sediment dynamics due to such human activities. In this study multi-decadal development of sediment dynamics in the transitional zone of the two large estuaries was analyzed, based on the synchronous hydrographic data in the winter of 2023, 2014 and 1983. The results revealed significant changes in regional hydrodynamics and suspended sediment transport, despite the continuous good correlations between the current velocity, suspended sediment concentration (SSC), water/sediment fluxes and tidal range. Specifically, the current velocity has been decreased by 8 - 21% after 2014, mainly due to the land reclamation (implemented around 2016) with several groins stretching into deep water and altering alongshore hydrodynamics. The SSC has decreased further by 29 - 38% in addition to the significant decrease during 1983 - 2014. The SSC changes are related to the combination of river damming which induced sediment load reduction and land reclamation which enclosed a large amount of sediment. Furthermore, the sediment transport from Changjiang Estuary to Hangzhou Bay decreased by 36% - 53%, explaining the observed bed erosion in the northern bay mouth in recent years. The findings are also relevant for studies on sediment dynamics in other large estuaries worldwide.

The largest estuary on the planet is not spared from plastic pollution: Case of the St. Lawrence River Estuary

The St. Lawrence River, one of the world's largest estuaries, drains >25 % of the world's freshwater reserves and is affected by various anthropogenic effluents. Although previous studies reported micro- and nanoplastics contamination in the Estuary and Gulf of St. Lawrence (EGSL), this study provides a first evaluation of macroplastic pollution along the north and south shores of the EGSL. Plastic debris categorization was performed according to the OSPAR protocol completed by polymer identification using Fourier-transform infrared spectroscopy. The EGSL appeared ubiquitously contaminated by plastic debris, dominated by single-use plastics primarily made of polypropylene (28 %), polyethylene (25 %) and polystyrene (17 %). The EGSL shores exhibited a mean contamination level of 0.17 ± 0.11 items/m2 and distance to Montreal significantly influenced the distribution of plastic debris. This study provides an essential baseline for implementing local waste reduction and management actions in the St. Lawrence watershed to reduce plastic pollution.

VARIATIONS OF THE COASTLINE IN A MEDIUM PERIOD (1985-2017) OF A TROPICAL ISLAND: THE CASE OF SOURE (MARAJÓ - PARÁ/BRAZIL)

The margins of Marajó Island, influenced by large estuaries (rivers Amazonas and Pará), are con- ditioned to constant morphological changes. This article shows the multitemporal dynamics of the coastline (1985-2017 - 32 years) and the quantification of the areas in accretion and erosion in the Northern Margin (Sector I), Cabo Maguarí (Sector II) and Eastern Margin (Sector III) of the Soure municipality. The method- ology included the acquisition of medium resolution satellite images LANDSAT 5 TM (bands 5, 4 and 3 - 1985) and LANDSAT 8 OLI (bands 6, 5 and 4 - 2017), orthorectified and georeferenced, with spatial resolu- tion of 30m. They were processed using the ArcToolbox tool. The visual interpretation of the images and the vectorization of the coastline were done in ArcMap, using the ArcGis 10.6 software. The overlapping of the vectors of the years 1985 and 2017 allowed the caCLulation of areas and rates. The results show the predom- inance of the accretion of the coastline. The under accretion totaled 25,382,783.18 m² ± 575.28 m² (4.72 m year-1) and mean linear advance of the coastline of 151.10 m. However, the eroded areas totaled 6,696,780.64 m² ± 575.28 m² (3.03 m year-1) and the mean linear retreat of the coastline was 97 m.

Decoupled shifts of dominant and rarer fish species as a response to warming and extreme events in a large estuary

AbstractDisentangling community responses to multiple stressors in a warming context is one of the most challenging tasks ecologists face. Taking advantage of a long‐term (1989–2017) and intensive fish monitoring survey (N = ~900K from 804 seines‐day), we present a comprehensive analysis on the dynamics of coastal fish communities in Jamaica Bay, New York, by addressing multiple dimensions of community change that although closely related are rarely considered in a single work. Specifically, we tested hypotheses about changes in composition, composition variability and community functional attributes, and the role of environmental drivers acting at different temporal scales. Our analyses suggest two decoupled community dynamics triggered by two different extreme events. First, the 1999 Atlantic Multidecadal Oscillation phase switch caused an abrupt shift from a single‐species dominance to intermittent co‐dominance, with species preferring higher temperatures, a shift that has persisted in time but that may be cyclical at a multidecadal scale. Second, the 2012 heat wave promoted an abrupt collapse of rare and cold‐water species, and the sudden arrival of warmer‐water species in a portion of the community that was already increasingly variable due to long‐term warming. Functionally, the entire community subtly shifted toward species with faster turnover and lower trophic levels. Our work sheds light on the complex responses of biological communities to warming in terms of the impacts on composition, its temporal variability, and its functional dimension, by disentangling the interplay of long‐term environmental trends such as warming, multidecadal cycles, and extreme events on different portions (i.e., dominant and rare thermal species) of the community.

Environmental factors have stronger effects than biotic processes in patterns of intertidal populations along the southeast coast of Brazil

Rocky shore communities are shaped by complex interactions among environmental drivers and a range of biological processes. Here, we investigated the importance of abiotic and biotic drivers on the population structure of key rocky intertidal species at 62 sites, spanning ∼50% of the Brazilian rocky shoreline (i.e., ∼500 km). Large-scale population patterns were generally explained by differences in ocean temperature and wave exposure. For the gastropod species Lottia subrugosa, differences at smaller scales (i.e., 0.1–1 km) were better explained by other abiotic influences such as freshwater discharge and substrate roughness. Based on the general population patterns of intertidal species identified, three main oceanographic groups were observed: a cold-oligotrophic grouping at northern sites (Lakes sub-region), a eutrophic group associated with large estuaries and urban zones (Santos and Guanabara bays); and a transitional warm-water group found between the two more productive areas. Larger individuals of Stramonita brasiliensis, L. subrugosa and Echinolittorina lineolata were generally found in the cold-oligotrophic system (i.e., upwelling region), while small suspension feeders dominate the warm-eutrophic systems. Evidence of bottom-up regulation was not observed, and top-down regulation effects were only observed between the whelk S. brasiliensis and its mussel prey Pernaperna. Environmental drivers as compared to biotic interactions, therefore, play a key role determining the population structure of multiple intertidal species, across a range of spatial scales along the SW Atlantic shores.

Characteristics of demersal fish community structure during summer hypoxia in the Pearl River Estuary, China.

In recent decades, hypoxic areas have rapidly expanded worldwide in estuaries and coastal zones. The Pearl River Estuary (PRE), one of China's largest estuaries, experiences frequent seasonal hypoxia due to intense human activities and eutrophication. However, the ecological effects of hypoxia in the PRE, particularly on fish communities, remain unclear. To explore these effects, we collected fish community and environmental data in July 2021 during the summer hypoxia development period. The results revealed that bottom-layer dissolved oxygen (DO) in the PRE ranged from 0.08 to 5.71 mg/L, with extensive hypoxic zones (DO ≤ 2 mg/L) observed. Hypoxia has varied effects on fish community composition, distribution, species, and functional diversity in the PRE. A total of 104 fish species were collected in this study, with approximately 30 species (28.6%) exclusively found in hypoxic areas. Species responses to hypoxia varied: species such as Sardinella zunasi, Coilia mystus, and Nuchequula nuchalis were sensitive, while Decapterus maruadsi, Siganus fuscescens, and Lagocephalus spadiceus showed higher tolerance. Within the hypoxia area, dissolved oxygen was the main limiting factor for fish community diversity. Functional diversity (FDiv) decreased with higher dissolved oxygen levels, indicating a potential shift in the functional traits and ecological roles of fish species in response to changing oxygen conditions. Further analysis demonstrated that dissolved oxygen had a significantly stronger effect on fish community structure at hypoxic sites than in the whole PRE. Moreover, other environmental variables also had significant effects on the fish community structure and interacted with dissolved oxygen in the hypoxia area. These findings suggest that maintaining sufficient dissolved oxygen levels is essential for sustaining fish communities and ecosystem health in the PRE. This study provides novel insights into the effects of hypoxia on fish communities in estuarine ecosystems and has significant implications for the ecological health and management of the PRE.

Spatial distribution and risk assessment of microplastics in surface waters of the St. Lawrence Estuary

Development of effective prevention and mitigation strategies for marine plastic pollution requires a better understanding of the pathways and transport mechanisms of plastic waste. Yet the role of estuaries as a key interface between riverine inputs of plastic pollution and delivery to receiving marine environments remains poorly understood. This study quantified the concentration and distribution of microplastics (MPs) (50–3200 μm) in surface waters of the St. Lawrence Estuary (SLE) in eastern Canada. Microplastics were identified and enumerated based on particle morphology, colour, and size class. Fourier Transform Infrared (FTIR) spectroscopy was used on a subset of particles to identify polymers. Generalized linear models (Gamma distribution with log-link) examined the relationship between MP concentrations and oceanographic variables and anthropogenic sources. Finally, a risk assessment model, using MP concentrations and chemical hazards based on polymer types, estimated the MP pollution risk to ecosystem health. Mean surface MP concentration in the SLE was 120 ± 42 SD particles m−3; MP concentrations were highest in the fluvial section and lowest in the Northwest Gulf of St. Lawrence. However, MP concentrations exhibited high heterogeneity along the length and width of the SLE. Microplastics were elevated at stations located closer to wastewater treatment plant outflows and downstream sites with more agricultural land. Black, blue, and transparent fibers and fragments ≤250 μm were most commonly encountered. Predominant polymer types included polyethylene terephthalate, regenerated cellulose, polyethylene, and alkyds. While the overall risk to ecosystem health in the entire estuary was considered low, several stations, particularly near urban centres were at high or very high risk. This study provides new insights into the quantification and distribution of MPs and first estimates of the risk of MP pollution to ecosystem health in one of the world's largest estuaries.

Use of multiple sampling techniques for cyanobacteria and cyanotoxin monitoring in the Sacramento-San Joaquin Delta under different hydrologic regimes

Cyanobacteria harmful algal blooms (CHABs) are a growing water quality problem in the upper San Francisco Estuary (California), also known as the Sacramento-San Joaquin Delta. We conducted cyanobacteria and cyanotoxin monitoring from 2020 to 2023, which spanned California's driest consecutive 3-year period and one of the wettest years on record (2023). To assess the impact of CHABs over this range of hydrologic conditions, we monitored invasive Asian Clams (Corbicula fluminea) for microcystin contamination and used molecular tools (qPCR and sequencing) to characterize cyanobacteria in the water column. We also used solid phase adsorption toxin tracking (SPATT) samplers to track microcystins (MCs) and other cyanotoxins in 2023. During the drought years, record breaking MCs, in excess of 1000 μg/L, were documented in water grab samples and Asian clams also accumulated higher MCs relative to the wet year. However, MCs were present in Asian clams during the entire study period. SPATT's confirmed MC presence during wet 2023 and sequencing results corroborated the integrative sampler findings. Yet, no MC was detected in water grab samples at our primary sampling sites during the drought year of 2022 or the wet year of 2023. This highlights the importance of using multiple sampling modalities to provide a more accurate assessment of MC contamination, especially in large estuaries where traditional discrete monitoring can easily miss episodic and transient CHAB events.

Environmental DNA metabarcoding reveals the effect of river slope on diadromous fish communities in island rivers

Understanding the natural factors that influence diadromous fish communities in river ecosystems is crucial for effective biodiversity management. However, this remains an unexplored and challenging task. Here, the relationship between river morphology and diadromous fish communities within rivers was explored. We focused on river slope and estuary size, as these may constrain their upstream migration and affect the availability of important nursery grounds for some species. Using environmental DNA metabarcoding, we investigated the diadromous fish communities within 12 rivers of subtropical Yakushima Island, southern Japan, and compared the community differences among rivers with different river slopes and estuary sizes. Fewer diadromous fish species were detected in rivers with steeper slopes and smaller estuaries. Additionally, the community similarity was greater among rivers with analogous gradients and estuary sizes. These results indicate that river slopes exert a species-selective effect on diadromous fish communities by affecting their upstream migration and estuarine habitat availability, thereby playing a crucial role in shaping the composition of these communities within rivers. Conserving gentle-gradient rivers with large estuaries would be crucial for preserving the diversity of diadromous fish species, which play a crucial role in shaping biodiversity especially in rivers of subtropical and tropical oceanic islands.

Large structural estuaries: Their global distribution and morphology

Estuaries are marine-flooded topographic depressions that receive sediment from both marine and fluvial sources. The morphology of the underlying drainage basin acts as a boundary condition for the spatial distribution of hydrodynamic energy within an estuary, and by extension the resultant depositional environment. In some large estuaries, bedrock structures at the mouth restrict oceanographic forcing, which has impacts on sedimentary infill. Such estuaries are termed here as Large Structural Estuaries (LSEs). For LSEs, the shape of the basin and associated bedrock morphology is fixed on the millennial scale having been formed by geological processes operating over timescales greater than that of the Holocene. However, the distribution and abundance of these LSEs are poorly understood. In this study, a global survey of large estuaries was conducted to identify and characterize LSEs from the worldwide population to understand regional drivers of coastal geomorphology. A total of 271 large (>50 km2) estuaries were identified, and these were clustered into 5 classes using Gaussian Mixture Modelling. These clusters were interpreted as Barrier Estuaries, Rocky Bays, Sandy Bays, Funnelled Estuaries, and LSEs. LSEs comprised 28.04 % of all large estuaries identified and were spatially heterogenous, associated with collision coastlines, and predominantly located at higher latitudes except for the Malay Archipelago. The global distribution of LSEs suggests that regional distribution of tectonic processes strongly influences their formation. LSEs are important global features comprising many of the major ports in the world and this work indicates they are unique landforms and traditional approaches to understanding sedimentation in these settings require adjustment to their unique geomorphology.

Hydrodynamic regimes and proposed structure for flood drainage and navigation stability in Thuan an estuary, Vietnam

Thua Thien Hue province is significantly affected by conditions of climate change and sea level rise. Located in central Vietnam, the province experiences various climate-related challenges that have a strong impact on its environment and communities. Furthermore, the complexity of hydrodynamic regimes at main estuaries is reasonably challenge for the development of socio-economic, especially in water transportation. As a large estuary of Thua Thien Hue province, Thuan An play a special role in flood drainage and waterway navigation of Huong River. Therefore, understanding the hydrodynamic regime of Thuan An estuary is crucial due to the continuous alterations in its morphology during monsoon seasons. Sedimentation within the estuary creates obstacles for the river mouth entering channel, while erosion affects the northern and southern coasts. These changes have direct implications for flood drainage and waterway navigation. To gain insights into the hydrodynamic regime, this study focuses on further interpretation the hydrodynamic regimes in Thuan An inlet using Delft3D model combined with ArcGIS tools. By combining scientific modeling and practical implications, this study contributes to a better understanding the hydrodynamics of Thuan An estuary, ultimately facilitating sustainable development and management of this vital waterway system. Importantly, proposed structure for flood drainage and navigation stability in Thuan An estuary is considered with the elongation of 2 jetties. The proposed measure shows that the flood drainage is insignificantly affected and the water level at mostly locations in the river is not changed.

Assessing spatiotemporal risks of nonpoint source pollution via soil erosion: a coastal case in the Yellow River Delta, China.

Nonpoint source pollution (NPSP) has always been the dominant threat to regional waters. Based on empirical models of the revised universal soil loss equation and the phosphorus index, an NPSP risk assessment model denoted as SL-NPSRI was developed. The surface soil pollutant loss was estimated by simulating the rain-runoff topographic process, and the influence of path attenuation was quantified. A case study in the Yellow River Delta and corresponding field surveys of soil pollutants and water quality showed that the established model can be applied to evaluate the spatial heterogeneity of NPSP. NPSP usually occurs during high-intensity rainfall periods and in larger estuaries. Summer rainfall increased pollutant transport into the sea from late July to mid-August and caused estuarine dilution. Higher NPSP risks often correspond to coastal areas with lower vegetation coverage, higher soil erodibility, and higher soil pollutant concentrations. Agricultural NPSP originating from cropland significantly increase the pollutant fluxes. Therefore, area-specific land use management and vegetation coverage improvement, and temporal-specific strategies can be explored for NPSP control during source-transport hydrological processes. This research provides a novel insight for coastal NPSP simulations by comprehensively analyzing the soil erosion process and its associated pollutant loss effects, which can be useful for targeted spatiotemporal solutions.

Anthropogenic eutrophication and stratification strength control hypoxia in the Yangtze Estuary

Many large estuaries are threatened by intensifying hypoxia. However, due to the limited duration of available observations, uncertainties persist regarding the level of contemporary hypoxia intensity in a longer-term context and the relative contributions of climate versus human factors. Here we present sediment records for the hypoxia intensity and associated environmental parameters in the Yangtze Estuary over the past three centuries. The results show that the hypoxia intensity has been increasing during the last half century due to anthropogenic eutrophication, but the current hypoxia condition is not as severe as some preindustrial periods due to weaker stratification in the water column. Our findings suggest that if anthropogenic and climatic forcing coincide in the foreseeable future, the hypoxia intensity of the Yangtze Estuary may reach unprecedented levels.

Prototyping structured decision making for water resource management in the San Francisco Bay-Delta

A structured decision making (SDM) approach can help evaluate tradeoffs between conservation and human-benefit objectives by fostering communication and knowledge transfer among stakeholders, decision makers, and the public. However, the process is iterative and completing the full process may take years. It can be difficult to initiate an SDM effort when problems seem insurmountable. Occasionally, SDM may not even be the best or correct approach for addressing the conservation problem at hand. We describe the implementation of an SDM process to help inform difficult decisions related to competing objectives. We convened a diverse stakeholder group from the largest estuary in the western United States; the San Francisco Bay and Sacramento-San Joaquin Delta (Bay-Delta). The stakeholder group consisted of representatives from local, state, and federal agencies, non-profit organizations, and recreational fishers. The stakeholder group agreed on a problem statement and identified four priority objectives related to Chinook salmon, delta smelt, water availability and reliability, and agricultural water use. Furthermore, they proposed 14 candidate management actions to achieve their objectives. The group then used existing quantitative models and data to evaluate trade-offs in proposed management actions to identify areas of agreement of proposed candidate actions. The clear communication of the problem statement and objectives among the stakeholder group, along with evaluation of tradeoffs and uncertainty via decision-support models suggest that a full SDM approach may work in the Bay-Delta. We further communicate lessons learned during our implementation of SDM to help guide future SDM efforts in the region and elsewhere.

A Morphological Assessment On The Effects Of Embankments On Sediment Transport In Sandy Estuaries

Many large estuaries around the world are engineered to some degree for both flood protection of coastal communities and maintenance of economically vital shipping routes. Engineering challenges arise due to the highly dynamic morphological nature of estuaries, where the complex processes of sediment transport are driven by an interplay between the turbulent flows of rivers and tides. Naturally, the planforms of sandy estuaries have the tendency to converge landwards exponentially, where deviations from this shape provide alternating space and constrictions where tidal sandbars and channels form (Leuven et al., 2018-a; 2018-b). In the case of embankment by bank protection and dikes, such topographically forced channels can develop deep scour holes that endanger bank stability. Our objective is to study effects of channel and planform dynamics on scour depth variation near protected banks. To this end, we conducted scale experiments of estuaries with completely fixed banks in the tilting tidal flume the Metronome (Kleinhans et al., 2017). We study the effects of these fixed banks on the morphological behaviour of the channel pattern through one experiment, two repeat experiments, and a control without fixed banks. By collection of much longer timeseries than available in nature, we use the experiments to shed light on the topographic forcing and channel dynamics of these systems, and on the repeatability of Metronome experiments.

Nutrient concentration, loads and retention in a semiarid micro-estuary: The relative contribution of baseflow and flood events

Estuaries are a significant source of nutrients to the marine environment. The magnitude of this source is a function of nutrients load reaching the estuary and removal (attenuation) within estuaries. Most estuarine research is conducted in large estuaries, which do not reflect the processes in small estuaries in urban and semi-arid regions where flood water is a substantial portion of the annual discharge and the estuarine baseflow is often low and dominated by wastewater. To improve the understanding of nutrient attenuation and load into the Mediterranean, we conducted high-resolution nutrient sampling in the eutrophic Alexander micro-estuary as a test case. We sampled once per month during baseflows (years 2014–2019) and hourly during floods (years 2016–2018). The concentrations of inorganic nutrients (phosphorous (P) and nitrogen (N)) were extremely high during baseflows. Dissolved ammonium and particulate P were the only nutrients that were in the estuary (by 55 % and 30 %, respectively). Floods were rare, occurring ~4 % of the time, but contributed 62 % of the annual water discharge of the Alexander micro-estuary (14.7 ± 3.8 106 m3 y−1). The concentration of all dissolved nutrients decreased during floods but was higher than expected (DIN 584 ± 50 μmol L−1, phosphate 21 ± 2 μmol L−1), accounting for 42 % and 55 % of the overall annual DIN (123.5 ± 44.9-ton yr−1) and P (6.7 ± 1.9 ton yr−1) loads to sea, respectively. The N:P ratios were 16 and 34 during baseflow and flood events, respectively. Previously, nutrient loads were calculated by multiplying baseflow-measured concentrations by the total water volume of baseflow and floods. Our calculations, based on high-resolution sampling, revealed lower annual loads of P and N to the sea that were 56 % and 89 % of previous estimates, which is a considerable difference in an oligotrophic system such as the eastern Mediterranean.

Estuarine front dynamics drive ichthyoplankton assemblage in the tidal freshwater zone of the Río de la Plata, South America

Frontal zones are critical in the retention and dispersal of ichthyoplankton. This study evaluates the response of ichthyoplankton assemblage to frontal zone dynamics in one of the world's largest estuaries, the Río de la Plata, South America. Surveys were conducted between October 2014 and December 2017 in a nearshore habitat of the tidal freshwater zone of the estuary. Ichthyoplankton was collected using conical nets, and physico-chemical variables were registered in situ. The frontal zone dynamics were evaluated through the turbidity front (TF) location, determined by satellite imagery. Although most larval taxa belonged to the freshwater category, the estuarine category had the highest abundances. The highest species richness and abundance occurred during spring and summer, linked to the season and proximity of spawning areas. The dynamics of the TF emerged as the primary driver of interannual variability in ichthyoplankton assemblages within the tidal freshwater zone. A seaward shift of the TF correlated with a reduction in total abundance and the prevalence of freshwater larvae, whereas a retraction of the TF location corresponded to an increased abundance of marine larvae in the tidal freshwater zone. These interannual shifts in TF location and fish larvae assemblages were closely tied to significant variations in river discharges resulting from the ENSO events during the study period. The findings suggest that marine and estuarine fish eggs exhibit effective retention near the frontal zone and the outer regions of the estuary. In contrast, larvae from these categories disperse along the main axis of the estuary in response to the displacement of the frontal zone. Finally, we discuss into the role of the frontal zone in the Río de la Plata as a pivotal environmental cue for fish larvae, influencing their behavior to regulate dispersal patterns.