River Outflow Research Articles

Tracking the Dispersal of River Water, Atmospheric Deposition, and Shallow Sedimentary Trace Metal Inputs From the Congo Region Into the South Atlantic

AbstractRecent work has revealed the presence of an offshore near‐surface plume of dissolved trace elements in the South Atlantic Ocean (SAO). Dissolved Fe (dFe) supply from the Congo plume is equivalent to ∼40% of the annual atmospheric dFe supply to the SAO. However this plume is not captured by biogeochemical models, raising questions about its exact sources. To help understand the potential source mechanisms, we use particle tracking experiments to investigate elemental distributions. Results suggest that elevated concentrations of some elements in the Congo plume are primarily sourced from river discharge and wet atmospheric deposition with minimal influence from shelf sediments. River discharge is the main source in shelf regions and some off‐shelf regions, whereas atmospheric deposition dominates the area to the southwest of the Congo River outflow. A quantitative analysis along 3S specifically for dFe suggests a decrease in the contribution of river discharge from 90% to 30% moving off‐shelf, with a corresponding increase in the contribution of atmospheric deposition. Within the shelf zone, atmospheric deposition accounts for roughly 20%–40% and could be a major source of dFe around the river mouth. Integration of data from cruise GA08 reinforces the finding that wet deposition augments the concentrations of dFe, manganese (dMn), and cobalt (dCo) at distances over 1,000 km from the river mouth. Given present‐day patterns of nitrate, Fe, and Co limitation for primary producers in the SAO, changing rainfall patterns may have long‐term implications for both regional elemental budgets and ecologically dependent processes sensitive to trace element ratios.

Baroclinic Instability Induced Mesoscale and Submesoscale Processes in River Plumes: A Laboratory Investigation on a Rotating Tank

AbstractUnder the influence of buoyancy and the earth rotation, river outflows leaving an estuary typically have a two‐part structure, including a recirculating bulge near the river mouth and a coastal current propagating downstream. A continuous river outflow causes the bulge to expand in size and to accumulate freshwater, with the bulge eventually becoming unstable due to baroclinic instability. We conducted a series of laboratory experiments on a rotating tank to simulate an idealized river plume under various Coriolis frequencies, density differences, and shelf slopes. The horizontal velocity structures of the river plume were qutantified using particle imaginary velocimetry (PIV). We designed a velocity‐based vortex identification and tracking algorithm to capture anticyclonic eddies (ACEs) at the bulge center, cyclonic eddies (CEs) on the plume front, and coastal cyclonic return flow (CCRF) at the corner between estuary and coastal wall. Our results suggest a linear relationship between bulge instability parameter and bulge wavenumber, which can be used to predict the bulge instability patterns that are classified according to vortex stretching, splitting, and squeezing. Finally, we estimated the eddy kinetic energy contained within ACEs, CEs, and CCRF to explore the cross‐scale energy transfer and dissipation. Our results show that the bugle is more unstable in gentle slope cases, and its instability decreases with the inflow Rossby number and increases with the Froude number. The generation of CEs on the plume front may extract the energy from the larger scale anticyclonic core, which plays an important role in mass transport and frontal mixing of river plumes.

Influence of river runoff and precipitation on the seasonal and interannual variability of sea surface salinity in the eastern North Tropical Atlantic

Abstract. In tropical regions, the freshwater flux entering the ocean originates primarily from precipitation and, to a lesser extent when considering basin-scale averages, from continental rivers. Nevertheless, at the regional scale, river flows can have a significant impact on the surface ocean dynamics. Riverine freshwater modifies salinity and, therefore, density, stratification, and circulation. With its particular coastline and high cumulative river discharge, as well as its being in the vicinity of the intertropical convergence zone (ITCZ), the eastern part of the North Tropical Atlantic (e-NTA) region off northwestern Africa is a particularly interesting location to study the linkage between precipitation, river outflow, and sea surface salinity (SSS). Here, we focus on the regional e-NTA SSS seasonal cycle and interannual variability and on the impact of using various river runoff and precipitation forcing data sets to simulate SSS with a regional model. The simulated SSS values are compared with the Climate Change Initiative (CCI) satellite SSS values; in situ SSS values from Argo floats, ships, and a coastal mooring; and the GLORYS reanalysis SSS values. An analysis of the mixed-layer salinity budget is then conducted. Overall, the simulations reproduce the seasonal cycle and interannual variability well despite a positive mean model bias north of 15° N. The seasonal cycle is impacted by the phasing of the different runoff products. The mixed-layer SSS decrease during the rainy season is mainly driven by precipitation followed by runoff by means of horizontal advection and is partly compensated for by vertical mixing. In terms of interannual anomalies, river runoff has a more direct impact on SSS than precipitation. This study highlights the importance of properly constraining river runoff and precipitation to simulate realistic SSS values and the importance of observing SSS in coastal regions to validate such constraints.

Perspectives on Northern Gulf of Alaska salinity field structure, freshwater pathways, and controlling mechanisms

The biologically productive Northern Gulf of Alaska (NGA) continental shelf receives large inputs of freshwater from surrounding glaciated and non-glaciated watersheds, and a better characterization of the regional salinity spatiotemporal variability is important for understanding its fate and ecological roles. We here assess synoptic to seasonal distributions of freshwater pathways of the Copper River discharge plume and the greater NGA continental shelf and slope using observations from ship-based and towed undulating conductivity-temperature-depth (CTD) instruments, satellite imagery, and satellite-tracked drifters. On the NGA continental shelf and slope we find low salinities not only nearshore but also 100–150 km from the coast (i.e. average 0–50 m salinities less than 31.9, 31.3, and 30.8 in spring, summer, and fall respectively) indicating recurring mid-shelf and shelf-break freshwater pathways. Close to the Copper River, the shelf bathymetry decouples the spreading river plume from the direct effects of seafloor-induced steering and mixing, allowing iron- and silicic acid-rich river outflow to propagate offshore within a surface-trapped plume. Self-organized mapping analysis applied to true color satellite imagery reveals common patterns of the turbid river plume. We show that the Copper River plume is sensitive to local wind forcing and exerts control over water column stratification up to ∼100 km from the river mouth. Upwelling-favorable wind stress modifies plume entrainment and density anomalies and plume width. Baroclinic transport of surface waters west of the river mouth closely follow the influence of alongshore wind stress, while baroclinic transport east of the river mouth is additionally modified by a recurring or persistent gyre. Our results provide context for considering the oceanic fate of terrestrial discharges in the Gulf of Alaska.

Forecasting of sub-surface flow velocity through a temporarily open/close estuary under closed condition using machine learning

TOCE or temporarily open/closed estuary is a complex estuary system created by interaction of longshore transport and river outflows. Under the closed condition when an estuarine sandbar is formed, river flow is impeded creating backwater rise that could result in floods. At present, there are no hydrological models that could cater for river flows under closed estuary conditions. This paper proposes the application of a machine learning (ML) approach to model flow velocity in an estuarine sandbar which is crucial for flow estimations. A TOCE located at Mengabang Telipot, Terengganu, Malaysia was chosen for model development where river water and groundwater levels (inside the sandbar) were measured and tidal levels from an existing station were used. Average flow velocities were calculated using Darcy’s equation to represent observed data. Multilayer perceptron (MLP) neural networks were trained by the Levenberg-Marquardt algorithm and developed concerning the parameters above to predict groundwater average velocity. In this study, the optimal model was a neural network with four neurons in the hidden layer and the degree of influence of each parameter was determined by the results of the sensitivity analysis, where the R2 was obtained to be about 97–99% when predicted data was compared to observed data. Hence, the results indicated that neural networks were able to predict groundwater mean velocity quite well. The developed model was further verified using data from another TOCE from a different time. The results were very good having a R2 of 0.990. Hence, it was concluded that the ML approach was able to model flow in a TOCE and has great potential application in such an environment seeing that it is simpler than complex physics-based models.

Quantifying Surface Shelf Water Export in the Southern Middle Atlantic Bight Using a Lagrangian Particle Tracking Approach

AbstractShelf water is influenced by atmospheric forcing, river outflows, and the open ocean. Studying its variability is crucial for understanding anthropogenic impacts on coastal oceans and their transport to the open ocean. In the Middle Atlantic Bight (MAB), the interaction of the Gulf Stream with shelf/slope circulation leads to some of the complex exchanges between the shelf and open ocean along the U.S. East Coast. This study employs a Lagrangian particle tracking approach, grounded in a high‐resolution, data‐assimilative ocean reanalysis, to examine the export pathways of surface shelf water in the MAB. We analyzed over 700 daily images of simulated particle distributions using image clustering techniques. This revealed three distinct export patterns: abrupt entrainment to the Gulf Stream, gradual entrainment, and southern transport. Each pattern was observed roughly equally during the study period from January 2017 to December 2018. The observed export patterns are closely linked to the coastal circulation dynamics near Cape Hatteras. Understanding the timing and duration of these patterns is vital for assessing water quality and predicting the settlement of species that spawn in the region. Our study further underscores the influence of tropical cyclones, including Hurricanes Jose, Maria, and Chris, on these export patterns. These extreme weather events lead to significant shifts in coastal circulation near Cape Hatteras.

Ostracod turnover during the Carnian Pluvial Episode (Late Triassic) in the Western Neotethys

The Carnian Pluvial Episode (CPE) caused a biotic crisis and turnover in terrestrial and marine ecosystems in the Western Neotethyan realm. The expansion of deltaic systems on land and the increased siliciclastic input affected the distribution of benthic ostracods, both nearshore and offshore. Diversity changes of Carnian ostracod faunas from seven successions in the Transdanubian Range (TR), Western Hungary tracked the effects of the CPE, despite its distal hemipelagic depositional setting and position within an intraplatform basin. At the onset of CPE in the early Julian 2, the composition of the studied benthic ostracod assemblage was similar to that of the late Julian 1 communities albeit with decreased abundance of smooth bairdiids and healdiids. The ratio of healdiids and smooth bairdiids is likely related to the detrital supply with bairdiids colonizing primarily in intervals with decreased terrestrial input and carbonate platform progradation. During the maximum expansion of CPE in late Julian 2, the predominance of platycopid Bektasia indicates a shallowing trend, infilling of the Carnian basins and stressed environment due to clastic input and salinity stress. The Julian–Tuvalian boundary is characterized by high extinction rates among marine invertebrates simultaneously with the occurrence of a unique low diversity but abundant brackish water ostracod community with Simeonella and Renngartenella that can be traced along the entire Western Neotethys and northern Gondwana shelf. The appearance of this community is primarily linked to salinity variations during the CPE and the formation of restricted basin habitats in the TR with water stratification where the special community lived or was transported there with storms or river outflow. During the Tuvalian, fully open marine conditions were re-established with increased carbonate productivity in the platforms, favouring the colonization of stable shallow marine habitats with the appearance of ornate bairdiids in the ostracod faunas.

Microbial ecology of northern Gulf of Mexico estuarine waters.

Estuarine and coastal ecosystems are of high economic and ecological importance, owing to their diverse communities and the disproportionate role they play in carbon cycling, particularly in carbon sequestration. Organisms inhabiting these environments must overcome strong natural fluctuations in salinity, nutrients, and turbidity, as well as numerous climate change-induced disturbances such as land loss, sea level rise, and, in some locations, increasingly severe tropical cyclones that threaten to disrupt future ecosystem health. The northern Gulf of Mexico (nGoM) along the Louisiana coast contains dozens of estuaries, including the Mississippi-Atchafalaya River outflow, which dramatically influence the region due to their vast upstream watershed. Nevertheless, the microbiology of these estuaries and surrounding coastal environments has received little attention. To improve our understanding of microbial ecology in the understudied coastal nGoM, we conducted a 16S rRNA gene amplicon survey at eight sites and multiple time points along the Louisiana coast and one inland swamp spanning freshwater to high brackish salinities, totaling 47 duplicated Sterivex (0.2-2.7 µm) and prefilter (>2.7 µm) samples. We cataloged over 13,000 Amplicon Sequence ariants (ASVs) from common freshwater and marine clades such as SAR11 (Alphaproteobacteria), Synechococcus (Cyanobacteria), and acI and Candidatus Actinomarina (Actinobacteria). We observed correlations with freshwater or marine habitats in many organisms and characterized a group of taxa with specialized distributions across brackish water sites, supporting the hypothesis of an endogenous brackish-water community. Additionally, we observed brackish-water associations for several aquatic clades typically considered marine or freshwater taxa, such as SAR11 subclade II, SAR324, and the acI Actinobacteria. The data presented here expand the geographic coverage of microbial ecology in estuarine communities, help delineate the native and transitory members of these environments, and provide critical aquatic microbiological baseline data for coastal and estuarine sites in the nGoM.IMPORTANCEEstuarine and coastal waters are diverse ecosystems influenced by tidal fluxes, interconnected wetlands, and river outflows, which are of high economic and ecological importance. Microorganisms play a pivotal role in estuaries as "first responders" and ecosystem architects, yet despite their ecological importance, they remain underrepresented in microbial studies compared to open ocean environments. This leads to substantial knowledge gaps that are important for understanding global biogeochemical cycling and making decisions about conservation and management strategies in these environments. Our study makes key contributions to the microbial ecology of estuarine and coastal habitats in the northern Gulf of Mexico. Our microbial community data support the concept of a globally distributed, core brackish microbiome and emphasize previously underrecognized brackish-water taxa. Given the projected worsening of land loss, oil spills, and natural disasters in this region, our results will serve as important baseline data for researchers investigating the microbial communities found across estuaries.

Congo Basin Water Balance and Terrestrial Fluxes Inferred From Satellite Observations of the Isotopic Composition of Water Vapor

AbstractLarge spatio‐temporal gradients in the Congo basin vegetation and rainfall are observed. However, its water‐balance (evapotranspiration minus precipitation, or ET − P) is typically measured at basin‐scales, limited primarily by river‐discharge data, spatial resolution of terrestrial water storage measurements, and poorly constrained ET. We use observations of the isotopic composition of water vapor to quantify the spatio‐temporal variability of net surface water fluxes across the Congo Basin between 2003 and 2018. These data are calibrated at basin scale using satellite gravity and total Congo river discharge measurements and then used to estimate time‐varying ET − P over four quadrants representing the Congo Basin, providing first estimates of this kind for the region. We find that the multi‐year record, seasonality, and interannual variability of ET − P from both the isotopes and the gravity/river discharge based estimates are consistent. Additionally, we use precipitation and gravity‐based estimates with our water vapor isotope‐based ET − P to calculate time and space averaged ET and net river discharge within the Congo Basin. These quadrant‐scale moisture flux estimates indicate (a) substantial recycling of moisture in the Congo Basin (temporally and spatially averaged ET/P > 70%), consistent with models and visible light‐based ET estimates, and (b) net river outflow is largest in the Western Congo where there are more rivers and higher flow rates. Our results confirm the importance of ET in modulating the Congo water cycle relative to other water sources.

Diverse sources of fecal contamination in macroalgae wrack-affected environment adjacent to river outflow along the Baltic Sea coast

We investigated the dynamics of feces-associated microorganisms in areas with wrack accumulation in the southeastern part of the Baltic Sea. Our study covered single-day (2021 ) and multi-day (2022) observations during the recreational season. We collected water, sand, and wrack samples and assessed the abundance of fecal indicator bacteria (FIB), as well metagenomic analysis was conducted to monitor changes in microbial composition. Based on metagenomic data we identified taxa associated with feces, sewage, and ruminant sources. Human-related fecal pollution based on genetic markers correlated with the presence of Lachnospiraceae, Prevotellaceae and Rickenellacea abundance. Higher abundance and diversity of feces-associated and ruminant-associated taxa and the presence of enteric pathogens were observed when wrack accumulated near the river outflow in 2021, suggesting a potential link with fecal pollution from the river. As a preventive measure, it is recommended to remove the wrack to reduce the risk of exposure to potential enteric pathogens if it is accumulated next to the river outflow.

Statistical Modeling of Indus River Outflow at Tarbela Dam using Generalized Gumbel Type 2 Distribution

Statistical Modeling of Indus River Outflow at Tarbela Dam using Generalized Gumbel Type 2 Distribution

Seasonal Monitoring Method for TN and TP Based on Airborne Hyperspectral Remote Sensing Images

Airborne sensing images harness the combined advantages of hyperspectral and high spatial resolution, offering precise monitoring methods for local-scale water quality parameters in small water bodies. This study employs airborne hyperspectral remote sensing image data to explore remote sensing estimation methods for total nitrogen (TN) and total phosphorus (TP) concentrations in Lake Dianshan, Yuandang, as well as its main inflow and outflow rivers. Our findings reveal the following: (1) Spectral bands between 700 and 750 nm show the highest correlation with TN and TP concentrations during the summer and autumn seasons. Spectral reflectance bands exhibit greater sensitivity to TN and TP concentrations compared to the winter and spring seasons. (2) Seasonal models developed using the Catboost method demonstrate significantly higher accuracy than other machine learning (ML) models. On the test set, the root mean square errors (RMSEs) are 0.6 mg/L for TN and 0.05 mg/L for TP concentrations, with average absolute percentage errors (MAPEs) of 23.77% and 25.14%, respectively. (3) Spatial distribution maps of the retrieved TN and TP concentrations indicate their dependence on exogenous inputs and close association with algal blooms. Higher TN and TP concentrations are observed near the inlet (Jishui Port), with reductions near the outlet (Lanlu Port), particularly for the TP concentration. Areas with intense algal blooms near shorelines generally exhibit higher TN and TP concentrations. This study offers valuable insights for processing small water bodies using airborne hyperspectral remote sensing images and provides reliable remote sensing techniques for lake water quality monitoring and management.

Salinity Fronts in the South Atlantic

Monthly climatology data for salinity fronts in the South Atlantic have been created from satellite SMOS sea surface salinity (SSS) measurements taken from 2011–2019, processed at the Barcelona Expert Center of Remote Sensing (BEC), and provided as high-resolution (1/20°) daily SSS data. The SSS fronts have been identified with narrow zones of enhanced horizontal gradient magnitude (GM) of SSS, computed using the Belkin–O’Reilly algorithm (BOA). The SSS gradient fields generated by the BOA have been log-transformed to facilitate feature recognition. The log-transformation of SSS gradients markedly improved the visual contrast of gradient maps, which in turn allowed new features to be revealed and previously known features to be documented with a monthly temporal resolution and a mesoscale (~100 km) spatial resolution. Monthly climatologies were generated and analyzed for large-scale open-ocean SSS fronts and for low-salinity regions maintained by the Rio de la Plata discharge, Magellan Strait outflow, Congo River discharge, and Benguela Upwelling. A 2000 km-long triangular area between Africa and Brazil was found to be filled with regular quasi-meridional mesoscale striations that form a giant ripple field with a 100 km wave length. South of the Tropical Front, within the subtropical high-salinity pool, a trans-ocean quasi-zonal narrow linear belt of meridional SSS maximum (Smax) was documented. The meridional Smax belt shifts north–south seasonally while retaining its well-defined linear morphology, which is suggestive of a yet unidentified mechanism that maintains this feature. The Subtropical Frontal Zone (STFZ) consists of two tenuously connected fronts, western and eastern. The Brazil Current Front (BCF) extends SE between 40 and 45°S to join the subantarctic front (SAF). The STFZ trends NW–SE across the South Atlantic, seemingly merging with the SAF/BCF south of Africa to form a single front between 40 and 45°S. In the SW Atlantic, the Rio de la Plata plume migrates seasonally, expanding northward in winter (June–July) from 39°S into the South Brazilian Bight, up to Cabo Frio (23°S) and beyond. The inner Plata front moves in and out seasonally. Farther south, the Magellan Strait outflow expands northward in winter (June–July) from 53°S up to 39–40°S to nearly join the Plata outflow. In the SE Atlantic, the Congo River plume spreads radially from the river mouth, with the spreading direction varying seasonally. The plume is often bordered from the south by a quasi-zonal front along 6°S. The diluted Congo River water spreads southward seasonally down to the Angola–Benguela Front at 16°S. The Benguela Upwelling is delineated by a meridional front, which extends north alongshore up to 20°S, where the low-salinity Benguela Upwelling water forms a salinity front, which is separate from the thermal Angola–Benguela Front at 16°S. The high-salinity tropical water (“Angola water”) forms a wedge between the low-salinity waters of the Congo River outflow and Benguela Upwelling. This high-salinity wedge is bordered by salinity fronts that migrate north–south seasonally.

Spatial distribution of trace and rare earth elements of bottom sediments in Lake Ostrowite, Bory Tucholskie National Park, Poland

AbstractLake pollution has attracted concerns worldwide; especially the excessive accumulation of trace elements (TEs) and rare earth elements (REEs) in bottom sediments can pose a serious threat to ecosystem health. However, there is still a knowledge gap on the level of sediment pollution in lakes isolated from the direct impact of pollution sources, their spatial variability, and also on the factors influencing this state. The aim of this study is to investigate the level and spatial variability of TEs and REEs concentrations, as well as to determine their source and the factors determining their distribution in the bottom sediments of Lake Ostrowite. Lake Ostrowite is the largest and deepest water body located in the Bory Tuholskie National Park (in northern Poland), which completely isolates the lake from the direct impact of pollution sources. The study covered analyses of 32 surface samples of bottom sediments. The concentrations of 24 TEs and 14 REEs were determined using inductively coupled plasma mass spectrometry (ICP‐QQQ‐MS). The assessment of the enrichment of bottom sediments in TEs and REEs employed geochemical background values (GBV) that provided the basis for the calculation of relative concentrations and geochemical indices. The determination of their sources and supply routes was based on the cluster analysis and principal component analysis. The obtained results point to the enrichment of the bottom sediments with TEs and REEs. Relative concentration values of TEs and REEs in reference to geochemical background values were in ranges from 0.01 to 7.31, at an average of 0.99, and from 0.03 to 4.29, averaging 1.76, respectively. The enrichment factor values show moderately severe enrichment of sediments at the study sites. This was primarily determined by the concentrations of Ag (from the TEs group) and Lu (from the REEs group). The metal pollution index values showed an approximate spatial distribution of points in terms of the presence of TEs and REEs. The lowest concentrations of TEs and REEs occurred on the eastern shore of the western basin of Lake Ostrowite. TEs and REEs concentrations in sediments are positively correlated with the content of organic matter and depth and negatively correlated with distance of the sampling point from the river outflow from Lake Ostrowite. On the eastern shore of the western basin, TEs and REEs concentrations are additionally shaped by wind, predominantly from the western direction. With water wave action, organic matter is transported to the central part of the western basin, where it is accumulated. Since the lake is isolated from point and nonpoint pollution sources, relevant from a biogeochemical point of view are dry and wet depositions from the atmosphere as well as aquatic vegetation, shoreline vegetation, forest litter, soil, and groundwater.

Holocene Paleoclimate Records in Equatorial West Africa: Insights Based on the Characterization of Glycerol Dialkyl Glycerol Tetraethers

One gravity core retrieved from the Niger Delta was used to explore the origin of deposited organic matter (OM) and the paleo-climatic and environmental conditions over the Holocene in equatorial West Africa. The geochemical properties of sediments including glycerol dialkyl glycerol tetraethers (GDGTs) and elemental (%OC, %N, C/N) and isotopic (δ13Corg, δ15N) signatures were determined. The determination constrained the age of the column and revealed that the sediment OM was mainly derived from a marine source. The isoprenoid (iso)GDGTs were the dominant GDGTs, with a small amount of branched (br)GDGTs, which led to a low-branched and isoprenoid tetraether index (BIT, 0.02–0.21) and represented a low terrestrial input. Most isoGDGTs and OH-GDGTs were produced in situ by Marine Group I (MG-I) Thaumarchaeota, while the brGDGTs were mainly transported from land. A two-endmember model quantified the contribution of terrestrial OM, as 0.9–19.9% by BIT and 1.1–32.6% by δ13C. Accordingly, the millennium-scale sea surface temperatures (SSTs) were reconstructed based on the cyclopentane ring distribution (TEX86H) and the ring index of OH-GDGTs (RI-OH). The top core SSTs were lower than the modern mean annual SST due to the growth season and habitat depth of Thaumarchaeota. The reconstructed SSTs clearly revealed the four stages of paleoclimate change, in particular, the drought episode of 8.2 kyr and the following humid period. The above research has enhanced our understanding of the paleoclimate change in river outflow during the Holocene at the millennium scale.

Asynchronous application of modified biochar and exogenous fungus Scedosporium sp. ZYY for enhanced degradation of oil-contaminated intertidal mudflat sediment.

Intertidal mudflats are susceptible to oil pollution due to their proximity to discharges from industries, accidental spills from marine shipping activities, oil drilling, pipeline seepages, and river outflows. The experimental study was divided into two periods. In the first period, microcosm trials were carried out to examine the effect of chemically modified biochar on biological hydrocarbon removal from sediments. The modified biochar's surface area increased from 2.544 to 25.378 m2/g, followed by a corresponding increase in the hydrogen-carbon and oxygen-carbon ratio, indicating improved stability and polarity. In the second period, the effect of exogenous fungus - Scedoporium sp. ZYY on the bacterial community structure was examined in relation to total petroleum hydrocarbon (TPH) removal. The maximum TPH removal efficiency of 82.4% was achieved in treatments with the modified biochar, followed by a corresponding increase in Fluorescein diacetate hydrolysis activity. Furthermore, high-throughput 16S RNA gene sequencing employed to identify changes in the bacterial community of the original sediment and treatments before and after fungal inoculation revealed Proteobacteria as the dominant phylum. In addition, it was observed that Scedoporium sp. ZYY promoted the proliferation of specific TPH-degraders, particularly, Hyphomonas adhaerens which accounted for 77% of the total degrading populations in treatments where TPH removal was highest. Findings in this study provide valuable insights into the effect of modified biochar and the fundamental role of exogenous fungus towards the effective degradation of oil-contaminated intertidal mudflat sediments.

An Unprecedented Bloom of Oceanic Dinoflagellates (Karenia spp.) Inside a Fjord within a Highly Dynamic Multifrontal Ecosystem in Chilean Patagonia.

At the end of summer 2020, a moderate (~105 cells L-1) bloom of potential fish-killing Karenia spp. was detected in samples from a 24 h study focused on Dinophysis spp. in the outer reaches of the Pitipalena-Añihue Marine Protected Area. Previous Karenia events with devastating effects on caged salmon and the wild fauna of Chilean Patagonia had been restricted to offshore waters, eventually reaching the southern coasts of Chiloé Island through the channel connecting the Chiloé Inland Sea to the Pacific Ocean. This event occurred at the onset of the COVID-19 lockdown when monitoring activities were slackened. A few salmon mortalities were related to other fish-killing species (e.g., Margalefidinium polykrikoides). As in the major Karenia event in 1999, the austral summer of 2020 was characterised by negative anomalies in rainfall and river outflow and a severe drought in March. Karenia spp. appeared to have been advected in a warm (14-15 °C) surface layer of estuarine saline water (S > 21). A lack of daily vertical migration patterns and cells dispersed through the whole water column suggested a declining population. Satellite images confirmed the decline, but gave evidence of dynamic multifrontal patterns of temperature and chl a distribution. A conceptual circulation model is proposed to explain the hypothetical retention of the Karenia bloom by a coastally generated eddy coupled with the semidiurnal tides at the mouth of Pitipalena Fjord. Thermal fronts generated by (topographically induced) upwelling around the Tic Toc Seamount are proposed as hot spots for the accumulation of swimming dinoflagellates in summer in the southern Chiloé Inland Sea. The results here provide helpful information on the environmental conditions and water column structure favouring Karenia occurrence. Thermohaline properties in the surface layer in summer can be used to develop a risk index (positive if the EFW layer is thin or absent).

Observations of river-wave interactions at a small-scale river mouth

Small coastal rivers often discharge directly into the surfzone, where the fate of freshwater and river-borne materials (e.g., sediment, nutrients and contaminants) is primarily determined by interactions between buoyant plume processes and the nearshore wave forcing. Understanding river-wave interactions in the surfzone is essential to assess coastal water quality and impacts on ecosystem health along adjacent shorelines. These interactions are modulated by the variability in river discharge, offshore wave climate, tidal forcing, and surfzone dynamics, and therefore occur over a wide range of timescales. However, the different timescales at which river-wave interactions occur have not been fully investigated. Here we use in-situ and remote (drone-based) observations collected at the mouth of the Maipo River, a small-scale river system in central Chile, to investigate interactions between the river outflow and the incident wave forcing that may influence the distribution of freshwater along the coast. We focus on interactions occurring at infragravity (wave groups), tidal, and synoptic (offshore wave climate) timescales. The observations included inlet and surfzone deployments, and documented low river discharge conditions (QR∼ 20–27 m3s−1) with variable wave forcing (Hs∼ 1–3 m). We observed that the salinity signal at the river mouth is strongly modulated by the tide, with a freshwater plume forming solely during the ebb. Large oscillations associated to infragravity (IG) motions are continuously observed on top of the tidal variability. These oscillations are evidenced in surfzone salinity, inlet water levels, and inlet velocity during the early ebb when the freshwater plume starts to develop. As the plume evolves into a strong outflow jet during the late ebb, they are not observed in the inlet and become restricted to the surfzone. The combined analysis of drone imagery and in-situ observations suggests that the observed variability in surfzone salinity at IG frequencies is associated with the arrival of wave groups and the propagation of wave fronts in the plume area. In particular, during the late ebb, the large oscillations in surfzone salinity (amplitude of 15–20 psu) are explained by a contraction and expansion of the outflow plume in response to the variability in the onshore wave forcing at the timescale of wave groups. On synoptic timescales (hours to days), the surfzone salinity responded to the intensity of the offshore wave forcing (∼ 20 m depth), with higher waves promoting lower salinity as freshwater is mostly retained near the coast. A wave-river momentum comparison confirms that the wave forcing had the potential to trap freshwater in the surfzone. This study shows that nearshore waves may influence the dynamics of small river plumes and the fate of freshwater (and associated terrestrial materials) on a wide range of timescales, from infragravity (1–5 min) to hours and days.

Unravelling long-term impact of water abstraction and climate change on endorheic lakes: A case study of Shortandy Lake in Central Asia.

Endorheic lakes, lacking river outflows, are highly sensitive to environmental changes and human interventions. Central Asia (CA) has over 6000 lakes that have experienced substantial water level variability in the past century, yet causes of recent changes in many lakes remain unexplored. Modelling hydrological processes for CA lakes poses challenges in separating climatic change impacts from human management impacts due to limited data and long-term variability in hydrological regimes. This study developed a spatially lumped empirical model to investigate the effects of climate change and human water abstraction, using Shortandy Lake in Burabay National Nature Park (BNNP) as a case study. Modelling results show a significant water volume decline from 231.7x106m3 in 1986 to 172.5x106m3 in 2016, primarily driven by anthropogenic water abstraction, accounting for 92% of the total volume deficit. The highest rates of water abstraction (greater than 25% of annual outflow) occurred from 1989 to 1993, coinciding with the driest period. Since 2013, the water volume has increased due to increased precipitation and, more importantly, reduced water abstraction. Despite limited observational data with which to calibrate the model, it performs well. Our analysis underscores the challenges in modelling lakes in data-sparse regions such as CA, and highlights the importance and benefits of developing lake water balance models for the region.

Trace Metal Fluxes of Cd, Cu, Pb and Zn From the Congo River Into the South Atlantic Ocean Are Supplemented by Atmospheric Inputs

AbstractThe Congo River supplies vast quantities of trace metals (TMs) to the South Atlantic Ocean, but TM budgets for the Congo plume derived using radium isotopes for GEOTRACES cruise GA08 suggest additional input other than the river outflow. Considering the tight correlations between most dissolved TMs and salinity in the plume and the high rainfall during the wet season over the Congo shelf, we hypothesized that wet atmospheric deposition is a TM source to the Congo plume. Observed TM concentrations in rainwaters across the Congo shelf were mostly comparable to values from previous work in the North Atlantic and Mediterranean Sea. Wet deposition contributed the equivalent of 43% dCd, 21% dCu, 20% dPb and 68% dZn of the Congo River fluxes. Our findings show an important role of wet deposition in supplying TMs to the South Atlantic overlapping with the region that receives substantial TM fluxes from the Congo River.