Estuaries Of Large Rivers Research Articles

Regional Difference in Distribution Pattern and Morphological Characteristics of Embayed Sandy Beaches in Zhejiang Province, Eastern China

The distribution pattern and the morphology of sandy beaches have been extensively studied, while those in turbid coastal environments near large river estuaries are still unclear. This study analyzes the distribution pattern, morphological characteristics, and influencing factors of Zhejiang sandy beaches using statistical analysis, based on field data and historical records. Results show that the mean grain size distribution of Zhejiang sandy beaches ranges from fine sand to very coarse sand, and the beach slope and sediment grain size correspond well with the wave heights in the three regions of Zhejiang. The extent of beach headlands in central Zhejiang appeared the largest, suggesting an increased susceptibility to wave erosion due to the less sheltered headlands. Most sandy beaches in Zhejiang formed on the islands and the areas far from the estuaries, showing quantity difference in beach distribution. The comparison of the regional difference in Zhejiang sandy beaches shows that embayment is the main factor affecting the beach distribution pattern and morphological characteristics. The different embayment characteristics provide the space for beach formation and the interaction with the coastal process, the sediment supply, the nearshore hydrodynamic environment, and human intervention also have influence on the morphological characteristics of Zhejiang beaches.

Characterizing sedimentary black carbon in the Pearl River Estuary, Southern China

Sedimentary black carbon in coastal regions plays an important role in global carbon cycles due to its refractory nature in sediments. In this study, we conducted the first comprehensive investigation on black carbon (BC) in surface sediments of the Pearl River Estuary (PRE), a large tropical estuary system in China. We used two different analytical methods: IMPROVE-A thermal optical reflectance (TOR) and chemothermal oxidation (CTO-375) to constrain the distribution, and TOR can further constrain the composition (i.e., char and soot). The BC concentration determined by the TOR method (BCTOR) ranged from 0.07 to 7.13 mg/g, with char and soot contents ranging from 0.04 to 6.98 mg/g and 0.03–0.43 mg/g, respectively. The BC concentrations measured by the CTO-375 method (BCCTO) ranged from 0.27 to 1.88 mg/g. Char was the predominant constituent (87%) of sedimentary BCTOR in the PRE. Both char and soot had higher concentrations locating near the outlets and on the Shenzhen coast, indicating that they were sourced through direct fluvial transport and surface runoff. Soot particles were transported farther from fluvial sources than char particles. We estimated that the burial flux of BCTOR in the PRE sediments was 2.97 ± 1.61 mg/cm2/yr, acting as an important sink of BC. This study implies that large river estuaries are hotspots for BC burial, sinking a large amount of organic carbon from terrestrial reservoirs.

Dissolved organic matter composition and characteristics during extreme flood events in the Yangtze River Estuary

Understanding the molecular composition and fate of dissolved organic matter (DOM) during transport in estuaries is essential for gaining a comprehensive understanding of its role within the global biogeochemical cycle. In 2020, a catastrophic flood occurred in the Yangtze River basin. It is currently unknown whether differences in hydrologic conditions due to extreme flooding will significantly impact the estuarine to oceanic DOM cycle. We determined the DOM composition in the Yangtze River estuary (YRE) to the East China Sea by using Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) during the high discharge and the flood period (monthly average discharge was 1.2 times higher) on the same trajectory. Our study found that the composition of DOM is more diverse, and more DOM molecules were introduced to the YRE during the flood, especially in the freshwater end member. The result revealed that the DOM was significantly labile and unstable during the flood period. A total of 1840 unique molecular formulas were identified during the flood period, most of which were CHON, CHONS, and CHOS compounds, most likely resulting from anthropogenic inputs from upstream. Only 194 of these molecules were detected in the seawater end member after transporting to the sea, suggesting that the YRE served as a ‘filter’ of DOM. However, the flood enhances the transport of a group of terrigenous DOM, that is resistant to photodegradation and biodegradation. As a result, YRE experienced ~1.6 times higher terrigenous DOC flux than high discharge period. Considering the increased frequency of future floods, our study provides a preliminary basis for further research on how floods affect the composition and characteristics of estuarine DOM. With the help of the FT-ICR MS technique, we can now better understand the dynamic of DOM composition and characteristics in large river estuaries.

Holocene geomorphological evolution of a sediment-starved coastal embayment in response to sea level change: Insights from the Qing'ao Embayment, southern China

Aiming to gain a better understanding of the response of sediment-starved coastal systems to the climate-driven sea level change, this study examines the Holocene evolution history of Qing'ao Embayment, southern China, as a case study. Results suggest three distinct stages of the evolution history: Stage 1 (8400–6000 cal yr BP) records an initial sedimentation phase associated with the early Holocene marine transgression into the embayment. Stage 2 (6000–3000 cal yr BP) records early sediment deposition under relatively strong hydrodynamic inshore/open-bay environment during early marine regression. Stage 3 (3000 cal yr BP to present) records two substages: Stage 3a relates to the semi-enclosed bay, intertidal and lagoonal salt marsh facies between 3000 and 1300 cal yr BP, and Stage 3b records the final infilling of the embayment into a lagoonal plain. This evolution model suggests that the rates of sea level change and sediment accretion are the two major controlling factors for the Holocene geomorphological evolution of sediment-starving coastal systems.Comparison of the evolution of Qing'ao embayment with sediment-supply rich large-river estuary/deltas reveals unique features in the response to Holocene sea level change of this sediment-starved embayment: During the early Holocene marine transgression, sediment starts to accumulate in the small embayment around 8400 cal yr BP, hundreds of years later than in large river estuaries. During 8400–6000 cal yr BP, sediment accretion rate remains dramatically lower than sea level rise rate, which results in the quick growth of sediment accommodation space during this stage. Around 6000 cal yr BP, though sea level has remained relatively stable and large-river deltas have been growing for about 1000 years, the sediment-starved embayment was still under shallow marine and/or open bay conditions. At 3000 cal yr BP, shoreline started to prograde in sediment-starved embayment, which is about 3000 years later than large river deltas. Finally, agricultural activity started around 1300 cal yr BP in the embayment, thousands of years later than large river deltas. Limited or minimal fluvial sediment supply to the small embayment has constrained the land expansion during the sea level stabilization since mid-Holocene, and the limited living space might result in the much later agricultural activity in such coastal system.Due to the different balances between sea level change and sediment flux, sediment-starved coasts are expected to be under higher risks than large river deltas. However, earlier and more extensive human activities in large river basins have resulted in the reduction of sediment discharge that heighten the risks of large river deltas against future sea level rise. Thus, more attention should be paid to the coastal sediment supply budgets in the context of the background of rising sea level.

Dissolved organic matter cycling revealed from the molecular level in three coastal bays of China

As the widespread distributed and critical zones connecting the land and ocean systems, coastal bays are special units with semi-enclosed landforms to accommodate and process dissolved organic matter (DOM) in the context of increasing anthropogenic effects globally. However, compared to other common systems that have been paid much attention to (e.g., large river estuaries, wetlands), the roles of the coastal bays in coastal carbon cycling are less explored. To fill this knowledge gap, here we combined optical techniques and ultra-high-resolution mass spectrometry to systematically investigate the DOM chemistry of the three typical coastal bays in different nutrient levels, Xiangshan Bay, Jiaozhou Bay, and Sishili Bay, in China. Results show that terrestrial signals and anthropogenic imprints were observed in these three bays to various extents. Besides, Xiangshan Bay with a higher nutrient level had the DOM characterized by lower humification and aromaticity degree than Jiaozhou Bay and Sishili Bay, which not likely mainly resulted from the differences in the primary production or photochemical processing. Further examination reveals that microbial processing likely contributes to the differences in DOM chemistry among the three bays, as indicated by different proportions of potentially transformed nitrogen-containing molecules and relative abundances of the island of stability molecules. Considering the nutrient levels in different bays, we speculate that the lower nutrient concentrations would promote the efficiency of the microbial carbon pump (MCP), which hypothesized that heterotrophic microorganisms might contribute to the formation of marine recalcitrant organic carbon. Additionally, the enrichment of oxygen-rich compounds in the unique carboxyl-rich alicyclic molecule pool of Jiaozhou Bay and Sishili Bay suggests that the efficient MCP might preferentially form them in these two bays. This study emphasizes the importance of coordinating the land and ocean systems and controlling the nutrient discharge to coastal bays, thus, to potentially promote long-term marine carbon sequestration.

Difference and consistency of multi-sources land cover products in global large river estuaries

Many land cover products with different resolutions and types have been produced by researchers, which provide important information on changes in the terrestrial surface. However, multi-source land cover products around the world differ greatly at the coastal regional scale. Information on coastal land cover remains very limited for the large river estuaries around the world, which are one of the important ecosystems on Earth. There is an urgent need to review land cover products to provide a reference for understanding the exquisite evolutionary process and resource management of the delta ecosystems. Here, we evaluated the consistency and accuracy of the current mainstream land cover products through area deviation, area correlation, pixel superposition, and error matrix. We found that 1) the comprehensive land cover products had basically the same land cover type composition and distribution features in estuaries, but the area of different types in vary products vary to a degree; 2) the correlation coefficient of the comprehensive land cover products was 0.64–0.99, with the highest correlation being between GLC_FCS30 and GlobeLand30 and the lowest correlation being between Esri_Land_Cover and GlobeLand30; 3) among the comprehensive products, land cover types are inconsistent in their spatial distribution and wetlands were the most inconsistent, followed by bare land, vegetation, impervious area, cropland, and water body. Among the special products, tidal flats were the most inconsistent, followed by impervious areas and water bodies; 4) the special land cover products were highly consistent at the pixel-level in the delta regions, and the water body of special products were more consistent than impervious area and tidal flats; 5) the wetland classification accuracy of the comprehensive products in the delta areas must be improved and the overall accuracy of the Esri_Land_Cover was the highest. GLC_FCS30, the Global Surface Water dataset, and the Global Tidal Wetland Change dataset had higher accuracy than the special types, including for impervious areas, water bodies, and tidal flats. Our findings can help users choose the appropriate land cover data for their purpose, and supports the sustainable management of estuaries.

Coupling of Carbon and Oxygen in the Pearl River Plume in Summer: Upwelling, Hypoxia, Reoxygenation and Enhanced Acidification

AbstractAcidification and hypoxia are universal environmental issues in coastal seas, especially in large river estuaries such as the Pearl River estuary. In July and August of 2015, two legs of a field survey were conducted in the Pearl River plume. Leg 1 was sampled during the influence of upwelling favorable winds, while Leg 2 was during downwelling favorable winds. During both legs, instead of the typically observed dissolved inorganic carbon (DIC) consumption and dissolved oxygen (DO) over‐saturation, upwelling‐induced high DIC (>2,000 μmol kg−1), low pH (7.7–7.8) and low DO (140–150 μmol kg−1) values were observed in surface waters at the estuary mouth and the area off Hong Kong. In the bottom waters, hypoxia, acidification (pH 7.6–7.8) and DIC accumulation (DIC addition of ∼100–180 μmol kg−1) were observed. Hypoxia was less severe during Leg 2 compared to Leg 1. The stoichiometry of oxygen depletion to DIC addition was 0.89 for bottom water, suggesting remineralization was dominated by marine sourced organic matter. However, a comparison of data from the two legs showed that the stoichiometry of oxygen consumption to DIC accumulation was significantly higher during Leg 2 (0.73 ± 0.03 for Leg 1 vs. 0.80 ± 0.05 for Leg 2), although N/P ratios were the same (13.54 ± 1.93 for Leg 1 vs. 13.51 ± 2.04 for Leg 2). This phenomenon was attributed mainly to enhanced ventilation (re‐oxygenation) under the influence of the downwelling favorable winds during Leg 2. Although ventilation relieves some hypoxia, it might enhance acidification in bottom waters after a short‐term ventilation event. The enhanced acidification after short‐term ventilation is worthy of further study considering that most hypoxia and acidification are found in shallow coastal seas.

Editorial: Eutrophication, algal bloom, hypoxia and ocean acidification in large river estuaries, volume II

Editorial: Eutrophication, algal bloom, hypoxia and ocean acidification in large river estuaries, volume II

Features of the formation of the coasts of Chukotka and the Koryak highlands

Long-term expeditionary studies of the Geography Faculty of Lomonosov Moscow State University of the Arctic coast of Russia (1969 – 2015) have provided extensive material on hydrology, channel morphodynamics and geomorphology of the estuaries of large rivers from the Northern Dvina to the Kolyma. The joint work of the Moscow State University and the M. K. Ammosov North-Eastern Federal University made it possible to signifiantly expand the geography of research to the coasts of the Chukchi and Bering Seas. Based on the analysis of geological and geomorphological data and an array of published and archival materials, it was possible to establish patterns and specifi features of coastal and estuarine processes in the fial phase of stabilization of the post-glacial ocean transgression. Over the past 7-5 thousand years, floding of low-lying hilly coastal plains, river valleys and fjords has occurred. The active impact of the wave activity of the sea led to the erosion of the upper part of the underwater slope, the restructuring of its profie and the retreat of accumulative forms towards the land and the creation of modern outlines of the coastline. The sections of the coast extended into the sea, composed by a thickness of loose quaternary sediments, were partially eroded by waves, and sediments were involved in longitudinal and transverse movements, forming braids and embankments, which led to a general alignment of the coastline. Currently, in conditions of rising sea levels, the general trend towards the restructuring of the underwater slope and the coastal zone persists and can be extremely dangerous for coasts composed of strongly icy sandy-loamy alluvial-lacustrine deposits. A further increase in global temperature will signifiantly soften the climate of the Arctic coasts, cause the degradation of permafrost, thawing of underground ice and accelerate the process of retreat of coastal abrasion ledges, the sea edge of river deltas. Taking into account the regional geological and geomorphological features of the coasts of the Chukchi and Bering Seas, it should be expected that a possible rise in sea level will lead to thawing and degradation of permafrost rocks and vein ice.

Flocculation of suspended particles during estuarine mixing in the Changjiang estuary-East China Sea

Flocculation, triggered during estuarine mixing plays an important role in land-to-sea interactions and is a fundamental topic in near-shore oceanographic studies. Identifying in situ flocculation in large-river estuaries can be challenging due to the complex seawater circulation and heterogeneous suspended particulate matter (SPM) composition in these areas. In this study, three cruises were conducted in the Changjiang (Yangtze) River Estuary and the adjacent area in March, May, and July 2016. Vertical profiles of SPM total volume, mean size, and size spectra were determined using laser in situ scattering and transmissometry (LISST) measurements at 66–89 stations during the three cruises. Stable isotopic ratios of δ 13 C were also measured in the organic carbon contents of SPM collected at the surface, middle, and bottom layers of the sampling stations. LISST data were used to successfully identify the flocculation occurring in the field as well as to trace SPM size spectrum changes before and after the flocculation process. The δ 13 C values were utilized to study the response of biogeochemical parameters to the flocculation. Phytoplankton blooms occurring in May largely resulted in discontinuous variations in LISST parameters and δ 13 C from March to July. Although the pattern of SPM size spectra involved in flocculation differed among seasons, flocculation processes were always contributed by smaller particles with sizes of several tens of μm aggregating into larger ones >300 μm. Using LISST and δ 13 C measurements combined greatly improved our understanding of SPM dynamics in estuarine and coastal areas where estuarine flocculation was a critical component. • The SPM in the Changjiang Estuary could be classified into three sources. • Three sources were terrestrial sediments, biological particles, and flocculation. • LISST and OBS data could identify flocculation occurring in large-river estuaries. • The δ 13 C variations were more controlled by seasonal or spatial factors. • A combination of LISST and δ 13 C measurements could elucidate SPM dynamics.

Editorial: Eutrophication, algal bloom, hypoxia and ocean acidification in large river estuaries

Editorial: Eutrophication, algal bloom, hypoxia and ocean acidification in large river estuaries

Mapping Dynamic Turbidity Maximum Zone of the Yellow River Estuary from 38 Years of Landsat Imagery

Turbidity maximum zone (TMZ) plays a crucial role in estuarine ecosystems, exerting effects on erosion, environment evolution and socioeconomic activities in the coastal area. However, the long-term understanding of the TMZ in large river estuary such as the Yellow River estuary is still lacking. In this study, we focus on the TMZ distribution, variation and regulation mechanisms in the Yellow River estuary from different time scales. Based on time series Landsat images during the period 1984 to 2021 and Google Earth Engine (GEE), we proposed a TMZ extracting method in the Yellow River estuary to generate 322 TMZ maps. The overall accuracy of our algorithm reached 97.4%. The results show that there are clear decadal and seasonal TMZ variations during the 38-year period in the Yellow River estuary. Morphology, currents and wind speeds combined with seawater stratification have direct effects on TMZ at different time scales, while the direct impacts of tides and fluvial output of the Yellow River on TMZ are limited. In this article, the highly robust method provides a cost-effective alternative to accurately map the TMZ in global large river estuaries and systematically reveals the spatiotemporal evolution of TMZ, shedding light on the response mechanism of coastal geomorphology, marine ecological environment and biogeochemical cycle.

T-S and hydrodynamical structures within the deltaic regions and continental platforms adjacent to two northeastern Brazilian rivers

The delta-front estuaries of large rivers are crucial interfaces between lands and oceans, being considered hotspots of vulnerability to the impact of climate changes due to several causes: sea level rise; changes in the river discharges driven by climatic alterations in the hydrographic basins; high human concentration and other anthropic factors. This study focuses on the hydrodynamics/circulation and T-S structure within the shelf regions and continental platform adjacent to two among the major rivers of Northeastern Brazil: the Sao Francisco and Parnaiba. We aim to qualitatively and quantitatively describe the physical dynamics typical of these coastal regions and try to relate their main alterations to the effects of local mechanisms as tidal forcing. The choice of these deltas was motivated by the great potential that such, biodiverse regions have in acting as important proxies of environmental modifications affecting the Brazilian tropical zone, allowing to investigate the role of geographical variability in response to these changes. A series of regional, modelling simulations was realized, at increasingly higher resolutions, in order to assess the mean and seasonal spatio-temporal variability of physical properties (T-S distribution, salt transport, hydrodynamical features of the local circulation). The identification and analysis of the major scales of variability was also used to evaluate the performance and applicability of our numerical tool, the Regional Ocean Modelling System, in the study areas. Preliminary results on mesoscales processes indicate that both deltas are modulated by an intense turbulent activity. The continental platform near the Sao Francisco estuary is marked by the presence of subsurface-intensified vortices, and the one in proximity of the Parnaiba mouth is characterized by the appearance of small vortices, fine meanders and filaments probably triggered by non-linear interactions between the river plume and the North Brazilian Current rings.

Effects of climate change in the seas of China: Predicted changes in the distribution of fish species and diversity

Long-term habitat shifts in marine fishes under climate change have been discussed over the last few decades. However, there is still a limited understanding of how fish distribution and biodiversity patterns will change in the offshore areas of China over time. In the present study, the potential distributions of 21 important marine fishes under current conditions and two climate change scenarios up to the 2050 s were projected using the maximum entropy (MaxEnt) model. The predicted habitat shifts were further used to observe how future environmental changes would affect communities. By the 2050 s, nine out of 21 fish species may have reduced habitats, which were considered as potential loser species in adapting to climate change, while the other 12 fish species were considered as winner species. The habitats of 20 species were predicted to move northward, with a mean habitat centroid shifting distance from 110 to 206.5 km. A novel discovery is that the Yangtze River Estuary is probably a geographical barrier for the northward migration of some species. An asymmetry in the habitat range shift was observed within the study region, wherein the leading edge moved 1° faster than the trailing edge. The Beibu Gulf, Pearl River Estuary, Southwest Taiwan Strait, and Yangtze River Estuary were identified as fish refuges, indicating that large river estuaries and upwelling systems may have positive effects on reducing the vulnerability of biological communities to climate change. At the community level, species richness would be reduced at lower latitudes, while it would be increased at higher latitudes, and species turnover was the main component of temporal beta diversity. Our study provides an informative outlook on the impact of climate change on the distribution and biodiversity of marine fish species in China, supporting the adaptive conservation and management of fish habitats.

Marine pollen records provide perspective on coastal wetlands through Quaternary sea-level changes

The response of coastal wetlands to future sea-level rise remains uncertain. Palaeoecological data are essential to constrain the still conflicting ecological models. However, obtaining detailed palaeo-coastal stratigraphic records before Holocene is often difficult due to repeated Quaternary marine transgression-regressions. Here we utilize pollen data from a deep-sea sedimentary archive in northern South China Sea to explore the historical behavior of coastal wetlands at a large river estuary over the last 140-kyr. A recurrent wetland (Cyperaceae)-pioneer species (Selaginella)-zonal forest (Pinus) succession throughout the last glacial-interglacial cycle implies a coastal salt marsh origin of the Cyperaceae pollen. Comparing with global sea-level reconstructions, the increases in Cyperaceae pollen abundance, and hence the expansions of coastal salt marsh, were found to be closely linked with rapid large-scale sea-level rises. This finding indicates a resilience of coastal wetlands to future sea-level rise, and highlights the probable importance of conventionally ignored horizontal adaptability in long-term survival of coastal wetlands. Overall, marine pollen records provide an opportunity to supplement existing palaeoecological observations of coastal wetlands during the Quaternary.

Spatial and temporal variations in particulate organic carbon in Indonesian waters over two decades

Particulate organic carbon (POC) is one of the essential variables in oceanic biogeochemical processes. Assessing the spatiotemporal variability of POC will help deepen our understanding of the carbon cycle. In this study we collected moderate-resolution imaging spectroradiometer (MODIS) Aqua satellite data for surface POC in Indonesian waters during the period 2002–21, together with in situ data from the south-western Sumatran waters (SSW), Sunda Strait, Sumba–Savu waters and the Halmahera Sea and surrounds. These data were analysed to gain an understanding of the spatial and temporal variations in POC. Generally, high POC concentrations in Indonesian waters were found at coastal and continental shelves connected to the estuaries of large rivers; we assume that the POC concentrations in these areas are affected by terrestrial input. Meanwhile, in Indonesian waters apart from these estuaries, marine-derived materials are the main components of POC. The seasonal pattern of POC in Indonesian waters generally shows highest levels during the south-east monsoon and lowest levels during the north-west monsoon. Trends in POC concentrations are positively correlated with chlorophyll-a and negatively correlated with sea surface temperature. This study suggests that the combination of natural physicochemical processes and anthropogenic perturbations has significant effects on spatial and temporal variations in POC in Indonesian waters.

Using water age to study the biogeochemistry of nutrients in a large-river estuary and the adjacent shelf area

Whether a large-river estuary and the adjacent area behave as a water sink or as a source for important terrestrial biogeochemical compounds is a challenging question. Such aquatic systems are very complex and can be overwhelmed by physical circulation and estuarine mixing. In this study, physical oceanography and marine biogeochemistry were combined to address this question. Seven cruises were conducted in the Changjiang River (Yangtze) Estuary and the adjacent East China Sea shelf in February–March and in July over a three year period (2014 to 2016). During each cruise, nutrient concentrations (NO3−, SiO32−, PO43−, NH4+, and NO2−) and salinity were measured at the surface, middle, and bottom layers of 75–106 sampling stations, and water age was derived from numerical modeling. Two-variable regression analysis among salinity, water age, and nutrients was applied to calculate the specific variation rates of nutrient concentration versus water age. A positive rate value indicated that the water was a source of nutrients, and a negative value indicated the site as a sink. In both seasons tested, calculations for the five nutrients analyzed showed that all biogeochemical processes occurring in Changjiang Diluted Water (salinity <31.5) could distinctly change terrestrial fluxes from the Changjiang River in both seasons. Thus, the proposed method has the potential to elucidate the nonconservative behaviors, net fluxes, and budgets of important biogeochemical compounds through large-river dominated estuarine, coastal, and shelf areas worldwide.

Daily variability of copepods after successive tropical cyclones in the Ganges River estuary of India

Excessively large river estuaries of tropics can be impacted within months by successive tropical cyclones. The effect of these disruptions on the community ecology of such estuaries has been scarcely examined. On a daily basis for a week, the copepod community of the Ganges River estuary of India, was studied immediately after Cyclone Fani and Cyclone Bulbul in May and November 2019, respectively. The objective was to understand the ecological response of the copepod community of the Ganges River estuary facing successive extreme environmnetal changes in short interval. Differences in landfall sites and the amount of rain associated with each tropical cyclone led to different degrees of temporal changes of salinity, pH, and water-temperature of the estuary which might have caused the observed addition-deletion of species, rearrangements of abundance-hierarchy and expansion-shrinkage of omnivore-herbivore feeding guilds of the copepod community. The copepod Bestiolina similis maintained, however, its numerical dominant status. Tropical cyclone driven changes to the copepod community of an estuary may be temporary but the changes may affect benthic-pelagic linkages. Following an extreme environmntal change (e.g. cyclone, flood), an institution-backed monitoring (mix of high to low intensity sampling lasting for a few hours to years) with matching scales of zooplankton and biogeochemical data collection, encompassing freshwater and marine dominated sections of the Ganges River estuary is recommended. Nations bordering the Indian Ocean may consider a similar recommendation for monitoring their estuaries after any extreme environmental change, taking account of locale-specific modifications, and initiate a regional programme of estuaries on the United Nations Decade of Ocean (2020–2030) by collaborating through inter-governmental platforms.

Exchange Dynamics of Typical Emerging and Legacy Persistent Organic Pollutants at the Air‐Water Interface Over a Strongly Human‐Influenced Large River Estuary

AbstractTo explore the riverine or atmospheric influence on the source‐sink fates of typical persistent organic pollutants (POPs): polybrominated diphenyl ethers (PBDEs) and polychlorinated biphenyls (PCBs) in the large river estuary, atmospheric and seawater samples were collected simultaneously over a whole year at a remote small island in the Yangtze River Estuary (YRE), the East China Sea. A comparable net input of BDE‐209 from the atmosphere to the sea for both air‐water gas exchange and dry/wet deposition was observed. The gas deposition of BDE‐209 had a strong dependence on the gaseous concentration, which was in turn governed by atmospheric transport. For Br&lt;10‐BDEs, which have been prohibited in recent years, the gas exchange almost reached equilibrium at the air‐water interface during the whole observation period. However, a strong net gas volatilization was observed for legacy ΣICESPCBs especially in the flood season, and the volatilization was significantly associated with ambient parameter‐wind speed. These remarkably different exchange dynamics of PBDEs and PCBs at the air‐water interface over the YRE could be attributed to their primary emission intensity and inconsistent environmental transport pathways. The reversal of the source‐sink fates of BDE‐209 and legacy PCBs suggest that the strongly human‐influenced large river estuary water could act as a source‐sink buffer for POPs during the switch from approved to prohibited usage.

The current state and 125 kyr history of permafrost on the Kara Sea shelf: modeling constraints

Abstract. The evolution of permafrost on the Kara shelf is reconstructed for the past 125 kyr. The work includes zoning of the shelf according to geological history; compiling sea level and ground temperature scenarios within the distinguished zones; and modeling to evaluate the thickness of permafrost and the distribution of frozen, cooled and thawed deposits. Special attention is given to the scenarios of the evolution of ground temperature in key stages of history that determined the current state of the Kara shelf permafrost zone: characterization of the extensiveness and duration of the existence of the sea during stage 3 of the marine oxygen isotope stratigraphy (MIS-3), the spread of glaciation and dammed basins in MIS-2. The present shelf is divided into areas of continuous, discontinuous-to-sporadic and sporadic permafrost. Cooled deposits occur at the western and northwestern water zones and correspond to areas of MIS-2 glaciation. Permafrost occurs in the periglacial domain that is within a zone of modern sea depth from 0 to 100 m, adjacent to the continent. The distribution of permafrost is mostly sporadic in the southwest of this zone, while it is mostly continuous in the northeast. The thickness of permafrost does not exceed 100 m in the southeast and ranges from 100 to 300 m in the northeast. Thawed deposits are confined to the estuaries of large rivers and the deepwater part of the St. Anna trench. The modeling results are correlated to the available field data and are presented as a geocryological map. The formation of frozen, cooled and thawed deposits of the region is inferred to depend on the spread of ice sheets, sea level, and duration of shelf freezing and thawing periods.