Near-bottom Water Research Articles

Distribution of methane in the upper water layer of the northern Black Sea: Seasonal and daily trends and seawater-air emissions

We report on methane (CH4) concentration measurements in the northern Black Sea area conducted during 6 cruises with R/V Professor Vodyanitsky from 2017 to 2019. Our work is a multi-season study at a uniform station grid covering an area of 88 × 103 km2 and including three latitudinal transects that comprises both surface and vertical profile water-column measurements. The main goal of the work was to assess the seasonal patterns of vertical CH4 structure in the aerobic water column (upper 100 m) and its emission to the atmosphere.In surface waters, the mean dissolved CH4 concentration ranged from 2.6 nmol L−1 detected in November 2018 to 11.5 nmol L−1 measured in June–July 2018, respectively. Calculated CH4 seawater-air fluxes and saturations were mostly positive (i.e. net flux to atmosphere), and winter fluxes (2.6 μmol m−2 d−1) were higher than summer fluxes (1.6 μmol m−2 d−1) due to the higher wind speed. The integral CH4 flux from the whole study area (88 × 103 km2) ranged from 84 to 235 kM day−1.It was shown that, on average, the methane concentration in the upper layer for deep-water stations where the seabed is located at depths >160 m (σt >16.2) was lower compared to stations at shallow water depths (28–140 m, σt <16.2). The most distinct difference was obtained for the summer season (June–July 2018) and a less significant difference – for spring (April–May 2019) and winter season (November–December 2018). During these seasons the water column was also considerably less saturated in CH4 compared to the entire monitoring period. We observed subsurface maxima, which were generally located at the base of the thermocline and exceeded 100 nmol L−1 at some stations. Exceptions were observed in October 2019 (cruise 110), when vertical CH4 distributions were characterized by two-peaks at ∼20 and ∼50 m depth. The strong influence of the thermohaline structure on the water column CH4 distribution has also been shown in studies of daily dynamics of CH4 vertical profiles in the shallow water region. Despite the high variability of CH4 concentrations, significant similarities in vertical distributions of CH4 and chlorophyll-a for which sub-surface maxima coincided at some stations, are shown. Extremely high concentrations of CH4 (up to 351 nmol L−1) in the near-bottom water layer were revealed during all seasons at the station near the Dnieper paleo-channel at the northwestern edge of the study area. This enrichment is assumed to be caused by methane emissions from gas seeps densely located in this region.

Dissolved oxygen fluctuation alters dissolved organic carbon stability and can accumulate resistant fraction

Recurrent dissolved oxygen (DO) depletion is common in many estuaries and coastal areas worldwide. However, its impact on the accumulation of resistant dissolved organic carbon (DOC) remains controversial. To address how recurrent DO depletion affects the degradation and stability of marine DOC, a series of DOC degradation incubations in a particulate-dissolved coexisting system in April 2021 were conducted, followed by a field investigation in the East China Sea (ECS) in October. In the ECS, we found that DO ranged from 4.71 to 7.40 mg/L in the near-bottom waters, while the degradation index (DI), an indicator of amino acids molecules, increased from −0.78–2.84, suggesting an accumulation of labile DOC over DO depletion. This is further supported by the bacteria community composition that showed an increase in anaerobic metabolism families when DO <6.29 mg/L. Laboratory incubation revealed strong labile DOC release (∼ 243 ± 38 μmol/L) from particulate organic carbon (POC) over oxygen depletion, followed by significant DOC loss in either anoxic-oxic or oxic-oxic groups. The degraded original DOC was slightly less in the oxygen depletion-treated group compared to the oxic group, and the resistant proportion was estimated as 1% ∼ 8% after 1–14 days of anoxic treatments. The resistant DOC accumulation after DO alleviation for 30 days was much less in amount when compared to the big release of labile DOC over DO depletion (48 vs. 243 μmol/L) and we believed that the lack of observed resistant DOC accumulation in the field observation was due to the overlap of multiple anoxic-oxic cycling processes and the masking effect of newly labile DOC released from the POC.

Wintertime diffusion of sedimentary phosphorus – implications for under-ice phosphorus removal from eutrophic lakes

PurposeMany eutrophic lakes are located in regions where lakes become ice-covered during the winter. This study aimed to find out if phosphorus (P) could be withdrawn from such lakes by utilizing the wintertime accumulation of P to the near-bottom water.MethodsData for water quality and sediment characteristics were collected from two eutrophic boreal lakes with tube samplers and sediment corers. Diffusion rates of P across the sediment-water interface (SWI) and within the active sediment layers, and potential export of P via wintertime withdrawal were calculated.ResultsIn the stratifying Lake Kymijärvi, P concentration in the near-bottom water reached 66 µg L−1 and P diffusion across SWI in the hypoxic area 5.4 mg m−2 d−1. In the shallow Lake Savijärvi, maximum P concentration was 78 µg L−1 but P diffusion rate only 0.34 mg m−2 d−1. In Kymijärvi, the concentrations of Fe and Mn in the sediment were high relative to P. In Savijärvi, sediment P was bound to clay minerals and calcium carbonates, while Fe was bound in sulfides.ConclusionIn Kymijärvi, a theoretical14.3% reduction in epilimnetic TP concentration could be achieved in 20 years with 20 L s−1 winter withdrawal. In Savijärvi, 10 L s−1 withdrawal could theoretically cause a 5.8% reduction in TP concentration in 5 years, but the low P diffusion rate across SWI, and the low discharge of the lake may limit P removal. In Kymijärvi, where summertime withdrawal is already applied, additional winter withdrawal could accelerate lake recovery.

Insights into carbonate environmental conditions in the Chukchi Sea

Abstract. Healthy Arctic marine ecosystems are essential to the food security and sovereignty, culture, and wellbeing of Indigenous Peoples in the Arctic. At the same time, Arctic marine ecosystems are highly susceptible to impacts of climate change and ocean acidification. While increasing ocean and air temperatures and melting sea ice act as direct stressors on the ecosystem, they also indirectly enhance ocean acidification, accelerating the associated changes in the inorganic carbon system. Yet, much is to be learned about the current state and variability of the inorganic carbon system in remote, high-latitude oceans. Here, we present time series (2016–2020) of pH and the partial pressure of carbon dioxide (pCO2) from the northeast Chukchi Sea continental shelf. The Chukchi Ecosystem Observatory includes a suite of subsurface year-round moorings sited amid a biological hotspot that is characterized by high primary productivity and a rich benthic food web that in turn supports coastal Iñupiat, whales, ice seals, walrus (Odobenus rosmarus), and Arctic cod (Boreogadus saida). Our observations suggest that near-bottom waters (33 m depth, 13 m above the seafloor) are a high carbon dioxide and low pH and aragonite saturation state (Ωarag) environment in summer and fall, when organic material from the highly productive summer remineralizes. During this time, Ωarag can be as low as 0.4. In winter, when the site was covered by sea ice, pH was &lt;8 and Ωarag remained undersaturated under the sea ice. There were only two short seasonal periods with relatively higher pH and Ωarag, which we term ocean acidification relaxation events. In spring, high primary production from sea ice algae and phytoplankton blooms led to spikes in pH (pH &gt; 8) and aragonite oversaturation. In late fall, strong wind-driven mixing events that delivered low-CO2 surface water to the shelf also led to events with elevated pH and Ωarag. Given the recent observations of high rates of ocean acidification and a sudden and dramatic shift of the physical, biogeochemical, and ecosystem conditions in the Chukchi Sea, it is possible that the observed extreme conditions at the Chukchi Ecosystem Observatory are deviating from the carbonate conditions to which many species are adapted.

Widespread and increasing near-bottom hypoxia in the coastal ocean off the United States Pacific Northwest

The 2021 summer upwelling season off the United States Pacific Northwest coast was unusually strong leading to widespread near-bottom, low-oxygen waters. During summer 2021, an unprecedented number of ship- and underwater glider-based measurements of dissolved oxygen were made in this region. Near-bottom hypoxia, that is dissolved oxygen less than 61 µmol kg−1 and harmful to marine animals, was observed over nearly half of the continental shelf inshore of the 200-m isobath, covering 15,500 square kilometers. A mid-shelf ribbon with near-bottom, dissolved oxygen less than 50 µmol kg−1 extended for 450 km off north-central Oregon and Washington. Spatial patterns in near-bottom oxygen are related to the continental shelf width and other features of the region. Maps of near-bottom oxygen since 1950 show a consistent trend toward lower oxygen levels over time. The fraction of near-bottom water inshore of the 200-m isobath that is hypoxic on average during the summer upwelling season increases over time from nearly absent (2%) in 1950–1980, to 24% in 2009–2018, compared with 56% during the anomalously strong upwelling conditions in 2021. Widespread and increasing near-bottom hypoxia is consistent with increased upwelling-favorable wind forcing under climate change.

Use of Zeolite (Chabazite) Supplemented with Effective Microorganisms for Wastewater Mitigation of a Marine Fish Farm

A study was conducted to assess the efficacy of chabazite zeolite in mitigating ammonia levels in wastewater from a land-based marine fish farm in southern Tuscany (Italy). The fish farm discharges effluent into a lagoon, constituting an important eutrophication source. The experimental setup involved a pond/canal that received wastewater from three sea bream tanks (40 L/s). A 50 m canal section was divided into two parallel halves (T and B), each about 3 m wide. In T, a chabazite bed (granules about 3 cm ϕ) was placed that was about 6 cm thick; B was untreated and used as a control. Five sampling trials were conducted in both T and B to determine N-NH4, N-NO3, and P-PO4 levels, in surface and near-bottom waters at both input and output. Prior to the zeolite addition, T and B sediments were sampled for TN and TP determination. Results indicated the not-managed canal system released nutrients and the output values were higher than the input, overshadowing the zeolite effects. Significant zeolite effects were observed by comparing B and T for differences between input and output: in T, nitrate increased (p = 0.05), demonstrating a resumption of nitrification, and ammonium (p = 0.07) and SRP (p = 0.06) decreased, in contrast to B.

First data on the occurence and ecology of Giant hookear sculpin &lt;i&gt;Artediellus ingens&lt;/i&gt; (Cottidae) off the Kuril Islands

Based on materials collected during monitoring of bottom trawl fishing, the occurrence, some aspects of ecology and size composition of giant hookear sculpin Artediellus ingens were examined for the first time off the Middle Kuril Islands. It has been established that this species is a regular bycatch at the depths 250–488 m at the temperature of the near-bottom water layer of 1.8–3.9 °C. Most likely A. ingens belongs to mesobenthal ichthyocene. In the trawl catches, this species is represented by individuals of 10–19 cm TL. Comparison of the maximum body length with that of the other species of the genera Artediellus indicated this species is one of the largest.

Microbial communities in the fluid migration zone in the sediments of the Krasny Yar methane seep (South Baikal)

The diversity and structure of microbial communities were investigated using 16S rRNA gene barcoding in the sediments of the Krasny Yar methane seep, in the zone of near-surface occurrence of gas hydrates and the presence of oxidized and restored channels. The diversity of both bacteria and archaea along the core depth was similar to the diversity found in sediments from other areas: methylotrophic methanogens and microorganisms involved in different stages of the organic matter fermentation were detected in the sediment strata of all depths investigated. Migration flows of oxygen-rich and aerobic bacteria-rich near-bottom water influenced greatly the diversity of microbial communities in oxidized channels. Fluids migrating from the deep zone to the bottom surface provided transport from anoxygenic sediments of anaerobic archaea involved in the AOM process. The data obtained are consistent with geochemical and geothermal indicators defining the zone of active migration of near-bottom waters.

Стерические колебания уровня Балтийского моря

The main purpose of the paper is to estimate the capabilities of NEMO 3.6 regional reanalysis data for a realistic description of the Baltic Sea steric level oscillations, the spatio-temporal structure research and causes of modern sea level changes, induced by fluctuations in water density, in the range of interannual and seasonal variability scales. Comparison of the steric level oscillations series calculated from contact measurements of temperature and salinity at oceanographic stations and from regional reanalysis data is carried out by evaluating various statistical accuracy criteria. The paper shows that the reanalysis data allow reproducing the Baltic Sea steric level oscillations quite accurately. Estimates of linear trends indicate that in the period 1993–2020 in the open Baltic and in the west of the Gulf of Finland, the steric sea level is mainly decreasing, while in the Gulf of Bothnia the steric level is increasing. Harmonic analysis of steric level oscillations showed that the annual harmonic a in the range of seasonal variability makes a predominant contribution to steric level oscillations, its amplitude significantly exceeds the amplitudes of the harmonics sa, ta and qa. The thermosteric component has the main influence on seasonal steric level oscillations, and only in the south-west of the sea there is a significant influence of the halosteric component. During the period under review, positive linear trends were observed in the amplitudes changes of the harmonics a, sa, ta and qa in most regions of the Baltic Sea, however, in the south-west of the open Baltic, a decrease in the amplitudes of all four harmonics of seasonal steric level oscillations was observed in the south-west part of the Open Baltic Sea. In conclusion, based on the obtained results, it is assumed that the identified modern regional changes in the Baltic Sea steric level oscillations are associated with an increase in air temperature, an increase in precipitation, a decrease in wind intensity, a desalination of the upper quasi-homogeneous layer and an increase in deep and near-bottom waters salinity caused by water exchange with salty waters of the North Sea.

Microbial Community Composition as an Indicator of the State of Basins Located at the Sea Coast (Exemplified by the Kanda Bay, Kandalaksha Gulf, White Sea)

Abstract—Formation of the Kanda Bay resulted from construction of a railway dam (1916) and subsequent isolation of the sea lagoon from the main basin of the Kandalaksha Gulf, White Sea. Decreased action of tidal flows, which mix the water column of the lagoon, altered the hydrological regime of the basin. Decreased water exchange resulted in formation of oxygen-depleted near-bottom water and to sulfide contamination. A freshwater lake was, however, preserved in the southern part of the Kanda Bay. The composition of microbial communities was studied for the near-bottom water horizons at different sides of the Kanda Bay. The oxygen regime in this layer was found to change, with increasing concentrations of sulfide and methane and active processes of sulfate reduction and methane oxidation. The composition of the microbial community changed noticeably, with lower abundance of true marine and freshwater microorganisms and development of bacteria and archaea predominant in microbial communities of anoxic water in meromictic basins. Among the microbial diversity, indicator species with increased abundance were revealed. These are archaea of the genera Methanoregula and Methanosaeta (phylum Halobacterota). The sulfur cycle microorganisms, which were the indicators of stagnant marine water, included anoxygenic phototrophic bacteria of the class Chlorobia, (Chlorobium phaeovibrioides, Pelodictyon phaeoclathratiforme), Chloroflexi of the genus Chloronema, nonsulfur purple bacteria related to the genus Rhodoferax, colorless sulfur bacteria of the family Beggiatoaceae, and sulfur oxidizers of the genus Thiobacillus. Archaea of the genus Nitrosopumilus (phylum Crenarchaeota) and bacteria of the genus Woeseia may be considered opposites to the indicator microorganisms, since they were found only in the open sea water. In our opinion, stable water exchange through the dam will result in the stable composition of the Kanda Bay microbial community, with only seasonal variations and year-to-year fluctuations. The negative scenario supports prediction of conversion of the Kanda Bay into a stratified basin with anoxic near-bottom water and the microbial community similar to that found in meromictic lakes.

Anoxygenic Phototrophic Bacteria of the Meromictic Lake Bol’shie Khruslomeny (Oleniy Island, Kandalaksha Gulf, Murmansk Oblast, Russia)

Abstract—The composition and structure of the anoxygenic phototrophic bacterial (APB) community in the water column of Lake Bol’shie Khruslomeny during winter were investigated. The community developed at the depth of 4.25 m, and its activity during the ice-covered period was very low (6.2 µmol C L–1 day–1). The water in the zone of highest development of phototrophic bacteria was of an unusual lemon-yellow color, probably due to the production and accumulation of polysulfides. The near-bottom water was also of lemon-yellow color and was resistant to oxidation by the air oxygen. In the zone of peak APB development the content of BChl d from green-colored green sulfur bacteria was considerably higher than that of BChl e from brown-colored green sulfur bacteria: 77 and 23%, respectively. The cultures of green and purple sulfur bacteria were isolated from lake water, and their physiological and genetic characteristics were determined. Two strains of green sulfur bacteria (brown-colored BrKhr17 and green-colored GrKhr17) contained gas vacuoles in their cells. Phylogenetically they were most closely related to the green-colored strain Chlorobium phaeovibrioides DSM 265 and were identified as new Chlorobium phaeovibrioides strains.

Properties of Antarctic Bottom Water in the Western Gap (Azores-Gibraltar Fracture Zone, Northeast Atlantic) in 2021

We present the results of measurements of thermohaline and dynamical structures, as well as hydrochemical parameters of near-bottom water in the Western Gap (Azores-Gibraltar Fracture Zone, Northeast Atlantic). The data were collected during cruise 59 of the research vessel Akademik Ioffe in October 2021. The results of the analysis showed that water with potential temperatures lower than 2.00 °C had crossed the Entrance Sill, but was not detected inside the interior basin of the Western Gap. The velocities of currents at the Entrance Sill were high (up to 30 cm/s) leading to mixing of the water overflowing the sill. While Antarctic Bottom Water (AABW) flows from the Entrance Sill to the Exit Sill it is subjected to modification. Bottom water becomes less dense after the sill due to mixing during overflow and in addition not all cold water overflows the sill. The dissolved oxygen concentration in the bottom layer increases from the Entrance Sill (239–242 μmol/kg) to the Exit Sill (241–248 μmol/kg) indicating the transformation of AABW as it circulates in the Western Gap and mixes with North Atlantic Deep Water propagating from the north. All velocity measurements indicate that north velocity component is a characteristic property of the bottom flow. Thus, water with specific chemical composition of AABW is transported to the Iberian Basin despite that its potential temperature becomes higher than 2 °C.

Closed-circuit hypolimnetic withdrawal and treatment: impact of effluent discharge on epilimnetic P and N concentrations

Closed-circuit hypolimnetic withdrawal and treatment systems (HWTS) represent a novel lake restoration technique in which nutrient-rich near-bottom water is pumped through a treatment system and returned to the same lake. However, the design of such systems is not yet standardized and routing of effluent waters must be planned carefully to minimize the risk of adverse water quality impacts. Here we assessed the risk of HWTS effluent to elevate epilimnetic nutrient concentrations under a range of withdrawal and effluent discharge scenarios (4.5–45 L/s, sand filtration only and sand filtration combined with wetland) at Lake Kymijärvi, Finland. The filter of the HWTS removed most of the phosphorus (67%), but only a small fraction of nitrogen (14%). For both nutrients, filter effluent concentrations were elevated with respect to the lake epilimnion. However, the results of our calculations suggest only minor increases (0–12%) in epilimnetic phosphorus concentrations in all withdrawal and discharge scenarios. For nitrogen, somewhat higher increases (1–17%) are expected unless the filter effluent is first discharged into a wetland as part of the HWTS circuit. We conclude that the impacts of the filter effluent on the epilimnion do not mask the benefits gained in the treated lake by the closed-circuit HWTS, but use of a buffering system such as a wetland decreases the risks further.

Chemical warfare agents and their risk assessment in Daphnia magna and fish in the Baltic Sea – 15 years of measurements

The Baltic Sea is one of the world's largest brackish waters, it is drained through the Danish Straits into the Kattegat, and almost enclosed by nine countries. The Baltic Sea is one of the world's most polluted water bodies thus facing a wide range of environmental threats towards its water resources, such as fish stocks, and coastal environments and economics. Chemical warfare agents (CWAs) that were dumped following the Second World War are known to occur in intact or degraded states in sediments and are documented to affect benthic fauna and fish as well as having injured fishermen having accidentally caught lumps of CWAs in their nets. However, a thorough mapping of remaining CWAs and degradation products and an understanding of the environmental risks does not exist although more than 75 years have passed. This study compiles and analyzes published/peer-reviewed data, generated since 2005 in five selected comprehensive studies, on sediment measurements of known dumped CWAs and degradation products in the Baltic Sea. As a worst-case approach, sediment concentrations are transformed to concentrations in near-bottom water, which represents Predicted Environmental Concentrations (PECs) to marine biota. To investigate the accuracy and representativeness of toxicological values, which are fundamental in risk assessments, two cases are considered: Case 1 (specificity) uses toxicological data (EC50 or NOEC) for Daphnia magna and fish with applied assessment factors (AF) to derive Predicted No-Effect Concentrations (PNECs) for organoarsenical and non-arsenical CWAs; Case 2 (robustness) uses partly Danish Environmental Quality Standards (DK EQS) for arsenicals, and partly a Water Quality Criterion (WQC) for arsenicals, representing the marine environment. From 872 data points risk quotients (RQs=PEC/PNEC) are calculated. In Case 1 exceedances of risk for the sum of chemicals (sumRQ>1) occur 24 and 1 times for Daphnia magna and fish, respectively, without applying AFs. 263 and 120 exceedances are found for Daphnia magna and fish, respectively, when applying AFs. Case 2 shows 0 (WQC) and 25 (DK EQS) exceedances for arsenicals when using more robust toxicological values, however, at the expense of specificity of chemicals and target species. The results underline the importance of obtaining more representative and accurate toxicological data (lowering AFs) in order to increase the accuracy of the risk estimates. This quantitative state of risk towards representative marine species indicates that there are indeed potential risks, and it qualifies the understanding and debate on the challenges and future actions regarding dumped chemical munitions in the Baltic Sea.

Vertical distribution of microplastics in a river water column using an innovative sampling method.

Due to limitations of sampling methods, subsurface water is usually a less well-investigated compartment of the water column when scientists assess microplastic contamination. In this study, microplastic (MP) contamination was assessed in a freshwater river both in surface and subsurface using an innovative sampling method. Microplastic contamination in the lower part of the water column, i.e., near-bottom water and in sediments, was also studied. Three sampling campaigns were carried out during different weather conditions: stormy, rainy, and dry in order to observe their influence on the microplastics vertical distribution. No significant difference was observed between the abundance and types of MPs in surface and subsurface water. The proportion of polymer with theoretical density < 1 (polypropylene d = 0.9, polyethylene d = 0.91-0.95) and polystyrene (d = 0.1-1.06) in the surface and subsurface samples was 73.5%, and this proportion drops to 40.8% for the samples located in the near-bottom water and the sediments. Our results indicate that the MP concentration of the different compartments analyzed can be significantly influenced by rainfall during and prior to the sampling day. This study highlights that in shallow rivers, surface water sampling is representative of the water column MP contamination, but that sampling without taking environmental conditions into account may lead to erroneous estimation of MPs concentration and flux entering the marine environment.

Hydrobiological characteristic of small lakes in northern Karelia during the ice-covered period

The life of lakes in winter is still insufficiently studied. The purpose of this article is a comparative analysis of the quantitative development of planktonic and benthic communities and the abiotic conditions of their existence during the ice-covered period in two small (&lt;1 km2) northern lakes that differ greatly in morphometry and the degree of water humification. A significant difference was revealed in the seasonal dynamics of plankton communities in the studied lakes. In a deeper lake with water uncolored by humus, the quantitative development of phytoplankton at the end of the ice-covered period was almost as significant as during the open-water period. It is necessary to note the rapidity of changes in under-ice phytoplankton in early spring: in April 2020, the content of chlorophyll a in water changed more than six times during a week. There was no mass development of phytoplankton under the ice in a shallow highly humified lake. The zooplankton of the studied lakes was characterized by the opposite trend. In the deep-water lake, the winter zooplankton was noticeably inferior to the summer one in terms of the level of quantitative development; in the shallow lake, a high biomass (~1 g/m2) of zooplankton was noted throughout the entire study period. The macrozoobenthos of lakes in winter, on the whole, retained the same high level of quantitative development as in the open-water period. Seasonal differences were local and were caused by the redistribution of bottom animals due to changes in the oxygen and, possibly, temperature regime of bottom waters. In particular, due to hypoxic-anoxic conditions at the end of the ice-covered period, benthic animals migrated from the profundal zone of the deep-water lake. After the ice melted and the near-bottom waters were saturated with oxygen, the profundal community quickly recovered. Thus, the quantitative development of biological communities in winter can vary greatly even in closely located lakes, in particular, it depends on the degree of humification of the water body. Given the high dynamism of phytoplankton in early spring, the assessment of the role of the under-ice period in the life of lakes, apparently, requires long-term research, including detailed observations at the end of ice-covered period.

Methane Production in a Temperate Freshwater Lake during an Intense Cyanobacterial Bloom

Abstract—Seasonal cyanobacterial blooms have a negative impact on freshwater ecosystems. The role of cyanobacteria in methane production and their relationship with methanogenic archaea are not yet well understood. The goal of the present work was to identify the features of methanogenesis in the water column and sediments of a profoundal part of the freshwater Lake Senezh (Moscow oblast) during a period of cyanobacteria over-bloom. Analytical, radiotracer, microscopic, molecular biological, and incubation techniques were used. Alkalization and oxygen oversaturation of the 0–2-m water layer were caused by intensive photosynthesis. The near-bottom water (4 m) was pH-neutral and hypoxic; the sediments were reduced. Methane was detected throughout the water column; its concentration in the surface water was an order of magnitude lower than in the near-bottom water and 4 orders of magnitude lower than in the sediments. Cyanobacteria of the species Microcystis aeruginosa predominated in the photic zone (up to 30% of the total number of the 16S rRNA gene fragments). The sequences of cyanobacteria and freshwater members of the SAR11 clade, which can potentially be involved in aerobic methanogenesis via decomposition of methylphosphonates (MPn), were also detected. The sequences of hydrogenotrophic methanogens of the genus Methanoregula, which are potentially capable of methanogenesis in cooperation with cyanobacteria, were revealed in oxygen-supersaturated water. Hydrogenotrophic and aceticlastic pathways of methanogenesis predominated in reduced sediments. Sequences of methanogens of to the orders Methanomicrobiales, Methanobacteriales, Methanosarciniales, and Methanomassiliicoccales were detected there. Cyanobacterial bloom promoted methanogenesis both in the photic zone of Lake Senezh (due to MPn decomposition and anaerobic methanogenesis in association with cyanobacterial aggregates) and in the near-bottom water and sediments (due to oxygen depletion and excessive release of substrates caused by sedimentation and degradation of cyanobacterial mortmass).

Behaviors of redox-sensitive tungsten and molybdenum in the northern South China Sea: From the Pearl River to the continental slope

To increase the proxy toolbox for the reconstruction of redox conditions, the transition metal tungsten (W) has been increasingly suggested as a promising new candidate during the last few years. However, compared to other trace metals such as molybdenum (Mo), studies on the redox behavior of W in the brackish/marine environment are relatively rare and mainly limited to euxinic settings. To contribute to our understanding of the behavior of W in modern aquatic ecosystems, we here present a water column W and Mo dataset that follows the salinity gradient of the Pearl River Estuary towards the adjacent coastal and offshore waters of the northern South China Sea (water depths range from 36 m to 3735 m) with the latter also including mostly anoxic porewater and rather oxic sediment surface data. Except for deviations of W in the low-salinity reaches of the Pearl River Estuary, both metals are conservatively introduced to the coastal ocean. The contribution of dissolved W via the Pearl River plume was identified in surface waters along the coastline. In line with previous open ocean studies, the conservative behavior of W proceeds towards the remote parts of the study area with surface waters that are possibly also affected by the South China Sea Warm Current. Within the short core porewater profiles, W concentrations increased with sediment depth and reached the highest levels at the more distal sites. These trends are not linked to the formation of more soluble thiotungstate species as indicated by generally non-sulfidic porewaters. Along with porewater gradients of W, the W concentrations in near-bottom waters suggest effective trapping of W at the sediment/water interface by Mn oxides especially at greater water depths and limited contribution to the open water column W inventory. In contrast to W, Mo concentrations show little variation in the reducing porewaters and remain close to or slightly above the ocean level. Enrichments of W and Mo in the surface sediments from the upper and lower continental slope reflect the known affinity of both trace metals to Mn oxides. Positive correlations of W with the heavy mineral-associated metals titanium (Ti) and zirconium (Zr) in sand-rich deposits of the outer continental shelf subjected to stronger near-bottom currents, further indicate a relation of sedimentary W abundances to the depositional environment.

Nutrient Flux under the Influence of Melt Water Runoff from Volcanic Territories and Ecosystem Response of Vilyuchinskaya and Avachinskaya Bays in Southeastern Kamchatka

Nutrient fluxes with river runoff can largely determine the state of coastal water ecosystems. The Vilyuchinskaya and Avachinskaya Bays of Kamchatka Peninsula were surveyed on 4–5 July 2022, just after the peak of the spring–summer flood of the Vilyucha and Avacha Rivers associated with the snow melting in volcanic areas. Additionally, water sampling was performed in river water in December 2022, in the period of winter low water. A general undersaturation of CO2 of surface waters was observed in the Vilyuchinskaya and Avachinskaya Bays. Strong supersaturation of CO2 of the near-bottom waters was observed, due to microbial destruction of organic matter, especially in Avachinskaya Bay. This organic matter was formed as a result of photosynthesis in the water column, which subsequently gravitated to the bottom and concentrated in specific bottom depressions in the studied bays. In these depressions, black sediments were found, in which sulfate reduction occurred. The porewater of sediments had a very high concentration of organic carbon, dissolved and organic nitrogen, and phosphates, as well as CO2 partial pressure. The source of nutrients for the studied bays is tephra (volcanic suspended particles), which covers all the watersheds, including the soil and snow, as a result of volcanic activity in the study region. Based on the calculated nutrient fluxes in river runoff for the summer and winter seasons and the DIN/DIP ratios in river, sea, and pore waters, the causes of phytoplankton blooms, including the occurrence of red tides, are discussed. The maximum flux of nutrients and, accordingly, phytoplankton blooms should be expected in the years with the highest volcanic activity and high summer air temperatures, since the melting of nutrient-rich snow and ice will be maximum. This will determine the flux of nutrients in the river runoff of the Kamchatka Peninsula and contribute to phytoplankton blooms.

Аккумуляция ртути в двустворчатых моллюсках в гидроэкосистеме устьевой области реки Красная (Вьетнам)

The article presents analysis of mercury content in different tissues such as gills, mantle, hepatopancreas and body (leg) in 4 species of bivalve molluscs in the estuary of the Red River in Vietnam. The studies were carried out in 2016–2021 during the main hydrological seasons (during the summer flood and winter low water) at 37 stations in the delta and estuarine coast of the Red River. There are shown the dependences of mercury content in mollusc tissues on the age parameters (length and mass of tissues) and on the content of pollutant in dissolved, suspended forms of near-bottom water layers and in bottom sediments. The mercury content in the tissues of molluscs varies whithin 0,023 ± 0,001 to 0,459 ± 0,007 mg/kg wet weight. The highest content of mercury was found in the hepatopancreas &#x0D; of Austriella corrugata, and the lowest - in the leg of Lutraria rhynchaena. Significant close positive correlations were registered between the mercury concentrations in the mollusc tissues and the mercury content in the near-bottom water layer and in bottom sediments (r from 0.68 to 0.92, p &lt; 0.05). All studied species of molluscs are classified as macroconcentrators of mercury in water; Austriella corrugata can actively accumulate mercury not only from the water, but also from the bottom sediments. Bivalve molluscs can be used as an object of biomonitoring when studying the distribution of mercury in the estuary of the Red River.