Benthic Layer Research Articles

Effect of Climate and Socio-Economic Changes on the State of the Baltic Sea Ecosystems

In this study, calculations are performed to reproduce the main components of the Baltic Sea ecosystems based on the stationary, boxed SANBALTS model under various scenarios of climate change in combination with two scenarios of the development of socio-economic conditions. Under various warming scenarios, a decrease in the nutrient load will lead to a reduction in the area of hypoxic zones in the Baltic Sea, while the most significant decrease in the area is due to the scenario of a reduction in the agricultural sector and population. The phosphorus in the benthic layer will decrease, the concentration of inorganic nitrogen will increase, and nitrogen fixation will decrease. The most noticeable decrease in primary production and an increase in water transparency is associated with a moderate climate change scenario and with a significant decrease in the nutrient load. The results showed that under certain combinations of scenarios, an improvement in the ecological state of the sea can be expected even under the most unfavorable climatic scenario.

Reproduction of the Current Climatic State of the Lake Ladoga Ecosystem

A three-dimensional ecohydrodynamic model of Lake Ladoga based on the St. Petersburg Baltic Eutrophication Model (SPBEM) is proposed. Unlike existing models of the Lake Ladoga ecosystem, the proposed model is implemented on a high-resolution spherical grid (horizontal grid size ≈1 km), contains a benthic layer module and describes the cycles of nitrogen and phosphorus in the water column and bottom sediments. A run of the seasonal and interannual variability of the state of Lake Ladoga in the period 1979–2018 was carried out when setting as forcing the atmospheric influence and runoff of rivers flowing into Lake Ladoga for the hydrothermodynamic module and the supply of nutrients from the atmosphere and from land for the biogeochemical module. A comparison of the results of calculating the current climatic state of Lake Ladoga with the available satellite andexpeditionary observation data showed that the model correctly reproduces the climatic seasonal variation of the surface temperature field, its vertical distribution, average values and range of changes in the main characteristics of the lake’s ecosystem. The proposed model can be used to study the influence of external natural and anthropogenic factors on biogeochemical processes and the functioning of the Lake Ladoga ecosystem.

Unveiling microplastic distribution and interactions in the benthic layer of the Yangtze River Estuary and East China Sea

Microplastics (MPs), recognized as an emerging global environmental concern, have been extensively detected worldwide, with specific attention directed towards the Yangtze River Estuary (YRE) and East China Sea (ECS) regions. Despite their critical research significance, there remains a knowledge gap concerning the distribution of MPs in the benthic layer within this area, particularly regarding interactions governing their occurrence. Here we illuminate the distribution of MPs within the benthic layer and unravel the intricate interplay between bottom water and sediment in the YRE and ECS. We find that MPs are notably more abundant in bottom water, ranging from 8 to 175 times higher than in surface water. These MPs predominantly consist of polyester fibers, exhibit a size range between 0.5 and 5.0 mm, and display distinct coloration. Co-occurrence network analysis and Principal Coordinate Analysis confirm a robust correlation between MPs in bottom water and sediment, signifying the pivotal role of bottom water in mediating the distribution and transportation of MPs within the benthic layer. Furthermore, a positive correlation between MPs in sediment and bottom water turbidity underscores the impact of surface sediment resuspension and upwelling on MPs distribution. This study clarifies the intricate interactions within the benthic layer and highlights the crucial role of bottom water as a mediator in the vertical distribution of MPs, advancing our understanding of the “source-to-sink” transport processes governing MPs within water-sediment systems.

Degradation of starch-based bioplastic bags in the pelagic and benthic zones of the Gulf of Oman

The Gulf of Oman is becoming increasingly polluted with plastics, hence bioplastics have been considered ‘a substitute’, although their biodegradability in marine environments has not been well investigated. Most research has been performed on cellulose-based bioplastics, whereas starch-based bioplastics have proven to be a suitable, but less researched, alternative. This study is the first of its kind designed to investigate the degradability of two different types of starch-based bioplastic bags, available in the market and labeled as “biodegradable”, in the pelagic and benthic zones of one of the warmest marine environment in the world. Fourier-Transform Infrared Spectroscopy (FTIR) showed a clear reduction in the presence of OH, CH, and CO in the bioplastic bags after 5 weeks of immersion. Thermo-Gravimetric Analysis (TGA) indicated degradation of glycerol, starch, and polyethylene. The biofouling bacterial communities on bioplastic surfaces showed distinct grouping based on the immersion zone. Candidaatus saccharibacteria, Verrucomicrobiae, Acidimicrobiia and Planctomycetia sequences were only detectable on bioplastics in the pelagic zone, whereas Actinomyces, Pseudomonas, Sphingobium and Acinetobacter related sequences were only found on bioplastics in the benthic layer. We conclude that starch-based bioplastics are more readily degradable in the Gulf of Oman than conventional plastics, hence could serve as a better environmentally friendly alternative.

An experimental study on tolerance to hypoxia in tardigrades.

Introduction: Tardigrades are small aquatic invertebrates with well documented tolerance to several environmental stresses, including desiccation, low temperature, and radiation, and an ability to survive long periods in a cryptobiotic state under arrested metabolism. Many tardigrade populations live in habitats where temporary exposure to hypoxia is expected, e.g., benthic layers or substrates that regularly undergo desiccation, but tolerance to hypoxia has so far not been thoroughly investigated in tardigrades. Method: We studied the response to exposure for hypoxia (<1ppm) during 1-24h in two tardigrade species, Richtersius cf. coronifer and Hypsibius exemplaris. The animals were exposed to hypoxia in their hydrated active state. Results: Survival was high in both species after the shortest exposures to hypoxia but tended to decline with longer exposures, with almost complete failure to recover after 24h in hypoxia. R. cf. coronifer tended to be more tolerant than H. exemplaris. When oxygen level was gradually reduced from 8 to 1ppm, behavioral responses in terms of irregular body movements were first observed at 3-4ppm. Discussion: The study shows that both limno-terrestrial and freshwater tardigrades are able to recover after exposure to severe hypoxia, but only exposure for relatively short periods of time. It also indicates that tardigrade species have different sensitivity and response patterns to exposure to hypoxia. These results will hopefully encourage more studies on how tardigrades are affected by and respond to hypoxic conditions.

Sea‐Level Rise Impacts on Tidal Marshes and Estuarine Biogeochemical Processes

AbstractWe used a numerical model to investigate the effects of sea‐level rise (SLR) on the biogeochemical processes in the York River Estuary with extensive tidal marshes. The fully‐coupled hydrodynamic‐water quality‐marsh model accounts for the spatial and temporal variations of physical‐biogeochemical interactions between the tidal marshes and surrounding waters. This study focuses on an SLR scenario where the vertical accretion of tidal marshes keeps pace with the rising sea levels. Results show that SLR amplifies the tidal range and prolongs flooding duration, which results in enhanced porewater exchanges of materials between the tidal marshes and the surrounding waters. The increased availability of shallow‐water habitats and enhanced light utilization in the shallow areas under SLR promote phytoplankton production (PP) in the shallow‐water regions of the York River. Consequently, the organic carbon in the open water is fueled by the contributions from shallow waters and the enhanced export of organic carbon from the marshes. The change in the dissolved oxygen (DO) budget in the York River Estuary is attributed to changes in water column respiration, net metabolism of the benthic layer, reaeration, PP, and increased stratification under SLR. The net DO flux out of the York River increases at the York River mouth. Diel DO variation, especially in the marshes in the upper estuary, promotes phosphorus release from the sediment. The changes in dissolved nitrogen under SLR are relatively minimal.

An Improved Model for Water Quality Management Accounting for the Spatiotemporal Benthic Flux Rate

Although water quality models provide useful interpretations for water quality management, it is critical to accurately input and simulate the flux rate, which varies with space and time. In the Environmental Fluid Dynamics Code model, the flux rate value set does not consider spatiotemporal variability. The water quality of the Saemangeum freshwater lake in Korea is poor despite quality improvement measures. In this study, the model was improved by considering the characteristics of flux rates that change spatiotemporally based on environmental conditions and factors influencing the benthic layer. An exponential relational expression was generated and applied to the model while considering the aerobic, anaerobic, and influencing factors. Results from four important sites in the Saemangeum Reservoir were compared with the RSR, %Difference, and AME results of the previous model for evaluating the reproducibility of the improved model. Calibration and verification of the model were performed in 2013 and 2016, respectively. The improved model yielded values close to the optimal value after computing the evaluation functions of both models. It had excellent reproducibility and simulated water quality by reflecting a reasonable value for the benthic flux rate. The improved model can be extended to evaluate other water bodies in the future.

Hydrodynamic Control of Sediment‐Water Fluxes: Consistent Parameterization and Impact in Coupled Benthic‐Pelagic Models

AbstractBenthic oxygen dynamics and the exchange of oxygen and other solutes across the sediment‐water interface play a key role for the oxygen budget of many limnic and shallow marine systems. The sediment‐water fluxes are largely determined by two factors: sediment biogeochemistry and the thickness of the diffusive boundary layer that is determined by near‐bottom turbulence. Here, we present a fully coupled benthic‐pelagic modeling system that takes these processes and their interaction into account, focusing especially on the modulation of the sediment‐water fluxes by the effects of near‐bottom turbulence and stratification. We discuss the special numerical methods required to guarantee positivity and mass conservation across the sediment‐water interface in the presence of rapid element transformation, and apply this modeling system to a number of idealized scenarios. Our process‐oriented simulations show that near‐bottom turbulence provides a crucial control on the sediment‐water fluxes, the oxygen penetration depth, and the re‐oxidation of reduced compounds diffusing upward from the deeper benthic layers especially on time scales of a few days, characterizing oceanic tides, internal seiching motions in lakes, and mesoscale atmospheric variability. Our results also show that the response of benthic‐pelagic fluxes to rapid changes in the forcing conditions (e.g., storm events) can only be understood with a fully coupled modeling approach.

Impacts of Nile tilapia cage culture on water and bottom sediment quality: The ability of an eutrophic lake to absorb and dilute perturbations

AbstractEnvironmentally sustainable aquaculture depends on accurate understanding of the impacts of aquaculture‐derived organic matter (AOM) and the ability of aquaculture systems to absorb and dilute perturbations. To this end, the present study assessed the impacts of AOM from cage culture of Nile tilapia on the ecology of Lake Victoria, Kenya, using fish cages near Anyanga Beach in Siaya County from December 2018 to October 2019. Four locations were surveyed for organic loadings from cage cultures, located 0, 50, 150 and 500 m (as a control site) away from the cages. The cage aquaculture produced increased P and N concentrations near the cages and a decreased N:P molar ratio. These changes stimulated algal growth which, in turn, affected the water quality. The organic material accumulated on the bottom under the cages, increasing the benthic BOD (BOD, &gt;10 mg/g), a sensitive indicator of the ecological footprint of the cage aquaculture. Further, the negative ORP observed in the benthic layer suggested anoxic bacterial metabolism, possibly causing build‐up of sulphides and methane. These changes altered the abundance and composition of both limnetic and benthic communities. At the beginning of the study, 22 zoobenthic taxa existed around the cages and 18 at the reference sites. Only 3 saprophilous taxa, chiefly gastropods (Physellaspp.), bivalves (Sphaeriumspp.) and oligochaetes (Tubifexspp.) were present at the cage site and 17 at the reference site at the end of the culture period. The Shannon diversity index exhibited a declining tendency with the length of culture period at the cage site, signifying a negative impact of aquaculture on biodiversity. The water quality recovery after cage disturbance is rapid (&lt;4 months), noting there was no significant difference in the water quality recorded at the cage site and the other sampling sites after a fallow period of 4 months. However, the recovery of the sediment and meiofauna was far from complete at the end of this period. Moving the cages slightly (50–100 m) away from the former location may allow the benthic communities to recover and alleviate the problem. Further, the fallowing period, particularly for the Anyanga Beach site, should be extended from four to at least 5 months to allow for the environment to recover. With the rapid increase of cage fish farming in the African Great Lakes Region and with the potential for its occurrence in other lakes, there is a need to develop regulations to guide the industry, as well as the need for continuous monitoring of the environment, in order to provide information to guide investments and ensure sustainable cage farming.

Threatened skates exhibit abiotic niche stability despite climate change in the southwestern Atlantic Ocean

Climatic changes are disrupting distribution patterns of populations through shifts in species abiotic niches and habitat loss. The abiotic niche of marine benthic taxa such as skates, however, may be more climatically stable compared with the upper layers of the water column, which are more exposed to immediate impacts of warming. Here, we estimate climate change impacts in Riorajini, a tribe of four skates, as a proxy to evaluate the vulnerability of a temperate coastal zone in the Southwest Atlantic, and study niche dynamics in a scenario of environmental changes on this group of threatened species. We modelled the abiotic niche of each species under present climatic conditions (2000–2014), projected them to the future (2100), then measured distributional stability, expansion, and unfilling. Our results revealed abiotic stability between the scenarios modelled despite the advancement of climate change, suggesting that the benthic layers where these skates occur may be a refuge from the increasing thermal stress. However, the exposure of shallow waters to climate change may be detrimental to nursery habitats. Thus, although their abiotic niche may remain stable in the future, the loss of extension of occurrence might be a peril for them with climate change.

Characteristics and differences of microplastics ingestion for farmed fish with different water depths, feeding habits and diets

Microplastics have become a potential hazard to the quality and safety of aquatic products although there are few reports on microplastics distribution related to water depth, feeding habits and diets in aquaculture ponds. In this study, five aquaculture fish species (Ctenopharyngodon idella, Hypophthalmichthys molitrix, Cirrhinus molitorella, Oreochromis niloticus and Pelteobagrus fulvidraco) were selected and their accumulation of microplastics was measured and classified. We found that the water depth, feeding habits and diets led to large differences in microplastics accumulation. The bottom-dwelling omnivorous fish had the largest ingestion of microplastics and ranked as P. fulvidraco>O. niloticus>C. molitorella，the herbivore C. idella in the middle benthic layer and the upper-layer filter-feeder H. molitrix had the lowest ingestion. The detection frequency in the gastro-intestinal tracts for C. idella, H. molitrix, C. molitorella, O. niloticus, and P. fulvidraco were 93.33%, 91.67%, 87.50%, 92.86%, and 90.91%, and detection frequencies for gills were 86.67%, 100%, 87.5%, 100%, and 81.82%, respectively. Fibers presented the dominant shape (91.92%) and blue was the primary particle color in the majority of the samples (77.39%) and 0.5–2 mm particles dominated. The plastics were composed of olefins, acrylic, rayon, polyester, polyethylene and polypropylene. Our study first reported the relationship between MPs accumulation and water depth, feeding habits and diets in aquaculture ponds, which also has a certain correlation with the fish size.

Quantifying Marine Plastic Debris in a Beach Environment Using Spectral Analysis

Marine plastic debris (MPD) is a globally relevant environmental challenge, with an estimated 8 million tons of synthetic debris entering the marine environment each year. Plastic has been found in all parts of the marine environment, including the surface layers of the ocean, within the water column, in coastal waters, on the benthic layer and on beaches. While research on detecting MPD using remote sensing is increasing, most of it focuses on detecting floating debris in open waters, rather than detecting MPD on beaches. However, beaches present challenges that are unique from other parts of the marine environment. In order to better understand the spectral properties of beached MPD, we present the SWIR reflectance of weathered MPD and virgin plastics over a sandy substrate. We conducted spectral feature analysis on the different plastic groups to better understand the impact that polymers have on our ability to detect synthetic debris at sub-pixel surface covers that occur on beaches. Our results show that the minimum surface cover required to detect MPD on a sandy surface varies between 2–8% for different polymer types. Furthermore, plastic composition affects the magnitude of spectral absorption. This suggests that variation in both surface cover and polymer type will inform the efficacy of beach litter detection methods.

Seasonality modulates wind-driven mixing pathways in a large lake

Turbulent mixing controls the vertical transfer of heat, gases and nutrients in stratified water bodies, shaping their response to environmental forcing. Nevertheless, due to technical limitations, the redistribution of wind-derived energy fuelling turbulence within stratified lakes has only been mapped over short (sub-annual) timescales. Here we present a year-round observational record of energy fluxes in the large Lake Geneva. Contrary to the standing view, we show that the benthic layers are the main locus for turbulent mixing only during winter. Instead, most turbulent mixing occurs in the water-column interior during the stratified summer season, when the co-occurrence of thermal stability and lighter winds weakens near-sediment currents. Since stratified conditions are becoming more prevalent –possibly reducing turbulent fluxes in deep benthic environments–, these results contribute to the ongoing efforts to anticipate the effects of climate change on freshwater quality and ecosystem services in large lakes.

Acoustic evidences of the beginning of anchovy (Engraulis encrasicolus) schooling in the Northern Alboran Sea (Mediterranean Sea)

During the MEDiterranean International Acoustic Survey (MEDIAS) carried out in 2016 and 2017 in the Northern Alboran Sea in July, a benthic scattering layer located mainly in Malaga Bay was acoustically detected at multiple frequencies and biologically identified using different sampling devices (plankton net and pelagic trawl). This layer mainly corresponded to an anchovy larvae aggregation ranged 2 and 40 mm standard length. Our results pointed out that anchovy larvae massively migrate from the surface to the bottom near the coast, until the 70 m isobath, forming large aggregations acoustically detected as a scattering benthic layer, in which small schools form and emerge.This work has highlighted the potential of acoustic surveys such as MEDIAS to generate anchovy recruitment success indicators complementary and simultaneously to the traditional ones (biomass of the reproductive stock) and their great utility as a fisheries management tool in areas such as the Alboran Sea where the fishing resource is scarce.

Importance of salinity-induced stratification on flocculation in tidal estuaries

Flocculation of suspended particles in tidal estuaries exhibits large spatiotemporal variability due to an interplay of various physical and biogeochemical drivers. Salinity (S) is known to promote flocculation of fine-grained suspended particulate matter (SPM). However, the influence of salinity and salinity-induced stratification on flocculation has not been sufficiently investigated yet. This study aims to understand how these two factors, interactively with turbulent shear (G) and SPM concentration (C), control the vertical variation of floc size and flocculation process in different depth layers in a typical tide-dominated estuarine environment. Analysis of field observation data shows that flocculi (diameter < 20 μm) are mainly affected by C and originate primarily from local resuspension. Macroflocs (> 200 μm) are mainly controlled by stratification that greatly improves aggregate collision efficiency; Microflocs (20–200 μm), as a transition group between flocculi and macroflocs, are affected by dynamics of both sides. They are influenced jointly by C, G and stratification. Besides, the fresh water-dominated surface layer is dominated by small particles (flocculi and microflocs), confined in a relatively narrow particle size range between Ο (100) and Ο (101) as a result of the low level of both C (13–20 mg/L) and S (< 2 practical salinity units). Below the surface layer, floc size increases drastically along with an increased salinity-induced density gradient and achieves maximum particle size (Ο (102)) within the stratified layer. Because of its high efficiency in promoting flocculation and formation of macroflocs, the stratified layer around the halocline can be regarded as an optimal flocculation zone. The benthic layer is characterized by high C (> 30 mg/L), gentle G (~5/s), and periodic stratification, which result in a wide size range between Ο (101) and Ο (102) with microflocs as the dominant group. Finally, we found that the accuracy of flocculation modeling can be significantly improved by integrating a simple relationship between particle collision and stratification.

Assessing the contribution of the La Luna Sea to the global sink of organic carbon during the Cenomanian-Turonian Oceanic Anoxic Event 2 (OAE2)

Epicontinental seas were important features of the paleogeographic landscape during the Cretaceous; however, the role they played as sinks of organic carbon is still poorly understood. The La Luna Formation (Albian-Coniacian) is a series of organic-rich limestones deposited in northwestern South America on an epicontinental sea (the La Luna Sea). This formation offers a forty-million-year continuous record of environmental change characterized by periods of oceanic anoxia in an epicontinental sea. The La Luna Sea, may have played an important role –although so far unexplored– in carbon cycling through the ocean during the Cretaceous, specifically during short-term, global-scale disruptions in the carbon cycle known as oceanic anoxic events (OAEs). To evaluate the role of the La Luna Sea in global carbon cycle perturbations, we conducted a detailed lithological and chemostratigraphic analysis of two stratigraphic sections from the Upper Magdalena Basin of Colombia, both of which encompass the Oceanic Anoxic Event 2 (OAE2) at the Cenomanian–Turonian boundary. Compared to deposits in the modern ocean, the La Luna Formation has high total organic carbon (TOC) before, during, and after OAE2. Foraminifera and nannoplankton assemblages also imply a stressed upper water column during OAE2. Geochemical and paleontological evidence suggests that the sediment-water interface was anoxic across the late Cenomanian and early Turonian. Strata deposited just after OAE2, however, contain inoceramid bivalves, consistent with short-lived re‑oxygenation of the benthic layer. Estimates of primary productivity, the covariation of Mo and U enrichment factors, and relations between Cd, Mo, Co, and Mn also reveal that the La Luna Sea was biogeochemically similar to the modern Cariaco Basin. Despite high concentrations of organic carbon found in the La Luna Formation, mass-accumulation rates of organic carbon are low, a finding that can be explained by a reduction in the accumulation rate of sediments caused by the peak of sea-level transgression that took place at the Cenomanian–Turonian transition. Based on the areal extent of the La Luna Sea and mass-accumulation rates of organic carbon, 1.7 Eg of C were removed from the ocean over 500 ky and deposited in the La Luna Sea. Interestingly, although the La Luna Sea was one-third the size of the Western Interior Seaway (WIS), the amount of organic carbon buried in the WIS during OAE2 was similar (1.4 Eg of C). In these two epicontinental seas, 3.1 Eg of C were removed from the ocean during OAE2, accounting only for 3.4% of the total C needed to cause a perturbation of the carbon cycle similar to that observed during OAE2. The low amount of organic carbon buried in the La Luna Sea and the WIS suggests that neither of these inland seas were responsible for the efficient removal of organic carbon from the ocean during OAE2. This conclusion challenges the explanation that epicontinental seas were major sinks of organic carbon—and therefore they did not play a significant role in the carbon cycle during the Mesozoic OAEs and other disruptions of the carbon cycle in Earth's history.

New insights into the biogeochemical cycling of copper in the subarctic Pacific: Distributions, size fractionation, and organic complexation

AbstractThe chemical speciation of Cu is dominated by complexation with organic ligands. However, the size fractionation of dissolved Cu and its organic ligands are not well understood. In this study, we revealed new insights into the biogeochemical cycling of Cu in the subarctic Pacific based on both chemical speciation and size fractionation analyses. Our data show that the surface waters of the subarctic Pacific contain a pool of colloidal Cu that was not observed in the subtropical South Pacific Ocean. Both soluble and colloidal Cu in the surface waters were strongly bound to organic ligands. In the intermediate waters, strong soluble Cu‐binding organic ligands were present, which could be related to the Cu uptake mechanism of methanotrophs under low oxygen environments. In deep waters, dissolved Cu concentrations increased but soluble Cu concentrations remained mostly constant, indicating that there are sources of colloidal non‐labile Cu in the benthic layer. Dissolved Cu exhibited a positive linear relationship with silicate in the upper 1500 m while soluble Cu displayed a linear relationship with silicate throughout the water column. The results of this study show that dissolved Cu is bound by organic ligands of various sizes and binding strengths throughout the water column of the subarctic Pacific, and that the characteristics of these ligands change with depth.

The relation between temperature and silica benthic exchange rates and implications for near-seabed formation of diagenetic opal

This study calculated the dissolution rates of biogenic silica deposited on the seafloor and the silicic acid benthic flux for 22 Ocean Drilling Program sites. Simple models developed for two host sediment types—siliciclastic and carbonate—were used to explain the variability of biogenic silica dissolution and recycling under present-day low (−0.3 to 2.14 °C) bottom-water temperatures. The kinetic constants describing silicic acid release and silica saturation concentration increased systematically with increasing bottom-water temperatures. When these temperature effects were incorporated into the diagenetic models, the prediction of dissolution rates and diffusive fluxes was more robust. This demonstrates that temperature acts as a primary control that decreases the relative degree of pore-water saturation with biogenic opal while increasing the silica concentration. The correlation between the dissolution rate and benthic flux with temperature was pronounced at sites where biogenic silica is hosted in surficial sediments mostly composed of biogenic carbonates. This association is because the dissolution of carbonates provides the alkalinity necessary for both silica dissolution (also its reprecipitation as opal-CT) and clay formation; thus strongly reducing the retarding influence of clays on biogenic opal dissolution. Conversely, the silica exchange rates were modified by presence of aluminosilicates, which led to a higher burial efficiency for biogenic opal in detrital- than in carbonate-dominated benthic layers. Though model prediction of first-order silica early transformation suggests likely effects from surface temperatures (0–4 °C) on opal-CT precipitation over short geological times (< 4 Ma) near seabed in the Antarctic Site 751, the relationship between silica solubility and surface-area variability in time is a more critical control. Since silica solubility and surface area decrease with time, the < 4 Ma elapsed time aged opal-A to the point that changes in specific surface area caused minor effects on solubility, allowing for formation of opal-CT at low temperature settings near the seabed.

An integrated optic and acoustic (IOA) approach for measuring suspended sediment concentration in highly turbid environments

Accurate measurement of suspended sediment concentration (SSC) in highly turbid environments has been a problem due to optical or acoustic signal saturation and attenuation. The saturation returns a limited measurement range, and the attenuation raises an ambiguity problem that a low optical or acoustic output could mean a low or a high SSC. In this study, an integrated optic and acoustic (IOA) approach is proposed to (i) overcome the ambiguity problem; (ii) increase the measurement range to high SSC values; and (iii) obtain high-resolution SSC profiles. The IOA approach is a combination of Argus Suspension Meter (ASM), Optical Backscatter Sensor (OBS) and Acoustic Doppler Velocimeter (ADV). In this approach, the ASM-derived SSC is preferred because of its lowest relative error, followed by OBS and ADV. The ASM can produce high-resolution (1 cm interval) SSC profiles when it is not saturated (usually SSC < 9 g/L). When ASM is saturated, the SSC is recovered by OBS. Since the ambiguity problem is solved, the measurement range of OBS and ADV can be extended up to 300 g/L. The best way to use an ADV, however, is to have a rough estimation first and assist in the OBS conversion, because its estimates contain large uncertainty. To further mitigate the impact of sediment particle size on SSC retrieval, we suggest the usage of in-situ sediment samples for sensor calibration. The IOA approach was verified in the Yangtze Estuary which is a highly turbid system. Comparison of the IOA approach outputs against water sampling results demonstrates the reliability of the IOA approach with a relative error of 17–34%. The observed high SSCs were up to 63 g/L. The field data show that high SSCs were confined in the benthic layer (within 2 m above the bed) in the wet season under a high river discharge, whereas the suspension was better mixed throughout the water column in the dry season. • A combination of ASM, OBS and ADV resolves the ambiguity problem of two possible SSC values for an OBS or ADV output. • The integrated optic and acoustic (IOA) approach enlarges the measurement range of OBS and ADV to 300 g/L. • The IOA approach provides high-resolution (1 cm) SSC profiles by ASM. • The IOA approach was successfully applied in the Yangtze Estuary with SSC > 10 g/L.

Chemical Monitoring of Organic Ecotoxicants in Aqueous Systems

Monitoring of organochlorine pesticides (OCP), polychlorinated biphenyls (PCB) and polycyclic aromatic hydrocarbons (PAH) in surface waters, sediments and aquatic lives of the benthic layer from different water systems of Ukraine was carried out. Information about the levels of these organic contaminants was collected, summarized and presented according to universally accepted regulatory standards that determine the toxicity of compounds of these classes. Regularities of variations of ecotoxicants concentrations in five water systems of the Dnipro river and the Black Sea (Dnipro-Bug estuary, the Danube delta, Balaklava bay, Sevastopol bay) were established. It was found that in many bottom sediment samples the recorded levels of OCP, PCB, and PAH exceed the values of TEC (Threshold Effect Concentration) and PEC (Probable Effect Concentration). To assess the toxicity of hydrobionts, the content of OCP and PCB is given as relative to the toxicity equivalent factor (TEF) and toxicity equivalent (TEQ). The levels of PCB in hydrobionts are presented relative to TEQ and indicate that hydrobionts of the bottom layer are significantly contaminated with compounds of OCP and PCB and thus bioconcentration of such organic contaminants by high organisms along the trophic chain is possible.