One of the worst socio-environmental disasters to mark the history of Brazil and the world occurred in November 2015 and involved the mining sector. The collapse of the Fundao dam released approximately 43 million m³ of iron ore tailings, which moved downstream to reach the Doce River. This resulted in the contamination of water, soil, and sediments along the entire course of the river, which also affected its mouth in the Atlantic Ocean. Four years after the disaster, several socio-environmental problems continue to persist in the affected areas. In this context, the reservoirs built along the Doce River deserve special attention as they are artificial environments that are highly vulnerable to changes in water parameters. This study aims to determine water quality indicators of these reservoirs using remote sensing data and image processing methods, including semi-analytical algorithms, to comprehend the progress of eutrophication processes. Operational land imager/Landsat-8 data (from 2013 to 2019) were used to map the suspended particulate matter (SPM), euphotic zone (Zₑᵤ) and chlorophyll-a (Chl-a) before and after the collapse. The results showed significant changes in SPM and Zₑᵤ in the reservoirs after the collapse. Non-conformity of these parameters is observed even now, and they tend to intensify during rainy periods when resuspension processes of sediments occur. Moreover, there has been an increase in the eutrophication of reservoirs as noticed by the significant increase in Chl-a after the disaster, especially in January, July, and August.

Reservoirs are an important type of drinking water source for megacities, while lots of reservoirs are threatened by odor problems during certain seasons. The influencing factors of odor compounds in reservoirs are still unclear. During August 2019, a nationwide survey investigating the distribution of odor compounds in reservoirs used as drinking water sources was conducted on seven reservoirs. 2-methylisoborneol (2-MIB) and geosmin were detected in almost every reservoir, and some odor compound concentrations even exceeded the odor threshold concentration. The average concentration of 2-MIB was 2.68 ng/L, and geosmin was 3.63 ng/L. The average chlorophyll a concentration was 8.25 μg/L. The dominant genera of phytoplankton in these reservoirs belonged to cyanobacteria and diatom. Statistical analysis showed that odor compound concentration was significantly related to the chlorophyll a concentration and indicated that the odor compounds mainly came from phytoplankton. The concentration of odor compounds in the euphotic zone was significantly related to phytoplankton species and biomass. Therefore, the odor compound concentrations in the subsurface chlorophyll maxima layer was generally higher than in the surface layer. However, the odor compounds in the hypolimnion layer were related to the density current. This research suggests that both phytoplankton proliferation events and heavy storm events are important risk factors increasing odor compounds in reservoirs. Control of algal bloom, in-situ profile monitoring system and depth-adjustable pumping system will greatly reduce the risk of odor problems in reservoirs using as water supplies for large cities.

∑₃₀PAH gas phase concentrations (13–86 and 22–40 ng m⁻³ in the Mediterranean and Black Seas, respectively) dominated the atmospheric levels due to the high contribution of phenanthrene, dibenzothiophene and their alkylated derivates. The high variability of PAH atmospheric concentrations in the different sub-basins is due to several factors (i.e. air-mass trajectory, proximity to sources and losses by deposition). The ∑₃₀PAH atmospheric deposition (dominated by low MW PAH net air–water diffusive fluxes) is estimated to be ∼3100 ton y⁻¹ (Mediterranean) and ∼500 ton y⁻¹ (Black Sea). Net volatilization for certain PAHs was estimated. Deposition fluxes (1–2 orders of magnitude higher than reported PAH settling fluxes in the water column) confirm an important depletion/sink of water column PAH in the photic zone, especially for low MW PAHs. Degradation processes in the water column may be responsible for this decoupling. Conversely, high MW PAHs dry deposition fluxes are similar to their settling fluxes.

Coccolithophore calcite production (CP) was investigated for the first time in the Bay of Bengal. Against expectation, calcite production was not fueled by the nutrient-enriched cold eddy because of the reduced light penetration. CP rate was observed to be higher at the anticyclonic eddy possibly benefited from rare species production. The adjoining river-induced shallow mixed-layer depth and eddy activity co-influenced CP rate. On average, the integrated CP rates were 0.04, 0.15, and 0.07 mmol C m⁻² h⁻¹ for the cyclonic eddy, anticyclonic eddy, and outer area, respectively. In the upper photic waters, CP rates showed a strong correlation with primary productivity (PP). However, a decoupling of CP and PP was observed in the lower photic zone due to differential light and nitrogen preferences in calcification and carbon fixation. Our findings could help to understand the biogenic response of eddy and inorganic-organic carbon association in the pelagic biogeochemical cycles.

Marine particle-attached (PA) and free-living (FL) microbes play important roles in the biogeochemical cycling of organic matter along the water column. Deep-sea cold seeps are highly productive and chemosynthetic ecosystems, their continuous emission of CH₄, CO₂, and H₂S can reach up to 100 m in the above water, therefore would influence the distribution and potential metabolic functions of deep-sea prokaryotes. In this study, the vertical distribution profiles of both PA and FL microbes in the water column above two cold seeps of the South China Sea were investigated using Illumina sequencing and quantitative PCR (qPCR) based on 16S rRNA gene. Photosynthetic and heterotrophic prokaryotes were predominant in respective surface and deep layers below the photic zone. The typical cold seep chemosynthetic microbes, such as methanotrophs and sulfate-reducing bacteria were observed with low proportions in the two cold seeps as well. Distinct PA and FL microbial fractions were found in terms of abundance and diversity. FL fraction exposed to the bulk water was significantly affected by temperature and inorganic nutrients, whereas PA fraction relied more on the organic matter of the particles and less susceptible to the environmental variability. Our study highlights the importance of vertical geochemical gradients on the distribution and potential metabolic choice of marine microbes and extends our current knowledge of depth-associated microbial distribution patterns.

This study provides data concerning the hydrography and water chemistry of the Atlantic region between 29–38° N and 27–31° W, and establishes background values for dissolved Cu, Cd, Pb and As. Three water masses were identified: the Eastern North Atlantic Central Water (ENACW), the Mediterranean Water (MW) and the North Atlantic Deep Water (NADW). The ENACW exhibits a clear meridional gradient of temperature and salinity, with comparatively high values at the southern sites and lower values on the Azores platform. The ENACW, which includes the euphotic zone, also had comparatively high concentrations of oxygen and lower concentrations of nutrients and metals. The Cu, Cd and Pb results suggest that new background concentrations for OSPAR Region V (the Wider Atlantic) should be established as follows: 0.15–13nM for Cu, 0.05–1.4nM for Cd and 0.03–5nM for Pb. The background concentrations of As for OSPAR Region V should be 7–28nM.

Ocean-color remote sensing has been used as a tool to detect phytoplankton size classes (PSCs). In this study, a three-component model of PSC was reparameterized using seven years of pigment measurements acquired in the South China Sea (SCS). The model was then used to infer PSC in a cyclonic eddy which was observed west of Luzon Island from SeaWiFS chlorophyll-a (chla) and sea-surface height anomaly (SSHA) products. Enhanced productivity and a shift in the PSC were observed, which were likely due to upwelling of nutrient-rich water into the euphotic zone. The supply of nutrients promoted the growth of larger cells (micro- and nanoplankton), and the PSC shifted to greater sizes. However, the picoplankton were still important and contributed ∼48% to total chla concentration. In addition, PSC time series revealed a lag period of about three weeks between maximum eddy intensity and maximum chlorophyll, which may have been related to phytoplankton growth rate and duration of eddy intensity.

We analyzed the data obtained from field observations on a gas hydrate drilling area in Dongsha of northern South China Sea (SCS) in middle May (before drilling) and early October (after drilling) in 2013. The variation in the phytoplankton communities and biomass as well as the impacts of environmental factors including dissolved methane was studied. Results indicated that the gas hydrate drilling area in Dongsha, SCS exhibited a typical low-nutrients low-chlorophyll a (LNLC) environment accompanied with low phytoplankton abundance. A total of 103 taxa belonging to 52 genera of 5 classes were identified, with diatoms and dinoflagellates dominating the community. Both phytoplankton abundance and chlorophyll a (Chl a) were highest at the subsurface maximum layer. The subsurface chlorophyll maximum (SCM) before and after drilling were stabilized at 75 m (0.30 ± 0.06 mg/m³ and 0.51 ± 0.29 mg/m³, respectively), while the subsurface maximum of abundance after drilling went deeper to 75 m (604.17 ± 313.22 cells/L) from the surface (707.14 ± 243.98 cells/L) before drilling. After drilling, phosphate and Chl a increased significantly, but no significant differences were observed on abundance. Dominant species of diatoms were basically constant with dinoflagellates becoming more apparent in higher occurrence and abundance, while Cyanophyta was diverse after drilling. Redundancy analysis (RDA) and Spearman's correlation analysis both indicated that temperature, pH and phosphates were major factors causing fluctuation in phytoplankton community structure, while dissolved methane had non-significant impact directly. We clearly found both abundance and Chl a increased in particular water layers (between 50 and 75 m) and at stations (DS06, DS08 and DS15) where dissolved methane concentrations were also abnormally high. This study appeared to partly coincide with the findings of natural oil seeps in the Gulf of Mexico, which assumed that the turbulence from the natural oil and gas leaking zone could raise the bottom water through the rising bubbles and bring cold nutrient rich waters to the thermocline from the deep seeps. This plume-generated upwelling could then fuel a bottom-up effect on the photosynthetic species in the upper pelagic waters within the euphotic zone.

Phytoplankton assemblages related to environmental factors and ecological status of the Gemlik Gulf were investigated between June 2010 and May 2011. A total 155 phytoplankton species were detected and 6 taxa (Amphisolenia laticincta, Archaeperidinium minutum, Cochlodinium sp., Gynogonadinium aequatoriale, Heterocapsa rotundata and Metaphalacroma sp.) were new records for the Turkish Seas. The lowest and highest total phytoplankton abundance among the sampling units (depths) was recorded in April 2011 (7.4×103cellsL−1) and July 2010 (251.8×103cellsL−1). Local small patches of visible red tide events were detected especially in the gulf, although a phytoplankton bloom was not observed. The water column was well stratified in the early autumn and well mixed in the early spring according to stratification index values. Surface nutrient concentrations increased especially at stations located inside of the gulf. The limiting effect of silicate was observed in early, mid-summer and early winter periods while the nitrogen was the limiting nutrient in the gulf during the whole sampling period. In the Gulf, low water quality-high mesotrophic and bad water quality-eutrophic status, high quality and low trophic level were generally detected according to Chl a, dissolved oxygen and trophic index. However, indices developed to determine the trophic level and water quality of the Mediterranean Sea can give unexpected results about the current environmental quality status when it is applied to the Marmara Sea which has limited photic zone by the halocline-pycnocline and thermocline.

Simplified physical models and geometrical considerations reveal general physical and dynamical properties of microplastic particles (0.5–5mm) of different density, shape and size in marine environment. Windage of extremely light foamed particles, surface area and fouling rate of slightly positively buoyant microplastic spheres, films and fibres and settling velocities of negatively buoyant particles are analysed. For the Baltic Sea dimensions and under the considered idealised external conditions, (i) only one day is required for a foamed polystyrene particle to cross the sea (ca. 250km); (ii) polyethylene fibres should spend about 6–8months in the euphotic zone before sinking due to bio-fouling, whilst spherical particles can be retained on the surface up to 10–15years; (iii) for heavy microplastic particles, the time of settling through the water column in the central Gotland basin (ca. 250m) is less than 18h. Proper physical setting of the problem of microplastics transport and developing of physically-based parameterisations are seen as applications.