Mercury speciation in surface and deep waters of the Mediterranean Sea

Mercury speciation and its distribution in surface and deep waters of the Mediterranean Sea were studied during two oceanographic cruises on board the Italian research vessel URANIA in summer 2003 and spring 2004 as part of the Med Oceaneor and MERCYMS projects. The study included deep water profiles of dissolved gaseous Hg (DGM), reactive Hg (RHg), total Hg (THg), monomethyl Hg (MeHg) and dimethyl Hg (DMeHg) in open ocean waters. Average concentrations of measured Hg species were characterized by seasonal and spatial variations. Overall average THg concentrations ranged between 0.41 and 2.65 pM (1.32 ± 0.48 pM) and were comparable to those obtained in previous studies of the Mediterranean Sea. A significant fraction of Hg was present as “reactive” Hg (average 0.33 ± 0.32 pM). Dissolved gaseous Hg (DGM), which consists mainly of Hg⁰, represents a considerable proportion of THg (average 20%, 0.23 ± 0.11 pM). The portion of DGM typically increased towards the bottom, especially in areas with strong tectonic activity (Alboran Sea, Strait of Sicily, Tyrrhenian Sea), indicating its geotectonic origin. No dimethyl Hg was found in surface waters down to the depth of 40 m. Below this depth, its average concentration was 2.67 ± 2.9 fM. Dissolved fractions of total Hg and MeHg were measured in filtered water samples and were 0.68 ± 0.43 pM and 0.29 ± 0.17 pM for THg and MeHg respectively. The fraction of Hg as MeHg was in average 43%, which is relatively high compared to other ocean environments. The concentrations reported in this study are among the lowest found in marine environments and the quality of analytical methods are of key importance. Speciation of Hg in sea water is of crucial importance as THg concentrations alone do not give adequate data for understanding Hg sources and cycling in marine environments. For example, photoinduced transformations are important for the presence of reactive and elemental mercury in the surface layers, biologically mediated reactions are important for the production/degradation of MeHg and DGM in the photic zones of the water column, and the data for DGM in deep sea indicate the natural sources of Hg in geotectonicaly active areas of the Mediterranean Sea.     

Seasonal and inter-annual dynamics of mesoplankton in the northwestern Japan Sea

Based upon four decades of observations in the northwestern part of the Japan Sea, the seasonal and inter-annual variations of zooplankton abundance and species composition in the epipelagic layer are considered. Seasonal characteristics of the zooplankton community are described in detail for five domains that occur within this part of the Japan Sea. Inter-decadal variation was not significant, but inter-annual variation was considerable and generally opposite to water temperature changes in the upper layer. Exceptional years such as 1996 were noted when a maximum abundance of zooplankton in summer occurred along with a slowing of the seasonal pattern of succession.     

Transient Eastern Mediterranean deep waters in response to the massive dense-water output of the Aegean Sea in the 1990s

We present a detailed account of the changing hydrography and the large-scale circulation of the deep waters of the Eastern Mediterranean (EMed) that resulted from the unique, high-volume influx of dense waters from the Aegean Sea during the 1990s, and of the changes within the Aegean that initiated the event, the so-called ‘Eastern Mediterranean Transient’ (EMT). The analysis uses repeated hydrographic and transient tracer surveys of the EMed in 1987, 1991, 1995, 1999, and 2001/2002, hydrographic time series in the southern Aegean and southern Adriatic Seas, and further scattered data. Aegean outflow averaged nearly 3 × 10⁶ m³ s⁻¹ between mid-1992 and late 1994, and was largest during 1993, when south and west of Crete Aegean-influenced deep waters extended upwards to 400 m depth. EMT-related Aegean outflow prior to 1992, confined to the region around Crete and to 1800 m depth-wise, amounted to about 3% of the total outflow. Outflow after 1994 up to 2001/2002, derived from the increasing inventory of the tracer CFC-12, contributed 20% to the total, of 2.8 × 10¹⁴ m³. Densities in the southern Aegean Sea deep waters rose by 0.2 kg/m³ between 1987 and 1993, and decreased more slowly thereafter. The Aegean waters delivered via the principal exit pathway in Kasos Strait, east of Crete, propagated westward along the Cretan slope, such that in 1995 the highest densities were observed in the Hellenic Trench west of Crete. Aegean-influenced waters also crossed the East Mediterranean Ridge south of Crete and from there expanded eastward into the southeastern Levantine Sea. Transfer into the Ionian mostly followed the Hellenic Trench, largely up to the trench’s northern end at about 37°N. From there the waters spread further west while mixing with the resident waters. Additional transfer occurred through the Herodotus Trough in the south. Levantine waters after 1994 consistently showed temperature–salinity (T–S) inversions in roughly 1000–1700 m depth, with amplitudes decreasing in time. The T–S distributions in the Ionian Sea were more diverse, one cause being added Aegean outflow of relatively lower density through the Antikithira Strait west of Crete. Spreading of the Aegean-influenced waters was quite swift, such that by early 1995 the entire EMed was affected. and strong mixing is indicated by near-linear T–S relationships observed in various places. Referenced to 2000 and 3000 dbar, the highest Aegean-generated densities observed during the event equaled those generated by Adriatic Sea outflow in the northern Ionian Sea prior to the EMT. A precarious balance between the two dense-water source areas is thus indicated. A feedback is proposed which helped triggering the change from a dominating Adriatic source to the Aegean source, but at the same time supported the previous long-year dominance of the Adriatic. The EMed deep waters will remain transient for decades to come.     

Iodine chemistry in deep anoxic basins and overlying waters of the Mediterranean Sea

Iodate (IO₃⁻) is the predominant dissolved species of iodine in the oxygenated waters of the Mediterranean Sea. Iodide (I⁻) is present in significant quantities (up to 65 nM) in oxygenated waters in the photic zone and near the interface above the anoxic and saline Bannock Basin. Lesser quantities of I⁻ (< 10 nM) are found throughout the rest of the oxic water column. An additional unidentified dissolved iodine species is present immediately above the anoxic interface.Total dissolved iodine (ΣI) increases dramatically across the seawater/brine interface. Part of this increase is undoubtedly the result of the dissolution of iodine-rich evaporites during formation of the brine bodies at the Tyro and Bannock Basins. The vertical distribution of ΣI and other dissolved chemical species (particularly PO₄³⁻) in the Bannock Basin brine, however, suggests an additional, present-day, diagenetic source of dissolved iodine to the brine. Based on the increase in the concentration of the most soluble major ions across the seawater/brine interface, 5–7 μM of the 11.5-μM increase in ΣI concentration must be attributed to diagenesis.     

Reproduction in rare bathyal octopods Pteroctopus tetracirrhus and Scaeurgus unicirrhus (Cephalopoda: Octopoda) in the east Mediterranean as an apparent response to extremely oligotrophic deep seas

Reproductive patterns of two benthic bathyal octopods, Pteroctopus tetracirrhus and Scaeurgus unicirrhus have been studied in extremely nutrient-poor environment of the deep-sea Eastern Mediterranean. Both species were found to exhibit a reproductive tactics of producing eggs much larger than in the western part of the sea which likely results in larger hatchlings with higher viability. P. tetracirrhus exhibited a typical “deep-sea” spawning strategy of simultaneous maturation of a single batch of large eggs with atresia of excessive oocytes, whereas reproductive strategy of S. unicirrhus is particular for shelf octopodids: asynchronous maturation of numerous batches of small eggs with no obvious regulatory atresia. Existence of these two types of ovary development and utilisation of fecundity are closely related to two types of evolutionary stable reproductive strategies based on existence of either very large or very small eggs with a few species occupying the “intermediate” position.     

Observations and measurements of wave-induced drift of surface inextensible film in deep and shallow waters

The present study investigates the drift of two-dimensional floating surface films by both deep and shallow water waves in the laboratory. The focus is on the observations and measurements on the drift behavior and to clarify some outstanding issues. Thin polyethylene sheets with reflective markers were attached to simulate the inextensible surface films. Upon the initiation of a wave train, two infrared cameras recorded continuously the instantaneous position of the markers at 50 Hz. The temporal variation of the drift velocity was then determined by processing the recorded data. In all experiments, the drift velocity increased quickly in the beginning and reached a quasi-steady mean value. The magnitude of the surface drift typically increased with the longitudinal length of the polyethylene sheet until the sheet length was approximately the same as the wavelength. Lengthening the sheet length further did not lead to significant changes in the drift velocity. Under the shallow water condition, the wave-induced drift velocity increased linearly with the Ursell number and with a lower water depth.     

Seasonal dynamics in colored dissolved organic matter in the Mediterranean Sea: Patterns and drivers

Two autonomous profiling “Bio-Argo” floats were deployed in the northwestern and eastern sub-basins of the Mediterranean Sea in 2008. They recorded at high vertical (1 m) and temporal (5 day) resolution, the vertical distribution and seasonal variation of colored dissolved organic matter (CDOM), as well as of chlorophyll-a concentration and hydrological variables. The CDOM standing stock presented a clear seasonal dynamics with the progressive summer formation and winter destruction of subsurface CDOM maxima (YSM, for Yellow Substance Maximum). It was argued that subsurface CDOM is a by-product of phytoplankton, based on two main characteristics, (1) the YSM was located at the same depth than the deep chlorophyll maximum (DCM) and (2) the CDOM increased in summer parallels the decline in chlorophyll-a. These observations suggested an indirect but tight coupling between subsurface CDOM and phytoplankton via microbial activity or planktonic foodweb interactions. Moreover, the surface CDOM variations observed both by floats and MODIS displayed different seasonal dynamics from what recorded at subsurface one. This implies that CDOM standing stock can be hardly detected by satellite. It is worthnoting that surface CDOM was found to be more related to the sea surface temperature (SST) than chlorophyll-a concentration, suggesting its physical origin, in contrast to the biological origin of YSM and subsurface standing stocks.     

Seasonal variations in the distribution of Fe and Al in the surface waters of the Arabian Sea

Concentrations of dissolved Al and Fe in the surface mixed layer were measured during five cruises of the 1995 US JGOFS Arabian Sea Process Study, Concentrations of both Al and Fe were relatively uniform between January and April, the NE Monsoon and the Spring Intermonsoon period, ranging from 2 to 11 nM Al (mean 5.3 nM) and 0.5 to 2.4 nM Fe (mean 1.0 nM). In July/August, after the onset of the SW Monsoon, surface water Al and Fe concentrations increased significantly (Al range 4.5–20.1 nM; mean=10 nM, Fe range 0.57–2.4 nM; mean=1.3 nM), particularly in the NE part of the Arabian Sea, as the result of the input and partial dissolution of eolian dust. Using the enrichment of Al in the surface waters, we estimate this is the equivalent to the deposition of 2.2–7.4 g m⁻² dust, which is comparable to values previously estimated for this region. Approximately one month later (August/September), surface water concentrations of both Al and Fe were found to have decreased significantly (mean Al 7.4 nM, mean Fe 0.90 nM) particularly in the same NE region, as the result of export of particulate material from the euphotic zone. Fe supply to the surface waters is also affected by upwelling of sub-surface waters in the coastal region of the Arabian Sea during the SW Monsoon. Despite the proximity of high concentrations of Fe in the shallow sub-oxic layer, freshly upwelled water is not drawn from this layer and the NO₃/Fe ratio in the initially upwelled water is below the value at which Fe limitation is through to occur. Continued deposition of eolian Fe into the upwelled water as it advects offshore provides the Fe required to raise this ratio above the Fe limitation value.     

Fluorescence characteristics of dissolved organic matter in the deep waters of the Okhotsk Sea and the northwestern North Pacific Ocean

Fluorescent dissolved organic matter (DOM), a fraction of chromophoric DOM, is known to be produced in the deep ocean and is considered to be bio-refractory. However, the factors controlling fluorescence properties of DOM in the deep ocean are still not well understood. In this study, we determined the fluorescence properties of DOM in the deep waters of the Okhotsk Sea and the northwestern North Pacific Ocean using excitation-emission matrix (EEM) fluorescence and parallel factor analysis (PARAFAC). One protein-like, two humic-like components, and one uncertain component, which might be derived from a fluorometer artifact, were identified by EEM-PARAFAC. Fluorescence intensity levels of the protein-like component were highest in the surface waters, decreased with depth, but did not change systematically in the bathypelagic layer (1000 m - bottom). Fluorescence characteristics of the two humic-like components were similar to those traditionally defined as marine and terrestrial humic-like fluorophores. The fluorescence intensity levels of the two humic-like components were lowest in the surface waters, increased with depth in the mesopelagic layer (200 - 1000 m), and then slightly decreased with depth in the bathypelagic layer. The ratio of the two humic-like components remained in a relatively narrow range in the bathypelagic layer compared to that in the surface layer, suggesting a similar composition of humic-like fluorophores in the bathypelagic layer. In addition, the fluorescence intensities of the two humic-like components were linearly correlated to apparent oxygen utilization (AOU) in the bathypelagic layer, suggesting that both humic-like components are produced in situ as organic matter is oxidized biologically. These findings imply that optical characteristics of humic-like fluorophores once formed might not be altered further biologically or geochemically in the deep ocean. On the other hand, relationships of fluorescence intensities with AOU and Fe(III) solubility were different between the two humic-like components in the mesopelagic layer, suggesting different environmental dynamics and biogeochemical roles for the two humic-like components.     

Mercury methylation,demethylation and reduction rates in coastal and marine surface waters of the Mediterranean Sea

In situ experiments using isotopically labeled mercury species (¹⁹⁹Hg(II) and Me²⁰¹Hg) are used to investigate mercury transformation mechanisms, such as methylation, demethylation and reduction, in coastal and marine surface waters of the Mediterranean Sea. The aim of this work is to assess the relative contribution of photochemical versus biological processes to Hg transformation mechanisms. For this purpose, potential transformation rates measured under diurnal and dark incubation conditions are compared with major biogeochemical parameters (i.e. hydrological and biological data) in order to obtain the relative contribution of various biotic and abiotic mechanisms in both surface (high light) and bottom (low light) waters of the euphotic zone. The results demonstrate that coastal and marine euphotic zones are significant reactors for all Hg transformations investigated (i.e. methylation, demethylation, reduction). A major outcome demonstrates that Hg methylation is taking place in oxic surface seawater (0.3–6.3% day^− 1) and is mainly influenced by pelagic microorganism abundance and activities (phyto- and bacterioplankton). This evidences a new potential MeHg source in the marine water column, especially in oligotrophic deep-sea basins in which biogeochemistry is mostly governed by heterotrophic activity. For coastal and marine surface waters, although MeHg is mainly photochemically degraded (6.4–24.5% day^− 1), demethylation yields observed under dark condition may be attributed to microbial or chemical pathways (2.8–10.9% day^− 1). Photoreduction and photochemical reactions are the major mechanisms involved in DGM production for surface waters (3.2–16.9% day^− 1) but bacterial or phytoplanktonic reduction of Hg(II) cannot be excluded deeper in the euphotic zone (2.2–12.3% day^− 1). At the bottom of the euphotic zone, photochemical processes are thus avoided due to the attenuation of UV-visible sunlight radiation allowing biotic processes to be the most significant. These results suggest a new potential route for Hg species cycling in surface seawater and especially at the maximum biomass depth located at the bottom of the euphotic zone (i.e. maximum chlorophyll fluorescence). In this environment, DGM production and demethylation mechanisms are thus probably reduced whereas Hg methylation is enhanced by autotrophic and heterotrophic processes. Experimental results on mercury species uptake during these investigations further evidenced the strong affinity of MeHg for biogenic particles (i.e. microorganisms) that correspond to the first trophic level of the pelagic food web.     

Acoustic and optical variations during rapid downward motion episodes in the deep north-western Mediterranean Sea

An Acoustic Doppler Current Profiler (ADCP) was moored at the deep-sea site of the ANTARES neutrino telescope near Toulon, France, thus providing a unique opportunity to compare high-resolution acoustic and optical observations between 70 and 170 m above the sea bed at 2475 m. The ADCP measured downward vertical currents of magnitudes up to 0.03 m s⁻¹ in late winter and early spring 2006. In the same period, observations were made of enhanced levels of acoustic reflection, interpreted as suspended particles including zooplankton, by a factor of about 10 and of horizontal currents reaching 0.35 m s⁻¹. These observations coincided with high light levels detected by the telescope, interpreted as increased bioluminescence. During winter 2006 deep dense-water formation occurred in the Ligurian subbasin, thus providing a possible explanation for these observations. However, the 10-20 days quasi-periodic episodes of high levels of acoustic reflection, light and large vertical currents continuing into the summer are not direct evidence of this process. It is hypothesized that the main process allowing for suspended material to be moved vertically later in the year is local advection, linked with topographic boundary current instabilities along the rim of the ‘Northern Current’.     

黄海西部海洋湍流的季节变化特征分析

在2006—2007年开展的"中国近海海洋综合调查与评价"项目中,作者利用自由下降湍流剖面仪MSS60在南黄海海区分别进行了夏、冬、秋季三个航次的微尺度湍流观测,并计算分析了该海区的湍动能耗散系数ε,湍扩散系数κ等。通过与温度、流速分布图对比,结果表明三个季节的湍流混合趋势大体一致。在沿岸浅水区,混合作用比较强烈。而深水区湍流混合的垂直分布明显地表现出三层结构,混合较强的上混合层和底混合层,及相对较弱的中层。风混合和潮混合是黄海湍流混合的主要形式。风的影响主要表现在海洋上层,潮流的影响则表现于底层。     

Application of deep learning technique to the sea surface height prediction in the South China Sea

A deep-learning-based method, called ConvLSTMP3, is developed to predict the sea surface heights(SSHs).ConvLSTMP3 is data-driven by treating the SSH prediction problem as the one of extracting the spatial-temporal features of SSHs, in which the spatial features are "learned" by convolutional operations while the temporal features are tracked by long short term memory(LSTM). Trained by a reanalysis dataset of the South China Sea(SCS), ConvLSTMP3 is applied to the SSH prediction in a region of the SCS east off Vietnam coast featured with eddied and offshore currents in summer. Experimental results show that ConvLSTMP3 achieves a good prediction skill with a mean RMSE of 0.057 m and accuracy of 93.4% averaged over a 15-d prediction period. In particular,ConvLSTMP3 shows a better performance in predicting the temporal evolution of mesoscale eddies in the region than a full-dynamics ocean model. Given the much less computation in the prediction required by ConvLSTMP3,our study suggests that the deep learning technique is very useful and effective in the SSH prediction, and could be an alternative way in the operational prediction for ocean environments in the future.     

楚科奇海海冰周年变化特征及其主要关联因素

利用1999年美国国家冰雪资料中心的各种卫星遥感综合分析数据对楚科奇海海冰周年变化进行详细分析，将全年的海冰变化过程分成密集冰封期、东岸融化期、单湾结构期、双湾结构期、三湾结构期、全线北撤期、南进封闭期、全面冻结期8个阶段。海冰冻结期仅2个月，海冰融化期持续4～5个月，说明融冰过程的吸热是个漫长的过程。太平洋与北冰洋海面高度差形成的正压压强梯度力是白令海水进入北冰洋的主要动力，白令海水进入冰下形成的暖水海冰边缘区是海冰融化的重要机制。白令海水在楚科奇海扩散过程受到海底地形产生的Taylor柱效应的显著影响，使其产生绕过浅滩，沿海谷流动，在海谷的方向上输送更多的水体和热量的现象，形成海冰融化的湾状结构。楚科奇海的局地风场也是海冰形态变化的重要因素之一。局地风场在冬季阻碍白令海水的入流，而在夏季促进白令海水的入流。     

Bathymetric variations in vertical distribution patterns of meiofauna in the surface sediments of the deep Arctic ocean (HAUSGARTEN,Fram strait)

Deep-sea benthic communities and their structural and functional characteristics are regulated by surface water processes. Our study focused on the impact of changes in water depth and food supplies on small-sized metazoan bottom-fauna (meiobenthos) along a bathymetric transect (1200–5500 m) in the western Fram Strait. The samples were collected every summer season from 2005 to 2009 within the scope of the HAUSGARTEN monitoring program. In comparison to other polar regions, the large inflow of organic matter to the sea floor translates into relatively high meiofaunal densities in this region. Densities along the bathymetric gradient range from approximately 2400 ind. 10 cm^-2 at 1200 m to approximately 300 ind. 10 cm^-2 at 4000 m. Differences in meiofaunal distribution among sediment layers (i.e., vertical profile) were stronger than among stations (i.e., bathymetric gradient). At all the stations meiofaunal densities and number of taxa were the highest in the surface sediment layer (0–1 cm), and these decreased with increasing sediment depth (down to 4–5 cm). However, the shape of the decreasing pattern differed significantly among stations. Meiofaunal densities and taxonomic richness decreased gradually with increasing sediment depth at the shallower stations with higher food availability. At deeper stations, where the availability of organic matter is generally lower, meiofaunal densities decreased sharply to minor proportions at sediment depths already at 2–3 cm. Nematodes were the most abundant organisms (60–98%) in all the sediment layers. The environmental factors best correlated to the vertical patterns of the meiofaunal community were sediment-bound chloroplastic pigments that indicate phytodetrital matter.     

Spectral contributions of the components of seawater to the beam attenuation coefficient in surface waters of the Mediterranean Sea

We compute model spectra of the beam attenuation coefficient in surface waters of the Mediterranean Sea. These spectra are used to determine the contribution of the components of seawater (suspended matter, yellow substance, pigments of phytoplankton, and pure water) to the beam attenuation coefficient in different types of seawater. For the surface waters, we establish the relationship between the light scattering coefficient and the attenuation coefficient at a wavelength of 547 nm and determine the background (limiting minimum) value of the coefficient of absorption by the yellow substance in waters of the Mediterranean Sea. It is compared with the values of the same parameter for some other basins (Black Sea, Lake Baikal, Baltic Sea, and oceanic waters).     

滨州市海域海冰的时空分布和冰级特征

文章根据2005—2017年滨州市海域冬季各类海冰监测数据资料,对冰日、冰期、冰型、冰厚、浮冰密集度和结冰范围等冰情要素进行分析,总结海冰的时空分布和冰级特征,为进一步开展相关海域的海冰灾害研究和防灾减灾工作提供必要的参考。研究表明,滨州市海域的总冰期一般为60~80d,其中严重冰期约为25d,河口和浅滩等近岸海域冰情较重;冰型、冰厚和结冰范围等在不同冰期和冰级具有不同特点,对海上设施和海上活动造成不同程度的影响。     

In situ video observations of benthic megafauna and fishes from the deep eastern Mediterranean Sea off Egypt

Remotely operated vehicle (ROV) video observations were used to document benthic fauna at a hydrocarbon drilling location, at 2 720 m depth, in the poorly studied deep water off northern Egypt. The decapod Chaceon mediterraneus was the most common organism at the site and the only benthic megafaunal invertebrate observed. Three species of fish, Coryphaenoides mediterraneus, Cataetyx laticeps and Bathypterois mediterraneus, were also encountered. These findings confirmed these three species as the deepest-distributed benthic fish in the eastern Mediterranean, and confirmed that the deep Mediterranean, in particular the eastern basin, is one of the world's poorest deep-sea ecosystems in terms of diversity. The behaviour exhibited by the species observed was consistent with their natatory capacity, deduced from their feeding intensity (gut fullness) and diet (mainly suprabenthic prey were consumed), and conservative life strategies adapted to an extremely low energy environment.