Deep-Sea Research Part I-Oceanographic Research Papers最新文献

Understanding variations in marine phytoplankton primary production (PP) is crucial for assessing the response of the marine environment to climate change and for quantifying the ocean carbon cycle. However, the spatiotemporal variability of size-fractionated PP in the South China Sea (SCS) remains ambiguous. Our study investigated the monthly, seasonal, and inter-annual variability of size-fractionated PP in the SCS using satellite observations from 2002 to 2022. There were noticeable seasonal and monthly variations in size-fractionated PP, with notably high PP values appearing during the cold season. The disparities in distribution and the distinct fluctuation patterns between size-fractionated PP suggest that total PP alone is not a comprehensive indicator of marine ecosystem health. Over the past two decades in the SCS, there were more pronounced decreases in total, pico-, and nano-PP, whereas micro-PP displayed no significant trend. The most pronounced decline occurred in the northern SCS, contrasted by increases in coastal areas. These size-fractionated PP anomalies showed strong correlations with climate change indices, highlighting the impact of environmental factors on these anomalies, such as sea surface temperature, mixed layer depth, and wind speed. Our findings emphasize the importance considering size-fractionated PP to gain a more nuanced understanding of the ocean carbon cycle and the marine ecosystem's response to climate changes.

Understanding the trophic ecology of marine ecosystems is a key challenge given that they are subject to anthropogenic pressures that can alter the integrity of the food web. The bathyal zone of the Central Tyrrhenian Sea is characterized by an high level of community biodiversity, heavy anthropogenic pressure and thus represents a fundamental environment for the study of trophic relationships between demersal fish species which live in sympatry, but this basin still remains data deficient. To fill this information gap, we investigated the trophic niche overlap of four deep-sea fish species, Galeus melastomus, Helicolenus dactylopterus, Lepidorhombus boscii, and Trisopterus capelanus, in the Central Tyrrhenian Sea using an integrated approach of stomach content and stable isotopes analysis (δ¹³C and δ¹⁵N). Our analysis revealed that the blackmouth catshark G. melastomus displays a wide trophic niche with considerable overlap with other fish species, as it consumed diverse prey including fish, crustaceans, and cephalopods, thus proving a generalist and opportunistic feeding behavior. The blackbelly rosefish H. dactylopterus exhibits a narrower isotopic trophic niche comprising mainly benthic crustaceans and so suggesting a marked specialism in feeding strategy. The diet of the four-spotted megrim L. boscii showed a marked overlap with that H. dactylopterus, as these species shared a similar benthic habit and relied upon a similar pool of resource. The poor cod T. capelanus showed a diverse diet comprising both prey captured close to the bottom, especially decapod and mysid crustaceans, and bathypelagic prey, mainly Osteichthyes, Myctophidae, and amphipod Hyperiidae, with an intermediate partially segregated isotopic niche width. The combination of stomach content analysis and stable isotopes analysis provided the first characterization of the trophic relationships, shedding light on the trophic niche overlap of these four ecologically important deep sea fish species. The diets of the four investigated species showed similarities in the composition of prey, but also differences which allows them to partially reduce competition. This information may be of crucial relevance for the development of effective management and conservation strategies for the bathyal Mediterranean environment.

An inflow of warm and salty Atlantic origin waters (AW) from the Nordic Seas to the Arctic Ocean interior is in the list of the major external factors, which control the hydrologic regime of the East-Atlantic sector of the Arctic Ocean. Rapid decay of the sea ice cover in 2000–2010s raised questions on the possible changes in the fate of the inflow under present ice-depleted conditions: whether warm and salty AW will cool and freshen slower (due to less ice to be melted underway), or intensified heat loss to the atmosphere through the open surface will lead to the faster AW cooling on shorter distance? We present rare hydrologic data, collected during the late winter 2019 in the international expedition “Transarktika-2019” in the northern part of the Barents Sea and at the adjoining continental slope of the Nansen Basin. On the basis of field data, supported by the oceanic reanalysis product and numerical modelling we have studied the transformation of AW on its transit between Svalbard and Franz Joseph Land. We show that the observed cooling and desalination of the fraction of AW over the continental slope around this area is controlled by lateral mixing with colder and fresher waters, outflowing from cross-slope canyons, which cut the continental slope: Kvitøya Trough, Franz Victoria Trough and probably, the British Channel. The obtained results demonstrate more intensive transformation of AW on this transit compared to previous studies. Possible explanations of this contrast, which are supported by the earlier studies, may include: the season of the survey when dense water outflow through canyons is at its maximum and gradual “atlantification” of the East-Atlantic sector of the Arctic Ocean, which favours intensification of dense water formation in winter polynyas under conditions of increased seasonality of sea ice. In the latter case, faster cooling and desalination of AW en route in this part of the Arctic Ocean may be considered as a reasonable hypothesis, provided the atlantification of the Nansen Basin is progressing.

Mesoscale eddies are quasi-circular ocean currents accompanied by dynamic and thermodynamic variations, and are classified as anticyclonic (AEs) and cyclonic eddies (CEs) according to their rotation direction. Eddies transition between surface cold CEs/surface warm AEs (“conventional” eddies) and surface warm CEs/surface cold AEs (“unconventional” eddies) during their lifetimes. However, the characteristics and mechanisms of these transitions remain unclear. In this study, satellite and in-situ data were used to explore the spatiotemporal variation and vertical structures of “conventional” and “unconventional” eddies during the evolution of long-lived eddies (>1 year) in the global ocean from 1993 to 2015. On average, long-lived eddies are “unconventional” for about 40% of their lifetimes, and these “unconventional” eddies are concentrated in the South Atlantic Ocean and off the western and southern coasts of Australia. “Unconventional” eddies show distinct temporal variation and subsurface temperature structures. Generally, cold AEs and warm CEs last <3 months and are more active in summer than in winter. They have cold or warm cores within the mixed layer depth of the water column, which is affected by eddy–wind interactions via eddy-induced Ekman pumping. However, some cold AEs in the South Atlantic and warm CEs in the South Indian Ocean can last for >3 months and display weak seasonal variation. In addition, their cold and warm cores can extend to ∼200 and ∼300 m, respectively, and are related to subduction and the Leeuwin Current System.

The Brazil Current (BC) experiences intense mesoscale activity from its origin on the northeastern Brazilian coast till it reaches the Brazil-Malvinas Confluence. The BC meandering behavior near the Cape Santa Marta (27°S – 31°S), has not been well described in the literature. In this study, in situ observations and satellite images are used to describe the BC meanders and cyclonic eddies near the cape. The results obtained from two Lowered Acoustic Doppler Current Profilers (LADCP) sections show that these structures are surface intensified but extend through the whole water column. Conductivity Temperature and Depth (CTD) profiles show evidence of coastal water trapping inside an eddy. Altimeter data show two hot spots of cyclones, one at 28.5°S and the other at 30.5°S, with an average of ∼5 and 7 eddies per year, respectively. The eddies generated on both sites have a mean lifetime of 37 days, and no significant differences were found between their surface properties. Satellite images of chlorophyll show that coastal water horizontal advection is a recurrent phenomenon in these eddies, imprinting an eddy stirring signal into their average composites. Furthermore, using Empirical Orthogonal Functions to analyze the BC flow in a mooring line, we observed that the mesoscales meanders are responsible for explaining at least 1/3 of the flow variability in this area.

Bottom trawling significantly impacts benthic communities, reducing habitat complexity and biodiversity. The distribution and abundance of key species in the northeastern Gulf of Cádiz, Spanish waters of Atlantic Ocean, such as soft-bottom octocorals and burrowing megafauna, were examined using video observations and Spatial Distribution Models developed with Random Forest. The study was conducted in the Site of Community Importance “Volcanes de fango del golfo de Cádiz" focusing on depths between 300 and 950 metres depth. The effects of environmental variables and bottom trawling on octocoral distribution and abundance were assessed, and management strategies for habitat protection were proposed. Bottom trawling was found to decrease habitat suitability for large octocoral species while the abundance of Norway lobster was positively correlated with certain values of bottom trawling effort. The study suggests that bottom trawling causes significant reduction in octocoral distributions, substrate homogenization, and habitat differentiation. A 200 km² area is recommended for a specific bottom trawling regulation to protect soft-bottom octocorals and support the recovery of commercial burrowing megafauna.

The exponential increase in plastic production coupled with variable global waste management system efficiencies has resulted in large amounts of plastic waste entering the ocean every year. Although we know millions of tonnes of plastic have entered the oceans, we do not yet understand the patterns of its accumulation across space nor the drivers of these patterns. The deep ocean is expected to be a resting place, or reservoir, for most plastic pollution. Here, we conducted a rigorous, systematic review of previously published datasets to synthesize our understanding of macroplastic pollution (>5 mm) on the ocean floor. Using extracted data, we built predictive additive models to estimate the amount and distribution of plastic on the ocean floor. We built two models: one using data from remote operated vehicles (ROVs) and another using data from bottom trawls. Using the model built with ROV data, which was better-constrained, we estimate that 3 to 11 million metric tonnes (MMT) of plastic pollution resides on the ocean floor as of 2020. This is of similar magnitude to annual inputs from land and one to two orders of magnitude greater than what is predicted to be floating on the ocean surface. To improve future estimates and our understanding of global patterns, we provide recommendations for ocean floor monitoring of plastic pollution.

When the internal tides encounter topography during propagation, the scattering effect will induce the baroclinic energy to transfer from the low mode to the higher mode, which may cause the internal tide to become unstable or even enhance dissipation. The characteristics of scattering of mode-1 M₂ internal tide in the northern South China Sea (SCS) and its impact on baroclinic energy dissipation are explored in this study based on numerical simulations. The results show that the energy flux of the mode-1 M₂ internal tide gradually weakens during propagation into the South China Sea. In the northern SCS, which has more complex topography, such as the continental slope, scattering will occur, resulting in a vertical mode increase and shear enhancement, and finally, the baroclinic energy is dissipated. Next, the effect of different topographies on mode-1 M₂ internal tide scattering is analyzed by ideal experiments. The mode-1 M₂ internal tide is scattered by topography, and the propagation as well as dissipation are also modified. The results show that the strong energy flux is mainly distributed on the surface of the continental slope and the area near the slope break. With the increase in criticality, the energy flux around the seamount and on the shelf gradually weakens, and the dissipation rate increases continuously. In addition, the slope topography is more likely to induce internal tide scattering than the seamount topography.

The acknowledgement of the importance of small-scale turbulent mixing for the redistribution of heat, nutrients and suspended matter in the ocean has led to renewed interest in the breaking of internal waves at underwater topography. This follows from observations that turbulence intensity increases from the ocean interior to the seafloor. As two-dimensional models require reduction of turbulent buoyancy flux in the vicinity of the seafloor to allow for up-welling flows, the question is how thin such a layer of reduced turbulence above the seafloor can be. From an observational study in this subject, we present 400-day moored high-resolution temperature measurements in a Rockall canyon between 0.9 < h < 152 m from the steeply sloping thalweg-seafloor. In the area, Thorpe-scale calculated turbulence dissipation rate is predominantly governed by the breaking of semidiurnal internal tides. Tidal-mean turbulence profiles increase with depth, together with inertial-subrange temperature-variance. A distinct further increase in turbulence is found for the lower 4 m across which inertial-subrange temperature variance decreased. This was observed during most of a tidal phase, except during the warming phase, when a decrease in turbulence was found in the lower few meters. The thin layer above the seafloor showed a distinct change in distribution of small-scale stratification and a transition from little inertial-subrange variance at h = 0.9 m, via dominant convection-turbulence at h < 5 m to dominant shear-turbulence at h > 30 m, as established from spectral information. The lack of an observed mean near-seafloor buoyancy-flux reduction is hypothesized to be compensated by 3D-effects, temporary effects, less steep slope effects, or none at all.

Three-dimensional deep-sea structures enhance substrate complexity and can shape numerous ecological hotspots that play critical roles in the diversity and distribution of benthic faunal assemblages. The characteristics of these features are not fully understood due to the logistical challenges of exploring at great depths and remote locations. One such deep-sea feature, the Wallaby-Zenith Fracture Zone (WZFZ) located in the East Indian Ocean, was mapped using a full-ocean depth multibeam echosounder system. Additionally, twelve baited landers and five crewed submersible dives were conducted in water depths between 4709 and 6591 m. We use seafloor bathymetry, bathymetric derivatives and video footage from the submersible dives to characterise the benthic structures, substrate types, habitat diversity, and the distribution of associated functional groups of megafauna for the WZFZ. The Benthic Terrain Modeler toolbox was used as an initial semi-automated step to generate benthic structural classes, which were further characterized through the inclusion of bottom salinity and temperature data collected by the scientific landers and integrated with the observations of seafloor substrate from the submersible video footage. This resulted in identification of nine benthic habitat classes characterised by unique seafloor morphological structure and substrate texture. A polymetallic nodule field, a possible extension of the previously reported Cape Leeuwen nodule field, was also observed along submersible transects conducted on the slopes of the WZFZ. The distribution and diversity of the functional megafauna groups observed are influenced by the deep-sea benthic habitats, defined by seafloor structures and substrate heterogeneity, especially the presence of hard substrates such as outcropping bedrock and polymetallic nodules within the WZFZ.