Deep-sea Research Part Ii-topical Studies in Oceanography最新文献

The marine region around Christmas Island and Cocos (Keeling) Islands in the eastern Indian Ocean includes a large number of under-researched seamounts. Here novel geomorphological and oceanographic data from the RV Investigator IN2021_V04 and IN2022_V08 expeditions to the region are presented to demonstrate the variety of form, size and height of these seamounts, and their spatial and bathymetric relationship to the various water masses present in the region. This includes the discovery of a large extinct caldera (‘Eye of Sauron’) at 3100–3700 m depth southeast of Christmas Island and a circular volcanic crater south of Cocos (Keeling) Islands at 3800–4000 m. Eleven seamounts were mapped for the first time, and mapping was completed or extended for a further 23, including the massive Muirfield seamount that summits at 17 m below sea-level. The presence is documented of another relatively shallow seamount (‘Green-eye’, summit 438 m) at the southernmost end of the Cocos (Keeling) Islands Territory. The new data emphasises the scarcity of shallow habitat (above 2000 m) for benthic biodiversity from the region, and the eastern Indian Ocean in general, and its importance for conservation.

The abundance, characteristics, and distribution of microplastics in almost all aquatic environments have been well documented. However, the distribution of microplastics in open oceans, especially in the western Indian Ocean, is rarely studied. Hence, for the first time, we determined the meridional distribution characteristics of microplastics in the surface mixed layer of the western Indian Ocean during the boreal summer monsoon in 2017. We used the multiple opening and closing nets and environmental sampling system to collect microplastics from 11 stations between 5°N and 16°S along 67°E in the western Indian Ocean; the surveyed stations with similar temperature and salinity profiles were clustered into four groups. Microplastics were detected at all sampling stations. The mean abundance of microplastics in the study area was 0.23 ± 0.04 particles/m³, and black-colored polyester fibers 1.0–5.0 mm in size were the most abundant type of microplastics among the samples collected. However, the mean abundance of microplastics differed among the clustered groups due to the influence of different ocean currents associated with each group, indicating meridional heterogeneity in the distribution of microplastics in the surface mixed layer of the western Indian Ocean. These findings provide reliable information for a better understanding of the characteristics of microplastics in the surface waters of the western Indian Ocean.

Many fish species undergo ontogenetic habitat shifts as they grow to fulfill new biological, ecological and environmental requirements. While relationships between fishes and large hard-substrate cold-water corals (CWC) (e.g., Desmophyllum pertusum reefs) have frequently been studied, there are relatively fewer studies examining the relationships of fish with habitats specifically provided by smaller corals (e.g., sea pens) in soft-bottom environments. Despite this knowledge gap around soft-bottom corals, growing evidence of their importance has nonetheless justified their inclusion as conservation targets in numerous Marine Protected Areas (MPA), including the Canadian Laurentian Channel MPA. Here, we performed ROV and near-seabed drift-camera system surveys within the Laurentian Channel MPA in 2017 and 2018 to assess the influence of fish body size and habitat type on fish small-scale distribution in a low-relief deep-sea soft-sediment environment. We compared the local size structure of the four most abundant deep-sea demersal fish taxa of the channel (Redfish (Sebastes spp.), Witch Flounder (Glyptocephalus cynoglossus), Marlin-Spike Grenadier (Nezumia bairdii) and Longfin Hake (Phycis chesteri)) across one barren and five structural benthic habitats defined by the presence of nine dominant epibenthic invertebrates (actiniarians and CWCs). We used generalized additive models to identify biotic (benthic habitats) and abiotic (depth, bottom types) covariates of size for each taxon. We observed 15,381 fish within the 43.6-ha study area, of which 7,511 fish were measured. Juveniles represented 99% of all fish measured, with a notable increase in average fish size in 2018. While we did not find any associations between benthic habitats and fish life stages, the analysis revealed a significant increase in fish size within sea pen habitats for all four taxa. Conversely, we found a taxon-specific influence of bottom type on fish size for all taxa. In addition, Redfish and Longfin Hake size was positively correlated with depth. For deep-sea demersal fish taxa of the MPA, our results suggest that 1) sea pens provide nursery habitat for early-life stages, 2) fish undergo ontogenetic shifts in micro-habitat use and specialization, and 3) fish-habitat associations appear to be facultative rather than obligate. Through the use of in-situ video data, this study provided evidence that small and large fish do not use the same micro-habitats, and that sea pens contribute significantly to fish habitat despite providing less habitat heterogeneity than reef-forming scleractinians or large gorgonians. These results contribute to empirical understanding of fish-habitat relationships at different fish life stages and may inform fisheries management, as well as monitoring efforts in the MPA and other protected deep-sea environments.

Scleractinian corals create habitat that persists on geological timescales and supports some of the most diverse ecosystems on Earth. Throughout their depth range, these ecosystems are threatened by anthropogenic disturbances, including global ocean change and hydrocarbon extraction. While numerous studies have focused on how shallow-water corals respond to stressors, there is a paucity of research on deep-sea corals. Here, we analyze the gene expression patterns from a reef-forming deep-sea coral, Desmophyllum pertusum (previously Lophelia pertusa), exposed to oil and dispersant mixtures under current and projected future temperature and pH conditions. The overall gene expression patterns varied among coral colonies (genets), but dispersant exposure induced the strongest response. A Weighted-Gene Correlation Network Analysis (WGCNA) identified networks of co-expressed genes in response to different stressors. Gene ontology (GO) enrichment analysis revealed that D. pertusum exhibited the coral cellular stress response (CSR) during exposure to oil, dispersant, and a decrease in pH. Dispersant exposure elicited a response that included up-regulation of apoptosis, immune system, wound healing, and stress-related pathways. Coral nubbins exposed to oil exhibited signs of resource reallocation and a reduction in growth to maintain cellular homeostasis. The decrease in seawater pH resulted in a less severe stress response than dispersant exposure. These results support the idea that there is an underlying environmental stress response (ESR) shared by scleractinians, but that this response varies depending on the type and intensity of the stress with which they are challenged.

The Benguela Upwelling System (BUS) exerts a major oceanographic and sedimentary influence on the Namibian and western South African continental margin. Previous literature has distinguished between the Northern Benguela Region (NBR) and Southern Benguela Region (SBR) due to the varying sedimentary, oceanographic, upwelling and faunal nature of the two areas. A comprehensive understanding of how upwelling and palaeoceanography between these two subsystems relate to, or differ between each other, is limited. Late Quaternary foraminifera are extremely useful in recording and tracking past oceanographic and climatic changes in the region. Planktic foraminifera from two cores recovered from the western continental slope of South Africa, at a water depth of ∼3500 m, were analysed and compared to previous studies from the region to determine any variations in the water mass, oxygenation, upwelling and productivity history between the NBR and SBR, and during glacial-interglacial periods. Results from this study indicate that influence from subpolar, subtropical and upwelled surface waters play a major role in the faunal distributions and oceanographic history of the area. Abundances of subpolar species are lowest off western South Africa during interglacial periods, indicating obstruction from the subtropical convergence and restriction of the polar fronts during those periods. Benthic and planktic foraminifera indicate strengthened upwelling and the extension of upwelling cells further slopeward during glacial periods. Increased productivity associated with stronger upwelling during glacial periods led to increased nutrient and organic matter delivery to the seafloor, resulting in eutrophic conditions. These environments are further amplified by nutrient-rich bottom water masses that are strengthened during these periods. Upwelling intensity and productivity also exert controls on the benthic foraminifera, where epifaunal taxa abundances and epifaunal-infaunal ratios increase from the NBR to SBR, and glacial epifaunal-infaunal ratios decrease compared to interglacial periods. Higher abundances of tropical-subtropical species were recorded in the NBR and Agulhas retroflection relative to western South Africa, indicative of the influence of the Angola-Benguela Front to the north of upwelling zones and the inflow of warm Agulhas Current waters from the South Indian Ocean to the southwest of South Africa. This study therefore reveals that the NBR and SBR vary in their palaeoceanographic record as a result of their position to important southeast Atlantic oceanographic features such as the Angola-Benguela Front, the Agulhas Current, the Benguela Current and polar fronts.

The Terra Nova Bay (TNB) Polynya in the Western Ross Sea of Antarctica is a major producer of High Salinity Shelf Water (HSSW), a precursor to Antarctica Bottom Water (AABW). Processes occurring in and around the polynya can therefore effect change in the lower limb of overturning circulation in this region. Here, we use data from a densely-instrumented upper-ocean mooring, deployed for 1 year in a region of active HSSW formation within TNB, to examine the coupling of surface brine rejection and vertical mixing to katabatic wind forcing. We find a high correlation between salinity and winds during the wintertime HSSW production season at the mooring site, with a lag-response of 20 h in near-surface (∼47 m) salinity to winds measured at the nearby Automatic Weather Station (AWS) Manuela. Salinity and temperature measurements show a fully destratified water column by June, with a lag-response of near-seabed (∼360 m) salinity to near-surface salinity of just 5 h. Measurements of turbulent kinetic energy (TKE) dissipation rate (ɛ) from moored pulse-coherent acoustic Doppler current profilers (ADCPs) show general agreement with classic boundary layer scaling (BLS), and calculations of a vertical mixing timescale using the Obukhov length scale average to ∼2.5 h during austral winter, consistent with the 5-h lag time observed in the salinity data. Comparisons to data from concurrent mooring deployments along the southern boundary of TNB, as well as to previously published assessments of model simulations and data from Climatic Long-term Interaction for the Mass-balance in Antarctica (CLIMA) moorings, allow us to explore spatial variability in the coupling of winds and salinity across TNB and to speculate on possible HSSW circulation pathways.

We describe the first year-long hydrographic mooring timeseries from a location just to the south of the Drygalski Ice Tongue – the ice margin that forms the southern boundary of the Terra Nova Bay Polynya in the western Ross Sea. The region is where any northward flowing component of the Victoria Land Coastal Current encounters the ice tongue and supports an occasional polynya. The hydrographic mooring was deployed nearby Geikie Inlet from February 2017 through to March 2018, and was coupled with several contemporaneous oceanographic moorings to the north of the Drygalski Ice Tongue. This provides data with which to examine the water column dynamics in the context of local circulation and interaction with the ice tongue. The Terra Nova Bay region is subject to strong katabatic winds, however the polynya to the south of the Drygalski Ice Tongue operates at different times through the annual cycle when compared to the Terra Nova Bay Polynya to the north, as the sea ice in the south-side region is far more constrained in its motion yet, temperature and salinity are broadly consistent north and south of the ice tongue. Sub-surface Ice Shelf Water is observed south of the ice tongue. Transients in near-bed temperature and salinity are observed on both sides of the ice tongue, albeit with the northside leading by ∼8–9 days. Notably, the temperature transient precedes that of salinity by around 40 days. This suggests that, at this near-coastal position, the circulation beneath the ice tongue is primarily southward.

Literature on Ross Sea zooplankton is limited, although it is the most productive system and has the highest biomass of phytoplankton in the Southern Ocean. Energy transfer within the food web and response of continental shelf food web to climate change depend on the knowledge of the density and distribution of zooplankton. We evaluated its density, composition, spatial distribution and their potential relations with environmental factors and specific water mass presence. Particular attention was given to copepods. Mesozooplankton samples were collected between 14 January and February 11, 2017 from 14 stations in the western Ross Sea and Terra Nova Bay, while other micronekton samplings were completed at 6 stations. Results highlighted three pools of stations: one inside Terra Nova Bay with the highest densities, one on the platform with lower mean density, and the third represented by a N–S transept at 175°E with the lowest mean density. This partitioning of the region fit with the grouping of stations according to a ternary plot based on the different percentages of water masses in each station and reflected the similarity of zooplankton. The presence of specific water masses and their contribution in the upper 200 m influences zooplankton biodiversity and density. A phylogenetic tree of the zooplankton was constructed to analyse the distance among the observed taxa. The area located near the coast exhibited the highest degree of phylogenetic overdispersion. In coastal waters, typical copepod species constituted the main part of the neritic zooplankton of Terra Nova Bay and Ross Sea. Oithona similis was the most abundant species both in coastal and offshore sectors, followed by pteropods, polychaetes and euphausiids.