Progress in Oceanography最新文献

A large data set on stomach content (over 27,000 individual fish) – collected in the Barents Sea in 2015 – was used to examine ontogenetic and interspecific changes in diet with size for 35 fish species. The analysis combined the use of hierarchical clustering and random forest. Two different diet metrics were used (% weight of prey types per weight of stomach content or per weight of fish, the latter reflecting also feeding intensity), and data were analysed based on average diet of predator groups (species and size groups) across the entire sample series (from the whole Barents Sea in different seasons), or for data broken down by geographical areas and seasons. Similar trophic groups (clusters) were identified for the various data sets suggesting that the results on trophic structure were robust. The trophic groups including size information were broadly similar to groups found earlier with data averaged at species level. Hierarchical clustering produces a hierarchy of trophic groups (or trophic guilds) at various levels of diet dissimilarity. With 12 clusters identified, one cluster had fish as dominant prey, and 5–6 others had either plankton or benthos as dominant prey. The clusters tended to be distinct and homogenous with one dominant prey category in average diet (often > 60 %). This was especially the case for the plankton clusters, with copepods, euphausiids, hyperiid amphipods, or gelatinous zooplankton as dominant prey in each of 4 trophic groups. The benthos clusters tended to be less dissimilar with more overlap in diet composition, with predominance of either hard-shelled forms (echinoderms and molluscs) or softer prey (polychaetes and crustaceans) for groups of clusters. There were clear patterns of ontogenetic shifts in fish diet. Fish that clustered as piscivores at larger size tended to grow from being planktivores when smaller. Smaller species of planktivores or benthivores shifted position among trophic groups but remained within the categories of planktivores or benthivores, respectively. Taxonomy (species) tended to be more important than size for explaining the diet composition of the different clusters of fish predators.

Our extensive research spanning four decades (1979–2023) investigated the spatial and vertical distributions, dynamics of catch rates (CPUEs), and basic biological features of the understudied deep-water species whiteblotched skate (Bathyraja maculata), which inhabits the North Pacific Ocean. The data used in our study were gathered from publicly accessible databases such as OBIS, GBIF, and AFSC and supplemented by the results of Russian bottom trawl surveys. Our investigation revealed regions with the densest aggregations of whiteblotched skate, notably along the eastern Bering Sea slope, off the Aleutian Islands, in the eastern Sea of Okhotsk, and in the Pacific waters off southeastern Kamchatka and the northern Kuril Islands. Typically, these areas showed the greatest abundance of this species at depths ranging from 400 to 700 m. Remarkably, whiteblotched skates exhibited a migratory pattern toward greater depths during colder months for overwintering, while in the warm period, they predominantly fed at shallower depths. The species also displayed a strong preference for bottom temperatures in the range of 3.1–4.5 °C. Individual whiteblotched skates had a total length spanning from 18 to 127 cm, with the bulk falling within the 50–100 cm range. An intriguing observation was the absence of significant differences in length and body weight between males and females. Additionally, our data revealed that condition factor values reached their peak during the autumn–winter period. Over the years, we observed a substantial increase in the catch rate of whiteblotched skates across most parts of their range. In these areas, the catch rates (CPUEs) initially decreased from the 1980 s to the early 2000 s before increasing.

Large icebergs (>20 km long) are responsible for most of the freshwater discharged into the Southern Ocean. We report on in situ and satellite observations made during the break-up phase around South Georgia of the giant tabular iceberg A-68A. The in situ measurements were obtained during a 4-day visit by a research vessel in February 2021, where physical, chemical and biological measurements were made at a range of distances away from the main and subsidiary icebergs. These results were compared to a far-field station 133 km away. Upstream of the iceberg field, water column structure was similar to ambient water although there was evidence of iceberg-associated phytoplankton as a likely remnant of the passage of the icebergs. Nevertheless, enhancement of primary productivity along the path of the icebergs was not resolved in either in situ or monthly mean satellite observations. There was a considerable brash-ice field moving ahead of the icebergs which limited the number of downstream sampling stations. One downstream station within 2 km of iceberg A-68P showed several ice-melt influenced features that distinguished it from most other stations. Firstly, there was a strong stratified meltwater influenced layer that reached to around 120 m. This had the effect of deepening underlying water masses, with the core of the temperature minimum layer around 50 m deeper than elsewhere. Secondly, there was evidence of rapid downward displacement of both particulate material and certain phytoplankton taxa that may be a further result of this water mass deepening. Thirdly, macronutrient profiles were altered, with concentrations of nitrate, silicic acid and phosphate characteristic of deeper layers being found closer to the surface and a dilution of the ambient nutrient pool just above the iceberg draft that we ascribe to meltwater released from basal melting. Meanwhile, nutrient recycling processes associated with organic matter remineralisation were also modified by the physical restructuring of the water column and biotic components. Finally, the ice-associated phytoplankton taxa Pseudo-nitszchia/Nitszchia, found in both upstream and downstream locations, were abundant at this < 2 km-distant station through melting out from the iceberg and subsequent rapid growth. Overall, we resolved alterations to water column structure, nutrient profiles and phytoplankton community composition at fine to medium scales around the iceberg field. Nevertheless, although there may have been longer term and larger scale impacts, the dynamic oceanographic environment, including the presence of a strong oceanographic front and shelf-edge processes, dominated during the collapse of A-68A.

Past commercial whaling depleted the humpback whale in the Southern Ocean; however, its population has recovered since hunting was terminated in 1966. Information on abundance is essential not only managing target animals but understanding the ecosystem because the species is one of the main consumers of Antarctic krill. Abundance of this species has not been reported in the eastern Indian sector (80–150°E) of the Southern Ocean since the mid-2000s. A multidisciplinary ecosystem survey focusing on Antarctic krill was conducted in the area by the Japanese research vessel Kaiyo-maru in 2018/19 austral summer. An opportunistic cetacean sighting survey (platform of opportunity (POP) survey) was conducted as part of the survey. Four baleen and five toothed whale species were sighted during the sighting survey. Humpback whales (113 schools/178 animals) were most frequently observed, followed by fin whales (15/36). Abundance of humpback whales was estimated as 22,395 individuals (CV = 0.270, 95 % confidence intervals: 13,310–37,663) based on the design-based line transect method. Krill consumption by humpback whales in this area was tentatively estimated as 1.336 million tons, corresponding to 30.9 % of krill biomass, although the amount varied greatly depending on the consumption rate’s assumption. The estimated abundance was comparable with previous estimates, however, there are some caveats in interpreting the results. As no approaching whale sightings were made during this POP survey, the proportion of sightings of unidentified large whales was high, possibly leading to an underestimation of the abundance. Dedicated sighting surveys are desirable to obtain more accurate and unbiased abundance estimates, especially for management purposes, although the results of this study could be used for ecological studies in combination with other data sets recoded by the multidisciplinary survey.

The occurrence, abundance, biomass and size composition of six boreal fish species (walleye pollock Gadus chalcogrammus, Pacific cod G. macrocephalus, northern wolffish Anarhichas denticulatus, beaked redfish Sebastes mentella, Greenland halibut Reinhardtius hippoglossoides, glacier lanternfish Benthosema glaciale) in the Siberian and Pacific Arctic are presented based on the analysis of open databases (OBIS, GBIF and FishBase) and the results of scientific surveys of TINRO 1977–2020. In open databases, information on the records of these species is available mainly for the eastern Chukchi Sea and the Beaufort Sea and is practically absent for the seas of the Siberian Arctic, which is due to technical reasons. Data from long–term surveys of TINRO indicate the extension of the ranges of North Atlantic species eastward and North Pacific species westward indicating the ongoing borealization of the Siberian Arctic due to a changing climate. At the species level, it can lead to the overlap of the ranges of the “western” and “eastern” populations of Greenland halibut, as well as the ranges of Pacific and Greenland cods, which will make it possible to exchange genetic material between these populations/species and will contribute to the disappearance of genetic differences between them.

The Seychelles-Chagos Thermocline Ridge (SCTR) is a biologically important region of open ocean upwelling within the south west Indian Ocean (5–10°S and 45–90°E), driven by the tropical gyre. The SCTR refers to an elongated feature that joins two local minima in thermocline depth; the Seychelles Dome (SD) and Chagos Dome (CD). Entering the ocean basin from the east, the Indonesian Throughflow (ITF) has been shown to interact with the upwelling region, although the relationship between the phytoplankton bloom associated with the SCTR and the ITF are so far unexplored. Using in situ observations and remotely sensed data, the buoyancy fluxes from the ITF are shown to strongly condition surface chlorophyll-a (chl-a) concentrations over the Chagos Dome, the eastern extreme of the SCTR, at seasonal and interannual scales. Accordingly, we find a significant inverse correlation (r = −0.43) between the altimeter-derived volume transport of the ITF and the surface chl-a concentrations. This inverse correlation increases (r = −0.61) when only the 10th and 90th percentile of the ITF volume transport anomalies are considered, indicating the influence of the ITF may be overcoming other physical drivers, especially under extreme ITF events. We hypothesise that the buoyancy flux of a strong ITF input ‘caps’ the Chagos Dome with warm, less saline waters, suppressing surface phytoplankton and reducing the surface chl-a concentrations. This hypothesis is supported by a strong, significant correlation (r = 0.66) between remotely sensed surface salinity and surface chl-a over the region. This relationship is not found over the Seychelles Dome, where the ITF has a weaker direct impact over the bloom. These results suggest that the westward travel of ITF waters may condition the eastward expansion of the SCTR and, therefore, the zonal extent of the associated chl-a bloom. This happens at seasonal and interannual time-scales concomitantly with the propagation of downwelling Rossby waves, deepening the thermocline and facilitating the westward advance of ITF waters. This is visible through a combination of remotely sensed and in situ observations at depth from the RAMA mooring array at the eastern domain of the SCTR, where intrusions of warm, less saline waters, typical of ITF waters, coincide with downwelling Rossby waves deepening the thermocline. Thus, both the westward travel of ITF waters and the propagation of downwelling Rossby waves shape the eastward expansion of the SCTR and, therefore, the zonal extent of the associated surface chl-a bloom on a year-to-year basis.

Sedimentary properties and accumulation rates on the continental shelf and in the deep sea reflect temporal oceanographic, biological and chemical processes occurring in the water column and the sediment surface. We used the radionuclides ²¹⁰Pb, ²²⁶Ra, and ¹³⁷Cs activities to estimate sedimentation rates during the last century at nine stations in the northern Barents Sea region. Elemental (C, N) and stable isotopic composition (δ¹³C, δ¹⁵N) were also analysed from the nine stations sampled in August 2018, and, for five other stations sampled in August and December 2019, and in March and May 2021. Sediment accumulation rates varied between 130 and 1 410 g m⁻² y⁻¹. The < 63 μm normalized total organic carbon (TOC₆₃) and the total nitrogen from the sediment surface varied between 0.90–2.56 % and 0.13–0.33 %, respectively. Ice-free shelf stations had higher TOC₆₃ and possibly fresher organic matter (high δ¹³C, low δ¹⁵N) than ice-covered more northern stations. The opposite trend was observed for total inorganic carbon. We found that these trends in biogeochemical parameters were spatially structured by the winter sea ice concentration and biological production differences, and exhibited a south-north separation of the Polar Front region. The low and stable organic carbon accumulation rate (1.7–13.4 g C_org m^-2 y⁻¹; AR_toc) is a function of slow sedimentation rates, and high degradation and residence time in the water column and at the sediment–water interface. Overall, the AR_toc has been stable for the past 100 years, with a slight increase from the early 1970s to the present at the shelf and slope stations. Our results highlight that spatial scales of variability of the studied sedimentary parameters are linked to spatial patterns of important environmental variables (e.g., chlorophyll-a, sea ice concentration) in the region. In contrast, no seasonal differences were observed in the sediment parameters of revisited stations, and the dated sediment geochemical profiles did not exhibit substantial longer-term variation. This means that climate-induced changes in variables that modify the sedimentary geochemistry of the environment may affect benthic community activity and structure before leaving a record in AR_toc.

Full-depth hydrographic sections of the BROKE experiment in 1996 (across the Antarctic margin from 80 to 150°E; Bindoff et al., 2000) were revisited for the first time during the 2018/2019 austral summer. We describe the subsurface physical oceanography in 2019 and the hydrographic changes between 1996 and 2019 not documented in earlier studies. The survey captured decadal changes in ocean structure from the southern flank of the Antarctic Circumpolar Current (ACC) to the continental shelves. In five cross-slope meridional sections, where 1996 and 2019 measurements are comparable (112, 120, 128, 140, and 150°E), the poleward shift of the southern boundary of the ACC (50–120 km) prevailed near the continental rise. The simultaneous displacement of barotropic ACC fronts and poleward migration of deep water contributed to full-depth warming (0.1–1.6 °C) and potentially to a reduction in the bottom water volume. Freshening was widely observed from the deep to bottom layers (∼0.02 g/kg), with the signal extending from the upper continental slope. Bottom-intensified freshening was accompanied by an oxygenation of 10–20 μmol/kg, indicating that freshening-driven oxygenation of bottom layers counteracted the deoxygenation effect of the poleward barotropic frontal shift. Westward transport of the Antarctic Slope Current decreased by more than 10 Sv from 1996 to 2019 in the five cross-slope sections; its frontal features and current axis shifted offshore by more than 20 km in 112–140°E. Additionally, subsurface warming along modified Circumpolar Deep Water by up to 0.4 °C was commonly detected across the upper continental slope. For the 2019 hydrography, shelf water sufficiently dense to form bottom water (>28.35 kg/m³) was found to the east of Mertz Polynya (142–148°E), implying a pathway for dense shelf water export from the eastern margin of Mertz Polynya. Our findings underscore the importance of sustained efforts for in-situ observations that widely cover the East Antarctic margin.