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

The present study investigates the interannual variability of the advective pathways and transit times of the Red Sea Overflow Water (RSOW) in the western Arabian Sea using virtual particles as a proxy indicator for the poorly understood RSOW spreading. The Lagrangian simulations are based on the GLORYS12 eddy-rich reanalysis (1/12°), which assimilates most satellite and in situ observations from 1993 to 2018. Statistical analysis of particle positions reveals the Gulf's mouth is always the main RSOW export route out of the Gulf of Aden. Moreover, there is substantial interannual variability in the three RSOW pathways in the western Arabian Sea, which are consistent with in-situ salinity variability at the RSOW layer. The faster Socotra pathway is strongest for particles released in 1998–1999 and 2012 and almost non-existent for the ones released in 2006–2007. The strongest state of the Socotra pathway co-occurs with some of the most powerful El-Nino/Southern Oscillation and Indian Ocean Dipole events in history. A decadal seesaw stands out between the Northwest pathway, which advects RSOW northward offshore the Arabian Peninsula, and the Southwest pathway, which advects RSOW southward to the Somali Basin along the eastern side of Socotra. While the Northwest pathway strengthened from 1996 to 2011, the Southwest weakened. These changes are associated with interannual variability in the western boundary undercurrents and subsurface eddy kinetic energy. Interestingly, the Northwest pathway trajectories are eddy-dominated, in striking contrast with the Socotra and Southwest pathways, in which western boundary undercurrents are major players. This fact suggests that eddy-induced transport is likely to have a significant role in spreading the RSOW northward. No considerable interannual variability in transit times is detected for any pathway.

Estimates of the partial pressure of CO₂ (pCO₂) derived from biogeochemical Argo floats have the potential to improve our knowledge of the highly variable and partially observed Southern Ocean carbon sink through sampling at improved temporal and spatial resolution. Here we use the data from six biogeochemical Argo floats to characterise near-surface dissolved inorganic carbon (DIC) concentrations and fluxes at the site of an intense (Chl-a >3 mg m⁻³) mesoscale diatom bloom situated northwest of South Georgia. Concurrently, we provide independent analysis and validation of the methodology used by the Southern Ocean Carbon and Climate Observational and Modelling (SOCCOM) project for deriving surface pCO₂ from float-based pH and oxygen measurements. We compare the float observations with co-located ship data from bottle samples over a month-long period. When compared to data sampled within 24 h and 25 km of each float profile, we find good agreement with a mean offset of −0.005 ± 0.018 (1σ) between float pH and bottle-derived pH. This translates to comparable pCO₂ estimates between ship measurements and floats with a mean difference of 2.6 ± 12.8 (1σ) μatm, providing support for the use of biogeochemical Argo float data to supplement shipboard pCO₂ measurements in the Southern Ocean. Based on float-derived pCO₂ we calculate a sizeable local flux of CO₂ of 24 ± 7 mmol C m⁻² d⁻¹ (over a 27-day period) from the atmosphere into the surface mixed layer, driven by a large air-sea pCO₂ gradient and strong but variable winds. Despite the considerable air-sea flux, the local mixed layer carbon budget appears to be dominated by entrainment and detrainment of carbon-rich waters into and out of the mixed layer. However, given the large uncertainties associated with these fluxes and the significant challenges associated with closing the mixed layer budget, further research is required to refine float-based mixed layer DIC fluxes.

Arctic and subarctic ecosystems are transitioning due to ocean warming, resulting in conditions that will lead to shifts in phytoplankton communities, their nutritional compositions, and production of fatty acids (FAs). FA biomarkers are useful indicators of changing phytoplankton community composition and provide insight into basal resource quality for higher trophic level consumers such as zooplankton, fish, birds and marine mammals, yet phytoplankton FA information is largely lacking from the Bering and Chukchi Sea regions. Therefore, we analyzed suspended particulate matter (seston) FAs, chlorophyll-a (Chl-a) and environmental data collected from four surveys in the northern Bering and Chukchi Seas, two during June of 2017 and 2018 and two during August and September 2017 and 2019. Our objectives were to determine 1) whether seston FA composition was correlated with phytoplankton taxonomic composition analyzed using imaging microscope (FlowCAM) techniques, 2) if there were seasonal differences in seston FA concentrations, and 3) how FA concentrations varied with environmental variables. We found significant seasonal differences in seston FA compositions, with diatom biomarkers more prevalent in spring, followed by a community shift to dinoflagellate and small flagellate FA biomarkers in late summer. These results were confirmed by FlowCAM analyses. FA biomarkers were correlated with total and large size-fractioned Chl-a concentrations, nitrogen concentration and temperature. Lastly, we used a model framework to predict availability of the diatom-associated essential FA, eicosapentaenoic acid (EPA, 20:5n-3). Our analysis provides new information on phytoplankton FA dynamics and the important nutritional role of phytoplankton for higher trophic level consumers in the northern Bering and Chukchi Sea regions.

Elevated but variable phytoplankton biomass and productivity is often associated with the subtropical front (STF) where nitrogen-limited subtropical and iron-limited subantarctic waters mix. To understand variability within the STF east of New Zealand, we assessed phytoplankton community structure, growth, and grazing dynamics in relation to physico-chemical conditions across 23 stations distributed along the Chatham Rise region during late autumn-early winter. Serial dilution experiments were coupled with size-fractionated chlorophyll a (Chla) analysis (Total and <20 μm) and flow-cytometry (Synechococcus and picoeukaryote numbers, <2 μm) to estimate phytoplankton growth and microzooplankton grazing rates. Within the broad STF zone, subantarctic influenced waters (SAIW), frontal zone (FZ), and subtropical influenced waters (STIW) were delimited based on salinity, temperature and nutrient gradients. The chlorophyll a biomass (TChla) of phytoplankton and the abundance of larger sized cells (>20 μm chlorophyll a) peaked in FZ waters but declined steadily southwards into the colder SAIW, and rapidly reduced north into the STIW. Chlorophyll a <2 μm peaked in the northern STIW. Phytoplankton growth (TChla) was higher in warmer STIW (μ = 0.49 ± 0.07 day⁻¹) than in iron limited SAIW (μ = 0.29 ± 0.06 day⁻¹) but was on average moderate (μ = 0.42 ± 0.05 day⁻¹) when compared to previous studies in the region. Microzooplankton grazing on TChla was lower (m = 0.17 ± 0.04 day⁻¹) than growth and accounted for half of daily primary production (m:μ, 0.47 ± 0.06). Growth in the <20 μm Chla size fraction was higher (μ = 0.52 ± 0.06 day⁻¹) but a larger proportion was consumed by microzooplankton (m:μ = 0.64 ± 0.06). Picoeukaryotes showed the fastest growth (1.49 ± 0.13 day⁻¹) and grazing (1.43 ± 0.11 day⁻¹) rates on average, which peaked in the FZ but remained closely balanced across different waters (m:μ = 1.00 ± 0.02). Conversely, Synechococcus rates peaked in STIW and decreased southwards, with growth (μ = 0.42 ± 0.08 day⁻¹) generally exceeding grazing (m = 0.28 ± 0.06 day⁻¹) across all regions. Our results indicate differences in grazing together with nutrient (likely iron) availability were the primary factors controlling phytoplankton dynamics in the STF zone. These factors also affected the accumulation of larger phytoplankton biomass in the FZ, including the potential for export or transfer to higher trophic levels.

Species of the marine nematode of the genus Desmodora have been found to dominate (up to 78%) in the nematode fauna from the hydrothermal vent communities of submarine Piip volcano, Bering Sea. The morphological characteristics and molecular genetic data of Desmodora specimens from different habitats of the volcano were studied and three new and one known species have been described: Desmodora hydrothermica sp.nov., Desmodora marci, Desmodora spongiophila sp.nov., Desmodora spongiocola sp.nov. The species often lived together, but a pronounced spatial specialization was observed. Desmodora spongiophila sp.nov. and Desmodora spongiocola sp.nov. were abundant inhabitants of the vulcanellids and some hexactenellids. Desmodora hydrothermica sp.nov. dominated in microbial mats on the carbonate chimneys from the South Summit, while Desmodora marci was found on stones near vents and in bottom sediments with Calyptogena pacifica (Bivalvia: Vesicomyidae). The last two species were already known in deep-sea reduced environments of the Pacific and Atlantic oceans, in similar habitats. The δ¹³C and δ¹⁵N values and fatty acids composition of Desmodora samples from hydrothermal habitats confirmed the consumption of chemosynthetically derived organic matter. SSU and D2-D3 of LSU phylogenetic trees largely agree with those of previous analyses indicating that Desmodora is not monophyletic. Moreover, phylogenetic relationships within the subfamily Desmodorinae remained unresolved.

Three species of the amphipod family Caprellidae were collected in the area of the submarine Piip Volcano, the northwestern Bering Sea, during 75-th and 82-nd research cruises aboard the R/V Akademik M.A. Lavrentyev in 2016 and 2018 using the ROV Comanche-18. Two morphospecies of the genus Caprella (Caprellinae) were found on the Northern Summit of the Piip Volcano (depth 373 m) and on its northern slope (749–986 m). A new species, Cercops caecus Kireev and Golovan sp. nov. (Paracercopinae), was sampled in the abyssal zone adjacent to its southern slope (3362 m), where it was found on hydroids growing on a hexactinellid sponge. The new species differs from all other species of the genus by the lack of eyes and the presence of two pairs of gills in contrast to three paired gills found in the other congeners. This is the first record of an abyssal species of the subfamily Paracercopinae, whose other representatives are known from the shelf and the upper bathyal zones of the North Pacific, Arctic, and North Atlantic. This finding demonstrates the parallel processes of colonization of deep-sea habitats occurring in different subfamilies of Caprellidae. An updated diagnosis and a key to the Cercops species are provided. For the first time for Paracercopinae, data on the molecular marker (18S rRNA) was obtained.

Surface wave propagation and the modulations of wave parametric and spectral properties over the Gulf Stream (GS) are studied using a high spatial resolution (1 km) wave model that considers an idealized GS. While simulation results are generally consistent with a previous modelling study, we found that for following-current (FC) cases, reflection from the GS substantially increases wave height on the offshore side of the GS center by up to 25%, and decreases wave height on the landward side of the GS by as much as 80%. In the counter-current (CC) cases, the wave height profile is more symmetrical relative to the GS centerline, and the maximum 33% increase of wave height is predominantly driven by straining. The GS also causes an increase (decrease) in wavelength and directional spreading in the FC (CC) case. Additional model sensitivity experiments that further consider realistic shelf-ocean topography show that current modulation and bottom dissipation work in concert as low- and high-pass filters on the wave frequency spectra. Wave parameters and spectral modulations imposed by the GS have significant impacts on ocean-atmosphere momentum flux and wave energy resource.

Despite concern around plastic pollution in subtropical gyres, our understanding of the biological component of the neustonic layer (upper 20 cm of the ocean) in these areas is poor. Here we investigate the neuston (excluding copepods) assemblages across the Southern Indian Subtropical Gyre using triplicate samples collected by manta-trawls from 28 stations along a transect at latitude 20 S during June/July 2015. The vertical structure of the water column at each station was assessed using a CTD. With the exception of siphonophores, all non-copepod neuston were identified to morphospecies. Assemblages were dominated by ostracods, mysids, larval chaetognaths and siphonophores. The majority of collected specimens comprised facultative neuston, which were more common at night than during the day. Neuston assemblages from the east were quite distinct from the balance of samples, and their distribution reflected that of the warm, low salinity water associated with the Indonesian Throughflow. Two anticyclonic eddies had been documented at the time of the survey between 70 and 85° E, and this area was associated with a distinct neuston assemblage of generally low abundance. The key environmental factors that were linked to assemblage structure were associated with longitude – salinity, fluorescence and temperature at 200 m, emphasising the interaction between Indonesian Throughflow Water and the South Equatorial Current, eddy-related process and settlement of meroplankton. The study highlights the value of using morphospecies in studies of plankton assemblages.