Progress in Oceanography最新文献

Large pelagic crustaceans are a main component of the micronekton community in the deep-sea having an important role in the food webs and the biological carbon pump. However, they are scarcely studied in comparison to other groups such as mesopelagic fish. Here, we analyse day/night and bathymetric variability in taxonomic composition, abundance, and biomass across a latitudinal transect in the Atlantic Ocean from off Brazil (15°S) to the Canary Islands (25°N). A total of 95 species were identified belonging to 9 different families, of which Euphausiidae was the most abundant family and Acanthephyridae the family contributing the most to the total biomass. We found distinct assemblages associated with Atlantic ecoregions related to the environmental variables. Diel vertical migrations were detected along the entire transect, even crossing the oxygen minimum zone, likely due to the metabolic adaptations of these organisms.

Phytoplankton phenology describes yearly algal growth cycles and characterises the timing, duration, and magnitude of bloom occurrences. This study used satellite chlorophyll-a data from 1998 to 2020 and the Hierarchical Agglomerative Clustering method to define phenoregions based on phytoplankton phenology spatial patterns over the British Columbia and Southeast Alaska coastal oceans. The defined phenoregions were used to simplify the spatial complexity of the heterogenous study region and thus better describe phytoplankton seasonality across the target area. The cluster analysis allowed the delineation of four coherent regions: two coastal regions and northern and southern shelf/offshore regions. Results showed that each phenoregion had distinguishable phytoplankton phenological characteristics, likely due to different physical forcings acting in these areas. Moreover, the interannual variability of the spring bloom initiation was evaluated considering interactions between sea surface temperature (SST) anomalies and the El Niño Southern Oscillation Index (ENSO). Early spring blooms were associated with positive SST anomalies and El Niño conditions; conversely, average or late spring blooms occurred in years with negative SST anomalies and La Niña conditions, with the strongest relationship occurring in the southern shelf/offshore phenoregion. This study provided new insights into the regionalisation of the British Columbia and Southeast Alaska coastal oceans based on phytoplankton phenology patterns. Given the critical role of phytoplankton as the base of the marine food web, such phenoregions have implications for regional zooplankton biomass and fish production. The link between phytoplankton phenology and climate drivers points to the importance of environmental change in phytoplankton bloom dynamics. Further research into the connection between phytoplankton bloom indices and zooplankton community structure and production would be an important step towards using these indices for ecosystem monitoring and fisheries management.

Climatic fluctuations have been documented to strongly affect Arctic marine ecosystems. Plankton assemblages serve as the most sensitive indicators of such environmental forcing. We conducted a study to investigate the spatial variability of chlorophyll a (Chl-a) concentration during two pre-bloom periods (March–April 2021 and February–March 2022) in relation to the distribution of different water masses and associated properties. The upper 50 m layer of the water column was homogeneous and stable, characterized by high nutrient concentrations. Our mapping of the Barents Sea based on Chl-a concentrations revealed low estimates during the winter period. In contrast, two distinct Chl-a peaks were observed in the spring. The first region with high Chl-a concentrations was identified in Murmansk Coastal Water and Atlantic Water (0.7–1.4 mg m⁻³), reflecting the positive impact of the frontal zone between these interacting water masses. The second region with elevated Chl-a concentrations (0.9–1.1 mg m⁻³) was located in Kolguev-Pechora Water near the southeastern ice edge. Cold water regions (Barents Sea Water, Arctic Water, Novaya Zemlya Coastal Water) exhibited low spring Chl-a concentrations (0.03–0.3 mg m⁻³). Generalized additive models identified hydrological variables (temperature and salinity), dissolved oxygen content, and nutrient concentrations (nitrite, nitrate, phosphate) as significant predictors explaining a substantial portion of the Chl-a variability.

Tracking the processes of the spread of Fukushima-derived ¹³⁷Cs (¹³⁷Cs_F) contributes to a better understanding of North Pacific water dynamics. In this study, the vertical distributions of ¹³⁷Cs and ⁹⁰Sr in the Kuroshio-Oyashio confluence region were investigated in May 2018, and ¹³⁷Cs_F was separated from the background ¹³⁷Cs by exploiting the constant global fallout ¹³⁷Cs/⁹⁰Sr ratio. To the north of 35°N, ¹³⁷Cs_F peaked in the upper 100 m layer, whereas in and just south of the Kuroshio Extension (KE), ¹³⁷Cs_F exhibited subsurface peaks at depths of 300–500 m. The T/S diagram indicated that the ¹³⁷Cs_F maxima were distributed mainly within the range of lighter central mode water (L-CMW) during May 2018, even in and just south of the KE. We found that anticyclonic (cyclonic) eddies can promote (prevent) the intrusion of ¹³⁷Cs_F into the ocean interior. In addition, the high activity of regional anticyclonic eddies in the upstream KE resulted in the modification of ¹³⁷Cs_F-rich subtropical mode water (STMW) to L-CMW. Temporal changes in the ¹³⁷Cs_F vertical profiles and inventories revealed that ¹³⁷Cs_F in transitional and subarctic regions has increased since July 2014, implying the existence of additional sources of ¹³⁷Cs_F after July 2014, whereas ¹³⁷Cs_F in and just south of the KE has remained constant since July 2014, indicating that the ¹³⁷Cs_F entrained by STMW has recirculated in the western subtropical gyre. The comparison between surface ¹³⁷Cs_F concentrations in transitional and subarctic regions and those observed in Oyashio waters during 2018 did not support the return of ¹³⁷Cs_F to our study area via the western or whole subarctic gyre by May 2018. In contrast, the sea surface height distributions from 2016 to 2017 provide clear evidence that the warm-core rings and quasistationary Isoguchi western jet generated from the Kuroshio Current and KE intruded into the transitional region and even into the subarctic region. Therefore, we concluded that a portion of the ¹³⁷Cs_F that subducted into the subtropical western North Pacific during 2011–2012 have entered the transition zone and even the subarctic region since 2016. These results not only enhance our understanding of the protracted spread and fate of ¹³⁷Cs_F in the North Pacific but also provide important insights into North Pacific water mass circulation and mixing patterns.

Shallow seamounts are becoming increasingly recognised as key habitats for conservation due to their role as biological refuges, particularly throughout oligotrophic oceans. Traditionally, Taylor caps have been invoked as the mechanism driving biomass aggregation over seamounts but emerging evidence based on higher resolution measurements highlights the importance of internal waves (IW) to the local ecosystem. These waves can flush the benthic habitat with cool water from depth and impact on nutrient supply over short time scales through turbulent mixing that may also influence fish behaviour. They are dependent on the regional stratification, however, and thus influenced by planetary-scale variability in oceanographic conditions. We present here detailed observations of the internal wave regime over a shallow seamount, called Sandes, in the central Indian Ocean throughout different phases of the Indian Ocean Dipole (IOD) that modulated the regional stratification. A deep thermocline, caused by the 2019 IOD event precluded internal wave activity over the summit, whereas a thermocline collocated with the summit during 2020 when the IOD reversed polarity resulted in a 30 m amplitude internal tide signal (t ∼ 12.5 h). A shallow thermocline, observed during 2022, resulted in propagation of IWs over the summit with less visible internal tide. Harmonic analysis shows the presence of high frequency waves (t ∼ 15 min) on both flanks of the seamount during 2020 & 2022, which are likely a result of local shear instability, whereas 2019 shows an asymmetric response, potentially due to the strong background current and suppression of the thermocline beneath the depth of the summit. The potential importance of the waves over the summit to the local ecosystem may be attributed to the elevated turbulence measured at the thermocline during internal wave propagation, with ε > 10^-5 W kg-1 routinely observed. Our results highlight the ability of thermocline depth to act as a gating condition for internal wave evolution over the summit. These results show that, whilst the water column exhibits variability at short spatiotemporal scales compared to the frequently cited Taylor cap dynamics, it is also regulated by the wider basin scale processes. Thus, a more integrated approach is needed when assessing these dynamic and environmentally critical habitats to include the effects of physical oceanographic controls across multiple spatiotemporal scales.

Baffin Bay is an Arctic marginal sea connected to the North Atlantic via Davis Strait and the Labrador Sea. While the exchange of heat and freshwater through Davis Strait is known to strongly influence the subpolar North Atlantic, there are significant gaps in our understanding of the circulation and water mass distribution and transformation throughout Baffin Bay, in part due to limited direct velocity observations. In this study, high-resolution hydrographic, nutrient, oxygen isotope, and velocity data from two shipboard surveys in late-summer to early-fall 2021 are used to address these gaps. During the time period of observation, Baffin Bay was dominated by cold, fresh, nitrate-depleted Polar Water (PW) in the upper 300 m, with the coldest and freshest PW distributed along the western shelf and slope adjacent to Baffin Island. Only a small amount of warm and salty Atlantic-origin water was measured entering the southeastern bay at depth, which is diluted rapidly when passing through Davis Strait. Pacific-origin freshwater was dominant in the upper 200 m on the western side, with relatively small amounts of meteoric water on both sides of the bay. The circulation in Baffin Bay was generally cyclonic, consisting of a strong, surface-intensified western boundary current and a slower, weakly baroclinic eastern boundary current. Much of the eastern boundary current bifurcated to the west at the northern end of the Labrador Sea, and, as the remaining flow progressed through Davis Strait, it transitioned from surface-intensified to bottom-intensified. Basin-scale recirculation of the PW was documented using the shipboard data, which was also evident in the velocity field of an ocean reanalysis product for the same time period. Examination of the reanalysis fields from 1993 to 2021 indicates that the circulation in Baffin Bay was anomalously cyclonic during summer/fall 2021. Such basin-scale circulation anomalies can arise due to both the local wind stress curl pattern and remote wind forcing associated with the Arctic Oscillation index.

Phytoplankton communities and production in Arctic fjords undergo strong seasonal variations. Phytoplankton blooms are periods with high primary production, leading to elevated algal biomass fueling higher trophic levels. Blooms are typically driven bottom-up by light and nutrient availability but may also be top-down controlled by grazing. While phytoplankton spring blooms are common across all Arctic systems, summer and autumn blooms and their drivers are less predictable. Here we compare the long-term (≥4 years) bloom phenology and protist community composition in three Arctic fjords: Nuup Kangerlua in western Greenland, Ramfjorden in northern Norway, and Adventfjorden in western Svalbard. While Nuup Kangerlua is impacted by tidewater glaciers, Ramfjorden and Adventfjorden are impacted by river-runoff. We discuss and contrast the presence and predictability of spring, summer, and autumn blooms in these fjords and the main physical, chemical, and biological drivers. Spring blooms occurred in all three fjords in April/May as soon as sufficient sunlight was available and typically terminated when nutrients were depleted. Chain-forming diatoms together with the haptophyte Phaeocystis pouchetii were key spring bloom taxa in all three fjords. Summer blooms were found in Nuup Kangerlua and Ramfjorden but were not common in Adventfjorden. In Nuup Kangerlua nutrient supply via subglacial upwelling was the key driver of a diatom-dominated summer bloom. This summer bloom extended far into autumn with strong winds resupplying nutrients to the surface later in the season. In Ramfjorden runoff from a vegetated catchment provided organic nutrients for a flagellate-dominated summer bloom in 2019. A late autumn bloom dominated by Skeletonema spp. and other chain-forming diatoms was present after nutrients were resupplied by wind mixing. In Adventfjorden, we observed only minor summer blooms in 2 of the 8 years, while autumn blooms were never observed. With global warming, we suggest that summer blooms will be negatively impacted in fjords where tidewater glaciers retreat and become land terminating. In fjords with rich vegetated catchments, harmful algal blooms may occur more frequently as summers and autumns become warmer and wetter. However, for fjords in high-Arctic latitudes (>78 N), the day length will continue to restrict the potential for autumn blooms.

The subarctic Pacific is generally perceived as relatively homogeneous since the North Pacific Subpolar Gyre dominates the water circulation in the area. However, previous research showed significant spatial differences in phytoplankton abundance and community structure. This study aimed to identify regions associated with distinct phytoplankton phenology and composition to comprehensively describe the main phytoplankton variability patterns across the subarctic Pacific. To this end, satellite GlobColour time series observations and an extensive in situ phytoplankton pigment dataset were used in the analysis. Five bioregions were identified, based on the Self-Organized Mapping technique, using a greater than 20-year satellite data series. The bioregions in the open Pacific waters were dominated by green algae, haptophytes, and pelagophytes and were divided into the areas affected by the North Pacific Transition Zone and beyond. The other bioregions were defined around the Pacific basin margins where the diatom contribution was generally higher, with a particular distinction of waters surrounding the Kuril and the Aleutian Islands. Our bioregion designations allow for future evaluation of the processes controlling the physical and biological dynamics within each bioregion, which has direct implications for foraging conditions available to higher trophic levels, including potential food resource competition.

Submarine canyons act as hotspots of biodiversity, hosting vulnerable marine ecosystems, and playing a fundamental role in bridging coastal zones with deeper areas. Here, we investigated the suprabenthic and Deep Scattering Layer (DSL) zooplankton fauna, that play a key role in deep-sea food webs, as main resources for both mobile and sessile megafauna, in two submarine canyons (Squillace and Amendolara) of the Ionian Sea (Central Mediterranean Sea). Our results highlighted different taxonomic and functional diversity between the two adjacent canyons: (i) biomass and abundance of suprabenthos followed an opposite trend in the two canyons, increasing both with depth in Amendolara (higher abundance and biomass in the lower part of the canyon), and decreasing with depth in Squillace (greater in the head of the canyon); (ii) DSL zooplankton abundance and biomass followed a spatial distribution, decreasing with increasing distance from the coast for both canyons (i.e. lower offshore than at the head of the canyon). Food-web structure investigated by means of stable isotope analysis of δ¹³C and δ¹⁵N showed a more diverse trophic niche for suprabenthos than for zooplankton. Furthermore, possible feeding modes of species with unknown feeding behaviour have been proposed. The results of the current article highlight the different ecological processes occurring within each canyon. Understanding the spatial variations of communities inhabiting submarine canyons, especially those at the base of deep-sea food webs which can act as driver of megafaunal communities (both sessile and mobile-commercial species), is essential to focalise future conservation efforts.