Progress in Oceanography最新文献

Deep-sea ecosystems are particularly important to the cycling of matter and energy in the oceans and therefore in regulating Earth’s climate. The Atlantic Ocean is already experiencing significant abiotic changes, with expected warmer temperatures coupled with decreased particulate organic carbon (POC) export flux. However, there is yet a large gap in our understanding of warming impacts on deep benthic ecosystems and in the organic matter processing by benthic organisms in the seafloor. This study employed an experimental approach to assess the single and cumulative effects of two climate change stressors, temperature and POC quality, on macrofaunal benthic assemblages in the Cabo Verde Basin (CVB, Equatorial Atlantic) bathyal continental slope. Incubation enrichment experiments with ¹³C and ¹⁵N labelled diatoms Phaeodactylum tricornutum simulated climate projections for the next century with a balanced design, studying the effect of either increased temperature (+2°C), reduced POC quality (dialysed labile fraction), or both, against a control treatment. We found that echinoderms and polychaetes rapidly ingested labelled algae at rates between 0.02 and 21.9 µg C m⁻² d^-1. Given a strong spatial variability in macrofaunal biomass, the carbon and nitrogen incorporation by macrofauna was not affected by a + 2 °C warming, by a decreased organic matter quality, or the combination of both factors. Our study provides valuable insights into the biodiversity, biomass, and ecosystem functioning (C and N uptake rates) of deep-sea benthic ecosystems in the N Atlantic, and stress that potential effects of warmer temperatures and POC quality on carbon and nitrogen incorporation by macrofauna remain uncertain. We highlight the value of these experiments to better understand the effects of climate change on deep-sea ecosystems.

The Borderland Basins off Southern California are semi-isolated sea-floor depressions with connections to each other and to the open Pacific Ocean over narrow sills. A high-resolution, multi-year simulation is analyzed for its currents, stratification, and dissolved oxygen, with a focus on the mean conditions, intrinsic variability, and exchange rates with surrounding waters. The three shallowest, closest basins are given the most attention: Santa Barbara, Santa Monica, and San Pedro. Below the basin sill depths, the water masses in the basins are distinct from surrounding waters at the same density indicating a degree of dynamical isolation. The mean circulations are anti-clockwise around the topographic edges of the basins, consistent with eddy-driven flows (i.e., topostrophy). The mesoscale eddy variability is stronger than the mean flow, and at least partially it is comprised of topographic Rossby waves circuiting the edge slopes. Its magnitude is similar to the high-frequency currents (mostly tidal). There are recurrent cross-sill flows driven by an unbalanced pressure-gradient force, and these intermittently cause water mass flushing of the basins. The oxygen levels in the basins are occasionally anoxic, and they are maintained by a balance of downward physical transport from above, local respiration, and flux into the sediments. From a combination of multiple means of estimation, the deep basin water mass renewal times are on the order of a year or more, and this time is somewhat shorter in the Santa Barbara Basin than the others. The renewal processes are by intermittent sill overflows and by vertical exchanges through eddies and tides.

The microbial plankton community is an integral part of the pelagic ecosystem. It hosts essential functional groups that play a vital role in organic carbon production, release, uptake, and degradation within open-ocean ecosystems. Given its significance, carbon biomass estimates are urgently needed, especially in oligotrophic regions, to provide and enhance our knowledge of biogenic carbon pools. They also aid in validating biogeochemical models that characterize the functioning of these extensive marine ecosystems within the global carbon cycle. This study addresses the temporal variability of microbial community biomass in two oceanic zones: the west-central (Perdido) and southern (Coatzacoalcos) areas of the Gulf of Mexico. During three seasonally contrasting periods (nortes, rainy, and dry seasons), seawater samples were collected from the euphotic zone in both regions to estimate the carbon biomass of different pico- (<2–3 µm), nano-, and microplankton groups (>3–200 µm). Carbon biomass assessments for the microbial groups were based on their abundance and carbon conversion factors. Overall, we found a significant contribution of pico-prokaryotic components (heterotrophic bacteria, Prochloroccocus, and Synechoccocus) to the total microbial carbon stock of the euphotic zone (84–89 % global estimates). The finding suggests these microorganisms are key functional groups that drive carbon production and fate in the Gulf of Mexico ecosystem. Pico-cyanobacteria, especially Prochloroccocus, were the dominant primary producers (68–82 % total autotrophic carbon), mainly in the upper layer of the oligotrophic euphotic zone. This vertical pattern implies that the deep chlorophyll-a maximum (DCM) depth level was unrelated to a net increase in phytoplankton biomass in the three study periods. The distribution of microbial carbon biomass exhibited striking differences associated with winter mixing (the nortes season), high river discharge accompanied by cross-shelf transport (the rainy season), and the dynamics of mesoscale structures. Ecological aspects, such as the habitat preference of the organisms and the seasonal complementary development of mixotrophic and heterotrophic grazers and their prey, were also essential drivers in regulating the microbial carbon pool of both oceanic regions. The microbial carbon assessments conducted in this study contribute to identifying and quantifying key planktonic functional groups involved in the biogeochemical carbon cycle in the Gulf of Mexico open-ocean ecosystem.

The basin-wide phytoplankton succession and community behaviour in response to varying nutrient patterns during various upwelling phases are detailed, for the first time, in the eastern Arabian Sea (EAS, ∼6^◦ to 22^◦N) during the summer monsoon (SM) of 2018. Three consecutive observations were carried out during early SM (June-July), peak SM (August), and late SM (September-October), representing different phases of upwelling. During the early phase of upwelling, high phytoplankton biomass was observed in the south (column-integrated chlorophyll a: 74.09 ± 60.05 mg m⁻²) and moderate levels in the central (25.75 ± 6.51 mg m⁻²) and north (30.31 ± 12.32 mg m⁻²) EAS coastal waters. Diatoms were the dominant group (60–90 %) in the coastal stations throughout the upwelling period. Offshore regions characterised by deeper nutriclines (>50 m) had pico-phytoplankton dominance, including cyanobacteria (14–30 %), chlorophytes (19–24 %) and prochlorophytes (12–15 %); however, due to low nitrogen to phosphorous ratio (N/P: 2.6 ± 1.31) during this period, the contribution of diatoms decreased to less than 20 % in the offshore waters compared to the coastal EAS. During peak SM, upwelling induced shoaling of nutriclines and high N/P conditions (8.4 ± 5.25) in the mixed layers of south EAS coastal waters substantially enhanced phytoplankton biomass (chlorophyll a: 129.06 ± 96.24 mg m⁻²). Additionally, the shallow nutriclines supported diatoms dominance in offshore waters, particularly in the central EAS (up to 65 %), relative to the south and north EAS (22 to 33 %), where the upwelling intensity was weaker. The withdrawal of upwelling led to a deepening of nutricline and low N/P conditions (3.33 ± 2.77 in coastal and 3.35 ± 2.26 in offshore waters) during late SM. This supported the occurrence of cyanobacteria and dinoflagellates, as the contribution of diatoms to the total phytoplankton community sharply decreased to 50 %. In other words, upwelling in the EAS brings nitrogen-deficient (denitrified) waters; the available nitrogen is immediately consumed by the diatom community, resulting in low N/P conditions that favour the dominance of the cyanobacterial population towards late SM. Overall, substantial intra-seasonal variability was observed in nutrient stoichiometry, strongly modulated by the intensity of physical processes affecting the phytoplankton populations. Continuous monitoring is required to understand the phytoplankton populations, their impact on higher trophic levels, and the overall health of aquatic food web structure in the EAS.

Trophic links between the epipelagic (< 200 m) and mesopelagic layers of the Indian Ocean were investigated by carbon and nitrogen stable isotope ratios of 2405 samples collected from 2002 to 2016, and that encompass the base of trophic webs, and primary, secondary and tertiary consumers. The samples include particulate organic matter, gastropods, gelatinous organisms such as salps and pyrosomes, crustaceans, mesopelagic fishes, micronektonic and nektonic squids, tuna and swordfish. Stable δ¹³C and δ¹⁵N values were used to investigate trophic and resource partitioning between epipelagic vs mesopelagic (migrators and non-migrators), feeding patterns (zooplanktivorous vs micronektivorous), and at seamounts and off-seamount locations. We also investigated how contrasting environmental conditions within two biogeochemical provinces, the ISSG (Indian South Subtropical Gyre) and EAFR (East African Coastal Province), influenced stable isotope patterns. Our data suggest that broad-scale biogeochemical differences and local environmental conditions significantly shape trophic and resource partitioning. In oligotrophic systems, epipelagic migrating and non epipelagic-migrating organisms rely on food webs where suspended particles are ¹⁵N-enriched and organic matter recycled/re-processed. We show that seamounts form strong isotopic topographic barriers (which we define as “isobiome”) that impact the trophic linkages/connections between epipelagic migrants and non-epipelagic migrants, and those with zooplanktivorous feeding patterns. This study reveals that the trophic and resource partitioning in the ocean is more complex than initially thought, when environmental variability, bathymetric gradients, and a wider range of samples are taken into account compared to earlier studies. We also showed that a warmer ocean led to a reduction in productivity, lower values of δ¹³C and δ¹⁵N, and potential shifts in food web trophic structure that remain to be investigated further. Finally, we discuss how important it is to unravel this complexity on a global scale given the vulnerability of epipelagic and mesopelagic communities due to anthropogenic pressures in the Anthropocene.