Oligotrophic Regions Research Articles

Satellite estimation of the spectral power exponent of particulate backscattering coefficient in the global ocean

The spectral power exponent of the particulate backscattering coefficient (bbp), η, is a crucial parameter for assessing particle size distribution in the ocean, particularly for high-precision estimation of marine biogeochemical cycles. The development of remote sensing inversion models for η has been constrained by insufficient in situ data or faulty model mechanisms. In this study, biogeochemical Argo (BGC-Argo) data were effectively utilized to address the limitations of traditional observations, and the superior XGBoost method was employed to construct a global ocean η remote sensing inversion model (XGB model). In the 5-fold cross-validation, the Pearson correlation coefficient (R), root mean square error (RMSE), mean absolute error (MAE), and median absolute percentage deviation (MAPD) of the XGB model remained stable at approximately 0.65, 0.3, 0.25, and 16%, respectively, demonstrating high robustness. Via independent sample verification and comparison with major inversion models from the literature, the XGB model exhibited a high R value of 0.66 and the lowest RMSE, MAPD, and MAE, with values of 0.33, 16.81%, and 0.25, respectively, indicating its superior inversion capability. High η values were primarily found in oligotrophic regions and the iron-limited Southern Ocean, whereas low values were observed in coastal waters, exhibiting seasonal variation. The distribution of η in other ocean areas remained relatively stable, with nonsignificant seasonal changes. This study demonstrates the significant contributions of BGC-Argo observations in enhancing remote sensing estimation of global ocean η values, thereby providing valuable support for biogeochemical cycling research.

Spatiotemporal distribution of chlorophyll-a concentration in the south China sea and its possible environmental regulation mechanisms.

In this paper, the spatial and temporal distribution of chlorophyll-a (Chl-a) concentration in the South China Sea (SCS) and its major environmental regulator mechanisms were studied by using satellite remote sensing data sea surface temperature (SST), sea surface wind (SSW), and aerosol optical depth (AOD) spanning from January 2000 to December 2022. The results show that Chl-a in the SCS exhibit notable spatio-temporal variations: they peak in winter (∼0.234mgm-3) and autumn (∼0.156mgm-3), and decline in spring (∼0.144mgm-3) and summer (∼0.136mgm-3). Spatially, Chl-a near the coast and in upwelling areas are generally higher than those in offshore areas. A monthly average time series correlation analysis across the entire SCS shows that Chl-a significantly correlate with SST (R=-0.78, P<0.01) and SSW (R=0.78, P<0.01), and moderately correlate with AOD (R=0.29, P<0.01). The regulator of environmental factors also shows seasonal differences: during the winter monsoon period, Chl-a has the highest partial correlation with SSW (R=0.73, P<0.01), followed by SST (R=-0.55, P<0.01), and no significant partial correlation with AOD (R=0.14, P>0.05); during the summer monsoon period, Chl-a has the highest partial correlation with SST (R=-0.63, P<0.01), followed by AOD (R=0.40, P<0.01), and no significant partial correlation with SSW (R=0.12, P>0.05). A comprehensive analysis indicates that the mixing and upwelling processes regulated by the winter monsoon and SST exert a greater influence on nutrient variations. The enhanced mixing caused by the winter monsoon and the cold environment promote the growth of phytoplankton, leading to higher Chl-a concentrations in winter compared to other seasons. In contrast, the increased temperature in the summer monsoon period significantly weakens the mixing effect of wind speed and nutrients influx from deep layers to surface layers. Consequently, the external nutrient sourced from aerosol becomes crucial in determining Chl-a distribution, especially in oligotrophic regions near the southern SCS and the basin. However, in regions where other nutrient sources significantly contribute, such as the coastal areas influenced by seasonal upwelling, the contribution of aerosols is negligible.

The dominant marine Synechococcus clade II exhibits a non-canonical transcriptional response to cope with thermal stress

Temperature significantly impacts the growth and distribution of marine cyanobacteria, which employ diverse strategies to cope with temperature variations. However, the molecular mechanisms underlying thermal acclimation in cyanobacterial taxa that dominate vast oligotrophic regions of the ocean, such as the Synechococcus clade II, remain unknown. Here, we analysed the physiological and global transcriptional response of an ecologically relevant clade II strain (Synechococcus sp. RS9907) to long-term thermal acclimation at temperatures from 20 to 33 °C. The growth rate in RS9907 increased linearly with temperature up to the warm threshold (33 °C), but genes related with the heat-shock response were upregulated within the range of 28 to 33 °C, indicating significant cellular stress under warm conditions. Carbon fixation and assimilation genes (rcbLS, cbbA, zwf, glgP) showed minimum expression values at 20 °C and 33 °C, but the diel expression patterns of these genes remained unaffected by temperature conditions, being consistently upregulated during day-time. By contrast, some genes involved in photosynthesis (acpA, psbABO, psaABD) were strongly upregulated during night-time under warm conditions, suggesting regulatory imbalances during the diel cycle. Notably, the expression of genes related to the synthesis of osmolytes against salt stress and fatty acid desaturases, which are typically upregulated under cold temperature in other cyanobacteria, was induced in warm acclimated RS9907 cells. These results highlight distinct transcriptional mechanisms of thermal acclimation in members of the Synechococcus clade II compared to other cyanobacterial lineages. This emphasizes the importance of understanding the diverse and intricate ways in which marine cyanobacteria adapt to temperature fluctuations.

Horizontal distribution of marine microbial communities in the North Pacific Subtropical Front.

Microbial communities are crucial for important ecosystem functions in the open ocean, such as primary production and nutrient cycling. However, few studies have addressed the distribution of microplankton communities in the remote oligotrophic region of the Pacific Ocean. Moreover, the biogeochemical and physical drivers of microbial community structure are not fully understood in these areas. This research aims to investigate the patterns of prokaryotic and protists communities' distribution in the North Pacific Subtropical Front (NPSF). The NPSF is a vast oligotrophic region with layered surface water and strong ocean currents. Despite its considerable size, its community distribution and function are poorly studied. We used a 16S and 18S rRNA gene sequencing approach to identify and characterize the water column microbial communities at two depths, the surface (3-5 m) and the deep chlorophyll maximum (DCM, 108-130 m). We aimed to elucidate the horizontal distribution patterns of these communities and to dissect the factors intricately shaping their distribution in the NPSF. Results showed that the community structure of both prokaryotes and protists was significantly influenced by depth, temperature, and longitude. Regarding alpha diversity, both communities presented a higher diversity at the surface. The prokaryotes also demonstrated to have a higher diversity in samples placed further east. The prokaryotes were dominated by Proteobacteria and Cyanobacteria, and the eukaryotic communities were dominated by Syndiniales. Combining biological and hydrographic data analysis showed the influence of vertical currents near the frontal jet in shaping the vertical distribution of both prokaryotic and protist communities. Even though most studies do not consider anomalies that emerge at each depth, these occurrences are capable of having a strong impact and influence on community structure. This study marks a significant advance in unraveling the intricate community structure and distribution dynamics of marine microbial communities within the North Pacific Ocean.

Seasonal Water-Column Structure Drives the Trophic Niche of Fish Communities on a Temperate Continental Shelf.

In seasonally stratified marine environments, the dynamics of benthic-pelagic coupling plays a crucial role in shaping food web structures and fisheries production. We examined fish food web structures across three distinct shelf areas in the Southern Sea of Korea (SSK) during both stratified (summer) and mixed (spring) water conditions using stable isotopes of carbon (δ13C) and nitrogen (δ15N). In spring, fish communities exhibited a broader range of δ13C values compared with summer, indicating more diverse feeding strategies. Seasonal variations in the proportion of benthic and pelagic prey in consumer diets highlighted shifts in benthic-pelagic coupling, illustrating how consumers adjust their reliance on benthic or pelagic resources. The relative importance of the benthic pathway varied among species groups throughout the year. During stratified conditions, reduced benthic-pelagic coupling led to increased reliance on benthic prey, particularly in the oligotrophic region influenced by the Tsushima Warm Current (TWC). The food web spanned five trophic levels, with a median of 3.6. Several species, notably benthic ones, declined in their trophic positions during the summer stratification. These results suggest that fish food webs in the SSK are shaped by temperature-driven seasonal bottom-up control. Our findings further offer insights into how increased water-column stratification could impact the trophic niches of shelf-food webs in the TWC region.

Beyond the Surface: Mesophotic Reefs as Potential Refuges for Shallow Fish Assemblages.

The deep reef refugia hypothesis suggests that the effects of disturbance decrease as depth increases; thus, reefs in the mesophotic zone potentially serve as refuges for communities in shallower zones. This study challenged this hypothesis by evaluating fish diversity in shallow and mesophotic reefs in a marine protected area in the Gulf of California. During 2021-2022, we conducted 189 5-min video transects using remotely operated vehicles to document species richness and abundance. We evaluated six biological traits for each species (length, mobility, position, gregariousness, diet, and activity period) to estimate four functional indices (number of entities, richness, originality, and divergence), one phylogenetic index (Δ*), and Hill's numbers for taxonomic and functional indices. Benthic organisms were analyzed to explore relationships with ichthyofauna, while monthly water turbidity satellite data products were transformed into a light attenuation coefficient to identify the mesophotic zone (area between 10% and 0.1% of the incident light at the surface). At the study site, the mesophotic zone was identified to extend to 21 m under optimal conditions, which is shallower than what is typically observed in oligotrophic regions. Generalized linear models revealed significant variations in reef fish composition across spatial (site and zone) and temporal (season and year) dimensions. Additionally, generalized linear mixed models of functional richness and taxonomic Hill's numbers exhibited significantly higher values in the shallow zone. However, functional and phylogenetic indices showed similarities in fish assemblages. Despite differences in fish taxonomic diversity among zones that could be related to less environmental variation and resource availability in deep strata, mesophotic reef fish assemblages presented similar functions. Functions were maintained in mesophotic reefs, which suggests that the two zones are connected and that mesophotic reefs have the potential to act as partial refugia in the face of current and near-future climate change-related disturbances that could affect shallow zones.

Zooplankton fecal pellet flux and carbon export: The South China Sea record and its global comparison

Zooplankton fecal pellets constitute a major component of passively sinking particles in the ocean. The sinking of zooplankton fecal pellets provides an efficient vehicle for the transfer and sequestration of particulate organic carbon (POC) in the deep sea, which has been widely reported in different regions. However, most existing studies focus on the sinking flux of fecal pellets within the upper ocean, while lower mesopelagic and bathypelagic zones are rarely investigated. Here, we report the spatiotemporal flux variation of zooplankton fecal pellets collected by two sediment traps deployed in mesopelagic and bathypelagic zones (500 m and 2190 m, respectively) of the South China Sea from June 2020 to May 2022, and compare it with deep-sea (>500 m) fecal pellet flux data reported in the global ocean. In the South China Sea, fecal pellet fluxes display distinct seasonal patterns due to the control of the East Asian monsoon system, with higher fluxes in winter and spring, and lower fluxes in summer and autumn. Small fecal pellets (width < 100 μm) dominate the overall pellet numerical flux (more than 98 %), while rare large pellets (width > 100 μm) account for averaging 20 % of fecal pellet carbon flux. Both large and small pellet fluxes appear to be higher at 2190 m, mainly due to the in-situ reworking and repackaging of deep-dwelling zooplankton communities, as well as the input of lateral advection from high productive continental coasts and shelves. Identifiable zooplankton fecal pellets constitute approximately 10 % to the total POC flux in the deep South China Sea. Comparing the eutrophic polar and upwelling regions with mesotrophic and oligotrophic regions, we find a good correlation between marine primary production and fecal pellet carbon export. On the global scale, carbon fluxes through zooplankton fecal pellets to the deep sea are mainly constrained by the grazing impacts of zooplanktons, influenced by temperature, zooplankton biomass, and zooplankton size spectrum.

Yucatan shelf’s larval density and distribution of Auxis spp. and Caranx crysos are primarily driven by regional upwelling and seasonality

The extensive Yucatan Shelf (YS) in the southern Gulf of Mexico provides habitat for many marine species and supports important fisheries. A striking feature is the regional upwelling along its eastern margin due to the interaction of the Yucatan Current with the western slope of the Yucatan Channel. Coupled with easterly winds, the upwelled water fertilizes the shelf and contributes to high productivity in an otherwise oligotrophic region. However, an understanding of the role that regional upwelling plays on fish spawning and larval fish distributions is limited. We describe the distribution, frequency of occurrence, and densities of two commercially important neritic species (Auxis spp. and Caranx crysos) with contrasting life histories (different adult habitat, neritic vs. neritic and oceanic, and with spawning peak vs. year around spawner). The relationship between larval density and environmental variables, including upwelling indicators, was examined for three oceanographic cruises that covered the entire YS and which were held during summer through fall (2015, 2016 and 2018). None (Auxis spp.) or very few (C. crysos) larvae were caught during the late fall cruise in November. The highest densities of both taxa were found during the summer cruise in July, consistent with their spawning periods. Both species were mostly distributed beyond the 40 m isobath, and the highest densities were found over the central and eastern shelf. Generalized additive models indicated that Auxis spp. and C. crysos densities were correlated with low chlorophyll a concentration (located in the outer shelf) and a shallower 22.5 °C isotherm (a proxy for upwelling), indicating that regional upwelling was associated with higher larval fish densities.

Spatial and temporal distribution of chlorophyll a concentration in the Sunda Shelf

The Sunda Shelf, characterized as an oligotrophic region, exhibits relatively low chlorophyll concentrations, with interannual variations primarily driven by winter chlorophyll levels exceeding 0.2 mg m−3. Utilizing multisensor satellite data spanning from 1999 to 2019, we harnessed the Data Interpolating Empirical Orthogonal Functions (DINEOF) method to perform a comprehensive reconstruction of chlorophyll a concentration in the southern South China Sea (SSCS). The findings reveal pronounced seasonal and interannual variations in chlorophyll levels, characterized by a peak in January and a subsequent decline until April. Additionally, under the prevailing southwest monsoon, a secondary chlorophyll peak was observed in August, followed by a slight decrease until October. Wind speed (WS) and sea surface temperature (SST) emerged as significant influencing factors at the seasonal scale, displaying notable correlations with spatial and temporal changes in chlorophyll concentrations. Importantly, the monsoon’s role outweighs that of SST in regulating seasonal chlorophyll dynamics on the Sunda Shelf within the SSCS. The study highlights a strong negative correlation between chlorophyll and SST, indicating that nutrient inputs from winter upwelling stimulate phytoplankton growth. The strength of the Sunda Shelf upwelling is influenced by cyclonic circulation in the South China Sea, while El Niño-Southern Oscillation events modulate monsoon intensity, impacting cold water masses and cyclonic circulation strength. These findings contribute to a better understanding of seasonal and interannual chlorophyll dynamics in the SSCS, particularly on the Sunda Shelf, and their ecological implications.

Drivers of microbial carbon biomass variability in two oceanic regions of the Gulf of Mexico

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.

Geographic distribution of modern dinoflagellate cysts in surface sediments of the Arabian Sea: significance of taxa and morphometry as potential ecological indicators.

Studies on dinoflagellate cysts in the Arabian Sea (AS) are limited to the coastal waters, but no information from the deeper depths. The dinoflagellate cyst assemblages in surface sediment samples (0-2cm) from the deeper depths (up to ~ 4500m) of central (oxygen minimum zone (OMZ)) and southeastern (oligotrophic) AS revealed that the relatively good numbers of cyst concentrations reach deeper depths of OMZ (3505m) and oligotrophic (4368m) regions, but the former harbored more cyst concentrations than the latter. The cyst concentration and species count (including HAB species) recorded here are lower compared to the eastern (EAS) and western (WAS) AS, but the autotrophic cyst dominance (74-83%) at deeper depths is in contrast with the heterotrophic dominance in coastal AS. Of the recorded 41 cyst species (belonging to 18 genera), four species (cyst of Cladopyxis sp., Operculodinium janduchenei, Stelladinium bifurcatum, and Protoperidinium monospinum) from the deepest part of oligotrophic AS form the first report. In contrast, Spiniferites and Lingulodinium cysts were common occurrences. Taxonomic comparison with literature revealed (i) the prevalence of more cosmopolitan species (32 species) which could be due to the prevalence of large and small-scale lateral transport of cysts in oligotrophic regions followed by OMZ and coastal regions, respectively, and (ii) very few region-specific species, i.e., cyst of Protoperidinium latissimum, Lejeunecysta sabrina, cyst of Protoperidinium denticulatum in EAS and Impagidinium patulum, and I. strialatum, in WAS. Interestingly, variability in the morphometry was evident between the coastal and open oceans in some cosmopolitan cysts, e.g., Operculodinium centrocarpum and Lingulodinium machaerophorum. These findings from the less studied pelagic regions will contribute to the growing knowledge of dinoflagellate cyst distribution patterns and highlight the significance of cyst taxa and morphology as potential ecological indicators for AS.

Substrate availability may limit the response of tropical bacterioplankton biomass to warming

AbstractThe response of heterotrophic bacterioplankton to the addition of macrophytic dissolved organic matter (DOM) and temperature was investigated in the Red Sea. We added 40 μmol C L−1 of leachates obtained from seagrass and mangrove leaves to natural bacterial communities, incubated them at three temperatures (25.5°C found in situ plus 3°C below and above that value) and monitored the microbial and biogeochemical responses over 4 d. Seagrass and mangrove DOM, important allochthonous sources in tropical oligotrophic regions, had distinct chemical characteristics compared to unamended seawater, with mangrove substrates containing comparatively more nitrogen and protein‐like fluorescent DOM than seagrass. Specific growth rates (μ) increased twofold in the seagrass and mangrove treatments (1.0 and 0.8 d−1, respectively) relative to the seawater control (0.4 d−1). The biomass of heterotrophic bacteria generally reflected μ changes, reaching maximum values of 16.8 and 17.3 μg C L−1 in the seagrass and mangrove treatments, respectively, compared to just 2.6 μg C L−1 in seawater. The increase in μ values due to experimental warming followed the metabolic theory of ecology, mostly because of enhanced exoenzymatic activity, while cell size decreased as predicted by the temperature–size rule (mean −3% per °C increase). Although the labile nature of the specific seagrass and mangrove DOM leachates was clearly demonstrated, we conclude that tropical heterotrophic bacteria may have limited capability to increase their biomass as a consequence of future warming, even in the presence of high loadings of macrophytic DOM.

Long‐Term Trends in the Distribution of Ocean Chlorophyll

AbstractThe concentration of chlorophyll‐a (CHL) is an important proxy for autotrophic biomass and primary production in the ocean. Quantifying trends and variability in CHL are essential to understanding how marine ecosystems are affected by climate change. Previous analyses have focused on assessing trends in CHL mean, but little is known about observed changes in CHL extremes and variance. Here we apply a quantile regression model to detect trends in CHL distribution over the period of 1997–2022 for several quantiles. We find that the magnitude of trends in upper quantiles of global CHL (&gt;90th) are larger than those in lower quantiles (≤50th) and in the mean, suggesting a growing asymmetry in CHL distribution. On a regional scale, trends in different quantiles are statistically significant at high latitude, equatorial, and oligotrophic regions. Assessing changes in CHL distribution has potential to yield a more comprehensive understanding of climate change impacts on CHL.

Biogeographical and biodiversity patterns of planktonic microeukaryotes along the tropical western to eastern Pacific Ocean transect revealed by metabarcoding.

Microeukaryotic plankton (0.2-200 µm), which are morphologically and genetically highly diverse, play a crucial role in ocean productivity and carbon consumption. The Pacific Ocean (PO), one of the world's largest oligotrophic regions, remains largely unexplored in terms of the biogeography and biodiversity of microeukaryotes based on large-scale sampling. We investigated the horizontal distribution of microeukaryotes along a 16,000 km transect from the west to the east of the PO. The alpha diversity indices showed a distinct decreasing trend from west to east, which was highly correlated with water temperature. The microeukaryotic community, which was clustered into the western, central, and eastern PO groups, displayed a significant distance-decay relationship. Syndiniales, a lineage of parasitic dinoflagellates, was ubiquitously distributed along the transect and dominated the community in terms of both sequence and zero-radius operational taxonomic unit (ZOTU) proportions. The prevailing dominance of Syndiniales-affiliated ZOTUs and their close associations with dinoflagellates, diatoms, and radiolarians, as revealed by SparCC correlation analysis, suggested that parasitism may be an important trophic strategy in the surface waters of the PO. Geographical distance and temperature were the most important environmental factors that significantly correlated with community structure. Overall, our study sheds more light on the distribution pattern of both alpha and beta diversities of microeukaryotic communities and highlighted the importance of parasitisms by Syndiniales across the tropical PO.IMPORTANCEUnderstanding the biogeographical and biodiversity patterns of microeukaryotic communities is essential to comprehending their roles in biogeochemical cycling. In this study, planktonic microeukaryotes were collected along a west-to-east Pacific Ocean transect (ca. 16,000 km). Our study revealed that the alpha diversity indices were highly correlated with water temperature, and the microeukaryotic communities displayed a distinct geographical distance-driven pattern. The predominance of the parasitic dinoflagellate lineage Syndiniales and their close relationship with other microeukaryotic groups suggest that parasitism may be a crucial survival strategy for microeukaryotes in the surface waters of the Pacific Ocean. Our findings expand our understanding of the biodiversity and biogeographical pattern of microeukaryotes and highlight the significance of parasitic Syndiniales in the surface ocean.

Dissolved rare earth elements in the North Pacific Subtropical Gyre: Lithogenic sources and water mass mixing control

In the North Pacific Subtropical Gyre, which is one of the largest oligotrophic regions, there is a lack of information regarding the sources and transport of trace metals through water mass mixing. Rare earth elements (REEs) are essential for tracing lithogenic sources and water mass transport. In this study, we present dissolved REE concentrations and the factors controlling their distributions in the northwest Pacific during a GEOTRACES cruise (GP09). In the surface water along 11°N, we observed input signals from the Philippine and the Hawaiian Islands, characterized by positive Eu anomalies and slightly elevated REE concentrations. By incorporating our data and published REE data from the northwest Pacific to the southeast Pacific (40°N–40°S), we demonstrated that the REE concentrations and Yb/Nd ratios can distinguish North Pacific Intermediate Water (NPIW), Antarctic Intermediate Water (AAIW), and modified AAIW. By estimating the ratio of water mass mixing, we suggest that heavy REEs are predominantly contributed by water mass mixing (e.g., 93 % ± 4% for Yb) and can be used as semi-conservative tracers to quantify the mixing of NPIW and modified AAIW at the potential density of 27.2 kg/m3. At ∼800 m depth at stations K12, K13, and K14, weak negative Ce anomalies (>0.1) were observed, indicating the lateral transport of water masses imprinted with sediment signals from the Philippine Islands. In the deep waters (>1500 m) at the Luzon Strait (station K1), we propose that the weak Ce negative anomalies (>0.08) and low Yb/Nd ratios (<5), combined with the low beam transmission, are the result of particle resuspension and release. In deep water (>2000 m), combining high-resolution REE measurements with water mass analysis, our research reveals that dissolved REEs (except Ce) are dominantly controlled by water mass mixing (e.g., 80 %∼100 % for Yb and 70 %∼100 % for Nd). For the non-conservative behavior, which is not explained by water mass mixing, the residual fraction of heavy REEs (<20 % for Yb) originates from release of sinking particles (organic matter and siliceous particles), while the remaining concentrations of light REEs (<30 % for Nd) may be influenced by a combination of particle release and scavenging processes. These findings highlight valuable information about lithogenic sources and the proportions of REE distributions that are controlled by physical and biogeochemical processes. Moreover, it emphasizes the applicability of heavy REEs as effective tracers for understanding basin-scale water mass mixing in the northwest Pacific.

Laminated diatomaceous ooze from southern Mariana Trench in Late Pleistocene and Holocene for paleoclimate and paleoceanography implication

This study offers a new record of laminated ooze for paleoclimate, paleoceanography and paleoecology interpretation in the deep ocean, promoting search for other similar records in the West Pacific Ocean to improve understanding of Ethmodiscus rex ooze formation and relative paleoenvironmental change. The laminated diatomaceous ooze includes large quantities of fragmented E. rex in a box core retrieved from the Challenger Deep, Southern Mariana Trench. Radiocarbon dating of the ooze revealed the studied sediment to originate during an interval of ca. 23 400 to ca. 6620 cal. yr. BP, inserting a hiatus during ca. 19 800 - 16 800 cal. yr. BP. This is the first geochronology data to constrain the E. rex ooze time span across the Pleistocene/Holocene boundary, compared to previous studies in this West Pacific Ocean trench. In this study, the laminated diatomaceous E. rex ooze type, with increased abundance of fragmented E. rex valves is mainly documented during ca. 23 400 - 19 800 cal. yr. BP. within the Last Glacial Maximum stage, at ca. 12 900 - 11 500 cal. yr. BP. in the Younger Dryas stadial, and around 9000–7700 cal. yr. BP. in the Postglacial.These data show a response to the paleoclimatic conditions with late Pleistocene cold periods that triggered E. rex diatom bloom that strongly competed for nutrients in this principally oligotrophic region. These phenomena are likely to be linked with terrestrial nutrients input through the aeolian dust transfer by winter Monsoon and the silicon supply from Southern Ocean via thermohaline circulation. In the Postglacial, the E. rex bloom generated a high ooze accumulation rate in terms of higher flux deposition with a complicated formation mechanism.

Measurement of nano molar ammonium with a cyclic olefin copolymer microchip and low-power LED

In oligotrophic regions, ammonium (NH4+) concentrations can be below 50 nM, however, few existing instruments can measure below this level with high confidence. This work, based on the o-pthaldialdehyde (OPA) fluorescence assay, is applied to measure nanomolar NH4+ in a novel optofluidic Cyclic Olefin Copolymer (COC) microchip using a low-power (20 mW) Light Emitting Diode (LED) as the excitation source. The optical arrangement was first modeled using ray tracing software to determine the initial detection volume size. Ammonium standards made with artificial seawater of 5 nM to 1000 nM, were run in triplicates. The limit of detection (LOD) obtained was 1.5 nM (3 x σ of the blank) or a LOD of 15 nM when the y-intercept and the vertical variation of each measured concentration on the calibration curve were taken into consideration (y-intercept +3. S y/x). Precision at 5 nM and 1000 nM was 3.3% and 0.5% respectively. The optofluidic system was also compared to an off-the-shelf fluorometer (Jasco FP2020) and an existing high-resolution shipboard analyser using five different standard concentrations. The LOD and the ammonium concentrations uncertainty for the Jasco FP2020, shipboard analyser, and current microsystem were 217 nM, 39 nM, and 15 nM and ± 232 nM, ± 48 nM, and ± 16 nM respectively. The optical setup was also validated using real samples from the Atlantic. This optical design, without optical fibres, makes the system simple and suitable for use with other fluorescent assays when compact, rugged, low-cost, and low-power consumption instrumentation is required.

Bacterial biogeography of the Indian Ocean.

Historically, our understanding of bacterial ecology in the Indian Ocean has been limited to regional studies that place emphasis on community structure and function within oxygen minimum zones. Thus, bacterial community dynamics across the wider Indian Ocean are largely undescribed. As part of Bio-GO-SHIP, we sequenced the 16S rRNA gene from 465 samples collected on sections I07N and I09N. We found that (i) there were 23 distinct bioregions within the Indian Ocean, (ii) the southeastern gyre had the largest gradient in bacterial alpha-diversity, (iii) the Indian Ocean surface microbiome was primarily composed of a core set of taxa, and (iv) bioregions were characterized by transitions in physical and geochemical conditions. Overall, we showed that bacterial community structure spatially delineated the surface Indian Ocean and that these microbially-defined regions were reflective of subtle ocean physical and geochemical gradients. Therefore, incorporating metrics of in-situ microbial communities into marine ecological regions traditionally defined by remote sensing will improve our ability to delineate warm, oligotrophic regions.

Plume dispersal of a hyper-eutrophic micro-estuary to the eastern mediterranean during base flow and flood events

Rivers are the primary source of dissolved and particulate materials entering the oceans. Recent studies conducted in Mediterranean streams showed a decrease in freshwater flow which indicates that this oligotrophic region, and probably other semi-arid oligotrophic regions, are likely to become more affected by small stream discharge and an increase in flood event intensity and frequency which indicates the increasing importance of studying flood conditions. However, little is known about the dispersal patterns of small streams and micro-estuaries and their influence on the coastal environment. We investigated the discharge from the Alexander Stream micro-estuary into the eastern Mediterranean as a case study for the fate of nutrients and pollutants discharged from micro-estuaries. Our objective is to characterize the Alexander Stream plume dispersal during base flow and flood events using salinity as a tracer for solute dispersal and by measuring the concentrations of suspended matter. The advection pattern during base flow was characterized by a current that clings to shore and mostly flows southward as a thin layer. This steady influx enriches the coastal water with high nutrient loads and carries pollutants toward Beit Yannai recreational beach, chronically exposing the shore and its users year-round. During floods, the plume disperses mostly northward along the coastline, and a considerable volume of suspended sediments settles near the estuary outfall, creating a hotspot of potentially contaminated sediment. At the peak discharge of the flood, the high concentrations of suspended sediments form hyperpycnal flows seaward near the seafloor and along the shelf. These results show the influence of small streams on the marine and coastal environments in semi-arid regions, which is predicted to expand to more temperate zones in the future.

Quantifying N2 fixation and its contribution to export production near the Tonga-Kermadec Arc using nitrogen isotope budgets

The spatial distribution of marine di-nitrogen (N2) fixation informs our understanding of the sensitivities of this process as well as the potential for this new nitrogen (N) source to drive export production, influencing the global carbon (C) cycle and climate. Using geochemically-derived δ15N budgets, we quantified rates of N2 fixation and its importance for supporting export production at stations sampled near the southwest Pacific Tonga-Kermadec Arc. Recent observations indicate that shallow (&amp;lt;300 m) hydrothermal vents located along the arc provide significant dissolved iron to the euphotic zone, stimulating N2 fixation. Here we compare measurements of water column δ15NNO3+NO2 with sinking particulate δ15N collected by short-term sediment traps deployed at 170 m and 270 m at stations in close proximity to subsurface hydrothermal activity, and the δ15N of N2 fixation. Results from the δ15N budgets yield high geochemically-based N2 fixation rates (282 to 638 µmol N m-2 d-1) at stations impacted by hydrothermal activity, supporting 64 to 92% of export production in late spring. These results are consistent with contemporaneous 15N2 uptake rate estimates and molecular work describing high Trichodesmium spp. and other diazotroph abundances associated with elevated N2 fixation rates. Further, the δ15N of sinking particulate N collected at 1000 m over an annual cycle revealed sinking fluxes peaked in the summer and coincided with the lowest δ15N, while lower winter sinking fluxes had the highest δ15N, indicating isotopically distinct N sources supporting export seasonally, and aligning with observations from most other δ15N budgets in oligotrophic regions. Consequently, the significant regional N2 fixation input to the late spring/summer Western Tropical South Pacific results in the accumulation of low-δ15NNO3+NO2 in the upper thermocline that works to lower the elevated δ15NNO3+NO2 generated in the oxygen deficient zones in the Eastern Tropical South Pacific.