Deep Chlorophyll Maximum Research Articles

Microfossils, High-Resolution Stratigraphy, Geochemistry and Lithology of the Upper Jurassic and Lower Cretaceous (Urdyuk-Khaya and Paksa Formations) in the Nordvik Peninsula, Anabar Bay, Laptev Sea

Abstract —The goal of this study was to improve the high-resolution biostratigraphy of the Upper Jurassic and Lower Cretaceous in the type section on the Nordvik Peninsula, Anabar Bay, Laptev Sea. The results were used to identify a succession of 13 foraminiferal biostratigraphic units, 9 dinocyst units, and 8 palynofloral biostratigraphic units in the rank of zones and local zones. Based on new data, the stratigraphic continuity of this section is proved for the Lower and Middle Volgian, where a major stratigraphic hiatus was previously assumed. The established zones have variable correlation potential. Different versions of the subdivision of the section based on ammonites are discussed and the proposed scale is justified. A reference level based on calcispheres/calcareous dinocysts is defined providing global correlations and calibration of beds near Jurassic–Cretaceous boundary in the Tethyan and Arctic regions. The magnetostratigraphic subdivision of the studied section and its comparison with the Tethyan regions are discussed. Together with biostratigraphic data, the proposed δ13Corg curve in the Upper Jurassic and lower part of the Valanginian can be a tool for detailed correlations in different regions of the Northern Hemisphere. The cyclicity of carbon isotope composition in the Upper Volgian and Boreal Berriasian and isotope events in the Volgian and Lower Valanginian are determined within the Arctic region. The distribution of geochemical parameters (Corg, δ13Corg and hydrogen index) is determined in studied section, geochemical stratification of the section and variations in major oxide compositions are demonstrated. A sharp change in geochemical parameters is identified at the base of the Paksa Formation (Upper Volgian). The distribution interval of the “subsurface chlorophyll maximum” is determined. The study shows that high concentrations of isotopically light organic carbon in the first diasterene geochemical subhorizon were caused by high phytoplankton productivity and were deposited under reducing conditions favorable for preservation of organic matter. The analysis of major oxide compositions of the studied rocks revealed that variations in the chemical composition of the studied rocks reflect changes in their mineralogy and petrography.

Responses of Phytoplankton Communities to Internal Waves in Oligotrophic Oceans

AbstractUnderstanding the potential impacts of internal waves on phytoplankton communities in oligotrophic oceans remains an important research challenge. In this study, we elucidated the impact of internal waves on phytoplankton communities through a comprehensive 154‐hr time‐series of observations in the South China Sea (SCS). We identified distinctive variations in phytoplankton pigment biomass and composition across the upper, middle, and lower layers of the euphotic zone, which we attributed to the perturbations triggered by internal waves. Phytoplankton other than Prochlorococcus in the lower, nutrient‐replete layer likely benefitted from allochthonous nutrients introduced by internal waves, but their growth rates were constrained by light limitation, and their pigment biomass was held in check by microzooplankton grazing. In contrast, in the upper, nutrient‐depleted layer, the relative abundance of Prochlorococcus increased, likely because of the ammonium regenerated by zooplankton. The middle layer, characterized as the deep chlorophyll maximum layer, exhibited a dynamic equilibrium characterized by nutrient and light co‐limitation. This equilibrium resulted in high nitrate assimilation and growth by phytoplankton. The balancing of those rates by significant grazing losses maintained total chlorophyll a concentrations at a high level. Based on these findings, we proposed a three‐layer euphotic zone structure characterized by distinct physiological conditions, nutrient‐light dynamics, grazing pressure, and phytoplankton responses to internal waves. This three‐layer paradigm elucidated the intricate interplay between internal waves and phytoplankton communities and provided insights into the mechanisms that govern primary production and carbon cycling in oligotrophic oceanic ecosystems.

Seasonal and vertical distribution of living planktonic foraminifera in the western tropical Pacific Ocean

To investigate the seasonal and vertical distributions of planktonic foraminiferal species in the western tropical Pacific Ocean, we collected planktonic foraminifera seasonally and diurnally from eight water depths from 0 to 600 m using the Multiple Opening/Closing Net and Environmental Sensing System. We identified 23 living foraminiferal species (size fraction 212–1000 μm) and examined their abundances at the sites of oligotrophic areas. Seasonal changes were characterized by an increase in the diversity and abundance of planktonic foraminifera in summer. We estimated the habitat depth of planktonic foraminifera based on their vertical distribution. Planktonic foraminifera can be classified into species living in the surface mixed layer, uppermost thermocline, upper thermocline, and lower thermocline in the water column. The uppermost thermocline layer is characterized by the occurrence of subsurface chlorophyll maximum (water depth, 100–150 m). The boundary between the upper and lower thermocline layers (approximately 300 m) is at the depth at which the dissolved oxygen concentration begins to rapidly decrease. The species inhabiting the surface mixed layer were Globigerinoides sacculifer and Globigerinoides ruber white in spring. In summer, G. sacculifer dwelled in the surface mixed layer during night time, whereas they existed in the uppermost thermocline during daytime. Many species were found in the uppermost thermocline layer in summer, including G. ruber white, Globigerinoides conglobatus, Globigerinella siphonifera, Pulleniatina obliquiloculata, Globoquadrina conglomerata, Orbulina universa, Neogloboquadrina dutertrei, Globigerinella calida, and Hastigerina pelagica. The species Globorotalia hirsuta dwelled in the upper thermocline in spring, whereas it occurred in the lower thermocline in summer. Globorotaloides hexagonus inhabited the lower thermocline in all seasons. Our results will provide useful information for reconstructing changes in water column structure using planktonic foraminiferal shells in marine sediments.

Potential of Mie-Fluorescence-Raman Lidar to Profile Chlorophyll a Concentration in Inland Waters.

Vertical distribution of phytoplankton is crucial for assessing the trophic status and primary production in inland waters. However, there is sparse information about phytoplankton vertical distribution due to the lack of sufficient measurements. Here, we report, to the best of our knowledge, the first Mie-fluorescence-Raman lidar (MFRL) measurements of continuous chlorophyll a (Chl-a) profiles as well as their parametrization in inland water. The lidar-measured Chl-a during several experiments showed good agreement with the in situ data. A case study verified that MFRL had the potential to profile the Chl-a concentration. The results revealed that the maintenance of subsurface chlorophyll maxima (SCM) was influenced by light and nutrient inputs. Furthermore, inspired by the observations from MFRL, an SCM model built upon surface Chl-a concentration and euphotic layer depth was proposed with root mean square relative difference of 16.5% compared to MFRL observations, providing the possibility to map 3D Chl-a distribution in aquatic ecosystems by integrated active-passive remote sensing technology. Profiling and modeling Chl-a concentration with MFRL are expected to be of paramount importance for monitoring inland water ecosystems and environments.

Novel RNA viruses from the Atlantic Ocean: Ecogenomics, biogeography, and total virioplankton mass contribution from surface to the deep ocean.

Marine viruses play a major role in the energy and nutrient cycle and affect the evolution of their hosts. Despite their importance, there is still little knowledge about RNA viruses. Here, we have explored the Atlantic Ocean, from surface to deep (4.296 m), and used viromics and quantitative methods to unveil the genomics, biogeography, and the mass contribution of RNA viruses to the total viroplankton. A total of 2481 putative RNA viral contigs (>500 bp) and 107 larger bona fide RNA viral genomes (>2.5 kb) were identified; 88 of them representing novel viruses belonging mostly to two clades: Yangshan assemblage (sister clade to the class Alsuviricetes) and Nodaviridae. These viruses were highly endemic and locally abundant, with little or no presence in other oceans since only ≈15% of them were found in at least one of the Tara sampling metatranscriptomes. Quantitative data indicated that the abundance of RNA viruses in the surface and deep chlorophyll maximum zone was within ≈106 VLP/mL representing a potential contribution of 5.2%-24.4% to the total viroplankton community (DNA and RNA viruses), with DNA viruses being the predominant members (≈107 VLP/mL). However, for the deep sample, the observed trend was the opposite, although as further discussed, several biases should be considered. Together these results contribute to our understanding of the diversity, abundance, and distribution of RNA viruses in the oceans and provide a basis for further investigation into their ecological roles and biogeography.

Impact of subsurface chlorophyll maxima on satellite-based Arctic spring primary production estimates

In recent years several regional and pan-Arctic ocean color algorithms, which consider the unique bio-optical properties of the Arctic Ocean, have been developed to accurately extract surface chlorophyll a (Chl a), a key input into net primary production (NPP) algorithms, from spectral remote sensing reflectance (Rrs). However, most satellite derived NPP (NPPSAT) algorithms used in the Arctic do not account for production at deep subsurface chlorophyll maxima (SCMs), a prominent feature in the Arctic Ocean during the summer months, leading to underestimations of NPP during the post-bloom period. In this study, a shallow SCM was observed in early summer (June/July 2016) in Baffin Bay at the time of the sea ice edge bloom, raising the question to what extent this SCM contributes to Rrs and how it is captured in NPPSAT estimates. Radiative transfer simulations based on in situ data of inherent and apparent optical properties and Chl a concentration in the water column were used to examine the effects of heterogeneous vertical Chl a profiles of various strength and depth on spectral reflectance, algorithm-derived surface Chl aSAT and NPPSAT. NPPSAT estimates for varying Chl a profile shapes were then compared to NPPSAT estimates of reference simulations for homogenous Chl a profiles, and to measured NPP calculated using observed Chl a profiles. Results show a significant impact of shallow SCMs on Rrs with the depth of maximum Chl a < 30 m, causing an increase in Chl aSAT by 17 ± 6% relative to a vertically homogenous ocean. Interestingly, SCMs significantly influencing Rrs were found down to the fifth light penetration depth. The increase in Chl aSAT reduced the difference between predicted and measured NPP estimates to −25 ± 12% at stations with a shallow SCM. Furthermore, due to the significant contribution of these shallow SCM stations to regional NPP, regional predicted NPPSAT was only 16% lower than measured NPP. Our results demonstrate that the partial representation of a shallow and very productive SCM in Chl aSAT minimizes errors in satellite-based NPPSAT estimates of the Arctic spring bloom during the sea ice melt period.

Subsurface phytoplankton responses to ocean eddies can run counter to satellite-based inference from surface properties in subtropical gyres

Phytoplankton contribute approximately half of the Earth’s primary production, playing a crucial role in marine ecosystem functioning and global carbon cycling. However, our understanding of how ocean dynamics regulate the distribution of phytoplankton remains limited due to the scarcity of depth-resolved observations. Here, we compared the simultaneous measurements of two biogeochemical-Argo floats that tracked a cyclonic eddy (CE) and an anticyclonic eddy (AE) in the subtropical South Indian Ocean for a period of 3 months. The observations revealed a decoupling of subsurface phytoplankton biomass from surface chlorophyll. The CE exhibited a subsurface phytoplankton bloom initiated by the springtime shoaling of mixed layer to euphotic layer, accompanied by a counter-intuitive decrease in surface chlorophyll due to physiological adaptation. In comparison, the AE exhibited vertically homogeneous phytoplankton biomass, with a weak subsurface chlorophyll maximum layer, as the mixed layer remained at a deep level in the spring. Despite a higher concentration of surface chlorophyll in the AE than in the CE, subsurface and depth-integrated phytoplankton chlorophyll, biomass, and productivity computed with a bio-optical model are greater in the CE than in the AE. Statistics on global biogeochemical-Argo measurements over the past decade confirm the generality of this phenomenon in subtropical gyres. Our findings demonstrate that high concentrations of chlorophyll near the surface in these regions are not necessary indicative of high phytoplankton abundance at the subsurface layer. The results have important implications for inferring subsurface phytoplankton abundance and productivity from satellite measurements.

Hydrocarbons in size-fractionated plankton of the Mediterranean Sea (MERITE-HIPPOCAMPE campaign)

Aliphatic and polycyclic aromatic hydrocarbons (AHs and PAHs, respectively) were analyzed in the dissolved fraction (<0.7 μm) of surface water and in various particulate/planktonic size fractions (0.7–60, 60–200, 200–500 and 500–1000 μm) collected at the deep chlorophyll maximum, along a North-South transect in the Mediterranean Sea in spring 2019 (MERITE-HIPPOCAMPE campaign). Suspended particulate matter, biomass, total chlorophyll a, particulate organic carbon, C and N isotopic ratios, and lipid biomarkers were also determined to help characterizing the size-fractionated plankton and highlight the potential link with the content in AHs and PAHs in these size fractions. Ʃ28AH concentrations ranged 18–489 ng L−1 for water, 3.9–72 μg g−1 dry weight (dw) for the size fraction 0.7–60 μm, and 3.4–55 μg g−1 dw for the fractions 60–200, 200–500 and 500–1000 μm. AH molecular profiles revealed that they were mainly of biogenic origin. Ʃ14PAH concentrations were 0.9–16 ng L−1 for water, and Ʃ27PAH concentrations were 53–220 ng g−1 dw for the fraction 0.7–60 μm and 35–255 ng g−1 dw for the three higher fractions, phenanthrene being the most abundant compound in planktonic compartment. Two processes were evidenced concerning the PAH patterns, the bioreduction, i.e., the decrease in concentrations from the small size fractions (0.7–60 and 60–200 μm) to the higher ones (200–500 μm and 500–1000 μm), and the biodilution, i.e., the decrease in concentrations in plankton at higher suspended matter or biomass, especially for the 0.7–60 and 60–200-μm size fractions. We estimated the biological pump fluxes of Ʃ27PAHs below 100-m depth in the Western Mediterranean Sea at 15 ± 10 ng m−2 day−1, which is comparable to those previously reported in the South Pacific and Indian Ocean.

Distribution patterns of phytoplankton groups along isoirradiance layers in oligotrophic tropical and subtropical oceans

A pigment chemotaxonomic approach was used to find the distribution of phytoplankton groups over a wide extension of the oligotrophic tropical and subtropical Atlantic, Indian and Pacific oceans. The six sampling depths corresponded to percentages of surface irradiance, i.e. “isoirradiances”. Most of the 139 stations sampled presented a subsurface deep chlorophyll maximum (DCM). Only in 19 of the 818 samples the total chlorophyll a (TChl a) concentration was >0.5 mg m−3 and only in one >0.75 mg m−3. The composition of the phytoplankton populations showed low variability. Haptophytes and Prochlorococcus contributed most to TChl a (36% and 35%, respectively), followed by green algae (11%), Synechococcus (6.5%), pelagophytes (6.7%), dinoflagellates (3.2%) and diatoms (1.6%). Specific non-polar Chl c2 pigments were analyzed to estimate the pigment biomass of three types of haptophytes (6, 7 and 8) using CHEMTAX. Haptotophytes-8 was the most abundant (18%), while haptophytes-7 and -6 contribution was 13% and 5.2%, respectively. There was a vertical partition of the groups along the water column. Prochlorococcus, haptophytes-6 and -8, and pelagophytes presented low values in shallow and intermediate layers and a strong increase in pigment biomass at the DCM. Dinoflagellates, haptophytes-7 and green algae also with low values in the upper layers showed a sharp increase at the layer above the DCM, with similar values as in the DCM or even higher for green algae. The pigment biomass of diatoms and Synechococcus were fairly homogeneous throughout the water column. These three patterns were best reflected by estimating the contribution of each group to the TChl a concentration. The Malaspina expedition crossed some well recognized ecosystem/areas of the open ocean such as the Atlantic and Pacific equatorial upwellings, the Costa Rica and Guinea domes, the low oxygen area of the northeastern Pacific and the Great Australian Bight, for which the composition of the phytoplankton populations is provided.

Fine-scale vertical relationships between environmental conditions and sound scattering layers in the Southwestern Tropical Atlantic.

Ocean dynamics initiate the structure of nutrient income driving primary producers, and these, in turn, shape the distribution of subsequent trophic levels until the whole pelagic community reflects the physicochemical structure of the ocean. Despite the importance of bottom-up structuring in pelagic ecosystems, fine-scale studies of biophysical interactions along depth are scarce and challenging. To improve our understanding of such relationships, we analyzed the vertical structure of key oceanographic variables along with the distribution of acoustic biomass from multi-frequency acoustic data (38, 70, and 120 kHz) as a reference for pelagic fauna. In addition, we took advantage of species distribution databases collected at the same time to provide further interpretation. The study was performed in the Southwestern Tropical Atlantic of northeast Brazil in spring 2015 and autumn 2017, periods representative of canonical spring and autumn conditions in terms of thermohaline structure and current dynamics. We show that chlorophyll-a, oxygen, current, and stratification are important drivers for the distribution of sound scattering biota but that their relative importance depends on the area, the depth range, and the diel cycle. Prominent sound scattering layers (SSLs) in the epipelagic layer were associated with strong stratification and subsurface chlorophyll-a maximum. In areas where chlorophyll-a maxima were deeper than the peak of stratifications, SSLs were more correlated with stratification than subsurface chlorophyll maxima. Dissolved oxygen seems to be a driver in locations where lower oxygen concentration occurs in the subsurface. Finally, our results suggest that organisms seem to avoid strong currents core. However, future works are needed to better understand the role of currents on the vertical distribution of organisms.

Wind‐driven currents and water masses shape spring phytoplankton distribution and composition in hydrologically complex, productive shelf waters

AbstractThe Northern Patagonian shelf (35–42°S SW South Atlantic) is a transitional area between the subtropical and subantarctic domains, where the shelf shifts from source to sink of atmospheric CO2. In the southeast portion (39–42°S, 61–55°W) of this area, winds interact with different water masses and picture a complex circulation that supports two productive frontal systems: the Mid‐Shelf Front (MSF) and the Patagonian Shelf‐break Front. We combined remote sensing (e.g., monthly surface chlorophyll a over the period: September–December 2002–2021), reanalysis (winds and surface currents), and in situ measurements from three cross‐shelf cruises in spring 2016, 2017, and 2019, to assess the underlying drivers of phytoplankton distribution. The anomalous high chlorophyll concentration in the MSF in spring 2016 was related to enhanced water mass retention by N‐NE winds. Contrasting phytoplankton assemblages emerged related to stratified waters in the shelf influenced by the Patagonian Current and vertically mixed waters at the continental edge (Malvinas Current). Large diatoms prevailed in the outer shelf followed by dinoflagellates, while nanoflagellates and microzooplankton dominated in the mid‐shelf. Furthermore, thickness of the subsurface chlorophyll maximum was broader in the early spring than in a more advanced state of the bloom where the coccolithophore Emiliania huxleyi was abundant. We showed that different time‐scale physical processes—winds, surface currents and vertical mixing—are underpinning the development of spring blooms with different taxa composition in the Patagonian shelf, where different water masses define the hydrological complexity that led to patchy phytoplankton.

Spatiotemporal variations of the oxycline and its response to subduction events in the Arabian Sea

The Arabian Sea is a significant hypoxic region in world’s oceans, characterized by the most extensive oxygen minimum zones (OMZs). Both physical and biological processes can alter the vertical and horizontal distribution of dissolved oxygen within the upper ocean and affect the spatial and temporal distribution of hypoxia within the OMZ. To identify the key physical and biological factors influencing the boundaries of oxycline, we analyzed an extensive dataset collected from the biogeochemical-Argo (BGC-Argo) floats during the period of 2010–2022. In particular, we investigated the impact of physical subduction events on the oxycline. Our results shows that the upper boundary of the oxycline deepened in summer and winter, and seemed to be controlled by the mixed layer depth. In contrast, it was shallower during spring and autumn, mainly regulated by the deep chlorophyll maximum. The lower boundary of the oxycline in the western Arabian Sea was predominantly controlled by regional upwelling and downwelling, as well as Rossby waves in the eastern Arabian Sea. Subduction patches originated from the Arabian Sea High Salinity Water (ASHSW) were observed from the BGC-Argo data, which were found to deepen the lower boundary of the oxycline, and increase the oxygen inventory within the oxycline by 8.3%, leading to a partial decrease in hypoxia levels.

Metagenomic Analyses Reveal the Influence of Depth Layers on Marine Biodiversity on Tropical and Subtropical Regions.

The emergence of open ocean global-scale studies provided important information about the genomics of oceanic microbial communities. Metagenomic analyses shed light on the structure of marine habitats, unraveling the biodiversity of different water masses. Many biological and environmental factors can contribute to marine organism composition, such as depth. However, much remains unknown about microbial communities' taxonomic and functional features in different water layer depths. Here, we performed a metagenomic analysis of 76 publicly available samples from the Tara Ocean Project, distributed in 8 collection stations located in tropical or subtropical regions, and sampled from three layers of depth (surface water layer-SRF, deep chlorophyll maximum layer-DCM, and mesopelagic zone-MES). The SRF and DCM depth layers are similar in abundance and diversity, while the MES layer presents greater diversity than the other layers. Diversity clustering analysis shows differences regarding the taxonomic content of samples. At the domain level, bacteria prevail in most samples, and the MES layer presents the highest proportion of archaea among all samples. Taken together, our results indicate that the depth layer influences microbial sample composition and diversity.

Microbial dynamics in shallow CO2 seeps system off Panarea Island (Italy)

Shallow-water hydrothermal vents are extreme environments characterized by high temperatures, low pH, and high CO2 concentrations; therefore, they are considered as suitable laboratories for studying the effect of global changes on marine microbes. We hypothesized a direct effect of vents on prokaryotic community structure and functioning in the Panarea Island’s hydrothermal system. Sampling was conducted along a 9-station transect characterized by three active emission points. The water column was stratified with a thermocline at 25 m depth and a deep chlorophyll maximum between 50 and 100 m. Prokaryotic abundance ranged from 0.2 to 1.5 × 109 cells L−1, prokaryotic carbon production from 2.4 to 75.4 ng C L−1 h−1, and exoenzymatic activities degrading proteins, phosphorylated compounds, and polysaccharides were on the order of 4–28, 2–31 and 0.2–4.16 nM h−1, respectively. While microbial abundance and production were shaped by the water column's physical structure, alkaline phosphatase and beta-glucosidase activities seemed to be enhanced by hydrothermal fluids. The 16S rRNA gene amplicon sequencing analysis identified a surface, a deep, and a vent-influenced microbial community. In terms of relative abundance members of the SAR11 group dominated the water column, alongside Synechococcus and Prochlorococcus in surface and bottom samples, respectively. Vent-influenced stations were characterized by the presence of Thiomicrorhabdus, a sulfur-oxidizer chemolithoautotroph. Overall, this study provides insights on the coupling between microbial community structure and the biogeochemical cycling of nutrients in low-pH conditions (CO2 and H2S-based), thus addressing some of the opened questions about the response of microbes to acidification.

Phytoplankton Diversity and Co-Dependency in a Stratified Oligotrophic Ecosystem in the South Adriatic Sea

The oligotrophy of the southern Adriatic Sea is characterized by seasonal stratification which enables nutrient supply to the euphotic layer. A set of interdisciplinary methods was used to elucidate the diversity and co-dependency of bacterio- and phytoplankton of the water column during the stratification period of July 2021. A total of 95 taxa were determined by microscopy: 58 diatoms, 27 dinoflagellates, 6 coccolithophores, and 4 other autotrophs, which included Chlorophyceae, Chrysophyceae, and Cryptophytes. Nanophytoplankton abundances were higher in comparison to microphytoplankton. The prokaryotic plankton community as revealed by HTS was dominated by Proteobacteria (41–73%), Bacteroidota (9.5–27%), and cyanobacteria (1–10%), while the eukaryotic plankton community was composed of parasitic Syndiniales (45–80%), Ochrophyta (2–18%), Ciliophora (2–21%), Chlorophytes (2–4%), Haptophytes (1–4%), Bacillariophyta (1–13%), Pelagophyta (0.5–12%) and Chrysophyta (0.5–3%). Flow cytometry analysis has recorded Prochlorococcus and photosynthetic picoeukaryotes as more abundant in deep chlorophyll maximum (DCM), and Synechococcus and heterotrophic bacteria as most abundant in surface and thermocline layers. Surface, thermocline, and DCM layers were distinct considering community diversity, temperature, and nutrient correlations, while extreme nutrient values at the beginning of the investigating period indicated a possible nutrient flux. Nutrient and temperature were recognized as the main environmental drivers of phytoplankton and bacterioplankton community abundance.

The Bay of Bengal exposes abundant photosynthetic picoplankton and newfound diversity along salinity-driven gradients.

The Bay of Bengal (BoB) is a 2,600,000 km2 expanse in the Indian Ocean upon which many humans rely. However, the primary producers underpinning food chains here remain poorly characterized. We examined phytoplankton abundance and diversity along strong BoB latitudinal and vertical salinity gradients-which have low temperature variation (27-29°C) between the surface and subsurface chlorophyll maximum (SCM). In surface waters, Prochlorococcus averaged 11.7 ± 4.4 × 104 cells ml-1 , predominantly HLII, whereas LLII and 'rare' ecotypes, HLVI and LLVII, dominated in the SCM. Synechococcus averaged 8.4 ± 2.3 × 104 cells ml-1 in the surface, declined rapidly with depth, and population structure of dominant Clade II differed between surface and SCM; Clade X was notable at both depths. Across all sites, Ostreococcus Clade OII dominated SCM eukaryotes whereas communities differentiated strongly moving from Arabian Sea-influenced high salinity (southerly; prasinophytes) to freshwater-influenced low salinity (northerly; stramenopiles, specifically, diatoms, pelagophytes, and dictyochophytes, plus the prasinophyte Micromonas) surface waters. Eukaryotic phytoplankton peaked in the south (1.9 × 104 cells ml-1 , surface) where a novel Ostreococcus was revealed, named here Ostreococcus bengalensis. We expose dominance of a single picoeukaryote and hitherto 'rare' picocyanobacteria at depth in this complex ecosystem where studies suggest picoplankton are replacing larger phytoplankton due to climate change.

Kedalaman Klorofil Maksimum Selama Musim Peralihan II dan Implikasinya untuk Perikanan Tuna di Laut Banda

The Deep chlorophyll maximum (DCM) in the Banda Sea during the second transitional season (October 2015) was relatively shallow (25 - 64 m) above the mixed layer and far from the base of the euphotic layer or compensation depth. Its presence was identical to the depth of nitraclin injected into the mixed layer as a result of upwelling generated by internal waves. The contribution of these waves to the increase in Banda Sea fertility calculated using ocean color data is 56.87%. This phenomenon is due to the shallow optical depth of the satellite (20-25 m) while high chlorophyll-a concentrations are below. DCM was also associated with the swimming layer of Madidihang (Thunnus albacares) (34 - 68 m) caught with longline gear during the same period.

Distinct vertical behavior of key Arctic copepods following the midnight sun period in the East Siberian continental margin region, Arctic Ocean

Diel vertical migration (DVM) of zooplankton plays a vital role in biological carbon pump and food web interactions. However, there is considerable debate about the DVM of zooplankton in response to environmental changes in the Arctic Ocean. We investigated DVM behavior in the key Arctic copepods Calanus glacialis, Calanus hyperboreus, and Metridia longa following the midnight sun period in the East Siberian continental margin region. The two Calanus species showed non-DVM behaviors, whereas M. longa showed a typical DVM pattern consistent with the solar radiation cycle. Additionally, these species showed different vertical distributions. Calanus glacialis was distributed at depths above 20 m in the warm fresh water, where the highest density gradient was observed. Calanus hyperboreus was distributed at depths between 30 and 55 m in the cold salty water, where a high contribution of micro phytoplankton and the subsurface chlorophyll maximum (SCM) layer were observed. M. longa was found across a broader range of temperature and salinity than both Calanus species, and it was distributed in the upper water column, where the SCM layer was observed at night and at depths between 100 and 135 m in the daytime. These results imply that M. longa can be well adapted to the changing Arctic Ocean environment, where sea ice loss and ocean warming are ongoing, whereas C. hyperboreus can be the most vulnerable to these changes. These findings provide important information for understanding variations in the vertical distributions of key copepod species in the rapidly changing Arctic marine environment.

Hydrography and deep chlorophyll maximum patterns of the Athos Basin and the Thracian Sea continental shelf (North Aegean Sea) combining glider and satellite observations

This work focuses on the North Aegean Sea deep chlorophyll maximum (DCM) dynamics, as revealed during the first glider mission over the continental shelf and slope of the Thracian Sea and along the Athos basin, and presents a conceptual model on the response of the DCM in relation to the prevailing stratification/mixing of the water column. Water temperature, conductivity, salinity, chlorophyll-a, CDOM, DO and particle backscatter data (bb700), were recorded during this underwater glider mission in summer 2019. Daily satellite data from Sentinel 3 complemented the glider collected data to validate the satellite-derived estimates of surface Chlorophyll-a levels. The high-resolution physical and chemical observations were analyzed emphasizing on thermal stratification, pycnocline strength, CDOM and dissolved oxygen vertical distribution and the presence of deep chlorophyll maximum (DCM). DCM statistical characteristics were derived by fitting a Gaussian model on the chlorophyll-a profiles. The mean DCM was observed at 76 ± 14.5 m depth, located below the euphotic layer, estimated at 64 ± 2.1 m, with the water column dynamics affecting directly its depth, thickness and peak prominence. The satellite-derived chlorophyll-a, integrated within the euphotic layer, was found in fair agreement to the integrated chlorophyll-a values from surface to optical depth recorded by the glider. Chl-a at the DCM were found up to 0.60 mg m−3, the vertically-integrated pigment content was estimated at 45 ± 14 mg m−2 and the values of Optical Community Index (OCI) illustrate the dominance of small-sized plankton. The increased variability in the particulate backscattering coefficient bb700 in Thracian Sea shelf and slope deep waters seems related to sediment resuspension. CDOM levels increased from 0.76 ppb at the sea surface to 1.11 ppb at 40 m depth, being positively correlated to Chl-a within the DCM. CDOM concentration increased steadily at intermediate and deeper waters up to 1.34 ± 0.07 ppb. DO profiles revealed the presence of an oxycline in the DCM layer, capable to oxygenate the intermediate and deeper waters (+3 × 10−3 ml/L/m). Bottom DO in Athos basin was measured at 5.0 ml L−1, indicating that the apparent oxygen utilization in Athos Basin is approximately 50 μmol L−1.

Hypoxia formation triggered by the organic matter from subsurface chlorophyll maximum in a large estuary-shelf system

This study reports, for the first time, the role of shoreward transport of organic matter (OM) from subsurface chlorophyll maximum (SCM) in triggering hypoxia off the Pearl River Estuary (PRE, an outstanding example of typical estuary-shelf systems) based on field measurements. Compared to frequently observed hypoxia driven by surface eutrophication and terrestrial OM during large river discharge, we demonstrate that the upslope-transported SCM played a critical role in forming offshore hypoxia during low river discharge. Together with the plume-sourced OM trapped below the surface plume front, upslope-transported OM originating from the SCM accumulated underneath the pycnocline and consumed dissolved oxygen (DO), enhancing the bottom hypoxia. The DO consumption induced by the SCM-associated OM was estimated to contribute ∼ 26% (±23%) of the DO depletion under the pycnocline. Based on coherent and consistent physical and biogeochemical evidence and reasoning, this study reveals the contribution of SCM to bottom hypoxia off the PRE, which is unreported and likely occurs in other coastal hypoxic systems.