Conductivity-temperature-depth Research Articles

An Acoustic Survey of Beaked Whales and Kogia spp. in the Mariana Archipelago Using Drifting Recorders

The distribution, abundance, and habitat of cryptic cetacean species such as beaked whales and dwarf/pygmy sperm whales (Kogiaspp.) are challenging to study due to their long dive times and/or very limited surface behavior. Even less is known in minimally studied and remote regions, including the Mariana Archipelago and parts of the broader western Pacific. In 2018, we deployed a network of eight Drifting Acoustic Spar Buoy Recorders (DASBRs) on the west side of the Mariana Archipelago with the goal of examining the distribution and habitat of beaked whales andKogiaspp. in this region using passive acoustic monitoring. Concurrently, conductivity-temperature-depth (CTD) data were collected within the drift area and combined with satellite oceanographic data to build Ensemble Random Forest Models to identify specific oceanographic features that determine the distribution of these species. DASBRs deployed at locations ranging from 13°N to 18°N generally drifted from east to west between the Mariana Archipelago and the West Mariana Ridge. Spectral and temporal characteristics of echolocation signals were used to identify the presence of beaked whales andKogiaspp. species. This dataset contained frequency modulated (FM) pulses characteristic of Longman’s (Indopacetus pacificus), Cuvier’s (Ziphius cavirostris), and Blainville’s (Mesoplodon densirostris) beaked whales, as well as the unidentified beaked whale FM pulse known as the “BWC,” along with narrow-band high frequency clicks fromKogiaspp. The detection rate was substantially higher for all species on the five tracks in the region north of 15.5°N than for those drifts occurring farther south. Species distribution models suggest that differences in the oceanographic characteristics between the northern and southern regions may impact foraging opportunities, possibly explaining the specific ecological niche for these species within this water mass. This is the first study of the distribution of cryptic cetacean species within the wider Mariana Archipelago region. We demonstrate that autonomous drifting acoustic recorders, combined with environmental sampling and remote satellite data are a powerful tool for studying the habitat dependent distribution of cryptic cetacean species.

Scientific access into Mercer Subglacial Lake: scientific objectives, drilling operations and initial observations

AbstractThe Subglacial Antarctic Lakes Scientific Access (SALSA) Project accessed Mercer Subglacial Lake using environmentally clean hot-water drilling to examine interactions among ice, water, sediment, rock, microbes and carbon reservoirs within the lake water column and underlying sediments. A ~0.4 m diameter borehole was melted through 1087 m of ice and maintained over ~10 days, allowing observation of ice properties and collection of water and sediment with various tools. Over this period, SALSA collected: 60 L of lake water and 10 L of deep borehole water; microbes >0.2 μm in diameter from in situ filtration of ~100 L of lake water; 10 multicores 0.32–0.49 m long; 1.0 and 1.76 m long gravity cores; three conductivity–temperature–depth profiles of borehole and lake water; five discrete depth current meter measurements in the lake and images of ice, the lake water–ice interface and lake sediments. Temperature and conductivity data showed the hydrodynamic character of water mixing between the borehole and lake after entry. Models simulating melting of the ~6 m thick basal accreted ice layer imply that debris fall-out through the ~15 m water column to the lake sediments from borehole melting had little effect on the stratigraphy of surficial sediment cores.

Sea Ice Reduction During Winter of 2017 Due to Oceanic Heat Supplied by Pacific Water in the Chukchi Sea, West Arctic Ocean

Over the past few decades, the areal extent of the Arctic sea ice cover has decreased. During the winter of 2017, negative sea ice concentration anomalies occurred mainly in the Chukchi Sea and adjacent seas. The properties of Pacific water through the Bering Strait have changed in recent years. To highlight the role of the Pacific inflow during the 2017 Arctic sea ice retreat, we used mooring measurements and conductivity–temperature–depth (CTD) data to quantify the effect of inflow on sea ice in the Chukchi shelf. In September 2017, the temperature of the Pacific inflow was relatively high compared with the multi-year average, especially in the shelf north of 69°N where the temperature anomaly was generally greater than 1°C. The average heat content of each CTD station in September 2017 ranged from 0.77 to 1.58 GJ m–2, where each station was 0.25 GJ m–2 higher than the multi-year average. In the central shelf of the Chukchi Sea, the temperature of the 25–40 m layer increased after late May, and decreased after mid-September. The Pacific inflow could have provided a large amount of heat to the Chukchi shelf, the accumulated convective heat transported to the surface from September to October was approximately 1.68 × 1018 J and it impacted the sea ice growth conditions.

Krill availability in adjacent Adélie and gentoo penguin foraging regions near Palmer Station, Antarctica

AbstractThe Palmer Deep canyon along the West Antarctic Peninsula is a biological hotspot with abundant phytoplankton and krill supporting Adélie and gentoo penguin rookeries at the canyon head. Nearshore studies have focused on physical mechanisms driving primary production and penguin foraging, but less is known about finer‐scale krill distribution and density. We designed two acoustic survey grids paired with conductivity–temperature–depth profiles within adjacent Adélie and gentoo penguin foraging regions near Palmer Station, Antarctica. The grids were sampled from January to March 2019 to assess variability in krill availability and associations with oceanographic properties. Krill density was similar in the two regions, but krill swarms were longer and larger in the gentoo foraging region, which was also less stratified and had lower chlorophyll concentrations. In the inshore zone near penguin colonies, depth‐integrated krill density increased from summer to autumn (January–March) independent of chlorophyll concentration, suggesting a life history‐driven adult krill migration rather than a resource‐driven biomass increase. The daytime depth of krill biomass deepened through the summer and became decoupled from the chlorophyll maximum in March as diel vertical migration magnitude likely increased. Penguins near Palmer Station did not appear to be limited by krill availability during our study, and regional differences in krill depth match the foraging behaviors of the two penguin species. Understanding fine‐scale physical forcing and ecological interactions in coastal Antarctic hotspots is critical for predicting how environmental change will impact these ecosystems.

Electrochemical sensors for in-situ measurement of ions in seawater

• CTD probes are the preamble for current in-situ seawater monitoring. • Relevant species to measure in seawater: trace metals, nutrients and carbon species. • Electrochemical sensors in the last five years for seawater analysis. • Lack of in-situ applications. • Learning from the current panorama to advance in the field. In the current water monitoring panorama, certain benefits are expected to arise when the modus operandum of analysis shifts from sampling-based to purely in-situ approaches. Since the appearance of the first submersible conductivity-temperature-depth (CTD) probe in the 80 s, clear efforts to move towards decentralized strategies have been reported in the literature, with some having even been brought to the stage of fruitful commercialization. Among the portfolio of available analytical techniques, only a handful of approaches offer clear potential for implementation in submersible devices, in terms of adequate analytical features, autonomy, feasibility of miniaturization, and ease of portability. Electrochemical sensors have demonstrated excellent characteristics for this purpose, particularly in the detection of ions. The present review analyses electrochemical sensors that have shown an aptitude for in-situ measurements of ions, including trace metals, nutrients, and carbon species in seawater. The previous 5 years have been selected as the main period for review, although in some instances comment is made upon earlier contributions to the field or commercialized devices, where these are deemed to exemplify crucial technological advancements. There is a notable lack of electrochemical sensors being deployed in in-situ applications, and this scarcity is even more stark when seawater is considered: only a very few cases have been demonstrated under such challenging conditions. With the aim of providing inspiration towards genuine advances in the field of in-situ seawater analysis, this work also highlights some laboratory scale research as well as studies concerning other environmental waters than seawater.

Deep learning techniques‐based perfection of multi‐sensor fusion oriented human‐robot interaction system for identification of dense organisms

For detection of dense small-target organisms with indistinct features in complex background, the efficiency and accuracy of traditional target detection methods are low. Multi-sensor fusion oriented human-robot interaction (HRI) system has facilitated biologists to process and analyse data. For this, several deep learning models based on convolutional neural network (CNN) are improved and compared to study the species and density of dense organisms in deep-sea hydrothermal vent, which are fused it with related environmental information given by position sensors and conductivity-temperature-depth (CTD) sensors, so as to perfect multi-sensor fusion oriented HRI system. Firstly, the authors combined different meta-architectures and different feature extractors, and obtained five object identification algorithms based on CNN. Then, they compared computational cost of feature extractors and weighed the pros and cons of each algorithm from mean detection speed, correlation coefficient and mean class-specific confidence score to confirm that Faster Region-based CNN (R-CNN)_InceptionNet is the best algorithm applicable to hydrothermal vent biological dataset. Finally, they calculated the cognitive accuracy of rimicaris exoculata in dense and sparse areas, which were 88.3% and 95.9% respectively, to analyse the performance of the Faster R-CNN_InceptionNet. Results show that the proposed method can be used in the multi-sensor fusion oriented HRI system for the statistics of dense organisms in complex environments.

Long-term analysis of ocean noise floor in the northeast Pacific Ocean

Characterizing the ocean noise floor is important for tracking long-term acoustic changes and creating environmental regulations to mitigate the effects of human activities on the ocean soundscape. Underwater noise levels have been measured at two sites in the northeast Pacific off the coast of Oregon between 2015 and 2019. The acoustic data were recorded continuously at a sample rate of 64 kHz at 81 m depth and 581 m depth at the continental shelf and slope, respectively. Sequential one-minute power spectral density estimates are computed and used to calculate spectral probability density functions (SPDFs) for every month of the measurement period. The ocean noise floor is then defined as the 5th percentile of the SPDF. Multi-year data are used to compute time series of the ocean noise floor at various frequencies, which are then examined for long-term trends and seasonal patterns. The frequency dependence of the noise floor is analyzed and the results are compared between two measurement sites. Environmental data from surface buoys and Conductivity, Temperature, Depth (CTD) instruments collocated with the hydrophones are used to study how the ocean noise floor changes with wind, temperature, and sound speed. [Work supported by ONR.]

Energetics‐Based Estimation of the Diapycnal Mixing Induced by Internal Tides in the Andaman Sea

AbstractThe Andaman Sea (AS) is characterized by surprisingly weak stratification in its deep basin in drastic contrast to the nearby Bay of Bengal (BoB), presumably due to strong diapycnal mixing fertilized by the dissipation of internal tides. Here, we report on the first estimates of tidal mixing in the AS using idealized numerical simulations resolving the generation and evolution of low‐mode internal tides. The estimation is based on a diagnostic analysis of the energetics of the simulated internal tides of both local and non‐local origins. The results highlight the dominant role of the internal tides generated in the channels along the western boundary island chain in energizing diapycnal mixing in the entire AS. Tidal dissipation and mixing in the deep basin of the AS are found to be highly elevated, with the depth‐integrated dissipation rate and the diapycnal diffusivity being O(10−2–10 ) W m−2 and O(10−3–10−2) m2 s−1, respectively, both of which are 1–2 orders of magnitude larger than those in the deep BoB. These model‐based estimates of tidal mixing are in general consistent with those from in situ conductivity‐temperature‐depth (CTD) measurements using the fine‐scale parameterization. Our results suggest that the oft‐used tidal mixing parameterization considering only local internal tide generation would underestimate the depth‐integrated tidal dissipation in the AS by several orders of magnitude, highlighting therefore the necessity of considering contributions of non‐local internal tides in parameterizing tidal dissipation and mixing in marginal seas like the AS.

Retreat of Humboldt Gletscher, North Greenland, Driven by Undercutting From a Warmer Ocean.

Humboldt Gletscher is a 100‐km wide, slow‐moving glacier in north Greenland which holds a 19‐cm global sea level equivalent. Humboldt has been the fourth largest contributor to sea level rise since 1972 but the cause of its mass loss has not been elucidated. Multi‐beam echo sounding data collected in 2019 indicate a seabed 200 m deeper than previously known. Conductivity temperature depth data reveal the presence of warm water of Atlantic origin at 0°C at the glacier front and a warming of the ocean waters by 0.9 ± 0.1°C since 1962. Using an ocean model, we reconstruct grounded ice undercutting by the ocean, combine it with calculated retreat caused by ice thinning to floatation, and are able to fully explain the observed retreat. Two thirds of the retreat are caused by undercutting of grounded ice, which is a physical process not included in most ice sheet models.

Long-Term Changes in the Water Mass Properties in the Balearic Channels Over the Period 1996–2019

The analysis of a 24-year time series of Conductivity-Temperature-Depth (CTD) casts collected in the Balearic Channels (1996–2019) has allowed detecting and quantifying long-term changes in water mass properties in the Western Mediterranean. For the complete period, the intermediate waters have experienced warming and salting at rates of 1.4°C/100yr and 0.3–0.6/100yr for the Western Intermediate Water, and 1°C/100yr and 0.3–0.4/100yr for the Levantine Intermediate Water. The density of these two water masses has not changed. The deep waters, defined as those denser than 29.1 kg/m3, showed positive trends in temperature, salinity, and density (0.8°C/100yr, 0.2/100yr, and 0.02 kg.m–3/100yr, respectively). The high temporal variability of the upper layer makes the detection of long-term changes more difficult. Nevertheless, combining CTD data with temperature data from the oceanographic station at L’Estartit and simulated data from the NCEP/NCAR reanalysis, it can be established that the Atlantic Water increased its temperature at a rate of 2.1–2.8°C/100yr and likely its salinity at a rate of 0.6/100yr. The water column absorbed heat at a rate equivalent to 1–1.2 W/m2. All these trends are much higher than those reported in previous works (more than double in some cases). The warming of the water column produced an increase in the thermosteric component of sea level. However, this increase was compensated by the decrease in the halosteric component. Besides these changes, other alterations related to the Western Mediterranean Transition have been observed over shorter periods. The temperature and salinity of the intermediate waters increased before the winter of 2004/2005 and then the temperature and salinity of the deep waters increased dramatically in 2005. The density of the deep water reached values unprecedented before 2005. Deep and intermediate waters were uplifted by the presence of such dense deep waters. The arrival of warmer and saltier intermediate waters from the Eastern Mediterranean is also observed, mainly after 2010.

Assimilating In Situ and Remote Sensing Observations in a Highly Variable Estuary–Shelf Model

AbstractTo improve the forecasting performance in dynamically active coastal waters forced by winds, tides, and river discharges in a coupled estuary–shelf model off Hong Kong, a multivariable data assimilation (DA) system using the ensemble optimal interpolation method has been developed and implemented. The system assimilates the conductivity–temperature–depth (CTD) profilers, time series buoy measurement, and remote sensing sea surface temperature (SST) data into a high-resolution estuary–shelf ocean model around Hong Kong. We found that the time window selection associated with the local dynamics and the number of observation samples are two key factors in improving assimilation in the unique estuary–shelf system. DA with a varied assimilation time window that is based on the intratidal variation in the local dynamics can reduce the errors in the estimation of the innovation vector caused by the model–observation mismatch at the analysis time and improve simulation greatly in both the estuary and coastal regions. Statistically, the overall root-mean-square error (RMSE) between the DA forecasts and not-yet-assimilated observations for temperature and salinity has been reduced by 33.0% and 31.9% in the experiment period, respectively. By assimilating higher-resolution remote sensing SST data instead of lower-resolution satellite SST, the RMSE of SST is improved by ~18%. Besides, by assimilating real-time buoy mooring data, the model bias can be continuously corrected both around the buoy location and beyond. The assimilation of the combined buoy, CTD, and SST data can provide an overall improvement of the simulated three-dimensional solution. A dynamics-oriented assimilation scheme is essential for the improvement of model forecasting in the estuary–shelf system under multiple forcings.

A novel hydrographic gridded data set for the northern Antarctic Peninsula

Abstract. The northern Antarctic Peninsula (NAP) is a highly dynamic transitional zone between the subpolar-polar and oceanic-coastal environments, and it is located in an area affected by intense climate change, including intensification and spatial shifts of the westerlies as well as atmospheric and oceanic warming. In the NAP area, the water masses originate mainly from the Bellingshausen and Weddell seas, which create a marked regional dichotomy thermohaline characteristic. Although the NAP area has relatively easy access when compared to other Southern Ocean environments, our understanding of the water masses' distribution and the dynamical processes affecting the variability of the region is still limited. That limitation is closely linked to the sparse data coverage, as is commonly the case in most Southern Ocean environments. This work provides a novel seasonal three-dimensional high-resolution hydrographic gridded data set for the NAP (version 1), namely the NAPv1.0. Hydrographic measurements from 1990 to 2019 comprising data collected by conductivity, temperature, depth (CTD) casts; sensors from the Marine Mammals Exploring the Oceans Pole to Pole (MEOP) consortium; and Argo floats have been optimally interpolated to produce maps of in situ temperature, practical salinity, and dissolved oxygen at ∼ 10 km spatial resolution and 90 depth levels. The water masses and oceanographic features in this regional gridded product are more accurate than other climatologies and state estimate products currently available. The data sets are available in netCDF format at https://doi.org/10.5281/zenodo.4420006 (Dotto et al., 2021). The novel and comprehensive data sets presented here for the NAPv1.0 product are a valuable tool to be used in studies addressing climatological changes in the unique NAP region since they provide accurate initial conditions for ocean models and improve the end of the 20th- and early 21st-century ocean mean-state representation for that area.

In situ measurements of bioluminescence response of Gonyaulax spinifera to various mechanical stimuli

A conspicuous bioluminescence during nighttime was reported in an aquaculture farm in the Cochin estuary due to Gonyaulax spinifera bloom on March 20, 2020. In situ measurements on bioluminescence was carried out during nighttime to quantify the response of G. spinifera to various mechanical stimuli. The bioluminescence intensity (BI) was measured using Glowtracka, an advanced single channel sensor, attached to a Conductivity–Temperature–Depth Profiler. In steady environment, without any external stimuli, the bioluminescence generated due to the movement of fishes and shrimps in the water column was not detected by the sensor. However, stimuli such as a hand splash, oar and swimming movements, and a mixer could generate measurable bioluminescence responses. An abundance of ~ 2.7 × 106 cells L−1 of G. spinifera with exceptionally high chlorophyll a of 25 mg m−3 was recorded. The BI in response to hand splash was recorded as high as 1.6 × 1011 photons cm−2 s−1. Similarly, BI of ~ 1–6 × 1010 photons cm−2 s−1 with a cumulative bioluminescence of ~ 2.51 × 1012 photons cm−2 (for 35 s) was recorded when there is a mixer with a constant force of 494 N/800 rpm min−1. The response of G. spinifera was spontaneous with no time lapse between application of stimuli and the bioluminescence response. Interestingly, in natural environment, application of stimulus for longer time periods (10 min) does not lower the bioluminescence intensity due to the replenishment of water thrusted in by the mixer from surrounding areas. We also demonstrated that the bioluminescence intensity decreases with increase in distance from the source of stimuli (mixer) (av. 1.84 × 1010 photons cm−2 s−1 at 0.2 m to av. 0.05 × 1010 photons cm−2 s−1 at 1 m). The BI was highest in the periphery of the turbulent wake generated by the stimuli (av. 3.1 × 1010 photons cm−2 s−1) compared to the center (av. 1.8 × 1010 photons cm−2 s−1). When the stimuli was applied vertically down, the BI decreased from 0.2 m (0.3 × 1010 photons cm−2 s−1) to 0.5 m (0.10 × 1010 photons cm−2 s−1). Our study demonstrates that the BI of G. spinifera increases with increase in mechanical stimuli and decreases with increase in distance from the stimuli.

Diapycnal mixing across the photic zone of the NE Atlantic

Abstract. Variable physical conditions such as vertical turbulent exchange, internal wave, and mesoscale eddy action affect the availability of light and nutrients for phytoplankton (unicellular algae) growth. It is hypothesized that changes in ocean temperature may affect ocean vertical density stratification, which may hamper vertical exchange. In order to quantify variations in physical conditions in the northeast Atlantic Ocean, we sampled a latitudinal transect along 17 ± 5∘ W between 30 and 63∘ N in summer. A shipborne conductivity–temperature–depth (CTD) instrumented package was used with a custom-made modification of the pump inlet to minimize detrimental effects of ship motions on its data. Thorpe-scale analysis was used to establish turbulence values for the upper 500 m from three to six profiles obtained in a short CTD yo-yo, 3 to 5 h after local sunrise. From south to north, average temperature decreased together with stratification while turbulence values weakly increased or remained constant. Vertical turbulent nutrient fluxes did not vary significantly with stratification and latitude. This apparent lack of correspondence between turbulent mixing and temperature is likely due to internal waves breaking (increased stratification can support more internal waves), acting as a potential feedback mechanism. As this feedback mechanism mediates potential physical environment changes in temperature, global surface ocean warming may not affect the vertical nutrient fluxes to a large degree. We urge modellers to test this deduction as it could imply that the future summer phytoplankton productivity in stratified oligotrophic waters would experience little alterations in nutrient input from deeper waters.

Reverse circulation in Bahía Santa Elena, North Pacific of Costa Rica

This work describes the water circulation of Bahía Santa Elena using velocity fields and hydrographic data obtained from a ship-mounted Acoustic Doppler Current Profiler and a vertical profiler Conductivity–Temperature–Depth sonde, during two campaigns in August and October of 2019. Data were taken in a cross-bay transect in the main channel of the Bahía Santa Elena during a complete tidal cycle. The hydrographic data obtained during the two sampling campaigns suggest a reverse estuarine circulation; tidally averaged values of density and water velocities revealed a dense outflow at the bottom layer in the bay. This is the first description of an inverse estuary circulation pattern in Costa Rica and the second in Central America. Understanding the hydrographic properties of the bay is crucial for undertaking future research projects on water quality and biological communities.

Properties of surface water masses in the Laptev and the East Siberian seas in summer 2018 from in situ and satellite data

Abstract. Variability of surface water masses of the Laptev and the East Siberian seas in August–September 2018 is studied using in situ and satellite data. In situ data were collected during the ARKTIKA-2018 expedition and then complemented with satellite-derived sea surface temperature (SST), salinity (SSS), sea surface height, wind speed, and sea ice concentration. The estimation of SSS fields is challenging in high-latitude regions, and the precision of soil moisture and ocean salinity (SMOS) SSS retrieval is improved by applying a threshold on SSS weekly error. For the first time in this region, the validity of DMI (Danish Meteorological Institute) SST and SMOS SSS products is thoroughly studied using ARKTIKA-2018 expedition continuous thermosalinograph measurements and conductivity–temperature–depth (CTD) casts. They are found to be adequate to describe large surface gradients in this region. Surface gradients and mixing of the river and the sea water in the ice-free and ice-covered areas are described with a special attention to the marginal ice zone at a synoptic scale. We suggest that the freshwater is pushed northward, close to the marginal ice zone (MIZ) and under the sea ice, which is confirmed by the oxygen isotope analysis. The SST-SSS diagram based on satellite estimates shows the possibility of investigating the surface water mass transformation at a synoptic scale and reveals the presence of river water on the shelf of the East Siberian Sea. The Ekman transport is calculated to better understand the pathway of surface water displacement on the shelf and beyond.

Comparison of In Situ and Remote-Sensing Methods to Determine Turbidity and Concentration of Suspended Matter in the Estuary Zone of the Mzymta River, Black Sea

The paper presents the results of a comparison of water turbidity and suspended particulate matter concentration (SPM) obtained from quasi-synchronous in situ and satellite remote-sensing data. Field measurements from a small boat were performed in April and May 2019, in the northeastern part of the Black Sea, in the mouth area of the Mzymta River. The measuring instruments and methods included a turbidity sensor mounted on a CTD (Conductivity, Temperature, Depth), probe, a portable turbidimeter, water sampling for further laboratory analysis and collecting meteorological information from boat and ground-based weather stations. Remote-sensing methods included turbidity and SPM estimation using the C2RCC (Case 2 Regional Coast Color) and Atmospheric correction for OLI ‘lite’ (ACOLITE) ACOLITE processors that were run on Landsat-8 Operational Land Imager (OLI) and Sentinel-2A/2B Multispectral Instrument (MSI) satellite data. The highest correlation between the satellite SPM and the water sampling SPM for the study area in conditions of spring flooding was achieved using C2RCC, but only for measurements undertaken almost synchronously with satellite imaging because of the high mobility of the Mzymta plume. Within the few hours when all the stations were completed, its boundary could shift considerably. The ACOLITE algorithms overestimated by 1.5 times the water sampling SPM in the low value range up to 15 g/m3. For SPM over 20–25 g/m3, a high correlation was observed both with the in situ measurements and the C2RCC results. It was demonstrated that quantitative turbidity and SPM values retrieved from Landsat-8 OLI and Sentinel-2A/2B MSI data can adequately reflect the real situation even using standard retrieval algorithms, not regional ones, provided the best suited algorithm is selected for the study region.

The Horizontal Distribution of Siliceous Planktonic Radiolarian Community in the Eastern Indian Ocean

The plankton radiolarian community was investigated in the spring season during the two-month cruise ‘Shiyan1’ (10 April–13 May 2014) in the Eastern Indian Ocean. This is the first comprehensive plankton tow study to be carried out from 44 sampling stations across the entire area (80.00°–96.10° E, 10.08° N–6.00° S) of the Eastern Indian Ocean. The plankton tow samples were collected from a vertical haul from a depth 200 m to the surface. During the cruise, conductivity–temperature–depth (CTD) measurements were taken of temperature, salinity and chlorophyll a from the surface to 200 m depth. Shannon–Wiener’s diversity index (H’) and the dominance index (Y) were used to analyze community structure. There was a total of 168 plankton species, composed of Acantharia, Phaeodaria, Polycystina, Collodaria and Taxopodida (monospecific—Sticholonche zanclea, Hertwig is the only recognized species). Hence, it included both celestine-based and siliceous organisms, which are also described here for the first time from this region. Total radiolarians ranged from 5 to 5500 ind/m−3, dominated by co-occurrences of Sphaerozoum punctatum and Stichonche zanclea species at the south-equator zone (SEQ)-transect 80° E and equator zone (EQ)-transect Lati-0. The possible environmental variables were tested through RDA analysis; although no result was obtained for the full species dataset, the samples from the equatorial transect related strongly to mixed-layer chlorophyll a concentration and those of a north–south transect to surface silicate concentrations or mixed-layer nitrate were significantly correlated (p < 0.01) to the radiolarian community. Our results indicate that the silicate and chlorophyll-a concentrations are the two major factors affecting the radiolarian distribution along two of the investigated transects (southern equator and equator) in the study area.

Water Mass Characteristic in The Eastern Inflow Region of The Indonesian Throughflow During Leg-3 of Nusa Manggala Expedition (December 2018)

The Nusa Manggala (ENM) leg-3 expedition in 2018 was held on December 6-16 2018. The survey achieved eighteen Conductivity Temperature Depth measurement points and centered at North of West Papua and Halmahera Sea as the inflow area of eastern part of Indonesian Throughflow (ITF). The results of this study prove that Pacific Surface Water is present on the surface up to 70 m with a temperature of 26 °C - 30°C and salinity of 24 psu – 24.5 psu. Then, South Pacific Subtropical Water and South Pacific Intermediate Water are simultaneously 70 m - 250 m and 250 m - 1000 m at the northern of Halmahera Sea. They not exceed till the southern part of Halmahera Sea.

Near-surface western boundary circulation off Northeast Brazil

In the tropical Atlantic Ocean, the North Brazil Undercurrent (NBUC) and the North Brazil Current (NBC) play important roles as near-surface northward paths for the Atlantic meridional overturning circulation. To investigate the meridional evolution of the near-surface western boundary circulation in that region, two surveys were carried out off Northeast Brazil during austral spring 2015 and fall 2017, periods that we show to be representative of typical conditions for the spring and fall seasons. Using conductivity temperature depth oxygen probe and ship-mounted acoustic Doppler current profiler measurements, altimetry data, and numerical reanalysis products, we examine the spatiotemporal variability of the NBUC, providing a comprehensive view of the circulation, including NBUC-NBC transition and the potential impact of mesoscale eddies. The NBUC originating south of 10°S flows equatorward over the continental slope. NBUC mean velocity, transport and vertical extent of the velocity core were higher in spring 2015 (0.81 m s−1, 15.6 Sv, and >400 m) than in fall 2017 (0.65 m s−1, 11.0 Sv, and ~300 m). The upper limit of the NBUC velocity core rose slightly from 160 m at 9°S to 105 m depth at 6°S. This pattern is associated with a northward increase in current velocity from 0.7 m s−1 at 9°S to 1 m s−1 at 5°S. The orographic effect shifts the flow from northeastward south of 7.5°S to northwestward north of 7.5°S, following the shoreline. Besides, altimetry data show that this flow can be locally influenced by mesoscale activity. Farther north, at ~4.8°S, data obtained in fall 2017 show that the central branch of the South Equatorial Current (cSEC) enters into the western boundary system where it coalesces with the NBUC to form the NBC, flowing toward the Caribbean Sea. Finally, in fall, the NBUC retroflection does not feed the South Equatorial Undercurrent (SEUC), which, instead originates from retroflections of the cSEC and the equatorial branch of the South Equatorial Current (eSEC).