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Photosynthesis is the most important bioenergy conversion process on Earth. Capturing instantaneous changes in in situ photosynthesis in open ocean ecosystems remains a major challenge. In this study, fast repetition rate fluorometry (FRRF), which can obtain nondestructive, real-time and in situ estimates of photosynthetic parameters, was used for the first time to continuously observe the spatial variation in in situ photosynthetic parameters in the eastern Indian Ocean (EIO). We further formulated new insights regarding abiotic and biotic factors of potential importance in determining photosynthetic performance. First, we found that the distributions of micro/nano- and picophytoplankton were opposite under the control of nutrient concentrations. Micro/nanophytoplankton had higher cell abundances in the nearshore and upwelling regions, while picophytoplankton had higher abundances in the open ocean, and Prochlorococcus was the dominant group. Second, based on the FRRF technology, we obtained the high-precision and high-density vertical profile map of photosynthetic parameters in the euphotic layer. It was observed that values of the maximum photochemical efficiency (Fv/Fm; 0.14–0.55, unitless) and the functional absorption cross-section of PSII (σ PSII ; 1.71–4.90 nm2 RCII−1) increased with increasing depth, while high values of the photosynthetic electron transfer rates (ETR RCII ; 0.0019–17.0292 mol e− mol RCII−1 s−1) and the nonphotochemical quenching (NPQ NSV ; 0.35–7.26, unitless) occurred in the shallow 50 m layer, and the values decreased as the depth increased. Finally, we discussed limiting factors that regulated the distribution of photosynthetic parameters and concluded that optical properties varied significantly with changes in the ocean physico-chemical parameters and taxonomic composition of phytoplankton assemblages in the EIO. Picophytoplankton (especially cyanobacteria), rather than the micro/nanophytoplankton community, was the dominant factor influencing photosynthesis. Among abiotic factors, photosynthetically active radiation (PAR) was the proximal limiting factor affecting photosynthetic efficiency, followed by temperature and dissolved inorganic nitrogen (DIN). Consequently, phytoplankton photosynthetic parameters exhibited great variability, allowing rapid responses to environmental condition changes. In this study, we established the basis for detecting future changes in primary production in this oligotrophic area. • In situ photosynthetic parameters in the eastern Indian Ocean are presented. • Fv/Fm increases with increasing seawater depth. • Picophytoplankton is the dominant factor influencing photosynthesis. • Photosynthetically active radiation is the proximal limiting factor affecting photosynthesis. [ABSTRACT FROM AUTHOR]

The forcing and mechanisms governing sea surface temperature (SST) variations in the Indo-Pacific Warm Pool (IPWP) are complex. Insight into the full spectrum of IPWP climate dynamics, however, is limited by the spatial and temporal coverage of the climate data. In particular, the relationships among the changes in the SSTs, the precipitation patterns mainly associated with rainfalls from Asian Monsoons (AM), and the Indian Ocean Dipole (IOD) are poorly understood. To help us further assess the climate linkages, we have reconstructed a more spatial SST pattern during the Holocene by using multiple SST proxies (alkenone unsaturation index U 37 k ' and Mg/Ca of planktic foraminifer) in the Eastern Tropical Indian Ocean (ETIO), the western margin of the IPWP based on three sedimentary cores from NW offshore of Sumatra, and offshore of Sumatra and Java (BS24, SO139-74KL, and SO184-100430). Stable hydrogen and carbon isotope records of terrestrial plant waxes from a nearby marine sediment core SO189-144KL and geochemical tracers measured from the coral reefs within the Mentawai Islands in the ETIO are used here as AM driven precipitation and IOD records in our data synthesis. Not surprisingly, our synthesis suggests that insolation plays a major role that has been responsible for the increased SSTs in ETIO since the early Holocene, while other mechanisms remain effective in determining the timing of our reconstructed SST variations. In particular, our SST pattern shares less similarity with that of coral Sr/Ca SST and is decoupled from the coral IOD events in the mid-Holocene. We interpret that our reconstructed ETIO SSTs are driven dominantly by the solar forcing, but are also affected by other internal climate mechanisms such as the local shifts in AM-controlled upwelling and precipitation, episodic reductions in the flow of warm western Pacific surface water into the Indian Ocean due to increased precipitation over the Indonesian archipelago, and long-term ENSO or IOD-like climate change. [ABSTRACT FROM AUTHOR]

Abstract Statistical analysis of rainfall data from 2005 to 2015 showed that atmospheric deposition supplied large amount of dissolved inorganic nitrogen (38–155 mg·m−2·month−1) in N-deficient South China Sea and Eastern Indian Ocean. To understand marine ecosystem responses to increasing nutrient disturbances, we implemented field mesocosm experiments to study phytoplankton community structure and biodiversity responses to nutrient treatments with nitrate, phosphate and iron across tropical seas. Our results showed that DIN supply would change phytoplankton community structure and stimulated the regime shift from cyanobacteria to diatoms (relative dominance R > 0). Phytoplankton communities were dominated by diatoms (relative abundance >50%) accompanied by high chlorophyll a content with 1.58–39.27 μg·L−1 in DIN-added cultures, whereas cyanobacteria dominated communities (relative abundance >60%) with low biomass of 0.12–0.18 μg·L−1 in undisturbed cultures. Simultaneously increased DIN loading from atmospheric deposition would decrease ecological diversity of tropical seas owing to species competition and succession (Shannon diversity H′ decreased to <1). Graphical abstract Unlabelled Image Highlights • Wet deposition supplied large amount of DIN in two tropical seas. • Regime shift from cyanobacteria to diatoms was stimulated following DIN supply. • The study indicated that phytoplankton diversity of two tropical seas would be reduced following exogenous nutrients supply. • Management practices should be advocated to protect ecological diversity. [ABSTRACT FROM AUTHOR]

Benthic foraminiferal assemblages from a ~300 m deep core from an outer carbonate-ramp site off Western Australia (International Ocean Discovery Program Core U1460A) were examined to reconstruct the paleoceanographic evolution of the Carnarvon Ramp and the warm surficial Leeuwin Current (LC) for the last 3.54 Ma. Of the identified 179 benthic foraminiferal species, occurrences of the 15 most abundant taxa were assessed using Q-mode Cluster Analysis and Non-Metric Dimensional Scaling. Diversity, equitability, planktonic/benthic index, microhabitat preference, and sedimentary parameters such as lithology and sponge spicule content were analyzed to gather information about past intermediate- and surface-water circulation. Relative abundances of infaunal and epifaunal species were applied to indicate changes in organic-matter supply and oxygenation at the sea floor. Influence of upwelling was recognized by a high infaunal species ratio, with dominance by Uvigerina peregrina , Lagena annellatrachia and Trifarina bradyi . Epifaunal species such as Hanzawaia nipponica and Hyalinea florenceae gradually became more abundant around 1.14 Ma, indicating increased ventilation and establishment of the paleo-LC. A more substantial change was initiated by 0.91 Ma as marked by key species Spirorutilus carinatus and Rotorbinella sp., together with increased faunal diversity, benthic foraminiferal accumulation rates, and evidence for suspension feeding sponges. With the LC flow suppressing upwelling, and better ventilated waters entering the shelf, the environment favored epifaunal agglutinates, rotalids, and miliolids, while buliminids decreased. Under high-flow conditions of the LC, sponge spicules and skeletal carbonate production reached an optimum at ~0.6 Ma before returning to modern conditions. Supported by these observations, we propose the following paleoceanographic evolution of the Carnarvon Ramp: During the late Pliocene to mid Pleistocene (3.54–0.91 Ma) conditions of deep-water upwelling from the Western Australian Current and Indian Ocean Gyre indicate the absence of the capping LC on the outer carbonate ramp. A transitional phase started in the mid Pleistocene (1.14–0.61 Ma). The paleo-LC triggered gradual oxygenation at the sediment-water interface, which coincided with an increase in carbonate sedimentation rates, and waning sea-surface productivity. During a third phase, mid Pleistocene to present (0.91–0 Ma), the LC's intensity and flow rates peaked at ~0.6 Ma. Benthic foraminiferal accumulation rates reached a high, then decreased to present-day rates. For short periods, sea-surface productivity was moderately enhanced, likely due to fluctuating LC persistence or landward shift during glacial maxima. [ABSTRACT FROM AUTHOR]

With sea level rise in the early Holocene, the warm and low-salinity sea water from the Java Sea was transported into the eastern Indian Ocean after the opening of the Sunda Strait. However, the impact of this process on sediment provenance and paleoproductivity is rarely known. In this study, we analyzed the grain size, Sr-Nd-Pb isotopic compositions and biogenic elements of core CJ01-185 (1538 m water depth) in the eastern Indian Ocean off the Sunda Strait. Our new results reveal a large volcanic eruption in the west of Java at 2.1 ka, and the Krakatau eruption at 1883 CE. On the basis of the Pb isotopic compositions, 87 Sr/ 86 Sr ratios, and ε Nd values, we infer that sediments from core CJ01-185 were derived primarily from Java Island. There is a distinct change in Sr-Nd-Pb isotopic compositions of the sediments from the last glacial period to the Holocene, indicating less radiogenic Nd isotopes from the Java Sea into the eastern Indian Ocean after the opening of the Sunda Strait at ∼10 ka. Moreover, the sedimentation rate increased significantly from 6.5 cm kyr −1 during the last glacial period to 20 cm kyr −1 in the Holocene. The input of additional terrigenous nutrients from the Java Sea induced paleoproductivity with higher TOC and TN concentrations after the opening of the Sunda Strait. Our results thus suggest that the paleoproductivity was mainly influenced by terrigenous materials input in the Holocene, other than southeast monsoon or upwelling in the last glacial period. [ABSTRACT FROM AUTHOR]

Under the background of global warming, the area extent of the oligotrophic tropical oceans has growing due to increased water-column stratification over the past decades. Picophytoplankton is usually the most dominant phytoplankton group in oligotrophic tropical oceans and substantially contribute to carbon biomass and primary production three. Understanding how vertical stratification governs the community structure of picophytoplankton communities in oligotrophic tropical oceans is important for comprehensively understanding the plankton ecology and biogeochemical cycle in these areas. In this study, the distribution of the picophytoplankton communities in the eastern Indian Ocean (EIO) was investigated during a period of thermal stratification in the spring of 2021. Prochlorococcus contributed most (54.9%) to picophytoplankton carbon biomass, followed by picoeukaryotes (38.5%) and Synechococcus (6.6%). Vertically, the three picophytoplankton groups showed quite different distribution pattern: the abundance of Synechococcus was highest in the surface layer, while Prochlorococcus and picoeukaryotes were usually located between 50 m and 100 m. The relationship between the abundance of picophytoplankton and environmental factors was analyzed, and the results revealed that picophytoplankton distribution was strongly correlated with the degree of vertical stratification of the water column. The density of Synechococcus was higher in strongly stratified waters, while Prochlorococcus was more abundant in regions of weaker stratification. This is mainly attributed to variation of physicochemical parameters such as nutrient structures and temperature resulted from water column stratification. Understanding the distribution patterns of these organisms and their relationship with stratification in the oligotrophic EIO is essential for comprehensive understanding on oligotrophic tropical ecosystem with increasing stratification in future. • Picophytoplankton community in the eastern Indian Ocean is presented. • Prochlorococcus contributes more biomass to the open ocean than the Bay of Bengal. • Picophytoplankton distribution is correlated with the vertical stratification index. • Values of vertical stratification index increase with increasing latitude. [ABSTRACT FROM AUTHOR]

We report high-resolution paleoclimatic reconstructions using isotope, alkenone sea surface temperature (SST), productivity, and terrigenous proxy variations over the past 35 kyr from core SO184-10043 (07°18.57′S, 105°03.53′E) in the eastern tropical Indian Ocean offshore of southernmost (S) Sumatra. The core was retrieved at 2171 m water depth, from the monsoon driven seasonal upwelling area off southernmost Sumatra. Our paleoclimatic reconstructions show that an enhanced marine productivity was closely linked to strengthening of coastal upwelling during the increases of boreal summer insolation and associated southeastern (SE) monsoon strength, with pronounced semi-precession cycles (∼11 kyr). In contrast, our record of alkenone SSTs shows glacial to interglacial, a sea-level variation, but this fluctuates at dominant precession cycles (∼21 kyr). We also observed four SST “plateaus” with relatively warmer hydrographic stability in our records over the past 35 kyr: 32-27 ka, 24-20 ka, 17-12 ka, and 9-4 ka. The time scale of each plateau is ∼4-6 kyr. Our study indicates that since the last glacial maximum (LGM), the sea level rise has been responsible for the Sunda shelf flooding and the Indonesian Throughflow (ITF) from the South China Sea (SCS) to the eastern Indian Ocean, leaving a dominant fingerprint of a glacial to interglacial increase in SSTs since ∼9.5 ka We further argue that the a most recent centennial to millennial scale cooling at ∼3.5 ka observed in our SST record is a widespread event in the tropical Pacific, which has implications for advancing our understanding of the climate dynamics of the tropical oceans at these time scales. [ABSTRACT FROM AUTHOR]

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Oxygen minimum zones (OMZs) are geographically widespread phenomena among world seas, while knowledge regarding planktonic ciliate (microzooplankton) community structure in the OMZ is lacking. In this study, we analyzed community structure of planktonic ciliates in the OMZ waters of the Bay of Bengal, eastern Indian Ocean. Both hydrological characteristics (temperature, salinity, chlorophyll a in vivo fluorescence (Chl a) and dissolved oxygen (DO)) and planktonic ciliate community showed obvious variations in vertical profile. An obvious higher ciliate abundance and biomass occurred in the thermocline zones (surface to 100 m) than in the OMZs (150–800 m). For aloricate ciliates, abundance proportions of small (10–20 μm) and large (> 30 μm) size-fractions showed increase and decrease characteristics from thermocline zones to OMZs, respectively. With regard to loricate ciliates (tintinnids), more species richness and higher abundance occurred in the thermocline zones than in the OMZs. Four vertical distribution patterns were classified for tintinnids. Three species only occurred in the OMZs. Three-dimensional representation of tintinnid lorica length, lorica oral diameter and abundance proportion revealed that small-sized species in thermocline zones decreased more sharply to OMZs compared to large-sized species. Relationship between environmental variables and ciliate abundance showed that temperature, DO and Chl a were main factors determining ciliate abundance. Our results provide fundamental data for better understanding on the ecological role of planktonic ciliates in the thermocline zones and OMZs of the Indian Ocean. • An obvious higher ciliate abundance and biomass occurred in the thermocline zones (3-100 m) than in the OMZs (150–800 m); • More tintinnid species richness and higher abundance occurred in the thermocline zones than in the OMZs; • Temperature and DO, and Chl a were main factors determining ciliate composition. [ABSTRACT FROM AUTHOR]

Sea surface planktonic assemblages were sampled using environmental DNA at 1.5° latitudinal increments (from 39.5 to 11.5°S) following the 110°E meridian in the Eastern Indian Ocean to reveal factors structuring eukaryotic diversity. Metabarcoding the v4 region of the 18S rRNA gene revealed a eukaryotic assemblage over-dominated by amplicon sequence variants representing just four taxa that comprised 50% of reads, which were from copepods. Eukaryotes were predominantly heterotrophs and mixotrophs, with the presence of individual taxa responding to nutrient and temperature/oxygen gradients. Assemblage richness and diversity was greater at the northern end of the transect, but this diversity change occurred mostly around the sub-tropical front (∼35°S) with an unusually high degree of uniformity and non-monotonic progression in the alpha and beta diversity over the majority of the transect despite extending through distinct water masses, i.e., Indonesian Throughflow with tropical, low salinity waters, Subtropical Surface Water of high salinity waters and varying temperature, and Sub-Antarctic Mode Water with cooler and low salinity water. However, piecewise linear models of environmental and biotic data and fuzzy c-means clustering (with k = 2, 3 or 4) identified some discontinuities in the transect that were clumped at the southern end of the transect, although these cluster boundaries lacked consensus, being often poorly defined between the different data types. This eDNA data was largely concordant with morphological analyses, that also found the eukaryotic assemblage became largely stable northwards of the southernmost sampling points. These results contrast to other observations of diversity increasing monotonically toward the equator; i.e., eukaryotic communities were contrary to the "latitudinal diversity gradient hypothesis". This accords with observations that extremely oligotrophic conditions can stabilise planktonic assemblages, despite significant latitudinal differences in temperature of as much as 16 °C. • Broad north to south decrease in eukaryote planktonic biodiversity. • Large oligotrophic region lacks biodiversity gradient despite temperature gradient. • Copepods, heterotrophs and mixotrophs dominate metabarcodes from surface water. [ABSTRACT FROM AUTHOR]

This study reported geochemical and Sr–Nd isotopic variation of fine-grained mineral components in a deep-sea piston core G6-4, south of the Sumba Island, Eastern Indian Ocean. The new data enables us to characterize dust contributions from Australian continent and volcanic eruption history in the adjacent volcanic arc over approximately the last 300,000 years. The geochemical and isotopic features of the fine-grained components (< 20 μm) in Core G6-4 are broadly consistent with mixing between aeolian dusts from the Australian continent and volcanic ashes of the Sunda arcs. Simple two-component modeling shows that the majority of the core samples probably are composed of 50–80% aeolian dusts derived from the Australian continent. Sr–Nd isotopic ratios are fairly uniform for samples older than 180 ka, in contrast with those of the samples younger than 180 ka, which are much more variable with three anomalies characterized by higher 143 Nd/ 144 Nd and lower 87 Sr/ 86 Sr values recognized. We interpret the three anomalies in the last 80 ka as recording episodes of eruptions from nearby volcanoes. Thus, it is likely that volcanism become more active in the last 80 ka. Excluding these anomalies, the average 143 Nd/ 144 Nd was generally lower, and average 87 Sr/ 86 Sr was higher in the post-180 ka section than in the older section, which might be associated with increased Australian continental aridity toward more recent time. [ABSTRACT FROM AUTHOR]

In this study we reconstruct sea surface temperatures (SSTs) using two lipid-based biomarker proxies (alkenone unsaturation index and TEX86 index based on glycerol dibiphytanyl glycerol tetraethers) in 36 surface sediment samples from the Indonesian continental margin off west Sumatra and south of Java and the Lesser Sunda Islands. Comparison of measured temperatures (World Ocean Atlas 09) to reconstructed temperatures suggests that SST estimates based on reflect the SE monsoon SST in the upwelling area south of Java and the Lesser Sunda Islands. Estimates based on TEX86 using the calibration for temperatures >20°C ( ) are up to 2°C lower than -based SSTs. This offset is possibly related to either one or a combination of two factors: (i) the depth habitats of the source organisms and (ii) different seasonal production and/or seasonality of export associated with phytoplankton blooming triggered by primary productivity. In the non-upwelling area off west Sumatra, the alkenone-based SSTs are cooler than measured temperatures during the entire year, likely reflecting the limitations of the proxy beyond 28°C, while reconstructed temperatures based on are consistent with mean annual SST. [ABSTRACT FROM AUTHOR]

Abstract: Assemblages of organic-walled dinoflagellate cysts (dinocysts) from 116 marine surface samples have been analysed to assess the relationship between the spatial distribution of dinocysts and modern local environmental conditions [e.g. sea surface temperature (SST), sea surface salinity (SSS), productivity] in the eastern Indian Ocean. Results from the percentage analysis and statistical methods such as multivariate ordination analysis and end-member modelling, indicate the existence of three distinct environmental and oceanographic regions in the study area. Region 1 is located in western and eastern Indonesia and controlled by high SSTs and a low nutrient content of the surface waters. The Indonesian Throughflow (ITF) region (Region 2) is dominated by heterotrophic dinocyst species reflecting the region's high productivity. Region 3 is encompassing the area offshore north-west and west Australia which is characterised by the water masses of the Leeuwin Current, a saline and nutrient depleted southward current featuring energetic eddies. [Copyright &y& Elsevier]

Abstract: On a seasonal cycle, the phytoplankton biomass off the North West Cape region of Australia surrounding Ningaloo Reef increased during the austral autumn and peaked in winter. In this study, historical field data sets, satellite-derived ocean color observations and output from a data-assimilating numerical ocean model are used to investigate physical mechanisms that influence the seasonal variability in phytoplankton biomass off this coast. The mixed layer depth (MLD) off Ningaloo deepened to ∼100m in autumn, partly due to the acceleration of the Leeuwin Current, which induced an increase in nutrient concentrations and is the key mechanism responsible for the seasonal enhancement in chlorophyll a concentrations observed in the waters off Ningaloo Reef. By comparing the MLDs and the critical depth it is suggested that while average irradiance throughout the MLD was low, this had less impact on net growth than grazing. [Copyright &y& Elsevier]

Abstract: Oceanic response to Madden–Julian Oscillations (MJOs) is studied with satellite data, mooring observations, and reanalysis products to demonstrate that oceanic intraseasonal variabilities are a direct response to the atmospheric intraseasonal forcing. They propagate eastward to the Sumatran coast and southward along the coast to the southeastern Indian Ocean (SEIO) and the maritime continent, as coastal Kelvin waves. MJOs contribute to about 20% of the intraseasonal variabilities in the SEIO and the maritime continent. In addition, MJOs reduce the annual mean Indonesian Throughflow (ITF) and the associated westward temperature advection. However, MJOs only have slight influences on the peak ITF in boreal summer. The importance of INSTANT data is obvious not only for understanding of ITF but also for improving ocean reanalysis and should eventually lead to improved predictive understanding of phenomena such as MJOs. [Copyright &y& Elsevier]

• A cruise was conducted along 88°E in the EIO during the intermonsoon period. • Phytoplankton growth and mortality rates were studied using dilution experiments. • Multiple phytoplankton blooms were observed along the meridional transect. • Island effect near the Maldives or a cyclonic eddy induced phytoplankton blooms. • EIO is not a stable oligotrophic marine ecosystem as affected by physical events. The Eastern Indian Ocean (EIO) is considered to exhibit low chlorophyll a (Chl a) concentrations. However, high Chl a levels were occasionally reported and attributed to physical events that frequently occur in this area. Because of limited in situ observations, the underlying formation mechanism of high Chl a through the responses of components within the ecosystem is not fully understood. In this study, we conducted a meridional cruise along the 88°E longitude in the EIO (16.5°N to 20°S) during the fall intermonsoon period (November 8–28th, 2018). For the first time, surface phytoplankton growth and mortality rates were measured using dilution experiments. Sensitive nutrient analysis was also applied to determine the nanomolar nutrient concentrations. Our cruise captured high surface Chl a concentrations (∼298 ng L−1 at 10 m) in the Bay of Bengal, at the Equator, and in the southern EIO, even under consistent nitrogen depletion (dissolved inorganic nitrogen of ∼58 nM). All the blooms observed were in the decay phase, as suggested by the satellite observations of Chl a distributions and the low or negative phytoplankton net growth rate. However, the phytoplankton growth rate was enhanced by ∼4.9 times due to nutrient enrichment in dilution experiments, indicating that the bloom was initiated by nutrient enrichment and ceased by the severe nutrient limitation. The shipboard acoustic Doppler current profiler measurements and satellite observations of sea surface winds, surface currents, and sea surface height collectively suggested that phytoplankton blooms were triggered by nutrient enrichments that were possibly supplied by either the island effect near the Maldives or a cyclonic eddy in the southern EIO. In the former case, the Chl a -rich water generated near the islands was then advected eastward toward the study area by surface currents associated with the Wyrtki Jet. Our study indicated that the EIO was not a stable oligotrophic marine ecosystem with consistently low Chl a concentrations, and was quite dynamic as affected by unique regional physical events. [ABSTRACT FROM AUTHOR]

Abstract: Surface seawater samples were collected along the track of the R/V Hakuho-Maru cruise (KH-96-5) from Tokyo to the Southern Ocean. The 137Cs activities were determined for the surface waters in the western North Pacific Ocean, the Sulu and Indonesian Seas, the eastern Indian Ocean, the Bay of Bengal, the Andaman Sea, and the South China Sea. The 137Cs activities showed a wide variation with values ranging from 1.1 Bq m−3 in the Antarctic Circumpolar Region of the Southern Ocean to 3 Bq m−3 in the western North Pacific Ocean and the South China Sea. The latitudinal distributions of 137Cs activity were not reflective of that of the integrated deposition density of atmospheric global fallout. The removal rates of 137Cs from the surface waters were roughly estimated from the two data sets of Miyake et al. [Miyake Y, Saruhashi K, Sugimura Y, Kanazawa T, Hirose K. Contents of 137Cs, plutonium and americium isotopes in the Southern Ocean waters. Pap Meteorol Geophys 1988;39:95–113] and this study to be 0.016 yr−1 in the Sulu and Indonesian Seas, 0.033 yr−1 in the Bay of Bengal and Andaman Sea, and 0.029 yr−1 in the South China Sea. These values were much lower than that in the coastal surface water of the western Northwest Pacific Ocean. This was likely due to less horizontal and vertical mixing of water masses and less scavenging. 239+240Pu activities and 240Pu/ 239Pu atom ratios were also determined for the surface waters in the western North Pacific Ocean, the Sulu and Indonesian Seas and the South China Sea. The 240Pu/ 239Pu atom ratios ranged from 0.199±0.026 to 0.248±0.027 on average, and were significantly higher than the global stratospheric fallout ratio of 0.18. The contributions of the North Pacific Proving Grounds close-in fallout Pu were estimated to be 20% for the western North Pacific Ocean, 39% for the Sulu and Indonesian Seas and 42% for the South China Sea by using the two end-member mixing model. The higher 240Pu/ 239Pu atom ratios could be attributed to close-in fallout Pu delivered from the Enewetak and Bikini Atolls by ocean currents of branches of the North Equatorial Current to the Southeast Asian seas. [Copyright &y& Elsevier]

Abstract: The Gascoyne continental shelf extends from North West Cape (21.3°S) to Shark Bay (26.5°S), Western Australia, and is dominated by the Leeuwin Current (LC), an anomalous oligotrophic eastern boundary current that transports tropical water poleward and generates large-scale downwelling. In summer, shelf dynamics are influenced by wind-generated countercurrents (the Ningaloo and Capes Currents) that flow equatorward and are associated with localized upwelling. Here, we examined phytoplankton responses (biomass levels and distribution, primary production rates, species composition) to small-scale upwelling in this ecosystem which is generally dominated by a poleward current, using physical, chemical and biological oceanographic field data collected during a two-week research cruise through the Gascoyne region in the early austral summer of 2000. We found LC and offshore waters to be associated with low phytoplankton biomass (21.4±6.9s.d. mg chl a m−2) and low primary production (110–530mg Cm−2 d−1); surface (&#60;50m) waters were nitrate-depleted (generally&#60;0.1μM), with a strong nutricline present at the base of the mixed layer. Upwelling associated with the Ningaloo Current (NC) sourced water from this nutricline, and in conjunction with mixing generated by seaward offshoots, resulted in nitrate levels of up to 2–6μM within the euphotic zone. Biomass in these NC waters (35.9±11.6mg chl a m−2) was significantly higher than in LC/offshore regions, with primary production in the range of 840–1310mg Cm−2 d−1. Capes Current (CC) water was also highly productive (990mg Cm−2 d−1), and with low silicate levels and a high proportion of centric diatoms, was typical of an aging upwelled water mass. Thus, the dominance of the oligotrophic LC along the Gascoyne region can be offset by these equatorward countercurrents, although we hypothesize that the biological impact of any upwelling on the inner shelf would be a function of: (a) the depth of the LC''s nutrient-depleted mixed layer, (b) the strength and duration of upwelling-favorable winds (i.e. the intensity of upwelling), and (c) geographical location, primarily with respect to the width of the continental shelf and resultant proximity of upwelling flows to deep nutrient pools. [Copyright &y& Elsevier]

Abstract: Relative abundances of benthic foraminifera in 57 core tops collected within a depth-range between 700 and 4335 m below sea level [b.s.l.] from the eastern Indian Ocean (mostly between Australia and Indonesia) were investigated quantitatively using Detrended Correspondence Analysis (DCA) to analyse species spatial-distribution. Canonical Correspondence Analysis (CCA) and correlation matrices were used to evaluate the relationships between the species distribution and environmental variables (temperature, salinity, dissolved oxygen, nitrate and phosphate concentrations, carbon-flux rate). Seven key-species proved useful for distinguishing environmental parameters. Two groups of species are identified by means of the first DCA ordination axis. The first group increases in relative abundances with depth and includes three taxa: Oridorsalis tener umbonatus, Epistominella exigua and Pyrgo murrhina. These three taxa prefer a cold (&#60; 3 °C) and well-oxygenated (> 3.5 ml/l) environment, with low carbon flux to the sea floor (&#60; 3 g C m− 2 year− 1). O. tener umbonatus and P. murrhina tend to indicate reduced food availability, whereas E. exigua may indicate periodic delivery (seasonal) of organic matter to the sea floor. The second group includes Nummoloculina irregularis and Cibicidoides pseudoungerianus, typical of upper-bathyal depths. C. pseudoungerianus is correlated with a warm (> 2.5 °C) environment characterised by high carbon-flux rate (> 2.5 g C m− 2 year− 1). N. irregularis is associated with high dissolved-oxygen concentrations (> 3 ml/l) and its distribution is limited to south of 20 °S. In this area, the contemporary presence of the low salinity and well oxygenated Antarctic Intermediate Water and low primary productivity at the sea-surface (which causes low oxygen consumption at the sea floor) create the ideal conditions for this species. The second ordination-axis scores identify another taxon, Uvigerina proboscidea. The distribution of this species is mainly limited to low latitudes (north of 25°S), where carbon flux rate is high (> 3.5 g C m− 2 year− 1), due to higher primary productivity levels at the sea surface, and oxygen levels are low (&#60; 3 ml/l) due to the organic matter oxidation and the presence of oxygen-depleted Indonesian Intermediate Water and North Indian Intermediate Water. [Copyright &y& Elsevier]

[Display omitted] • Atmospheric deposition and upwelling increase nitrogen budget of tropical seas. • Nitrogen inputs enhanced carbon flux through promoting primary productivity. • The competition for nutrients caused the regime shift from cyanobacteria to diatom. • The C/N ratio would alter along with changed nutrient status and community structure. Exogenous nutrient inputs increasingly impact the structure and function of marine food weds through abiotic and biotic ways. The influence of nutrient inputs from atmospheric depositions and intermediate seawater supply on the phytoplankton growth and stoichiometry was studied in the Eastern Indian Ocean through multifactorial mesocosm experiments. Our results indicate that nitrogen inputs from atmospheric deposition and hydrodynamic processes did not only promote chlorophyll a biomass (5.3–58.52 μg·L-1) to increase the photosynthetically fixed carbon content but also generate a regime shift from picophytoplankton cyanobacteria to microphytoplankton diatom indicated by high relative abundance of microphytoplankton (>50%). These alternations of nutrient status and phytoplankton community structure reduced biodiversity of marine ecosystems (N-cultures, 0.18 to 0.56; control, 0.63) and generated the attendant C/N ratio of 5.6 ± 0.2 in cultures with nitrogen addition that was significantly lower than cultures with only phosphorus or iron addition. These results provide a thorough insight about the influence of nutrient inputs on marine biogeochemical cycle. Considering the universality of phytoplankton community alteration induced by exogenous nutrient inputs, integrating community compositions with phytoplankton stoichiometry may be helpful for clarify mechanisms of environment events impacting marine food weds. [ABSTRACT FROM AUTHOR]

• High p CO 2 and P interactively increased the abundances of Syn, Pro and PEuks. • Rising p CO 2 alone decreased the abundances of Syn, Pro and PEuks. • Elevated p CO 2 alone facilitated the NPQ NSV process significantly. • There was a strong coupling of picophytoplankton and the charge separation rates. • P enrichment masked the negative effects of OA on picophytoplankton and photosynthesis. Dynamics of picophytoplankton and photosynthesis will be inevitably impacted by changing marine environment, such as ocean acidification and nutrient supply, but related studies are very scarce. Here we cultured the picophytoplankton-dominated surface water of the oligotrophic Eastern Indian Ocean (EIO; R/V Shiyan-3 , 20 March to 18 May 2019) at two levels of p CO 2 (400 and 1000 ppm) and phosphate (0.05 and 1.50 µM) to investigate the interactive effects of elevated p CO 2 and phosphate (P) on the dynamics of picophytoplankton and photosynthetic properties. High p CO 2 and P levels interactively increased the abundances of Synechococcus , Prochlorococcus and picoeukaryotes by 33%, 18%, and 21%, respectively, of which high P level had a major promoting effect. Conversely, rising p CO 2 alone decreased their abundances by 9%, 32%, and 46%, respectively. For the photophysiological responses in relation to the combination of high p CO 2 and P levels, there was an increase in the maximum (F v /F m) and effective (F q '/F m ') photochemical efficiency, the electron transfer rates (ETR RCII) and the charge separation rates (JV PSII , an indicator of primary production), but a decrease in the non-photochemical quenching (NPQ NSV). Elevated p CO 2 alone facilitated the NPQ NSV process significantly, ultimately leading to reduced light use efficiency (e.g., F v /F m , F q '/F m ' and ETR RCII) and primary production (JV PSII). There was a strong coupling of picophytoplankton and JV PSII , suggesting the EIO primary productivity was potentially controlled by picophytoplankton. Overall, our results indicate that the negative effects caused by ocean acidification may be masked or outweighted by the role that P availability plays in regulating growth and metabolism in this oligotrophic ecosystem. [ABSTRACT FROM AUTHOR]

Microplastics (MPs) have been documented in almost all marine environments, including coastal regions, the open ocean, and the deep sea. However, relatively little knowledge was available about MP pollution in the open ocean, especially the Indian Ocean. We conducted field observations at 36 stations in the Eastern Indian Ocean (EIO), using a typical manta trawl with a mesh size of 330 μm for surface water sampling. Ours is the first study to obtained comprehensive and comparable baseline data about MPs in the EIO, including abundance, spatial distribution and characteristics. Abundance of MPs in the EIO varied from 0.01 items m−2 to 4.53 items m−2, with an average concentration of 0.34 ± 0.80 item m−2. The mean concentration of MPs in the Bay of Bengal (BoB) was 2.04 ± 2.26 items m−2 and 0.16 ± 0.17 items m−2 in the open ocean of the EIO. These results illustrate the high spatial heterogeneity of MPs distribution. Micro-FTIR analysis of polymer composition showed that the vast majority of MPs consisted of polypropylene (PP, 51.11%) and polyethylene (PE, 20.07%). Our data show that MP pollution in the EIO, whether in the epeiric sea or the open ocean, is among the highest of the world's oceans. The BoB is likely to become a MP hotspot due to the vast input of land-based plastics and the presence of multiscale recirculation gyres. These results are absolutely thought provoking: The EIO needs more attention on MPs. Unlabelled Image • MP data based on typical manta trawl are available for the first time in the EIO. • MP pollution in the EIO is among the highest of the world's oceans, with an average 0.34 ± 0.80 items m−2. • Monsoon climate plays an important role in the transmission of MPs between the EIO and SCS. • The EIO needs more attention with respect to MP pollution. [ABSTRACT FROM AUTHOR]