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As the only low‐latitude connection along the return branch of the Great Conveyor Belt, the Indonesian Throughflow (ITF) plays an important role in the large‐scale ocean–atmosphere interaction in the tropical region. However, the heat transport processes of the ITF along the outflow pathway in the eastern Indian Ocean over the recent geologic period is still debated. In this study, by using Mg/Ca ratios of the surface‐dwelling and thermocline‐dwelling planktonic foraminifera Globigerinoides ruber and Pulleniatina obliquiloculata, we reconstruct surface and thermocline water temperatures and thermocline structure at two sites in the main outflow path of the ITF in the eastern Indian Ocean for the last 160 kyr, and compare these new data with those from two cores in the central Indo‐Pacific Warm Pool. Our results show that, on the orbital time scale, the thermocline structure changes above the equatorial 90°E Ridge mimic those of the eastern Timor Sea, indicating that the ITF vertical structure remained unchanged during its penetration into the Indian Ocean. The thermocline water temperature in the equatorial western Pacific and eastern Indian Ocean presents similar change trend, suggesting that ITF is likely to be an important route for thermocline water transport into the Indian Ocean. However, the vertical structure of the ITF varied through time, reflecting the effects of sea level and orbitally‐driven monsoonal activity. This impacted heat transport processes, resulting in changes in the surface water temperature along the outflow pathway in the eastern Indian Ocean.

The widespread distribution of Prochlorococcus can be attributed to the extensive genetic diversity that allows them to adapt to various oligotrophic environments. We investigated the adaptation of Prochlorococcus to nutrient environments in the surface eastern Indian Ocean (EIO, 16.5 degrees N to 20 degrees S, 88 degrees E) in November 2018. The growth rate of the Prochlorococcus population and its response to macronutrient enrichments (NH4+ and PO43-) and the abundance of functional gene modules related to nutrient utilization were examined by on-deck incubation experiments and metagenomic analysis, respectively. Although the dissolved inorganic nitrogen was depleted (similar to 58 nM) and the Prochlorococcus populations were dominated by the high-light-adapted II ecotype, Prochlorococcus populations showed distinct physiological patterns, especially the response to macronutrient enrichments, indicating their adaptation to local nutrient environments. At the northernmost station in the Bay of Bengal, the significant increase in growth rate with macronutrient enrichments and the highest abundance of the phosphate starvation response two-component regulatory system module indicated adaptation to phosphorus-limited environments. In the southern EIO, the insignificant increase in growth rate with macronutrient enrichment and higher abundance of the iron complex transport system module suggested adaptation to iron-limited environments. However, genomic characteristics are not always associated with physiological characteristics. The abundance of the nitrate/nitrite transport system module was higher in the southern EIO, where the growth of Prochlorococcus relied on regenerated nitrogen sources as revealed by incubation experiments. These results reflected the complexity of Prochlorococcus adaptation especially in chronically oligotrophic environments, which was better revealed by combining physiological and genomic analyses.

Marine picocyanobacteria Synechococcus exhibit highly diverse pigment types (PTs) and hence possess great advantage to utilize different spectrum of light effectively and to occupy a wide range of light niches. In this study, we explored the diversity of Synechococcus PTs in the eastern Indian Ocean (EIO), surface water of Strait of Malacca (SSM), and coastal waters of Sri Lanka (SSL). All the detected PTs were phycourobilin (PUB) containing PT 3 and showed distinct distribution patterns. Low PUB PT 3a and partial chromatic acclimater PT 3eA dominated in coastal and shallow waters (SSM and SSL). In contrast, high PUB and chromatic acclimaters PT 3dA and PT 3c/3dB were mainly distributed in open ocean (EIO). PT 3dA and PT 3c/3dB occurred at similar depths of the lower euphotic layers but showed distinct distribution pattern that are partially exclusive, indicating that they compete with each other for the same light niche. Interestingly, the newly described PT 3f was detected with high relative abundances at all stations and particularly dominated in the upper euphotic layer in EIO, which was confirmed with PT-specific quantitative polymerase chain reaction (qPCR). The relative abundance of PT 3f was negatively correlated with nutrient level, implying that PT 3f is adapted to oligotrophic waters. Pronounced niche partition of different PTs was observed in the upper and lower layers of euphotic zone in EIO and SSM/SSL. Light, nutrients, and strong stratification may play important roles in the niche partition of different PTs. Further analysis about ecologically significant taxonomic units revealed high diversity within each PT at different locations, which provided insights for understanding specific PT with wide range of niches.

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 consistent

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 consistent

Marine picocyanobacteria Synechococcus exhibit highly diverse pigment types (PTs) and hence possess great advantage to utilize different spectrum of light effectively and to occupy a wide range of light niches. In this study, we explored the diversity of Synechococcus PTs in the eastern Indian Ocean (EIO), surface water of Strait of Malacca (SSM), and coastal waters of Sri Lanka (SSL). All the detected PTs were phycourobilin (PUB) containing PT 3 and showed distinct distribution patterns. Low PUB PT 3a and partial chromatic acclimater PT 3eA dominated in coastal and shallow waters (SSM and SSL). In contrast, high PUB and chromatic acclimaters PT 3dA and PT 3c/3dB were mainly distributed in open ocean (EIO). PT 3dA and PT 3c/3dB occurred at similar depths of the lower euphotic layers but showed distinct distribution pattern that are partially exclusive, indicating that they compete with each other for the same light niche. Interestingly, the newly described PT 3f was detected with high relative abundances at all stations and particularly dominated in the upper euphotic layer in EIO, which was confirmed with PT-specific quantitative polymerase chain reaction (qPCR). The relative abundance of PT 3f was negatively correlated with nutrient level, implying that PT 3f is adapted to oligotrophic waters. Pronounced niche partition of different PTs was observed in the upper and lower layers of euphotic zone in EIO and SSM/SSL. Light, nutrients, and strong stratification may play important roles in the niche partition of different PTs. Further analysis about ecologically significant taxonomic units revealed high diversity within each PT at different locations, which provided insights for understanding specific PT with wide range of niches.

Coccolithophores are calcifying protists that have a significant role in marine biological pump through calcite production (CP). We determined the biological calcification rate and ecologically key species (e.g., Gephyrocapsa oceanica) contribution to total calcite stocks in the eastern Indian Ocean (EIO) during spring intermonsoon; this region was quite undersampled compared to other oceanic regions. Our results indicated that the numerically dominant species are of great importance to cell calcite even with low cellular calcite. CP rate in EIO ranged from 0.148 to 85.017 μmol C·m−3·day−1 in the entire data set, which was lower than those in the global ocean. Coccolithophores contributed 8.5% of total phytoplankton carbon fixation. CP acted as a function of light irradiance in the euphotic zone, while a decoupling between CP and primary productivity was found in most of the bottom of euphotic zone. To some degree, this decoupling indicated calcification was more nitrate dependent and less light dependent than photosynthesis. CP/primary productivity ratios were highly variable in our database and were possibly influenced by photoacclimation across various coccolithophore species. As roughly estimated, the turnover rate of coccolith calcite took values around 0.02–0.05 day−1. The association between particulate organic carbon and calcite stocks implied a potential increasing ballast as depth increase. The current profiles of calcification and associated biogeochemical elements obtained from this field study helps our understanding about characteristics of biological pump in the EIO and are valuable for estimating calcification rate in the region using satellite data and numerical modeling. ©2020. American Geophysical Union. All Rights Reserved.

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 degrees-96.10 degrees E, 10.08 degrees N-6.00 degrees 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 degrees 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.

Background: The Indo-Pacific region has the largest number of seagrass species worldwide and this region is considered as the origin of the Hydrocharitaceae. Halophila ovalis and its closely-related species belonging to the Hydrocharitaceae are well-known as a complex taxonomic challenge mainly due to their high morphological plasticity. The relationship of genetic differentiation and geographic barriers of H. ovalis radiation was not much studied in this region. Are there misidentifications between H. ovalis and its closely related species? Does any taxonomic uncertainty among different populations of H. ovalis persist? Is there any genetic differentiation among populations in the Western Pacific and the Eastern Indian Ocean, which are separated by the Thai-Malay peninsula? Genetic markers can be used to characterize and identify individuals or species and will be used to answer these questions. Results: Phylogenetic analyses of the nuclear ribosomal internal transcribed spacer region based on materials collected from 17 populations in the Western Pacific and the Eastern Indian Ocean showed that some specimens identified as H. ovalis belonged to the H. major clade, also supported by morphological data. Evolutionary divergence between the two clades is between 0.033 and 0.038, much higher than the evolutionary divergence among H. ovalis populations. Eight haplotypes were found; none of the haplotypes from the Western Pacific is found in India and vice versa. Analysis of genetic diversity based on microsatellite analysis revealed that the genetic diversity in the Western Pacific is higher than in the Eastern Indian Ocean. The unrooted neighbor-joining tree among 14 populations from the Western Pacific and the Eastern Indian Ocean showed six groups. The Mantel test results revealed a significant correlation between genetic and geographic distances among populations. Results from band-based and allele frequency-based approaches from Amplified Fragment Length Polymorphi

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To study the effect of hydrographic factors on the spatial distributions of chlorophyll a (Chl a), an investigation was carried out in the tropical eastern Indian Ocean (80A degrees-100A degrees E along 7A degrees S, and 7A degrees-18A degrees S along 80A degrees E) in December 2010. The fluorescent method was used to obtain total Chl a and size-fractioned Chl a at the 26 stations. The results show that surface Chl a concentration averaged at (0.168 +/- 0.095) mg/m(3) s.d. (range: 0.034-0.475 mg/m(3)), concentrations appeared to be higher in the west for longitudinal variations, and higher in the north for latitudinal variations. Furthermore, the surface Chl a concentration was lower (0.034-0.066 mg/m(3)) in the region to the south of 16A degrees S. There was a strong subsurface Chl a maximum layer at all stations and the depth of the Chl a maximum increased towards to the east and south along with the respective nitracline. The spatial variation of Chl a was significant: correlation and regression analysis suggests that it was primarily affected by PO (4) (3-) , N(NO3-N+NO2-N) and temperature. Size-fractionated Chl a concentration clearly showed that the study area was a typical oligotrophic open ocean, in which picophytoplankton dominated, accounting for approximately 67.8% of total Chl a, followed by nanophytoplankton (24.5%) and microphytoplankton (7.6%). The two larger fractions were sensitive to the limitation of P, while picophytoplankton was primarily affected by temperature.

To study the effect of hydrographic factors on the spatial distributions of chlorophyll a (Chl a), an investigation was carried out in the tropical eastern Indian Ocean (80A degrees-100A degrees E along 7A degrees S, and 7A degrees-18A degrees S along 80A degrees E) in December 2010. The fluorescent method was used to obtain total Chl a and size-fractioned Chl a at the 26 stations. The results show that surface Chl a concentration averaged at (0.168 +/- 0.095) mg/m(3) s.d. (range: 0.034-0.475 mg/m(3)), concentrations appeared to be higher in the west for longitudinal variations, and higher in the north for latitudinal variations. Furthermore, the surface Chl a concentration was lower (0.034-0.066 mg/m(3)) in the region to the south of 16A degrees S. There was a strong subsurface Chl a maximum layer at all stations and the depth of the Chl a maximum increased towards to the east and south along with the respective nitracline. The spatial variation of Chl a was significant: correlation and regression analysis suggests that it was primarily affected by PO (4) (3-) , N(NO3-N+NO2-N) and temperature. Size-fractionated Chl a concentration clearly showed that the study area was a typical oligotrophic open ocean, in which picophytoplankton dominated, accounting for approximately 67.8% of total Chl a, followed by nanophytoplankton (24.5%) and microphytoplankton (7.6%). The two larger fractions were sensitive to the limitation of P, while picophytoplankton was primarily affected by temperature.

To study the effect of hydrographic factors on the spatial distributions of chlorophyll a (Chl a), an investigation was carried out in the tropical eastern Indian Ocean (80A degrees-100A degrees E along 7A degrees S, and 7A degrees-18A degrees S along 80A degrees E) in December 2010. The fluorescent method was used to obtain total Chl a and size-fractioned Chl a at the 26 stations. The results show that surface Chl a concentration averaged at (0.168 +/- 0.095) mg/m(3) s.d. (range: 0.034-0.475 mg/m(3)), concentrations appeared to be higher in the west for longitudinal variations, and higher in the north for latitudinal variations. Furthermore, the surface Chl a concentration was lower (0.034-0.066 mg/m(3)) in the region to the south of 16A degrees S. There was a strong subsurface Chl a maximum layer at all stations and the depth of the Chl a maximum increased towards to the east and south along with the respective nitracline. The spatial variation of Chl a was significant: correlation and regression analysis suggests that it was primarily affected by PO (4) (3-) , N(NO3-N+NO2-N) and temperature. Size-fractionated Chl a concentration clearly showed that the study area was a typical oligotrophic open ocean, in which picophytoplankton dominated, accounting for approximately 67.8% of total Chl a, followed by nanophytoplankton (24.5%) and microphytoplankton (7.6%). The two larger fractions were sensitive to the limitation of P, while picophytoplankton was primarily affected by temperature.

Maps are presented in this report showing, (1) the mean difference between the vapour pressure at the sea surface and at a height at 10 m, and (2) the mean difference between the potential temperature at the ocean surface and at a height of 10 m, over the ocean surface from 5 deg S to 50 deg S and from 100 deg E to 140 deg E, for the months of February, May, August and November. The maps are derived from maps of relevant variables shown in the World Ocean Atlas (1976). The sizes of errors in the maps presented here are discussed. The largest errors arise from errors in the W.O.A. maps. (Author)