Impact of anthropogenic sources on aerosol iron solubility over the Bay of Bengal and the Arabian Sea

Ambient aerosols collected from the marine atmospheric boundary layer of the Bay of Bengal and the Arabian Sea have been studied to assess the fractional solubility of aerosol iron, defined as Fe_ws (%)?=?Fe_ws/Fe_Tot?×?100; where Fe_Tot is total aerosol iron and Fe_ws is water soluble iron. The mass concentration of Fe_Tot over the two oceanic regions is not significantly different. However, the fractional solubility is 1–2 orders of magnitude higher over the Bay of Bengal (1.4–24%) compared to that over the Arabian Sea (0.02–0.4%). The spatio-temporal variability in Fe_ws (%) is attributed to differences in the nature of the mineral dust over the two oceanic regions. The Arabian Sea receives coarse dust from desert regions; whereas transport of alluvial dust from the Indo-Gangetic Plain is a dominant source to the Bay of Bengal. The poor fractional solubility (<1%) of Fe from mineral dust, hitherto overestimated in the literature, is documented for the Arabian Sea. A significant linear relationship (P-value?<?0.001) between Fe_ws (%), Fe_Tot and nss-SO₄ ^2? over the Bay of Bengal provides evidence for the chemical processing of mineral dust. Furthermore, the role of anthropogenic sources (biomass burning and fossil-fuel combustion) in enhancing the Fe_ws (%) is discernible from the chemical composition of fine mode (PM_2.5) aerosols over the Bay of Bengal. The potential impact of these Fe-dust depositions on phytoplankton carbon fixation and surface ocean biogeochemistry is discussed.     

Planktonic foraminifera of the northern Indian Ocean: Distribution and preservation in surface sediments

In the northern Indian Ocean, planktonic foraminiferal tests accumulate in a wide variety of surface-water environments and depositional settings. This variability enables us to isolate the effects that surface-water ecology and differential dissolution have on the distribution of planktonic foraminifera from 251 geographically widespread surface sediment samples.Foraminiferal abundance varies from 0 to > 10⁴ whole foraminifera in the greater than 150 μm fraction per gram dry sediment. Values < 10 characterize the three deep basins of the equatorial Indian Ocean and the western Bay of Bengal. Foraminiferal tests are most abundant on carbonate covered Ninety-East and Carlsberg Ridges. Absolute abundance patterns are mainly controlled by non-ecological processes. Variations in dissolution resistant species (RSP) with water depth reveal that the foraminiferal lysocline (FL) varies regionally. The FL is deepest (3,800 m) in the equatorial region, rises abruptly to 3,300 m in the Arabian Sea, and varies from 2,600 m to near 2,000 m moving northward in the Bay of Bengal. Deep samples with anomalously low RSP (< 30%) suggest redeposition.Systematic geographic and depth-related variation is observed for the 17 most abundant foraminiferal species. Dissolution resistant species (G. menardii, G. tumida, G. dutertrei, P. obliquiloculata) generally exhibit a rapid and continuous increase in relative abundance at and below the FL. Susceptible species (G. ruber, G. bulloides, G. glutinata, for example) exhibit a rapid and continuous decrease in relative abundance at and below the FL. Moderately susceptible species (G. conglobatus, G. aequilateralis, G. conglomerata, for example) rapidly increase in abundance at the FL and systematically decrease with depth below the FL.Principal components analysis (PCA) of faunal data from minimally dissolved (< 30% RSP) samples reveals important ecologically related species intercorrelations. The major biogeographic gradient is the negative covarying relationship between aG. bulloides-G. glutinata species pair and a grouping ofG. sacculifer, G. conglobatus, G. aequilateralis, andG. ruber. PCA of all samples demonstrates how differential dissolution alters this and other species relationships. Species groupings that incorporateG. ruber, G. menardii, andG. dutertrei are particularly affected by dissolution.Comparison of average faunal data from minimally dissolved samples in the northern Indian Ocean with similar samples from other tropical regions suggests varying environmental factors produce distinct faunas within the tropical ocean. For example,G. bulloides, G. falconensis, andG. hexagona are significantly more abundant in northern Indian Ocean surface sediments while such species asG. ruber, G. sacculifer, G. dutertrei, andP. obliquiloculata dominate in other tropical regions.     

Mesozooplankton community in the Bay of Bengal (India): spatial variability during the summer monsoon

This study addresses the spatial variability in mesozooplankton biomass and composition in the Central and Western Bay of Bengal (India) during the summer monsoon season of 2001. Perennially warmer sea surface temperatures (>28°C), stratified top layer (sea surface salinity, 28–33 psu), high turbidity, and low nutrient concentrations due to weak/null upwelling and light limitation make the Bay of Bengal a region of low primary productivity. Despite this, mesozooplankton biomass values, i.e. 2.9–104 mg C m⁻³ in the Central Bay and 1.3–31 mg C m⁻³ in the Western Bay, observed in the mixed layer (2–51 m) during the summer monsoon were in the same range as reported from the more productive Arabian Sea. Mesozooplankton biomass was five times and density 18 times greater at stations with signatures of cold-core eddies, causing a higher spatial heterogeneity in zooplankton distribution. Among the 27 taxonomic groups recorded during the season, Copepoda was the most abundant group in all samples followed by Chaetognatha. The dominant order of Copepoda, Calanoida, was represented by 132 species in a total of 163 species recorded. Oncaea venusta was the key copepod species in the Bay. In the Central Bay, the predominant copepod species were carnivorous/omnivorous vis-a-vis mostly herbivores in the Western Bay. Pleuromamma indica increased to its maximum abundance at 18°N in the Central Bay, coinciding with the lowest dissolved oxygen concentrations. The Central Bay had higher mesozooplankton biomass, copepod species richness and diversity than in the Western Bay. Although zooplankton biomass and densities were greater at the eddy stations, correlation between zooplankton and chl a was not statistically significant. It appears that the grazer mesozooplankton rapidly utilize the enhanced phytoplankton production in cold-core eddies.     

Quantification of total and particulate dimethylsulfoniopropionate (DMSP) in five Bermudian coral species across a depth gradient

The symbiotic dinoflagellate microalgae of corals (Symbiodinium spp.) contain high concentrations of dimethylsulfoniopropionate (DMSP), a multifunctional metabolite commonly found in many species of marine algae and dinoflagellates. A photoprotective antioxidant function for DMSP and its breakdown products has often been inferred in algae, but its role(s) in the coral–algal symbiosis remains elusive. To examine potential correlations between environmental and physiological parameters and DMSP, total DMSP (DMSP_t, from the host coral and zooxanthellae), particulate DMSP (DMSP_p, from the zooxanthellae only), coral surface area, and total protein, as well as zooxanthellae density, chlorophyll concentration, cell volume and genotype (i.e., clade) were measured in five coral species from the Diploria-Montastraea-Porites species complex in Bermuda along a depth gradient of 4, 12, 18, and 24 m. DMSP_t concentrations were consistently greater than DMSP_p concentrations in all species suggesting the possible translocation of DMSP from symbiont to host. D. labyrinthiformis was notably different from the other corals examined, showing DMSP_p and DMSP_t increases (per coral surface area or tissue biomass) with increasing water depth. However, overall, there were no consistent depth-related patterns in DMSP_p and DMSP_t concentrations. Further research, investigating dimethylsulfide (DMS), dimethylsulfoxide, and acrylate levels and DMSP-lyase activity in correlation with other biomarker endpoints that have been shown to be depth (i.e., temperature and light) responsive are needed to substantiate the significance of these findings.     

Biodiversity and biogeography pattern of benthic communities in the coastal basins of India

Our understanding of coastal biogeography patterns is presently limited to certain regions and marine groups. Comprehending large-scale patterns and their underlying predictors is critical due to the changing environmental conditions. The Indian coast, bounded by two contrasting seas, the Bay of Bengal and Arabian Sea, offers an excellent opportunity to determine the most important factors influencing coastal biogeography patterns. Here we use distribution data of 2581 species belonging to three benthic groups (polychaetes, gastropods and bivalves) to evaluate the role of environmental factors and coastal currents on the biogeography patterns. Data were obtained from the bioSearch database, developed by CSIR-NIO, Goa under the Census of Marine Life programme. As measures of diversity, we used species richness as well as taxonomic distinctiveness indices. Multivariate analyses were used in identifying biodiversity patterns and correlated biogeography patterns with environmental factors. This paper points out a clear difference between the eastern and western basins of India, but also within the basins. Mainland and island fauna also showed a clear distinction. The biogeography of the Indian coast is best explained by temperature, habitat heterogeneity and coastline length. Spatiotemporal variability in the boundary currents also forms an effective barrier for the dispersal of planktonic larvae of benthic fauna. This study has identified a finer scale division of the Indian coast that can help to effectively improve conservation plans. Our study is the first to attempt to understand the benthic biogeography patterns of the Indian coast. The findings will be useful in guiding future biogeography studies of this unique and underexplored marine system.     

TEMPERATURE THRESHOLD AS A BIOGEOGRAPHIC BARRIER IN NORTHERN INDIAN OCEAN MACROALGAE1

The most eastern point of the Arabian Peninsula, Ras Al Hadd, marks the boundary between the Arabian Sea and the Gulf of Oman. This geographic landmark coincides with an abrupt floristic turnover, probably one of the sharpest biotic transitions known in marine biogeography. The floras of different Arabian localities across this floristic break were compared using macrophyte distribution data throughout the Indian Ocean and seasonal sea‐surface temperature (SST) data. The localities from the Arabian Gulf and Gulf of Oman differ significantly from those of the Arabian Sea based on their species richness, species composition, average distribution range per species, general temperature affinity of the composing species, and seasonal temperature data of the coastal waters. Pooling the temperature data into two groups (SST_3avg, average SST of the three warmest seasons; SST_min, minimum of the seasonal SSTs) revealed a temperature limit at 28°C using both the temperature affinity data of the floras and the seasonal temperatures recorded for the specific Arabian localities, which significantly separates the Arabian Sea from localities of both Gulfs. Finally, SST data of the Indian Ocean were analyzed using this upper temperature threshold of macrophytes at 28°C and the lower temperature limit of corals at 25°C, revealing general macrophyte diversity patterns.     

A review of flying fishes of the subgenus Hirundichthys (Genus Hirundichthys, Exocoetidae). Part 2. Nerito-oceanic species: H. oxycephalus, H. affinis

The second part of the review of the subgenus Hirundichthys s.str. dealt with two nerito-oceanic species of the subgenus which have weakly pronounced “mirror” on the pectoral fins: H. oxycephalus and H. affinis. The validity of H. coromandelensis (Hornell, 1923) as a subspecies of H. oxycephalus is restored. A comparison of local populations showed that H. oxycephalus is a polytypic species and forms three subspecies: nominative H. oxycephalus oxycephalus from the Western Pacific and Eastern Indian Ocean, H. oxycephalus coromandelensis from the Arabian Sea, Bay of Bengal and adjoining waters of the Indian Ocean, and known by the only specimen from the waters of New Guinea H. oxycephalus frereensis ssp.n. Populations of H. affinis from the Western and Eastern Atlantic differ in the color of pectoral fins. Maps showing the geographical distribution of species and subspecies in the World Ocean are drawn up. A key for identification of species and subspecies belonging to the subgenus is proposed.     

Phylogeography of <Emphasis Type="Italic">Ceriops tagal</Emphasis> (Rhizophoraceae) in Southeast Asia: the land barrier of the Malay Peninsula has caused population differentiation between the Indian Ocean and South China Sea

The genetic structure of mangrove species is greatly affected by their geographic history. Nine natural populations of Ceriops tagal were collected from Borneo, the Malay Peninsula, and India for this phylogeographic study. Completely different haplotype compositions on the east versus west coasts of the Malay Peninsula were revealed using the atpB-rbcL and trnL-trnF spacers of chloroplast DNA. The average haplotype diversity (Hd) of the total population was 0.549, nucleotide diversity (θ) was 0.030, and nucleotide difference (π) was 0.0074. The cladogram constructed by the index of population differentiation (G _ST) clearly separated the South China Sea populations from the Indian Ocean populations. In the analysis of the minimum spanning network, the Indian Ocean haplotypes were all derived from South China Sea haplotypes, suggesting a dispersal route of C. tagal from Southeast Asia to South Asia. The Sunda Land river system and surface currents might be accountable for the gene flow directions in the South China Sea and Bay of Bengal, respectively. The historical geography not only affected the present genotype distribution but also the evolution of C. tagal. These processes result in the genetic differentiation and the differentiated populations that should be considered as Management Units (MUs) for conservation measurements instead of random forestation, which might lead to gene mixing and reduction of genetic variability of mangrove species. According to this phylogeographic study, populations in Borneo, and east and west Malay Peninsula that have unique genotypes should be considered as distinct MUs, and any activities resulting in gene mixing with each other ought to be prevented.     

STORM-PETRELS OCEANODROMA SPP. IN THE INDIAN OCEAN

In the course of the International Indian Ocean Expedition two storm-petrels, thought to be Oceanodroma matsudairae and O. monorhis, were encountered in the western Indian Ocean in some numbers. Two specimens of each species were collected, monorhis in the Arabian Sea, matsudairae off N.W. Australia. O. matsudairae, which had not been recorded before from these waters, appeafed to be concentrated within 5° of the equator and the birds probably belonged to the spring-breeding population of Volcano Island. O. monorhis was found mostly in the Arabian Sea. Bulweria bulwerii, for which there was but one certain Indian Ocean record, was also encountered several times.     

Highly differentiated population structure of a Mangrove species, Bruguiera gymnorhiza (Rhizophoraceae) revealed by one nuclear GapCp and one chloroplast intergenic spacer trnF–trnL

To evaluate the genetic diversity of a mangrove species and clarify the genetic structure of its populations, we studied nucleotide polymorphism in two DNA regions of Bruguiera gymnorhiza collected from the southern islands of Japan, Thailand, Malaysia, Indonesia, Micronesia, and India. The two DNA sequences were the chloroplast (cp) intergenic spacer between trnL and trnF genes (ca. 300 bp), and a part (ca. 550 bp) of the nuclear gene coding for glyceraldehyde-3-phosphate dehydrogenase (GapCp). Little polymorphism was found within each of the three geographical regions, Pacific Ocean, Bay of Bengal and Arabian Sea. Throughout the vast regions east of the Malay peninsula including Indonesia, Thailand, Micronesia and the southern islands of Japan (Pacific Ocean), essentially only one haplotype (apart from variation in number of a T repeat) was present. A second haplotype was present on the western coast of Malay Peninsula and the eastern coast of India (Bay of Bengal). On the southwest of Malay Peninsula both of these haplotypes were present. Finally a third haplotype was found only on the western coast of India (Arabian Sea). When taken over all geographic populations, total nucleotide variation within the species was large (μ = 0.006, average of the two genes). Our results are consistent with the hypothesis that this low genetic diversity within any local population and differentiation between the different oceans or regions are caused by very low gene flow between each of the different oceans coupled with frequent fluctuation of population sizes due to the change in sea level. The significance of these results is discussed from evolutionary point of the mangrove forests.     

Macroscale patterns of the biological cycling of dimethylsulfoniopropionate (DMSP) and dimethylsulfide (DMS) in the Northwest Atlantic

The influence of the seasonal development of microplankton communities on the cycling of dimethylsulfide (DMS) and its precursor dimethylsulfoniopropionate (DMSP) was investigated along a South–North gradient (36–59^°N) in the Northwest (NW) Atlantic Ocean. Three surveys allowed the sampling of surface mixed layer (SML) waters at stations extending from the subtropical gyre to the Greenland Current during May, July and October 2003. Pools and transformation rates of DMSP and DMS were quantified and related to prevailing physical and biochemical conditions, phytoplankton abundance and taxonomic composition, as well as bacterioplankton abundance and leucine uptake. The South–North progression of the diatom bloom, a prominent feature in the NW Atlantic, did not influence the production of DMS whereas conditions in the N Atlantic Drift lead to a persistent bloom of DMSP-rich flagellate-dominated phytoplankton community and high net DMS production rates. Macroscale patterns of the observed variables were further explored using principal component analysis (PCA). The first axis of the PCA showed a strong association between the spatio-temporal distribution of DMSP and the abundance of several phytoplankton groups including dinoflagellates and prymnesiophytes, as well as with microbial-mediated DMSP_d consumption and yields and rates of the conversion of DMSP into DMS. The second axis revealed a strong association between concentrations of DMS and SML depth and photosynthetically active radiation, a result supporting the prominent role of solar radiation as a driver of DMS dynamics.     

Zoogeography of the coral reef fishes of the Socotra Archipelago

Fish communities and habitats were studied at the Socotra archipelago (Gulf of Aden, ≈12°N 54°E). Extensive and unexpected hermatypic coral communities were recorded, at the centre of a 2200 km gap in knowledge of species and habitat distributions which coincides with a change from a western Indian Ocean coral reef fauna to an Arabian one. The fish assemblage associated with the Socotra archipelago corals is predominantly south Arabian. An east African influence, minimal on the mainland coasts of Arabia, is more evident here, and results in previously unrecorded sympatry between Arabian endemic species and their Indian Ocean sister taxa. A study of distributions of Chaetodontidae (butterflyfishes) in the north-western Indian Ocean reveals a number of distinct patterns, with a trend for species replacement along a track from the northern Red Sea to the Indian Ocean. A major feature of the reef fish zoogeography of the region is found to be a distinct south Arabian area, characterized by a 'pseudo-high latitude effect' which results from seasonal cold water upwelling along the Arabian sea coasts of Yemen and Oman and the Indian Ocean coast of Somalia. This south Arabian feature is consistent across a wide range of fish families. It is most pronounced in Oman and Yemen, and although it is the dominant influence at Socotra it is slightly 'diluted' here by the east African influence. The south Arabian area wholly or partly accounts for most of the major marine zoogeographic features around Arabia, and is the principal feature fragmenting Arabian coastal fish assemblages, and separating them from those of the wider Indo-west Pacific.     

Description of a new species of deep-water crab of the genus Homolodromia A. Milne-Edwards, 1880 from the northern Indian Ocean (Crustacea: Decapoda: Brachyura: Homolodromiidae)

ABSTRACT

Homolodromia rajeevani, a new species of deep-water homolodromiid sponge crab, is described from the northern Indian Ocean (Arabian Sea, depth 957 m, and Bay of Bengal, 645 m), and is the first record of the genus from the area. This species resembles the western Indian Ocean species, namely, Homolodromia bouvieri Doflein, 1904, in having 2 terminal spines on the propodi of the last two pereopods, but can be easily distinguished from the latter species by the inflated carapace, simple long setae on carapace and appendages, slender pseudo-rostral spines separated by a U-shaped base, and a slender arched dactylus of cheliped with maximum elevation at proximal part which bears broadly circular depressions with sparse setae. The most diagnostic character is the higher number of spines on the occlusal surfaces of propodal thumbs and dactyli of the pseudochela of the last two pereopods as compared to H. bouvieri. A key for the identification of the species under the genus Homolodromia is also provided.

http://zoobank.org/urn:lsid:zoobank.org:pub:48894F49-5124-4723-9FF2-3D30FB536DA5     

Late Quaternary deep and surface water mass evolution in the northeastern Indian Ocean inferred from carbon and oxygen isotopes of benthic and planktonic foraminifera

New planktonic and benthic foraminiferal stable isotope records from core YDY05 (northeastern Indian Ocean) provide new insights into paleoceanographic changes in the northeastern Indian Ocean since the last glacial period. The distinct δ¹⁸O decrease was observed since the beginning of the deglaciation to the mid-Holocene (∼8–6 kyr BP), possibly reflecting a reduction in surface salinity in the central Bay of Bengal (BoB) water, which probably resulted from strengthened precipitation, concurrent enhanced river discharge and rising sea-level, related to the intensification of Indian Summer Monsoon (ISM). Variations in benthic δ¹³C and δ¹³C_{Planktonic-Benthic} in our core site reflect significant variations in source water characteristics over the LGM-Holocene. The large δ¹³C_{Planktonic-Benthic} offset during the glacial period suggests a more sluggish deep water circulation, and lower δ¹³C_{Planktonic-Benthic} from the deglaciation to the Holocene suggests an enhanced deep water circulation in the central BoB. The drastic depletion in benthic δ¹³C during the glacial period suggests a significant reduction of North Atlantic Deep Water (NADW) intrusion and a progressive influx of Antarctic Bottom Water (AABW) and ¹²C-rich Circumpolar Deep Water (CDW) into the central BoB. In contrast, since the deglaciation, the central BoB experienced a drastically increased intrusion of better ventilated and ¹³C-rich NADW. The differences in benthic δ¹⁸O between the LGM section and the Holocene exceeds the ice volume effect by ∼0.5‰, providing further evidence that the deep water mass of the central BoB was influenced by the less dense NADW, instead of the AABW, since the last deglaciation.     

Seasonal changes in fluxes of methane and volatile sulfur compounds from rice paddies and their concentrations in soil water

Rice paddies emit not only methane but also several volatile sulfur compounds such as dimethyl sulfide (DMS: CH₃SCH₃). However, little is known about DMS emission from rice paddies. Fluxes of methane and DMS, and the concentrations of methane and several volatile sulfur compounds including hydrogen sulfide (H₂S), carbonyl disulfide (CS₂), methyl mercaptan (CH₃SH) and DMS in soil water and flood water were measured in four lysimeter rice paddies (2.5 × 4 m, depth 2.0 m) once per week throughout the entire cultivation period in 1995 in Tsukuba, Japan. The addition of exogenous organic matter (rice straw) was also examined for its influence on methane or DMS emissions. Methane fluxes greatly differed between treatments in which rice straw had been incorporated into the paddy soil (rice straw plot) and plots without rice straw (mineral fertilizer plot). The annual methane emission from the rice straw plots (37.7 g m^-2) was approximately 8 times higher than that from the mineral fertilizer plots (4.8 g m^-2). Application of rice straw had little influence on DMS fluxes. Significant diurnal and seasonal changes in DMS fluxes were observed. Peak DMS fluxes were found around noon. DMS was emitted from the flood water in the early growth stage of rice and began to be emitted from rice plants during the middle stage. DMS fluxes increased with the growth of rice plants and the highest flux, 15.1 µg m^-2 h^-1, was recorded before heading. DMS in the soil water was negligible during the entire cultivation period. These facts indicate that the DMS emitted from rice paddies is produced by metabolic processes in rice plants. The total amount of DMS emitted from rice paddies over the cultivated period was estimated to be approximately 5–6 mg m^-2. CH₃SH was emitted only from flood water during the first month after flooding.     

Spatial and temporal variability of dissolved sulfur compounds in European estuaries

The spatial and temporal distribution of Dimethylsulfide (DMS),Dimethylsulfoniopropionate (DMSP) – its precursor –,Dimethylsulfoxyde (DMSO) – one of its oxidation products –Carbonyl sulfide (COS) and Carbonyl disulfide (CS₂) wereinvestigated during nine oceanographic cruises carried out in six majortidal European estuaries between July 1996 and May 1998. Low levels ofDMS were recorded (mean of 0.6 nM, standard deviation () 0.3 nM)during these 9 cruises. Positive correlations between DMS and salinitywere frequently observed, with the highest DMS concentrations in theplume of estuaries, which could be explained by change in phytoplanktonspeciation from estuarine to shelf waters. Strong correlation betweenDMSP and DMS was reported for most of the estuaries denoting a simpleconservative mixing of riverine and marine waters controlled by tide. Incontrast to DMS, significant levels of COS and CS₂ with meanconcentrations of 220 ± 150 (pM; pM =10^–12 M) and 25 ± 6 pM respectively wererecorded in four estuaries, indicating that estuaries could be asignificant source of these compounds.     

The first record of the sicklefin chimaera <Emphasis Type="Italic">Neoharriotta pinnata</Emphasis> (Chimaeriformes: Rhinochimaeridae) in the southern Bay of Biscay (North-East Atlantic)

One specimen of an adult female of the chimaera Neoharriotta pinnata was caught in the southern of Bay of Biscay (44° N). Until now, this species has been reported at the coast of West Africa, in the Arabian Sea, the Bay of Bengal and to the south-west of India. It is the first time that this chimaera has been recorded at the latitudes north of the parallel 22° N. This report extends the known habitat of this species considerably.     

Radiolarian biogeography in surface sediments of the eastern Indian Ocean

We analyzed recurrent groups of Radiolaria in 74 core top samples from a transect through the eastern Indian Ocean in order to supplement our previous results from the western Indian Ocean (Johnson and Nigrini, 1980). We now identify six distinct recurrent groups and nine radiolarian assemblages in the combined data set of 120 samples; this extended sample coverage has led to several re-interpretations of the oceanographic significance of the radiolarian distribution patterns. Assemblage boundaries closely reflect the presence of major oceanographic fronts and surface currents including the South Equatorial Divergence, Subtropical Gyre, Subtropical Convergence, and Antarctic Convergence. At least four major aspects of the assemblages in the eastern transect are notably different from those in the western transect, leading to a marked east-west asymmetry in faunal distribution patterns across the Indian Ocean. The assemblage formerly associated with strong upwelling near the Arabian Peninsula is present throughout the Bay of Bengal as well, and is interpreted to reflect high salinity and low oxygen in the subsurface waters of the Indian Ocean north of the Equator. A new assemblage has been identified associated with the westward-flowing Pacific water into the eastern Indian Ocean in low latitudes, and may be a potential stratigraphic and paleoclimatic marker for times of low sea level when this westward near-surface flow was shut off (i.e., glacial maxima). An extensive region in the core of the subtropical gyre between 25°S and 35°S is relatively barren of Radiolaria, yet is marked by a characteristic assemblage distributed asymmetrically, perhaps reflecting the lack of a strong boundary current off the west coast of Australia. Assemblage boundaries in the vicinity of the eastward circumpolar flow are not strictly zonal, and may indicate significant deviations from the mean eastward flow as a necessary condition for conservation of potential vorticity when the flow encounters topographic irregularities.