Abundance and relationship of bacteria with transparent exopolymer particles during the 1996 summer monsoon in the Arabian Sea

Bacterial abundance and production, numbers, sizes and concentrations of transparent exopolymer particles (TEP) and total organic carbon (TOC) were measured during the 1996 summer monsoon to understand the relationship between TEP, the most labile particulate organic carbon, and bacteria. While high regional variability in the vertical distribution of TOC was discernible, TEP concentrations were high in surface waters at 18–20°N along 64°E with concentrations well over 25 mg alginic acid equivalents I⁻¹ due to upwelling induced productivity. Their concentrations decreased with depth and were lower between 200 and 500 m. Bacterial concentrations were up to 1.99 × 10⁸ I^–1 in the surface waters and decreased by an order of magnitude or more at depths below 500 m. A better relationship has been found between bacterial abundance and concentrations of TEP than between bacteria and TOC, indicating that bacterial metabolism is fueled by availability of TEP in the Arabian Sea. Assuming a carbon assimilation of 33%, bacterial carbon demand (BCD) is estimated to be 1.017 to 4.035 g C m^–2 d^–1 in the surface waters. The observed TEP concentrations appear to be sufficient in meeting the surface and subsurface BCD in the northern Arabian Sea.     

Physical control of primary productivity on a seasonal scale in central and eastern Arabian Sea

Usingin situ data collected during 1992–1997, under the Indian programme of Joint Global Ocean Flux Study (JGOFS), we show that the biological productivity of the Arabian Sea is tightly coupled to the physical forcing mediated through nutrient availability. The Arabian Sea becomes productive in summer not only along the coastal regions of Somalia, Arabia and southern parts of the west coast of India due to coastal upwelling but also in the open waters of the central region. The open waters in the north are fertilized by a combination of divergence driven by cyclonic wind stress curl to the north of the Findlater Jet and lateral advection of nutrient-rich upwelled waters from Arabia. Productivity in the southern part of the central Arabian Sea, on the other hand, is driven by advection from the Somalia upwelling. Surface cooling and convection resulting from reduced solar radiation and increased evaporation make the northern region productive in winter. During both spring and fall inter-monsoons, this sea remains warm and stratified with low production as surface waters are oligotrophic. Inter-annual variability in physical forcing during winter resulted in one-and-a-half times higher production in 1997 than in 1995.     

The relationship between volatile halocarbons and phytoplankton pigments during a Trichodesmium bloom in the coastal eastern Arabian Sea

Eukaryotic phytoplankton such as diatoms and prymnesiophytes produce biogenic halocarbons in the ocean that serve as important sources of chlorine and bromine to the atmosphere, but the role of cyanobacteria in halocarbon production is not well established. We studied distributions of chloroform (CHCl₃), carbon tetrachloride (CCl₄), methylene bromide (CH₂Br₂) and bromoform (CHBr₃) in relation to phytoplankton composition, determined from pigment analysis complemented by microscopic examination, for one month in coastal waters of the eastern Arabian that experienced a Trichodesmium bloom that typically occurs during the Spring Intermonsoon season. High concentrations of zeaxanthin (23 μg l⁻¹), alpha beta betacarotene (6 μg l⁻¹) and chlorophyll a (67 μg l⁻¹) were found within the bloom whereas the marker pigment concentrations were low outside the bloom. CHCl₃ and CCl₄ occurred in relatively high concentrations in surface waters whereas CH₂Br₂ and CHBr₃ were restricted to the subsurface layer. Chlorinated halocarbons were positively inter-correlated and with CHBr₃. The observed spatial and temporal trends in brominated compounds appear to be related to the abundance of Trichodesmium although correlations between concentrations of brominated compounds with various marker pigments were poor and statistically non-significant. The results support the existence of multiple sources and sinks of halogenated compounds, which might obscure the relationship between halocarbons and phytoplankton composition.     

Neural Networks in GPS Navigation

Neural networks have been proposed as nonlinear filters in a variety of applications that involve nonlinear processing of input signals; examples include blind signal separation, image registration, and blind deconvolution. The Global Positioning System (GPS) navigation equations are nonlinear (quadratic) in nature, and a direct closed form solution of the GPS navigation equations does not exist. This article presents a new approach to solving the GPS pseudorange equations using three-layer neural networks. A three-layer radial basis function (RBF) neural network is designed, which solves the non-linear GPS pseudorange equations directly as opposed to the linear least squares or extended Kalman filter approaches in traditional GPS receivers. For training the neural network, a carefully selected cost function is minimized using a variation of the classical conjugate gradient algorithm such that training time for the neural network is reasonable. Simulations have been performed at SiRF Technology Inc. that show stable behavior even under bad geometry conditions where the traditional recursive least squares and extended Kalman filter approaches show high sensitivity to measurement errors. Under good geometry conditions the neural network solution shows slightly improved noise performance compared to the expected performance of traditional leas squares solution. Simulations have been performed with additive white Gaussian noise and correlated noise models to evaluate the performance of the trained neural network. ? 2000 John Wiley & Sons, Inc.     

Seasonal controls on surface pCO2 in the central and eastern Arabian Sea

The variability in partial pressure of carbon dioxide (pCO₂) and its control by biological and physical processes in the mixed layer (ML) of the central and eastern Arabian Sea during inter-monsoon, northeast monsoon, and southwest monsoon seasons were studied. The ML varied from 80–120 m during NE monsoon, 60–80 m and 20–30 m during SW- and inter-monsoon seasons, respectively, and the variability resulted from different physical processes. Significant seasonal variability was found in pCO₂ levels. During SW monsoon, coastal waters contain two contrasting regimes; (a) pCO₂ levels of 520–685 μatm were observed in the SW coast of India, the highest found so far from this region, driven by intense upwelling and (b) low levels of pCO₂ (266 μatm) were found associated with monsoonal fresh water influx. It varied in ranges of 416–527 μatm and 375–446 μatm during inter- and NE monsoon, respectively, in coastal waters with higher values occurring in the north. The central Arabian Sea pCO₂ levels were 351–433, 379–475 and 385–432 μatm during NE-inter and SW monsoon seasons, respectively. The mixed layer pCO₂ relations with temperature, oxygen, chlorophylla and primary production revealed that the former is largely regulated by physical processes during SW- and NE monsoon whereas both physical and biological processes are important in inter-monsoon. Application of Louanchiet al (1996) model revealed that the mixing effect is the dominant during monsoons, however, the biological effect is equally significant during SW monsoon whereas thermodynamics and fluxes influence during inter-monsoons.     

2015–16 ENSO contributed reduction in oil sardines along the Kerala coast,south‐west India

Fishery along the west coast of India largely depends on pelagic fish such as oil sardines, which are dominant during the south‐west monsoon. However, the response of sardine population to the warming caused by the climatic events such as El Niño/Southern Oscillation (ENSO) and Indian Ocean Dipole (IOD) is poorly studied. Here, we hypothesize that the ENSO‐related changes in biogeochemistry can adversely affect the oil sardines. We have used biogeochemical data collected along the Kerala coast during September 2015 (ENSO year) and September 2017 (a normal year) supported by catch per unit effort (CPUE) and fishery landing data to show the ENSO‐related bio‐physical dynamics and its impact on the oil sardine population along the south‐west coast of India. During the 2015 ENSO year, upwelling velocity decreased minimizing cooling of surface waters and resulted with an increase in sea surface temperature (SST) (~1.8°C). Consequent decrease in nutrient levels favoured nano‐phytoplankton and pico‐phytoplankton. On the other hand, during September 2017 when the ENSO effect was nullified, the decreased SST and availability of nutrients in the euphotic zone resulted in the dominance of micro‐phytoplankton. The hydrographic conditions (reduction in upwelling intensity), and reduction in micro‐phytoplankton abundance and zooplankton density in turn perhaps affected the fishery potential of the region. Oil sardines population (along the west coast) collapsed from 1.55 lakh tones (2014) to 0.46 lakh tones during 2015–2016 ENSO event, while in 2017 the conditions become favorable and their population was back to normal (1.27 lakh tones). Our results are in close agreement with our hypothesis and suggest that ENSO events could reduce phytoplankton productivity and disrupt the food chain, which in turn can affect the oil sardine population along the west coast of India.     

Spatial variations of DMS, DMSP and phytoplankton in the Bay of Bengal during the summer monsoon 2001

Data on the distribution of dimethylsulphide (DMS) and dimethylsulphoniopropionate (DMSP) in relation to phytoplankton abundance in different oceanic environments is important to understand the biogeochemistry of DMS, which plays an important role in the radiation balance of the earth. During the summer monsoon of 2001 measurements were made for DMS and DMSPt (total DMSP) together with related biological parameters in the Bay of Bengal. Both DMS and DMSPt were restricted to the upper 40 m of the water column. Diatoms accounted for more than 95% of the phytoplankton and were the major contributors to the DMS and DMSPt pool. The mean concentration of DMS in the upper 40 m was observed to be around 1.8+/-1.9 nM in the study area, while DMSPt concentrations varied between 0.7 nM and 40.2 nM with a mean of 10.4+/-8.2 nM. The observed lower DMSPt in the northern Bay in spite of higher mean primary productivity, chlorophyll a and phytoplankton cell counts seemed to result from grazing. Though salinity divides the Bay into different biogeochemical provinces there is no relation between salinity and DMS or DMSPt. On the other hand DMS was linearly related to chlorophyll a:phaeopigments ratio. The results suggest the need for deeper insight into the role of diatoms in the biogeochemical cycling of DMS.