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

Journal of Earth System Science (Impact Factor: 0.79). 04/2012; 109(4):433-441. DOI: 10.1007/BF02708331
Source: OAI

ABSTRACT 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.

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    ABSTRACT: The development of phytoplankton bloom and its association with physical forcing is examined through an interdisciplinary field-work conducted in the vicinity of the central trough of the southern Yellow Sea during March-April 2009, with the aid of a surface Lagrangian drifter deployed at the bloom site. Bloom patches were detected using an empirical value and two of them were traced by the drifter for a period of several days respectively. Both of them appears as thin-layer subsurface chlorophyll a maximum (SCM) throughout the tracing, although their dominant phytoplankton species are not identical at all. The magnitude as well as the onset of these two blooms is different from each other, but both found to be relevant to local oceanic and meteorological conditions. Both of them demonstrate that the changes in the stability of hydrographical structure, especially at layers around the SCM, take a substantial role in triggering or terminating the blooming processes. Those changes in meteorological conditions, like wind speed and directions, solar radiation, are short and cause daily or synoptic scale variations in phytoplankton concentrations, but the frequency of northerly wind events predating the bloom season has a positive effect on the occurrence of spring blooms. The horizontal advection is another contributing factor indicated by the drifter which accounts for the bloom extinction at the station B20. In addition, due to the weak orbital horizontal movement, the bloom above the central trough persists longer and larger.
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    ABSTRACT: The diversity, abundance, biomass of phytoplankton and primary productivity in the shelf waters of four stations along the southwest coast of India were studied for the first time during May - June 2005. In order to study the vertical distribution, five sampling depths starting from (5 to 35 m) were elected within the euphotic zone. Nutrients (nitrate, nitrite, phosphate and silicate) showed an increasing trend from 5 m to deeper waters with a significant positive relationship with salinity (r > 0.83, p < 0.01). Result indicates that Primary Productivity (PP) and phytoplankton standing crop were direct tune with chlorophyll a and nutrient concentrations. Further, a significant positive correlation was observed between PP and phytoplankton standing crop (r > 0.87, p < 0.01) in most of the stations and also with chlorophyll a and phytoplankton standing crop (p < 0.01, r > 0.96). Among stations, 35 m depth at Kodungallur station recorded the least phytoplankton standing crop (0.87 × 103 cells L–1) and 10 m depth at Mangalore recorded the maximum (31.53 × 103 cells L–1). Phytoplankton community composition revealed 67 species of phytoplankton belonging to different taxonomic groups, in which bacillariophyceae constituted 49, pyrrophyceae 17 and cyanophyceae 1. Chaetoceros lorenzianus invariably constituted maximum abundance throughout the water column in all stations except Mangalore 10 m depth. Depth integrated (5 to 35 m) primary productivity of Mangalore (1284.7 mg C m-2 d-1) was nearly three times as high at Kodungallur (431.8 mg C m-2 d-1). The maximum depth integrated chlorophyll a (49.70 mg m-2) was recorded at Mangalore and minimum of 13.25 mg m-2 were recorded at Kodungallur. Vertical accretion of phytoplankton species was remarkable and the species diversity was predominant at 10 – 15 m water column depth, which is in concord with maximum biomass (chlorophyll a) and pycnocline layer. In general, increased phytoplankton diversity, Margalef richness d’ (5.53 ± 0.23), Shannon- Wiener H’ (2.56 ± 0.37), Pielou’s evenness J’ (0.61 ± 0.44) in the northern shelf waters Karwar and Mangalore (12 - 14oN) relative to southern shelf waters Calicut and Kodungallur (10 - 11oN) with concomitant increase both in abundance and biomass. In this study an attempt was also made to distinguish the phytoplankton community in to its different assemblages according to their locations (e.g., depth and station-ways). Mangalore recorded the maximum species diversity (52) followed by Karwar (49), Calicut (42) and Kodungallur (19). Southwest coast of India, Phytoplankton abundance and biomass could be affected by its spatial variability in community structure with species specific association (e.g., depth ways) and largely with respect to differences in the hydrographical conditions. The study suggests that phytoplankton community structure was varied with depth and plays a crucial role on PP and quantity of phytoplankton biomass available to the marine food web.
    International Journal of Current Research. 01/2010; 8:16.
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    ABSTRACT: Hydrographic observations in the southeastern Arabian Sea (SEAS) have identified a warm (>30°C) and stratified water mass (stability >5 × 10 m) along the near shore area between 10°N and 15°N during spring intermonsoon. This water mass was relatively low saline (34.2) and nitrate-rich (0.5 µM), favoring moderate primary production (6.7 mg C md). Since the mixing of the Arabian Sea and the Bay of Bengal waters is an important process during this period, the enhanced primary production in the SEAS is attributed to the entrainment of unconsumed nitrate left over during the previous season from the northern Arabian Sea. The season was further characterized by the presence of a deep chlorophyll a maximum (0.5 mg.m) in the outer shelf below (>50 m) the subducted Arabian Sea High Saline Waters, which was photosynthetically less active (
    Advances in Oceanography and Limnology. 12/2012;

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