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 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.
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    ABSTRACT: The Bay of Bengal (BoB) and the Arabian Sea (AS), the two major ocean basins in the north Indian Ocean, share similarities in geographical and meteorological characteristics but differ in hydrographic characteristics and biological production. The BoB is traditionally considered a less productive region than the AS. Chaetognatha, a major taxon of carnivorous mesozooplankton, often rank second in abundance to copepods and thus play a pivotal role in the pelagic food web of this region. We studied the influence of environmental variables on the distribution and the diversity of chaetognaths in these two ocean basins. Of the 21 species observed during the study, Flaccisagitta enflata dominated in the mixed layer to the thermocline, Decipisagitta decipiens from the bottom of the thermocline to 500 m and Eukrohnia fowleri below 500 m. Mesoscale and basin-scale processes such as eddies, gyres and upwelling had a profound influence on the abundance in the upper layer, while the presence of an oxygen minimum zone in the mid-depth layers had a crucial effect on the vertical distribution of the chaetognath population. Despite the variation in the controlling factors on chaetognath populations in the BoB and AS, the patterns of depth-specific abundance were similar.
    Marine Biology Research 01/2015; 11(3):269-282. DOI:10.1080/17451000.2014.914224 · 1.13 Impact Factor
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    ABSTRACT: Dansgaard-Oeschger oscillations and Heinrich events described in North Atlantic sediments and Greenland ice are expressed in the climate of the tropics, for example, as documented in Arabian Sea sediments. Given the strength of this teleconnection, we seek to reconstruct its range of environmental impacts. We present geochemical and sedimentological data from core SO130-289KL from the Indus submarine slope spanning the last ~ 80 kyr. Elemental and grain size analyses consistently indicate that interstadials are characterized by an increased contribution of fluvial suspension from the Indus River. In contrast, stadials are characterized by an increased contribution of aeolian dust from the Arabian Peninsula. Decadal-scale shifts at climate transitions, such as onsets of interstadials, were coeval with changes in productivity-related proxies. Heinrich events stand out as especially dry and dusty events, indicating a dramatically weakened Indian summer monsoon, potentially increased winter monsoon circulation, and increased aridity on the Arabian Peninsula. This finding is consistent with other paleoclimate evidence for continental aridity in the northern tropics during these events. Our results strengthen the evidence that circum-North Atlantic temperature variations translate to hydrological shifts in the tropics, with major impacts on regional environmental conditions such as rainfall, river discharge, aeolian dust transport, and ocean margin anoxia.
    Paleoceanography 02/2014; 29(2):99-114. DOI:10.1002/2013PA002509 · 3.92 Impact Factor

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