International Journal of Marine Science2016, Vol.6, No.50, 1-9
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Decadal mean of phytoplankton biomass concentration averaged over a period of ten years from 1998-2007 in the
Arabian Sea and Bay of Bengal is shown in (Figure 2 a). Both annual and climatic mean images show high
concentration of phytoplankton biomass in the coastal, shelf and open ocean region of northern, eastern and
western basins of northern Indian Ocean. High spatial and temporal variability in the chlorophyll concentration is
observed for most parts of the northern Indian Ocean basin in the annual mean images as well as in the decadal
mean image. This is clearly depicted in the standard deviation image of the climatic mean chlorophyll (Figure 2 b).
Figure 2: a) Climatic mean chlorophylla map (1998-2007). b) Standard deviation chlorophyll a map (1998-2007).
However, two regions of low chlorophyll concentration (Figure 2a, b shown in dark purple) and minimum
standard deviation (<1 mg.m-3) can be identified in the southern oceanic regions of Arabian Sea, Bay of Bengal
(0-10
N) and central Bay of Bengal (8-15
N). These comprise the low chlorophyll (< 0.2 mg.m-3) oligotrophic
waters of the northern Indian Ocean basin. The overall concentration patterns are repetitive for all the years
indicating the presence of low nutrient low chlorophyll region (LNLC) with no pronounced inter annual
variability.
Two small areas (marked as red) in the Figure 1 were identified for further analysis. The bathymetry image
corresponding to LNLC regions indicate that both areas are situated above a deep ocean away from the continental
margins. The depths of the ocean floor below these regions are approximately between 3000-4000 m (Figure 3).
LNLC area identified in Arabian Sea is a large patch of approximately 62010 km
2
and LNLC area in Bay of
Bengal corresponds roughly to a circular patch of ~ 16641 km
2
(Figure 3 a). Figure 3 b shows the interannual
variability of chlorophyll concentrations in the LNLC area of Arabian Sea and Bay of Bengal. The LNLC area in
Arabian Sea has a low climatic mean chlorophyll value of 0.15 mg.m-3(SD
0.013). A CV (coefficient of
variation) of 8.5% indicates a homogenous area of very low phytoplankton abundance. Similar results are
obtained for the LNLC area in Bay of Bengal with a climatic mean chlorophyll value of 0.15 mg.m-3 (SD
0.005)
and CV of only 3.2 %.
