International Journal of Marine Science2016, Vol.6, No.50, 1-9
3
A primary requirement for conducting fertilisation experiments is the identification of oceanic deserts, the regions
with uniformly low chlorophyll concentration and without any seasonal or interannual variations.
Given the vast expanse of oceanic waters, ship-board observations are not only laborious, time consuming and
expensive but are also limited to small space and time scales and many regions of the ocean remain grossly under
sampled. Satellite remote sensing provides a viable alternative for data collection over a vast oceanic area. Global
ocean colour data has been available from missions like SeaWiFS and Oceansat1 & 2.
Remote sensing of ocean colour or visible spectral radiometry refers to reflectance properties of ocean in the
visible and near infrared regions caused by the interactions of incident light with constituents present in water.
Water molecules, phytoplankton, detritus (algal debris, inorganic components, bacteria) and colored dissolved
organic matter modify the light emanating from the sea surface through selective absorption and backscattering
(Morel and Prieur, 1977).
Chl-a (
Chlorophyll-a), the green pigment primarily involved in the process of photosynthesis, selectively absorbs
blue (440 nm) and red light (670nm) of the electromagnetic spectrum leaving the green region (520-620nm) of the
spectrum relatively less absorbed. The colour of the ocean progressively shifts from blue to green as the
chlorophyll bearing phytoplankton concentration increases. This property of colour change is quantitatively
related to the ratios of backscattered light at various wavelengths taken from satellite-borne instrument to produce
synoptic maps of phytoplankton biomass characterized in terms of surface
Chl-a
concentration (O’Reilly et al.,
1998). Thus ocean colour images represent the amount of phytoplankton indexed as
Chl-a
concentration (mg.m
-3
)
in space and time. The new generation ocean colour sensors have been specifically designed to provide
quantitative estimation of
Chl-a
concentration over the range of 0.05-50 mg.m
-3
(Rast, 1996).
In this study, satellite derived ocean colour data has been used to identify low nutrient low chlorophyll (LNLC)
regions (ocean biological deserts), suitable for artificial enrichment, in the northern Indian ocean (Arabian Sea and
Bay of Bengal).
3. Materials and Methods
The study area comprises of Arabian Sea and Bay of Bengal, northern Indian Ocean. The Indian sub-continent
divides the northern Indian Ocean region into North West Indian Ocean (NWIO) and North East Indian Ocean
(NEIO). NWIO comprises Arabian Sea and Lakshadweep Seas and the NEIO consists of Bay of Bengal and
Andaman Seas. Due to the asymmetric shape imposed by the existence of Asian sub-continent an unique
atmospheric circulation known as monsoon circulation develops, which affect the ocean surface circulation
extending beyond the equator up to 10
o
S. Consequently, Indian Ocean north of 10
o
experiences semi-annual
reversal of the surface circulation in response to the changing wind system (Wyrtki, 1973).The
seasonally-reversing monsoon winds are divided into south-west (June-September) and north-east
(December-March) monsoon phases with two transition periods, spring inter monsoon (April-May) and fall inter
monsoon (October-November). The large-scale open-ocean, seasonally-reversing currents are known as monsoon
currents. Contrasting oceanographic regimes in NWIO and NEIO are produced due to asymmetrical distribution
of fresh water and energy arising from the continental topography and monsoon dynamics. The NWIO region is
affected by intense upwelling during the south–west monsoon and convective mixing during the north-east
monsoon that enriches the surface waters with essential nutrients, resulting in high rates of primary production
(Prasanna Kumar and Prasad, 1996). In contrast, an excess of precipitation over evaporation characterizes the
NEIO. Most of the sub-continent’s rivers drain into this region, which results in a strong thermohaline
stratification of the upper water column. As a result, despite large terrestrial inputs of nutrients NEIO is
characterised by relatively low primary production.
Monthly ocean colour datasets derived from SeaWiFS Level 3 monthly composites of Arabian Sea and Bay of
Bengal have been used in this study. Data sets were obtained from Goddard Distributed Active Archive Center
(
) for the years January 1998 through December 2007 at 9 km pixel resolution. To
