In-situ measurements of the bio-optical properties of the seawater are important to develop algorithms for seawater
constituent estimation using satellite remote sensing. A data collection campaign was conducted for bio-optical
characterization of the open and coastal waters of the Arabian Sea during April 15-29, 2006. Bio-optical measurements
were made using the Satlantic hyper-spectral underwater radiometer (Hyperpro-II) for 13 sampling stations include
oligotrophic, Trichodesmium bloom dominated and coastal waters in 400-800 nm spectral range.
For open oceans stations 1% light was available at 50 to 70 meter depth, whereas, for coastal waters it varied from 18 to
35 meter. The deep chlorophyll maxima (DCM) was observed at 30 to 42 meter depth during the bloom conditions with
surface chlorophyll-a concentration ranging between 0.1 to 0.85 mg m-3 whereas, for open ocean and non-bloom
conditions the DCM depth varied from 35 to 60 m with surface chlorophyll ranging between 0.05 to 0.12 mgm-3.
Particulate back scattering coefficient at 700-nm vary from 0.0011 to 0.0031 for bloom waters and 0.00046 to 0.0012 for
open ocean waters. The normalized water leaving radiance computed from these spectra in the spectral bands of IRS-P4,
OCM bands were examined. The global ocean chlorophyll-2 (OC2), and 4 (OC4) algorithms performed reasonably well
for open ocean waters, however both the algorithms overestimated chlorophyll concentration for bloom dominated
waters.
Sequence of the images from IRS P4 / OCM satellite and extensive shipboard sampling programme are used to
understand the seasonal variation of phytoplankton abundance and types in the Northeastern (NE) Arabian Sea and
Lakshadweep Sea. An appreciable degree of spatial and temporal variability is observed in chlorophyll a distribution
from November to April months, as well as coastal and offshore stations, indicating marked seasonality in phytoplankton
distribution in NE Arabian Sea. During November month (fall intermonsoon) average chlorophyll a (Chl a) by
fluorometer was (0.799 mgm^-3) and by OCM it was 0.584 mgm^-3. The higher chlorophyll a observed was due to
Trichodesmium (cyanobacteria) blooms. During December the average chlorophyll a was 0.34 mgm^-3 also due to
Trichodesmium filaments in water column. During January onwards winter cooling led to increase in nutrients which
enhanced chlorophyll a value to 0.64 mgm^-3 due to growth of flagellates (as seen by high chlorophyll b besides
chlorophyll a) in water column. February, March and April supported moderately high chlorophyll value (0. 3 to 0.5
mgm^-3) due to growth of prasinophytes (as seen by pigment prasinoxanthin) and blooms of the Noctiluca miliaris. Time
series monitoring of Noctiluca bloom was also conducted using OCM based chlorophyll images in NE Arabian Sea.
During February chlorophyll a retrieved by OCM was 0.3 to 0.9 mgm^-3. Pigment analysis of water samples indicated the
equal important of accessory pigment like zeaxanthin, prasinoxanthin, beta-carotene. The relevance of these pigments
estimated by HPLC like zeaxanthin (cyanobacteria), fucoxanthin (diatoms), peridinin (dinoflagellates) is presented and
discussed. Similarly, exercise is conducted in Lakshadweep waters where Trichodesmium related peak in chlorophyll a
was observed during March onwards in OCM data. The average chlorophyll a in NE Arabian Sea at surface during
November was (0.726 mgm^-3), December (0.34 mgm^-3), January (0.723 mgm^-3), February (0.344 mgm^-3), March (0.963
mgm^-3) and April 0.665 mgm^-3. Similar trend was observed in primary productivity estimates. The attempt is made to
work out seasonality in the productivity of the Arabian Sea using OCM derived chlorophyll and relation of enhancement
in productivity due to development of winter blooms in the Arabian Sea. The environmental conditions (temperature,
wind, nutrients and mixed layer depth) affecting these blooms responsible for year to year variation in bloom biomass
and productivity is also presented in detail.
Chlorophyll-a maps derived from IRS-P4 Ocean Color Monitor (OCM) was used to study the distribution pattern of phytoplankton biomass in the eastern Arabian sea off Karnataka-Goa coast, southwest coast of India. The data was compared with in-situ measurements of chlorophyll-a concentration estimated for 100 stations covering an area of more than 4000 km^2 in the above region. The presence of dense algal blooms spread over an area of almost 100 km^2 representing Trichodesmium sp. 18km off Kumta-Gokarna in the eastern Arabian Sea depicts high value (30 to 40 mg/m^3) of chlorophyll concentration. Similarly, around the Nethrani Island, off Bhatkal, the surface concentration ranged from 5 to 10 mg/m^3. The secchi depth varied from 4 to 8 m near the island. The sea surface area enveloping this high bloom (around the island) depicts a normal distribution of chlorophyll-a ranging from of 0.1 to 5 mg/m^3. It is suggested here, that the low salinity value (35 to 35.2 %) around the Nethrani Island enhances the algal bloom due to enrichment of nutrients in the shallow marine environment through probable inputs of nutrient charged fresh water from the island aquifers. Near river mouths, the values are marginally high in the range of 3 to 5 mg/m^3, probably enforced by riverine nutrient inputs well depicted by the Tadri River. The satellite (IRS-P4 OCM) derived images of chlorophyll during summer also shows high values as a band parallel to the coast. During the occurrence of algal blooms this band, parallel to the coast, widens offshore and this phenomenon of widening is typically absent during non-bloom summer scenarios, as identified for summer 2005.
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