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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.
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