Detection and study of blooms of Trichodesmium erythraeum and Noctiluca miliaris in NE Arabian Sea
S.
G.
Prabhu Matondkar, National Institute of Oceanography, Dona-Paula, Goa, INDIA, sgpm@nio.org
R.
M.
Dwivedi, Space Application Centre, Ahmedabad, Gujarat, rmdwivedi@rediffmail.com
J.
I.
Goes, Bigelow Laboratory for Ocean Sciences, West Boothbay Harbor, Maine, 04575, USA, jgoes@bigelow.org
(Presenting)
H.
R.
Gomes, Bigelow Laboratory for Ocean Sciences, West Boothbay Harbor, Maine, 04575, USA, helgadorosario@gmail.com
S.
G.
Parab, National Institute of Oceanography, Dona-Paula, Goa, INDIA, sparab2002@nio.org
S.
M.
Pednekar, National Institute of Oceanography, Dona-Paula, Goa, INDIA, surakshap@nio.org
The Arabian Sea is subject to semi-annual wind reversal associated with the monsoon cycle that result in two periods of elevated phytoplankton productivity, one during the northeast (NE) monsoon (November-February) and the other during the southwest (SW) monsoon (June-September). Although the seasonality of phytoplankton biomass in these offshore waters is well known, the abundance and composition of phytoplankton associated with this distinct seasonal cycle is poorly understood. Monthly samples were collected from the NE Arabian Sea (offshore) from November to May. Phytoplankton were studied microscopically up to the species level. Phytoplankton counts are supported by Chl a estimations and chemotaxonomic studies using HPLC. Surface phytoplankton cell counts varied from 0.1912 (Mar) to 15.83 cell x104L-1 (Nov). In Nov Trichodesmium thiebautii was the dominant species. It was replaced by diatom and dinoflagellates in the following month. Increased cell counts during Jan were predominantly due to dinoflagellates Gymnodinium breve, Gonyaulax schilleri and Amphidinium carteare. Large blooms of Noctiluca miliaris were observed in Feb a direct consequence of the large populations of G. schilleri upon which N. miliaris is known to graze. In Mar and April, N. miliaris was replaced by blooms of Trichodesmium erythraeum. This succession of species was distinctly reflected in the phytoplankton pigments measured by HPLC. In Nov high concentrations of zeaxanthin (0.5 mgm-3) and β-carotene (0.52 mgm-3) were mainly due to T. thiebautii while high concentrations of peridinin (0.4 mgm-3) were due to the large populations of dinoflagellates observed in Jan. During the N. miliaris bloom both Chl b (3.76 mgm-3) and prasinoxanthin ( 0.11 mgm-3) were reasonably high. The large amount of organic matter produced by these blooms export organic matter in subeuphotic depths. The monitoring of this export production is done with the help of OCM (IRS-P4I) during present study.
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