Distribution and Ecology of the Trichodesmium spp. in the Arabian Sea: Ship and Satellite studies
نویسندگان
چکیده
-Trichodesmium is a filamentous cyanobacteria which forms dense blooms along the west coast of India. These are high biomass blooms which are mostly surface floating. The chlorophyll a content of water crossed >100 μgl -1 during active bloom and hence was ideal for the detection by space. Besides the Arabian Sea, the Lakshadweep Sea, the Andaman Sea and the Bay of Bengal are also well known sites for the Trichodesmium bloom. During our special cruises to the Arabian Sea we collected data to help us to understand the seasonality of Trichodesmium spp. Trichodesmium starts appearing just after the SW monsoon in the offshore waters of the Arabian Sea. We have noticed massive blooms of Trichodesmium thiebautii when water temperatures were low (26 0 C). T. thiebautii was also found to be distributed throughout the water column. It was also observed in the coastal waters during March and in the Lakshadweep waters during April. The OCM images taken during these observations have been used to map and estimate the concentration of T. thiebautii. Low temperatures and calm condition are required for Trichodesmium thiebautii to bloom. According to our study, November-December appears to be the time during which this bloom flourishes in offshore waters of the Arabian Sea. T. erythraeum blooms were noticed in the coastal waters during the March to May and in the offshore waters during the April-May period. During the months from March-April a mixed bloom of Trichodesmium with Noctiluca miliaris was also observed. We have generated a series of OCM imageries from January to March in order to locate the appearance of these blooms in different regions of the Arabian Sea. . However in a typical Trichodesmium bloom chlorophyll a values are in the range of 2-5.905 μgl -1 . We have recorded upto 99.49% of Trichodesmium in the phytoplankton population. The Trichodesmium filaments varied from 74 to 516400 filamentsl -1 . Goa was found to be one of the most prominent site for Trichodesmium occurrence and the chlorophyll a level during our study varied from 187.2 to 2080 μgl -1 which is extremely high. From our data collected on various cruises it appears that the depletion of nitrate by the diatoms supplied by the upwelling during the SW monsoon triggered the T. thiebautii bloom in the offshore waters of the Arabian Sea. Similar conditions developed in the coastal waters during March-April-May period, which witnessed with higher water temperatures causing the T. erythraeum grow in bloom. Meanwhile offshore waters are conducive for Trichodesmium after April as they support mixed blooms of the Trichodesmium spp. in the offshore region. These events have been depicted by satellite images and discussed.
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