Algal bloom may spare Indian waters

Degrading algae can cause fish mortality

April 01, 2017 05:44 pm | Updated 05:46 pm IST

The coastal States of India may not suffer from the massive algal bloom that has been reported from the Arabian Sea. Ocean-watchers had earlier reported that a bloom of the size of Mexico, which originated in the Gulf of Oman, had reached the Arabian Sea and feared that it could reach Indian shores.

Those at the Indian National Centre for Ocean Information Services, (INCOIS), Hyderabad, confirmed that the algal species green Noctiluca scintillans had bloomed. The presence of the green algae gives deep green colour to the ocean in the areas of spread.

Researchers at INCOIS say that the current bloom was unlikely to impact the coastal States of the country. The researchers use remote sensing technique for the identification of the bloom species. In-situ studies were also carried out earlier by deploying research vessels to understand the various phases of the algal bloom.

Analysis of the satellite imageries of the bloom areas indicated that it extended from Oman to Gujarat. “Satellite time series images also reveal that this offshore bloom initiates in the Oman waters by January and subsequently intensifies and spreads over almost half of the Arabian Sea till March end. Subsequently, it becomes weak and disappears as the waters start warming by April,” the marine researchers noted.

Though the extension of the bloom towards Gujarat coast varies annually, typically it remains about 15 km away from the shore.

According to Rashmin Dwivedi, a consultant of the INCOIS and an expert in algal bloom, “when Noctiluca cells degrade, associated detritus in the form of particulate organic carbon sinks to deeper waters. During this process, decomposition occurs by the microbes and oxygen that is dissolved in water is consumed for their oxidation.”

The decomposition reduces dissolved oxygen from the water column and causes adverse effect on fish. Secondly, degrading Noctiluca cells release ammonia in the water increasing toxic level and it causes fish mortality, he explained.

Earlier studies in the bloom area had indicated that there was no significant increase in ammonia or decrease in dissolved oxygen during degrading stage of the bloom in the off shore waters of Gujarat.

INCOIS has plans for generating harmful algal bloom advisories, which would keep the Indian fishermen aware of the process, said a communication from S.S.C. Shenoi, Director, INCOIS.

According to researchers, the bloom develops in the Arabian Sea “as a result of a continuous process of winter cooling and convective mixing.

Cool dry continental air from the northeast causes an increase in surface density of the sea water due to evaporative cooling and increased salinity.” The “vertical mixing of the water masses causes recharge of water column with nutrients from the deep. The resulting nutrient enrichment increases the biological production” of the water, they noted.

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