New Delhi, Apr 28 (PTI) Food available for the Bay of Bengal's marine life reduced by 50 per cent during extremely strong and extremely weak monsoon periods, according to a study that analysed monsoon and productivity from the water body over the past 22,000 years.

Researchers, including those from the US' University of Arizona and the Indian Institute of Technology Gandhinagar, said that drastic changes to India's monsoon patterns could disrupt Bay of Bengal's marine life, threatening food security and livelihoods for millions. Findings are published in the journal Nature Geoscience.

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Located in the northeastern part of the Indian Ocean, the Bay of Bengal is estimated to contribute more than seven per cent of the world's marine fisheries catch.

Extreme conditions during a strong or weak monsoon can prevent a mixing between the deep and surface regions of the ocean, thereby keeping nutrients from reaching the upper regions where marine life thrives, the team explained.

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For the analysis, the researchers studied the fossil shells of foraminifera.

A tiny single-celled plankton, foraminifera lives in the ocean and builds calcium carbonate shells, which preserve information of the environment they grew in.

The shells, therefore act as "natural recorders of past ocean and climate conditions", the researchers said.

"By analysing their chemistry and tracking the abundance of certain types that thrive in productive waters, we reconstructed long-term changes in rainfall, ocean temperatures and marine life in the Bay of Bengal," lead author Kaustubh Thirumalai, a geoscientist and an assistant professor at the University of Arizona, said.

"Together, these chemical signals helped us understand how the monsoon and ocean conditions responded to global climate changes over the past 22,000 years," Thirumalai said.

Under climate change, the researchers said warmer surface waters and stronger freshwater runoff could be expected -- conditions that match periods in the past when marine productivity dropped sharply.

Winds may not be strong enough to counter the separation of ocean layers that suppresses mixing, they said.

The authors wrote, "we find that (Bay of Bengal) productivity collapsed during both extreme states of peak monsoon excess and deficits -- both due to upper-ocean stratification."

They said rainfall during a monsoon controls freshwater discharged into the Bay of Bengal, which significantly changes the physical, chemical, biological and geological properties of the ocean and affects the feeding cycle of fish and plankton.

During an intense monsoon, a freshwater layer can cap the ocean surface, blocking nutrients from below.

However, a weak monsoon can suppress nutrient delivery to the surface by cutting ocean circulation and mixing of ocean layers driven by winds, the team said.

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