As global warming intensifies, monsoon patterns in South Asia are expected to become stronger and more unpredictable. Scientists are now warning that these changes could severely impact fish populations in the Indian Ocean, especially in the Bay of Bengal. A new study published in Nature looks to the past to understand what the future may hold — and the findings are concerning.
Fossil clues from the deep past
Researchers analysed the shells of foraminifera — microscopic plankton that record environmental data in their calcium carbonate shells — from sediment cores dating back 22,000 years in the Bay of Bengal. The data revealed that both extremely weak and strong monsoons in the past caused sharp declines in marine productivity.
“We argue that as the monsoon becomes stronger and more variable, productivity collapses,” said Kaustubh Thirumalai, the study’s lead author and a scientist at the University of Arizona.
Bay of Bengal: Small area, large impact
“We argue that as the monsoon becomes stronger and more variable, productivity collapses,” says Thirumalai. And this is vital because though the Bay of Bengal covers less than 1% of the global ocean area, it contributes about 8% to global fishery production.
“The hilsa fishery by itself sustains the protein needs of one of the most densely populated regions in the world,” he says, adding that the disruptions in this ecosystem could have serious consequences for food security.
How monsoons impact marine life
Monsoon rainfall over land leads to increased river runoff, especially from major rivers like the Ganga. This freshwater forms a surface layer in the ocean, which blocks the mixing of deeper, nutrient-rich waters with the surface. “If the water doesn’t mix, nutrients don’t reach the sunlit zone where plankton grow,” Thirumalai explained. Since plankton form the base of the marine food chain, their decline affects the entire ecosystem.
Parallels between past and future
The study found that weak monsoons occurred during colder periods between 17,500 and 15,000 years ago, while strong monsoons aligned with warming around 10,000 years ago. In both cases, marine productivity declined. Today, with the Indian Ocean already warming, similar conditions are expected — stronger monsoons, warmer surface water, and weaker winds — increasing the risk of ocean stratification and reduced nutrient mixing.
Findings supported by other research
This study, a collaboration between scientists from the US, India, and Europe, is echoed by a recent study from the Central University of Kerala. That research, based on sediment from the Andaman Sea and Bay of Bengal, arrived at similar conclusions about the threats of extreme monsoons to marine life.
