Changes in monsoon affect marine productivity in Bay of Bengal: study

The Hindu

29-04-2025

Strong and weak monsoons can influence marine productivity in the Bay of Bengal, a study exploring fluctuations in the Indian Summer Monsoon (ISM) over the past 22,000 years has found. Marine productivity is a proxy for plankton growth – the main source of nourishment for aquatic life. The study is significant given that several climate models warn of significant disruption to the monsoon, under the impact of human-caused warming.
The study, which appears in the peer-reviewed, Nature Geoscience, brought together scientists from India, China, Europe and the United States.
'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,' said Kaustubh Thirumalai, of the University of Austin and lead author of the study. 'Together, these chemical signals helped us understand how the monsoon and ocean conditions responded to global climate changes over the past 22,000 years.'
Despite covering less than 1% of the world's ocean area, the Bay of Bengal provides nearly 8% of global fishery production. Its nutrient-rich coastal waters are vital to the densely populated communities along its shores, many of whom rely heavily on fisheries for food and income.
'Millions of people living along the Bay of Bengal rely on the sea for protein, particularly from fisheries,' said Yair Rosenthal, of the Rutgers University and a co-author. 'The productivity of these waters – the ability of the ocean to support plankton growth – is the foundation of the marine food web. If ocean productivity declines, it will powerfully affect the ecosystem, ultimately reducing fish stocks and threatening food security for coastal communities.'
The study found that both abnormally strong and weak monsoons throughout history caused major disruptions in ocean mixing, leading to a 50% reduction in food for marine life in the surface waters. This occurs because extreme monsoon conditions interfere with the vertical movement of nutrient-rich waters from the deep ocean to the surface, where plankton—the base of the food chain—flourish.
To reconstruct past ocean conditions, scientists analysed fossilised shells of foraminifera, tiny single-celled marine organisms that record environmental data in their calcium carbonate shells. These microfossils were retrieved from seafloor sediments by scientists aboard the JOIDES Resolution, a research ship operating under the International Ocean Discovery Program.
The researchers found that marine productivity declined sharply during periods like Heinrich Stadial 1 (a cold phase between 17,500 and 15,500 years ago) and the early Holocene (about 10,500 to 9,500 years ago), when monsoons were either unusually weak or strong. Monsoon rainfall directly affects river run-off into the Bay of Bengal, altering ocean salinity and circulation. When too much freshwater builds up at the surface, it prevents nutrient mixing. Conversely, weak monsoons reduce wind-driven mixing, also starving surface waters of nutrients.
'Both extremes threaten marine resource availability,' Mr. Thirumalai said.
By comparing ancient patterns with modern ocean data and climate model projections, researchers identified 'worrying' similarities, the authors said in a statement. Future scenarios suggest warmer surface waters and stronger freshwater run-off—conditions linked to past drops in marine productivity. Additionally, weaker future winds may fail to break through ocean stratification and restore nutrient cycling.