Extreme monsoon changes threaten the Bay of Bengal's role as a critical food source

(Press-News.org) New research involving Rutgers professors has revealed that expected, extreme changes in India’s summer monsoon could drastically hamper the Bay of Bengal’s ability to support a crucial element of the region’s food supply: marine life.

The study, published in Nature Geoscience, was conducted by scientists from Rutgers University, the University of Arizona and collaborators from India, China and Europe. To reach their conclusions, the scientists examined how the monsoon, which brings heavy rains to the Indian subcontinent, has influenced the Bay of Bengal’s marine productivity over the past 22,000 years.

Although the Bay of Bengal covers less than 1% of the global ocean, it supplies nearly 8% of the world’s fishery production. Its coastal waters support densely populated regions that rely heavily on marine resources for food and livelihoods. 

“Millions of people living along the Bay of Bengal rely on the sea for protein, particularly from fisheries,” said Yair Rosenthal, a Distinguished Professor in the Department of Marine and Coastal Sciences and the Department of Earth and Planetary Sciences at Rutgers University and an author of the study. “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 monsoon is essential for providing freshwater to the region, but the researchers found that both extremely strong and extremely weak monsoon periods over the centuries caused a significant disruption – a 50% reduction in food available for marine life at the surface. This occurred because these extreme conditions inhibited mixing between the deep and surface zones of the ocean, preventing nutrients from reaching the upper region where marine life thrives.

With climate change expected to make the monsoon more intense and variable, and those extremes provoking stratification of the ocean layers, the food supply produced by the Bay of Bengal may be threatened, researchers said.

To understand how the Indian summer monsoon and ocean productivity have changed over time, scientists studied the fossil shells of foraminifera – tiny single-celled plankton that live in the ocean and build calcium carbonate shells. The shells preserve information about the environment they grew in, acting like natural recorders of past ocean and climate conditions. 

“By analyzing 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 geoscientist Kaustubh Thirumalai, an assistant professor at the University of Arizona 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.”

The sediments analyzed were recovered from the seafloor by scientists aboard the National Science Foundation-funded research vessel JOIDES Resolution as part of the International Ocean Discovery Program.

The researchers found that productivity of the Bay of Bengal’s waters collapsed during periods of very weak monsoons, such as Heinrich Stadial 1, and very strong monsoons, such as those in the early Holocene. The period known as Heinrich Stadial 1, a significantly cold period, occurred between 17,500 and 15,500 years ago. The early Holocene, a time marked by rapid warming and sea level rise because of melting glaciers, occurred between about 10,500 and 9,500 years ago.

The amount of monsoon rainfall controls the volume of river discharge into the Bay of Bengal. The freshwater significantly changes oceanographic conditions and affects the feeding cycle of fish and plankton. When monsoon rains are too intense, a freshwater layer can cap the ocean surface, blocking nutrients from below. Without nutrients, plankton growth drops – and with it, the entire food chain, including fish. Weaker monsoons also suppress nutrient delivery by reducing ocean circulation and wind-driven mixing.

“Both extremes threaten marine resource availability,” Thirumalai said.

When researchers compared ancient patterns with modern ocean data and model projections, they found an unsettling parallel: Future scenarios project warmer surface waters and stronger freshwater runoff, conditions that match past intervals when marine productivity dropped sharply. Compounding the risk, future winds may not be strong enough to counteract the stratification that suppresses mixing.

Looking at past climate patterns helps scientists understand how the interconnected components and processes that compose the physical Earth, including its atmosphere and biosphere, affect the climate, environment and all organisms over long timescales.

“The relationship between monsoons and ocean biology we have uncovered in the Bay of Bengal gives us real-world evidence of how marine ecosystems have reacted to warming and monsoon shifts and may do so in the future,” Rosenthal said. “These insights can help refine projections and inform sustainable management of fisheries and coastal resources as the impacts of climate change accelerate.”

Kaixuan Bu, an assistant research professor with the Department of Marine and Coastal Sciences at Rutgers-New Brunswick, also contributed to this study.

Explore more of the ways Rutgers research is shaping the future.

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