Long before industrialization, abrupt shifts in India’s summer monsoon slashed plankton levels in the Bay of Bengal, halving the ocean’s surface food supply. A new study¹, based on 22,000-year-old ocean sediments, suggests that future climate-driven monsoon variability could disrupt marine ecosystems and food security for millions.

An international team examined sediment cores from the bay’s Mahanadi Basin, revealing that both extreme droughts and intense rainfall events severely reduced marine productivity. They traced the problem to a disruption in water stratification.

Nutrient-rich freshwater from both Himalayan and peninsular rivers creates a highly productive layer in the Bay of Bengal. Monsoon rainfall directly affects river runoff into the Bay of Bengal, altering ocean salinity and circulation. When monsoons are strong, a thick surface layer of freshwater blocks the vertical mixing of deeper, nutrient-rich waters. Weak monsoons, meanwhile, reduce wind-driven circulation.

Both scenarios limit essential nutrients like nitrate and phosphate from reaching sunlit waters, where plankton, the foundation of the ocean’s food web, flourish. These swings crippled productivity, the ability of the ocean to support plankton growth, in the past. “We wouldn’t have gleaned these insights from human-made measurements alone,” says Kaustubh Thirumalai, a sedimentologist at the University of Arizona, and the study’s lead author.

Unlike the Arabian Sea, which benefits from strong seasonal upwelling, the Bay of Bengal—which receives approximately 3.1 million trillion liters of fresh water in a normal year—depends on wind mixing and eddies, making its stratified waters especially vulnerable to climate change, says geoscientist Sambuddha Misra at the Indian Institute of Science, Bengaluru.

Yet, until now, scientists have been split over how intensified monsoons affect the Bay’s ecosystem, or to what extent. “Predictive models for the Indian subcontinent and the Bay of Bengal disagree strongly regarding the direction of change in rainfall amounts,” says Misra, who was not associated with the study.

The study offers rare, high-resolution data from individual foraminifera — microscopic plankton that record seawater conditions in their shells. By analysing three species that lived at different depths Globigerinoides ruber, Trilobatus sacculifer and Neogloboquadrina dutertrei, during the contrasting monsoon states, the team reconstructed water column chemistry across thousands of years.

The use of individual shell isotopic data preserved subtle, seasonal signals that bulk sampling would have averaged out. Scientists, including Thirumalai, aboard the JOIDES Resolution drillship extracted the cores as part of the Indian Ocean Discovery Programme. These ocean-floor samples, comprising sediment layers deposited over thousands to millions of years, provide a detailed archive of past marine conditions.

A study in contrasts

Two major crashes in plankton productivity stood out. The first, during the Heinrich Stadial 1 cold period (17,500–15,500 years ago), saw weakened monsoons reduce river runoff and wind mixing, warming and salinizing surface waters while starving plankton of nutrients. The second, in the early Holocene (10,500–9,500 years ago), occurred during intense warming and glacial melt. Torrential rains produced a thick freshwater cap on the ocean, again blocking deep-water nutrients from reaching the surface.

Not every strong or weak monsoon caused such disruption. The bay appears to cross ecological tipping points only when specific runoff, wind, and ocean mixing thresholds are breached. “Monitoring slow baseline shifts that can unexpectedly lead to pronounced ecological impacts gives us a framework to understand how the system behaves and where it might break,” Thirumalai says.

That understanding has contemporary relevance. “We now see falls of up to 30% in chlorophyll levels — the plankton equivalent of a crop yield — in years with heavy monsoon rainfall,” says co-author Arvind Singh of the Physical Research Laboratory in Ahmedabad. “The stratification is already choking productivity.”