India’s changing monsoon patterns and continuously warming climate could drastically weaken the Bay of Bengal’s ability to support marine life – a crucial part of the region’s food supply. The latest study has long-term consequences for fisheries and coastal livelihoods.
Flanking India’s vast east coast, the Bay of Bengal supports densely populated regions that rely heavily on marine resources for food and livelihoods. Even though it covers just 1% of the global ocean, the Bay of Bengal supplies nearly 8% of the world’s fishery production.
“Millions of people living along the Bay of Bengal rely on the sea for protein, particularly from fisheries. 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,” said author Professor Yair Rosenthal from the Department of Marine and Coastal Sciences at Rutgers University.
The team from Rutgers University, the University of Arizona, and collaborators from India, China and Europe found that both extremely strong and extremely weak monsoon periods over the past 22,000 years caused a nearly 50% reduction in food available from marine life at the surface. They published their findings in the peer-reviewed Nature Geoscience.
This may occur because the monsoon, which provides over 75% of the annual rainfall over the country during four months from June to September, is also essential for providing freshwater to the region. However, due to global warming and rising temperatures, this is changing because the southwest monsoon is becoming more erratic and extreme over the years.
INDIA’S MONSOON TURNING MORE ERRATIC
According to researchers, the extreme conditions in the past stopped the deep and surface waters from mixing, blocking nutrients from reaching the upper layers where marine life depends on them. With climate change expected to make the monsoon more intense and erratic, those extremes provoking stratification of the ocean layers, the food supply produced by the Bay of Bengal may be threatened, they said.
This is likely because the 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.
The study showed that the productivity of the Bay of Bengal’s waters collapsed during periods of very weak monsoons – a period between 17,500 and 15,500 years ago, and very strong monsoons, such as those in 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 team’s future projections showed that warmer surface waters, strong freshwater run-off and weaker winds could create conditions for a sharp fall in marine productivity in the Bay of Bengal.
“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,” said Rosenthal highlighting the need for sustainable management of fisheries and coastal resources as the impacts of climate change accelerate.
The team studied the fossil shells of foraminifera – tiny single-celled plankton that live in the ocean and build calcium carbonate shells, since they preserve information about the environment they grew in, acting like natural recorders of past ocean and climate conditions.
By analysing their chemistry and tracking the abundance of certain types that thrive in productive waters, they reconstructed long-term changes in rainfall, ocean temperatures and marine life in the Bay of Bengal. “Together, these chemical signals helped us understand how the monsoon and ocean conditions responded to global climate changes over the past 22,000 years,” said lead author geoscientist Kaustubh Thirumalai from the University of Arizona.
The sediments analysed were recovered from the seafloor by scientists aboard the research vessel JOIDES Resolution, funded by the National Science Foundation-funded as part of the International Ocean Discovery Program.