Subscribe
Extremely weak or strong Indian summer monsoon (ISM) could reduce marine life in the Bay of Bengal, putting food security at risk for millions of people, warned a new study.
Both extreme strong or weak monsoon over centuries have caused a 50 per cent drop in food available for marine life at the surface, the study published in the journal Nature Geoscience noted.
In addition to being the prime source of freshwater, ISM can also affect marine ecosystems across the northern Indian Ocean.
Despite covering less than 1 per cent of the global ocean, the Bay of Bengal supplies nearly 8 per cent of the world’s fishery production.
“Millions of people living along the Bay of Bengal rely on the sea for protein, particularly from fisheries,” Yair Rosenthal, 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, said in a statement.
The productivity of these waters, which is the ability of the ocean to support plankton growth, is the foundation of the marine food web. Plankton include tiny plants called phytoplankton and weak-swimming animals called zooplankton. Phytoplankton produce their own food and release oxygen as a byproduct. All the other living organisms in the ocean either directly or indirectly rely on them for food or oxygen.
If ocean productivity declines, meaning when plankton growth drops, the ecosystem is expected to change, ultimately reducing fish stocks and threatening food security for coastal communities, the expert added. Studies have shown that climate change could make the monsoon intense and variable, with impacts on marine life.
“Previous observational studies have noted how monsoon swings on interannual timescales can impact marine productivity in the Bay of Bengal,” Kaustubh Thirumalai, an assistant professor at the University of Arizona and lead author of the study, told Down To Earth. But the future projections on the impacts in the Bay of Bengal is highly uncertain.
The new study focuses on centuries to millennia timescales, and makes projections for the future.
The researchers examined the interaction between summer monsoon runoff, stratification (separation of water into different layers) and marine primary productivity in the Bay of Bengal over the past 22,000 years by studying the fossil shells of foraminifera — tiny single-celled plankton that live in the ocean.
These organisms have calcium carbonate shells, which keep a record of environment (past ocean and climate conditions) they grew in. They specifically looked at the isotopic ratio of oxygen on the shells to reconstruct the sea surface temperature over the last 22,000 years. More Oxygen-18, for example, in the sediment indicates cooler climate.
Their analysis showed that Bay of Bengal’s productivity collapsed during very weak monsoons, such as Heinrich Stadial 1, a significantly cold period, occurred between 17,500 and 15,500 years ago.
The same pattern unfolded during very strong monsoons, such as those seen in the early Holocene, a period marked by rapid warming and sea level rise because of melting glaciers, occurred between about 10,500 and 9,500 years ago, according to the study.
The researchers explained that the amount of monsoon rainfall controls the volume of freshwater discharge from rivers into the Bay of Bengal, which can alter oceanographic conditions and the feeding cycle of fish and plankton.
During intense monsoon, the freshwater influx can form a layer on the ocean surface, blocking nutrients from below reaching the top sunlit layers, which can be detrimental for plankton growth. Plankton float near the upper water layer.
Without nutrients, plankton growth drops, which means the entire food chain, including fish, will not have enough food.
Weaker monsoons, on the other, reduces ocean circulation and wind-driven mixing. This, in turn, blocks nutrient supply to plankton.
Future projections, too, appear dire. With warmer surface waters and stronger freshwater runoff from rivers, conditions in the future could match past records. The researchers also warned that with climate change, future winds may not be strong enough to allow mixing of different layers of water in the sea for nutrient supply.
“Our findings show that the impact of global warming on the Bay of Bengal in the next hundred years can rival that observed over these century-scale to millennial-scale swings,” Thirumalai said.
The team next plans to study the Arabian Sea. “The drivers, controls and types of marine biology in the Arabian Sea are vastly different than those in the Bay of Bengal. The atmospheric and oceanic anomalies affecting these regions were also different in the past,” the expert explained. If the team were to find similar results in the Arabian Sea, it could mean that large, synchronous climate impacts are occurring across broader regions in the northern Indian Ocean.
