Global warming is expected to bring stronger and more unpredictable monsoons to the region. How might these changes affect fish life in the Indian Ocean? A team of researchers tried to answer this question by looking at what happened in the deep past, through evidence left behind in plankton shells dating back 22,000 years on the seabed of the Bay of Bengal. What they found is not reassuring.
Monsoon extremes over the past 10-20,000 years led to disruptions in ocean ecosystems and a crash in marine productivity in the Bay of Bengal, showed their study, published in Nature on Monday. Combining these historical patterns with climate projections, researchers found that intense monsoons caused by global warming could result in similar disruptions in marine productivity in the future.
“We argue that as the monsoon becomes stronger and more variable, productivity collapses,” said Kaustubh Thirumalai, the study’s lead author and a scientist at the University of Arizona. That’s important, he notes, because although the Bay of Bengal covers less than 1% of ocean area, it generates almost 8% of global fishery production. “The hilsa fishery by itself sustains the protein needs of one of the most densely populated regions in the world,” he said.
For the study, researchers analysed fossilised foraminifera, microscopic single-cell zooplankton that hold a record of environmental conditions in their calcium carbonate shells to reconstruct the history of the Indian summer monsoon. Interestingly, they found that both extremes of rainfall – low and high – led to similar disruptions in ocean ecosystems, suggesting that marine productivity depends on a specific range of environmental conditions.
Weak monsoons occurred in a cold phase between 17,500 and 15,000 years ago, while strong monsoons occurred during a period of warming in the early Holocene around 10,000 years ago. Marine productivity declined in both periods.
How does monsoon rainfall over land influence ocean processes? One way is through river runoff which brings fresh water from the Ganga and other rivers into the Bay of Bengal. This fresh water forms a layer on the surface of the ocean, preventing the mixing of water. “If the water doesn’t mix, you don’t have nutrients replenishing the surface ocean, the sunlit part of the ocean where plankton photosynthesise,” said Thirumalai. Plankton are the foundation of the ocean’s food chain.
Warmer oceans – and the Indian Ocean is already warming – can also increase this “stratification” of water layers, and prevent the mixing of nutrients-rich waters. “Climate projections under future boundary conditions show stronger monsoon rainfall, warmer surface waters, and weaker winds – all conditions that echo the past extremes we studied,” said Tirumalai.
The new study is a collaboration between scientists from US, India, and Europe.
Another study, from researchers at the Central University of Kerala published last week, came up with similar findings through the analysis of sediments from the Andaman Sea as well as the Bay of Bengal.
