Rising CO2 level taking toll on marine organisms in Sunderbans, say scientists

The rising carbon dioxide (CO2) level in the air is turning the water in the Sunderbans estuary more acidic and taking a toll on marine fish and animals, particularly those having shells such as oysters, clams, crabs and lobsters, say scientists.

A team of marine scientists working in the Sunderbans has found that the pH level of water in the estuary, located at the southern tip of West Bengal, dropped from around 8.3 in the early 1980s to around 7.9 in 2019.

“While in the western part of the Sunderban, which is closer to urban areas, the pH has dropped to around 7.9 since 1984, in the central part of the delta, the decline is slower and has dropped to 8.2 during the same period,” said Abhijit Mitra, former head of the marine science department of Calcutta University.

The acidity of a substance is measured using the pH scale. A substance with a pH of 7 is considered neutral. An acid has a pH of less than 7. The normal range for pH in packaged drinking water as per Indian Standard is between 6.5 and 8.5.

At Namkhana in western Sunderbans, the rate of decline is 0.024 units per decade in the pre-monsoon and monsoon times. In the post-monsoon time, it is 0.019 units per decade. At Ajmalmari in the central part of the delta, the decadal decline is 0.012 units during pre-monsoon and 0.017 during post-monsoon.

“But in the estuary, this slight decline in pH level is taking the toll. Not only has the population of shelled organisms, such as oysters, clams, crabs, and lobsters, declined, their shell size has become thinner and they have become smaller in size,” said Mitra. He added they have little data from east Sunderbans which is closer to Bangladesh.

The population of shelled organisms has dropped by around 35%-40% in the western part of the delta, where the decline in pH is more. The size of Saccostrea cucullata, a species of edible oyster around Kakdwip and Namkhana fishing harbour in the Sunderban, has dropped by over 40%. In the central Sunderbans, the size of the species has dropped by around 20%.

Mitra and his fellow researcher Sufia Zaman have written a book “Estuarine Acidification: Exploring the situation of mangrove dominated Indian Sunderban estuaries”.

Fishermen working in the Sunderban area too have found that the size of several shell organisms including crabs and prawns has diminished.

“It is true that the size of several fish and other animals found in the sea, rivers and creeks of the Sunderban such as crabs and prawns are getting smaller. Earlier we used to get big crabs. Now we hardly get any big ones. Most are smaller in size. But unbridled fishing and catching of these fish and animals could also be a reason,” said Bijan Maity of Kakdwip Matsya Unnayan Samity, the apex body of fishermen and trawler owners.

Scientists explained that human blood has a pH of 7.35 to 7.45. Just as the slightest drop (0.2 -0.3 units) in pH level can make you sick, the drop in pH level can also take a heavy toll on sea animals.

“Shelled sea animals such as oysters and mussels depend on the calcium and carbonate ions in the water to form their shells with calcium carbonate. But with rising acidity, these carbonate ions become scarcer and more inaccessible to the animals. The result is that they can’t form the shell properly and may become smaller in size,” said Punyasloke Bhadury, assistant professor of biological sciences at the Indian Institute of Science Education and Research.

Corals need calcium carbonate to build reefs. Acidification of seas would not just corrode old coral skeletons but would also slow down the growth of new ones. The reefs would in turn weaken and become vulnerable to erosion.

“Ocean acidification is a part of global warming and climate change. The biggest example of corals getting affected by all these is bleaching. Corals are very sensitive to their surroundings and the slightest difference would affect them. The only way to reduce this is to limit the carbon footprint,” said Chowdula Satyanarayana, a scientist working on corals.

But it is not just the Sunderban alone where the pH level has dropped. Mitra and his team of researchers have found that such changes are also going on in other estuaries too.