Climate change is causing toxic metals trapped in the sediment beds of the Hooghly estuary in the Indian Sunderbans to leach out into the water system due to changes in ocean chemistry, say scientists, warning of potential human health hazards.
They predict that after about 30 years, increasing ocean acidification - another dark side of spiked atmospheric levels of carbon dioxide - could in fact unlock the entire stock of metals like copper and lead gathered in the sediment layer, and release them into the water system, leading to health issues.
Sunderbans is the world's largest mangrove forest and home to the endangered Royal Bengal tiger. More than two-thirds of the forest lies in Bangladesh and the rest in West Bengal.
Oceans act as cleansers by taking up a chunk (around one-fourth) of carbon dioxide emitted into the atmosphere. As atmospheric carbon dioxide levels shoot up, the levels absorbed by oceans increase, lowering their pH (indicator of acidity) and making them more acidic (ocean acidification).
Through the water, toxic metals are finding their way into the muscles and tissues of certain edible finfish, popular in the Indian Sunderbans area in West Bengal and because of the food chain, they pose a threat to human health as well, say researchers.
"This ocean acidification is leading to release of the toxic, carcinogenic metals into the water. Our study based on 30 years of real-time data (from 1984 to 2013) forecasts a significant lowering of pH after a period of 30 years due to ocean acidification. This is an offshoot of climate change.
"As a result, this will lead to the movement of entire biologically available copper and lead (but not zinc) from the underlying sediment compartment to the overlying aquatic phase," Abhijit Mitra, advisor, Oceanography, Techno India University (TIU) here, told IANS.
Existing data shows global oceanic pH has decreased by 0.1 pH units (25 percent increase in acidity) since the onset of the industrial revolution and is projected to decrease by up to another 0.4 pH units by 2100.
Published in the Journal of Energy, Environment and Carbon Credits, it confirms the role of ocean acidification in the funnel shaped Hooghly estuary bordering the western fringes of Indian Sunderbans - the world's largest continuous mangrove forests.
The Unesco World Heritage Site is known for its exceptional biodiversity in flora and fauna with as many as 334 plant species and 693 species of wildlife which include 49 mammals, 59 reptiles, eight amphibians, 210 white fishes, 24 shrimps, 14 crabs and 43 mollusks species.
The selected study station lies 2.8 km off the Namkhana island, located almost at the confluence of the Bay of Bengal and the estuary.
The study was a collaborative effort between researchers TIU, Calcutta University, Jadavpur University, Dhruba Chand Halder College, Barasat, Chanchal College, Malda and University Putra, Selangor, Malaysia.
Researcher Sufia Zaman, who studied the bioaccumulation of the toxic metals in edible fish species such as Goldspot mullet (commonly called parshe), noted an aberration: though there was "no significant increase" in industrial activities in the area over the years, there was an "increasing trend" of toxic metals in the coastal water.
The explanation: ocean acidification causing the metals to leach out from the sediment bed.
"The other sources are trawlers, which have lead paints on their underside, shrimp farms that release ammonia from the waste water and tourism activities," said Zaman, associated with TIU.
Mitra warned high levels of copper and lead could cause intestinal disorders and brain damage.
"The forecast values of dissolved zinc, copper and lead will touch 698.98 ppb (parts per billion), 497.65 ppb and 76.60 ppb respectively after a period of 30 years," said Pardis Fazli of Malaysia's University Putra.
Often referred to as another carbon dioxide problem besides global warming, ocean acidification is being implicated across the world for its impact on coral reef formation.
"For Indian Sunderbans, the present forecast values strongly justify the consideration of acidification phenomenon in order to develop a sound management action plan in context to heavy metal pollution monitoring and control," said Prosenjit Pramanick of TIU.
(Sahana Ghosh can be contacted at sahana.g@ians.in)
