In a development that will ring alarm bells for both environmentalists and policy makers, the mangrove forest cover in the Indian Sunderbans has been depleting alarmingly over the past few decades.

Mangrove Forest Cover Changes in Indian Sundarban (1986-2012) Using Remote Sensing and GIS, a publication by the School of Oceanographic

Studies, Jadavpur University, reveals that from 1986 to 2012, 124.418 sq. km. mangrove forest cover has been lost.

The total forest cover of the Indian Sunderbans as assessed by remote sensing studies for the year 1986 was about 2,246.839 sq. km., which gradually declined by 2,201.41 sq. km. in 1996, then down to 2168.914 sq km in 2001 and to 2122.421 sq km in 2012. The loss in the mangrove forest in the Indian Sunderbans is about 5.5 %.

“The continuation of this process in response to climate change and sea level rise poses a serious threat to the carbon sequestration potential and other ecosystem services of this mangrove forest in future,” authors SugataHazra and KaberiSamata noted.

The paper also notes that the mean sea level rise at the Sagar Island Station, measured from 1985 onward till 2010, shows a rise by 2.6-4 mm a year, which can be considered a driving factor for coastal erosion, coastal flooding, and an increase in the number of tidal creeks.

The publication highlights a time series of the erosion of at least 18 mangrove forested islands of the Indian Sunderbans from 1986 to 2012. For instance, the loss in mangrove cover at Gosaba has been about 20%, down from 517.47 sq km in 1986 to 506.691 sq km in 2012.

Significant losses

In Dulibhasani West, the loss of mangrove cover has been about 9.7% — from 180.03 sq. km. in 1986 to 163.475 sq. km. in 2012. The mangrove forest cover of Dalhousie, another island, has depleted by 16%, from 76.606 sq. km. in 1986 to 64.241 in 2012. Bhangaduni has one of the highest erosion levels of mangrove forest land, from 40.4 sq. km. in 1986 to 24.9 sq km in 2012, taking the loss to over 37%.

Jambudwip, one of the smallest uninhabited islands at the mouth of the sea, also has reduced forest cover from 6.095 sq. km. in 1986 to 5.003 sq. km. in 2012, or about 10%.

Other islands like Sajnekhali North, Matla and Bulchery have also suffered significant mangrove loss.

Professor Hazra, who heads the School of Oceanographic Studies, Jadavpur University, explained how climate change and sea level rise has contributed to the phenomenon of losing land, including mangrove forests in the Sundarbans, in the last part of the 21st century.

“This is because there is less fresh water flow and sediment supply in the western (Indian) part of the delta, so we have starvation of sediment and the rate of sea level rise is higher than sediment supply. Hence we are losing land, including mangrove forest,” he told The Hindu.

According to Professor Hazra, the eastern (Bangladesh) side of the delta is gaining land because of the huge amount of sediment and water flow from the Brahmaputra and Meghna rivers. The loss of forest cover occurs despite significant addition of forest land as plantations.

Freshwater inflow

Ajanta Dey, joint secretary and project director of the Nature Environment and Wildlife Society (NEWS), an NGO that has been working in the Sunderban ecosystem, said that a critical minimal inflow of freshwater is necessary for the luxuriant growth of mangroves.

“When freshwater inflow is missing, there is a change in mangrove succession, and freshwater loving species of mangroves are replaced by salt-water loving ones,” she pointed out.

She said the immediate impact of salinity will be on the fishing community, where commercially sought after fish species will be replaced by fish that does not have as much market value.

Ms. Dey also referred to a report by the Indian Space Research Organisation (ISRO), which was presented before the Eastern Circuit Bench of the National Green Tribunal in 2015.

The report pointed out that the Sunderbans has lost 3.71% of its mangrove and other forest cover, while losing 9,990 hectares of its landmass to erosion in one decade.

While earlier studies also expressed concerns over the fragile ecosystem of the Indian Sunderbans that, other than being home to the Royal Bengal Tiger, also harbours a population of 4.5 million people, this study presents definite proof of the loss of land and mangrove cover.