In 2008, Punarbasu Chaudhuri, mangrove ecologist from the University of Calcutta spotted an interesting mangrove plant at the bank of river Hooghly inside Kolkata city. It was quite unusual, as mangroves require a cyclic supply of saline water, and this growth at an upstream zone was remarkable. He then started an investigation on their distribution in the Hooghly estuary, and his recent paper suggests that the mangroves have started moving upstream, growing in less-saline regions.
Redistributing plants
After surveying the banks near Kolkata, he was able to spot a few mangroves belonging to the genus Sonneratia. He says that over the years due to gradual environmental changes and anthropogenic activities, mangroves have started to redistribute. The paper, published in Marine Pollution Bulletin, notes that they have reclaimed even the upper course of the river, which was completely devoid of mangroves before 1995.
His team spent years mapping the distribution of mangroves and associated species using ground surveys and satellite data. They also studied the sediments and water samples along the river banks. “With the rapid growth of Kolkata city, sewage disposal has increased the pollution load in the river waters. Globally, there is also rapid mean sea-level rise. All these factors might have played a role in this upstream migration,” explains Dr. Chaudhuri who is with the University of Calcutta’s Department of Environmental Science.
Associates found
The team saw that between Barrackpore and Birlapur, in a non-saline region, about 239 mature trees and numerous saplings of Sonneratia caseolaris (commonly known as mangrove apple) have grown naturally. They were just four to five years old with fruits and flowers, exhibiting luxuriant growth. The team also found the redistribution of several other mangrove associate trees, shrubs and climbers in that region.
“This is not welcome news. They directly indicate changes in the micro-environment. The rate of sedimentation, quality of the sediment and biogeochemistry of the river has all been affected by elevated anthropogenic activities and global climate change events,” says Somdeep Ghosh, first author of the paper who completed his PhD under the guidance of Dr. Chaudhuri from the University of Calcutta.
Change in ecology
The team emphasised the fact that the construction of Farakka Barrage in 1975 has increased fresh water flow in River Hooghly, thereby causing change in ecology and chemistry of the river.
They also found high chemical oxygen demand in the river because of increased release of harmful chemicals from multiple point and non-point sources. Studies from China have shown that Sonneratia caseolaris grow well in the presence of high chemical oxygen demand of water.
“This shows the potential of Sonneratia caseolaris to act as a bio-indicator of regional environmental changes. The decline in the mangrove area along with this migration may increase the amplitude of coastal hazards such as storm surges, erosion and flooding. More studies are needed to understand in detail this new horizon of mangrove adaptation and dispersion ecology. We are also planning to study more rivers in this region to get a detailed picture of this migration,” adds Dr. Chaudhuri.
