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.
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
What are Mangroves?
A mangrove is a shrub or small tree that grows in coastal saline or brackish water.Mangroves occur worldwide in the tropics and subtropics, mainly between latitudes 25° N and 25° S.
The mangrove biome or mangal, is a distinct saline woodland or shrubland habitat characterized by depositional coastal environments, where fine sediments (often with high organic content) collect in areas protected from high-energy wave action.
Upstream migration of mangrove species such as Sonneratia caseolaris, Sonneratia apetala, Derris trifoliata, Hibiscus tiliaceus, and Thespesia populnea has been noticed in River Hooghly
Reasons for Mangrove Migration:
Over the years due to gradual environmental changes and anthropogenic activities, mangroves have started to redistribute.
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
With the rapid growth of Kolkata city, sewage disposal has increased the pollution load in the river waters.
High pollution load has an effect on the chemical oxygen demand and a higher chemical oxygen demand influences mangrove migration upstream.
Globally, there is also rapid mean sea-level rise. All these factors might have played a role in this upstream migration
Consequences of Mangrove Migration
The decline in the mangrove area along with this migration may increase the amplitude of coastal hazards such as storm surges, erosion and flooding.
Migration of mangrove upstream distirb the natural ecological balance and could interfere with the growth and presence of other communities in the food chain.
Now, presently, Change in ecology of mangroves distribution in the Hooghly estuary:
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 caseolarisgrow 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.