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.
Mangroves in India:
- Mangroves represent a characteristic littoral (near the seashore) forest ecosystem.
- These are mostly evergreen forests that grow in sheltered low lying coasts, estuaries, mudflats, tidal creeks backwaters (coastal waters held back on land), marshes and lagoons of tropical and subtropical regions.
- Mangroves grow below the high-water level of spring tides.
- The best locations are where abundant silt is brought down by rivers or on the backshore of accreting sandy beaches.
- Mangroves are highly productive ecosystems, and the trees may vary in height from 8 to 20 m. They protect the shoreline from the effect of cyclones and tsunamis.
- They are breeding and spawning ground for many commercially important fishes.
- Since mangroves are located between the land and sea, they represent the best example of ecotone.
- Mangroves are shrubs or small trees that grow in coastal saline or brackish water. Mangroves are salt tolerant trees, also called halophytes, and are adapted harsh coastal conditions.
- Mangrove vegetation facilitates more water loss. Leaves are thick and contain salt-secreting glands. Some block absorption of salt at their roots itself.
- They contain a complex salt filtration system and complex root system to cope with saltwater immersion and wave action. They are adapted to the low oxygen (anoxic) conditions of waterlogged mud.
- They produce pneumatophores (blind roots) to overcome the respiration problem in the anaerobic soil conditions.
- Mangroves occur worldwide in the tropics and subtropics, mainly between latitudes 25° N and 25° S.
- They require high solar radiation to filter saline water through their roots. This explains why mangroves are confined to only tropical and sub-tropical coastal waters.
- Mangroves occur in a variety of configurations. Some species (e.g. Rhizophora) send arching prop roots down into the water.
- While other (e.g. Avicennia) send vertical “Pneumatophores” or air roots up from the mud.
- Adventitious roots which emerged from the main trunk of a tree above ground level are called stilt roots.
- Mangroves exhibit Viviparity mode of reproduction. i.e. seeds germinate in the tree itself (before falling to the ground).
- This is an adaptive mechanism to overcome the problem of germination in saline water.
Redistributing plants: Reasons for moving Up-stream:
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.
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.
Importance of Mangroves:
Mangrove plants have (additional) special roots such as prop roots, pneumatophores which help to impede water flow and thereby enhance the deposition of sediment in areas (where it is already occurring), stabilise the coastal shores, provide a breeding ground for fishes.
- Mangroves moderate monsoonal tidal floods and reduce inundation of coastal lowlands.
- They prevent coastal soil erosion.
- They protect coastal lands from tsunami, hurricanes and floods.
- Mangroves enhance the natural recycling of nutrients.
- Mangrove supports numerous florae, avifauna and wildlife.
- Provide a safe and favourable environment for breeding, spawning, rearing of several fishes.
- They supply woods, firewood, medicinal plants and edible plants to local people.
- They provide numerous employment opportunities to local communities and augments their livelihood.
About Coringa Mangroves:
Recently, Andhra Pradesh Government has constituted a seven-member committee for fulfilment of norms required for proposing the Godavari Mangroves (Mada forests), at Coringa, as a World Heritage Site.
Godavari Mangroves at the Coringa Wildlife Sanctuary (CWLS) are touted to be the second largest mangroves in India. The largest mangrove forest in the world is Sundarbans, West Bengal.
The mangrove forests in Andhra Pradesh are located in the estuaries of the Godavari and the Krishna rivers. The Godavari mangroves are located in Godavari estuary of East Godavari district.
Along with the mangrove forest, the Hope Island, a naturally formed sandy stretch amidst the sea that turned into a synonym for biodiversity, too comes under the purview of the sanctuary.
The sanctuary also has a site where Olive Ridley Sea Turtles nest from January to March every year.
Once the Coringa sanctuary gets the heritage site tag, UNESCO will help develop tourism and protect the wildlife in the mangroves.
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 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.
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.
The decline in the mangrove area along with this up stream less saline areas migration may increase the amplitude of coastal hazards such as storm surges, erosion and flooding.
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.
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.