Restored mangroves in Adyar Estuary have substantial carbon storage capacity, finds a case study

A case study by the National Centre for Sustainable Coastal Management (NCSCM) has found that mangrove plantations in Adyar Estuary have stored significant amounts of carbon, despite facing challenges like industrial effluent pollution and urban encroachment. 

These restored mangroves have achieved an annual carbon sequestration rate of 1,195 metric tons of CO² equivalent, the study led by C. Viswanathan, Scientist, Conservation of Coastal and Marine Resources Division, NCSCM. 

The study focused on a 56.6-acre area of mangrove afforestation carried out by the Chennai Rivers Restoration Trust between 2008 and 2016 within the estuary, which had previously been degraded by urbanisation, industrial pollution, and encroachment. The afforested mangrove ecosystem, composed of six different species, has stored a total of 2,288 metric tons of carbon (or 8,397 metric tons of CO² equivalent). 

The dominant species, Avicennia marina, contributed significantly to the carbon storage, with above-ground biomass accounting for 72.89 Mg C (megagram of carbon) per hectare and below-ground biomass for 28.63 Mg C per hectare. 

These numbers far exceed the global median carbon reserves in similar mangrove ecosystems, which average 94.1 Mg C per hectare in above-ground biomass and 34.1 Mg C per hectare in below-ground biomass.

The restored site exhibited a median vegetation biomass of 222.26 Mg per hectare, which, the study said, is an indicator of the ecosystem’s capacity to trap carbon. Further, the mangroves’ carbon sequestration rate — 325.71 Mg C per year — showed their potential to help mitigate climate change.

This restoration effort is especially significant given the degradation of the Adyar Estuary. For decades, the ecosystem suffered from unchecked industrial discharges and untreated sewage, which led to a sharp decline in mangrove cover. 

While Chennai never had as extensive mangrove forests as other coastal districts like Cuddalore or Tiruvarur, it once supported notable stands in the Kosasthalaiyar, Cooum, and Adyar estuaries.

However, industrial and domestic effluents continue to pollute the Adyar River. A report by the Tamil Nadu Pollution Control Board from October 2024 highlighted severe contamination, with fecal coliform levels recorded at 1,026 MPN per 100 ml — more than ten times the permissible limit of 100 MPN per 100 ml. 

A resident living near the Adyar Ecopark, who wished to remain anonymous, said that sewage continues to flow freely into the river and sightings of dead fish floating near the eco-park have become increasingly common. “Addressing this issue would significantly support local biodiversity,” he said.