South Asia

  • Mangrove planting can rapidly silt up lagoons

    Tharaka Gamage

    17/05/10

[COLOMBO] Planting mangroves can disturb the natural flow of water in estuaries and lagoons and rapidly clog them with sediments, a study in Sri Lanka shows.

The December 2004 Asian tsunami which devastated coastal Sri Lanka was followed by a mangrove-planting drive as the best protection against similar catastrophes in the future.

The study, funded by the International Union for Conservation of Nature (IUCN) and due to be released later this month, has re-appraised mangrove planting in micro tidal barrier-built estuaries [characterised by limited opening into the sea] and lagoons.

"Mangroves grow at the edge of the water, make loose sediment stable and then march onwards into the water. When matters are left to nature the process of filling up is slow,'' Jaympathy Samarakoon, the lead scientist of the IUCN study, told SciDev.Net.

''By planting mangroves, without giving consideration to water flow, quantity and distribution, the natural balance is upset," Samarakoon, team leader of the Integrated Resources Management Programme on Wetlands in the Central Environmental Authority of Sri Lanka, said.

Lagoons provide food security and livelihood for a large number of poor households that depend on fishery resources. Additionally, during the rainy season, they store excess water before it is carried into the sea at low tide.

Because Sri Lankan coasts have a low tidal range [the difference between high tide and low tide] the estuaries and lagoons are vulnerable to silting.

Such facts have thrown a shadow over the mangrove-planting spree. "Mangroves have been planted for the purpose of extending the land … this is wrong,'' Ranjith Mahindapala, IUCN country  representative, said.

Mala Damayanthi Amarasinghe, head of botany at the University of Kelaniya, has a different view: "Lagoon filling does not take place because of mangroves. Siltation takes place whether mangroves exist or not, because of a natural phenomenon called flocculation [a condition in which small charged particles attach to each other],'' she said.

The IUCN study was based on siltation readings from 1990 to 1994 in the Negombo lagoon, north of Colombo, which shows areas with grasses and mangroves having a sedimentation rate of six millimetres per year compared to 1.5 millimetres in barren areas.