An immense underwater engine
Beneath the warm, shallow waters of Sri Lanka's Puttalam Lagoon lies a vast and powerful biological engine. This is one of the largest seagrass ecosystems in the Indian Ocean, a sprawling underwater meadow that is important for carbon sequestration. The lagoon itself covers a large area of 327 square kilometers, and its seagrass beds are a critical component of its ecology and economy, supporting the livelihoods of the 250,000 people living nearby.
These are not seaweeds, but true flowering plants that form dense meadows on the lagoon floor. At least eight distinct species have been recorded here, including the broad-leafed Enhalus acoroides, Thalassia hemprichii, and several species of Cymodocea and Halophila. Enhalus acoroides, a large and structurally complex species, is particularly effective at carbon storage. Studies show it has the highest organic carbon storage in its biomass among the lagoon's species, at approximately 1.34 megagrams of carbon per hectare. A significant portion of this carbon—over 80% in the case of Enhalus—is stored in the plant's belowground root and rhizome systems. This underground network anchors the plant and stabilizes the seafloor, while also trapping particles and organic matter from the water.
Blue carbon powerhouse
The true power of the Puttalam seagrass beds lies in the soil. As the plants die and decompose, the carbon they contain becomes buried in the sediment. The waterlogged, low-oxygen conditions of the lagoon floor prevent this organic matter from breaking down completely, effectively locking the carbon away for potentially thousands of years. This "blue carbon" sink is remarkably efficient. Globally, seagrass meadows are responsible for up to 18% of all carbon burial in the ocean, despite covering less than 0.2% of its surface.
The meadows in Puttalam are a key part of this global system. Research by organizations like the Blue Resources Trust shows their importance for both biodiversity and climate mitigation. These habitats are essential nurseries for a huge diversity of fish and invertebrates, many of which are commercially important. The local fishing industry is highly dependent on the health of these meadows, which provide feeding and breeding grounds for finfish and shellfish. The plants also are natural water filters, trapping sediment and pollutants, which keeps the lagoon's water clear. The meadows are also feeding grounds for migratory water birds and were historically a habitat for the threatened Dugong (Dugong dugon).
Unfortunately, these important ecosystems face numerous threats. Effluent from shrimp farms established in the 1990s, agricultural runoff, and damaging fishing practices like the use of push nets directly harm the seagrass. Increased boat traffic also causes physical damage, with propellers cutting scars into the meadows that can lead to erosion. Conservation efforts and scientific monitoring are underway to protect this natural asset.