|Sea grasses in Seychelles (Grida.no)|
The Seychelles archipelago has huge expanses of sea grasses, one of the largest in the Western Indian Ocean. Most people here mistake sea grasses for “seaweeds”. But sea grasses are flowering plants like plants on land unlike “seaweeds” which are algae.
For years many marine conservationists and agencies have given sea grasses stepdaughter treatment. Yet, these ecosystems have extremely important roles to play as habitats, food sources, and sediment stabilizers. Sea grass meadows are well frequented by fish and therefore are targeted by some trap fishermen. About 50 species of fish are found in the sea grasses around the granitic islands of Seychelles.
Sadly, sea grasses are declining not only in Seychelles but globally as well. In Seychelles, we have lost many hectares of sea grasses through coastal reclamation, siltation and freshwater runoff. A study found that river effluents had a major impact on sea grass meadows near the river mouths.
But now because carbon is such a key factor in climate change, sea grasses have assumed another very important function. They are an important store of ‘blue carbon’, the carbon stored by marine life. A paper published recently in Nature Geoscience estimates that around 3 tonnes of carbon are stored in every hectare of living sea grass. The paper says that an average of 140 tonnes of organic carbon are stored in the top metre of each hectare of sea grass soil, which is around twice that found in soils on land.
Sea grass meadows cover between 30-60 million hectares (around 0.2% of the area of the oceans) and between 4.2 to 8.4 petagrams of organic carbon (one petagram is equal to a thousand million tonnes) are stored in the top metre of seagrass soils. A less conservative estimate suggests the figure could be as high as 19.8 petagrams. Soils on land, by comparison, cover 15 billion hectares and contain between 1500-2000 petagrams of organic carbon. A further 75.5 to 151 teragrams of carbon are stored in seagrass itself (one teragram is equal to one million tonnes).
There are no estimates yet for the Indian Ocean region, but Mediterranean sea grass meadows have the largest stores of organic carbon currently known. In the Mediterranean, sea grass soils containing 11-metre thick organic carbon deposits that have built up over thousands of years have been found.
In Seychelles we have 8 species of sea grasses. Almost thirty percent of the inshore reef areas of Mahe, Praslin and La Digue are covered by sea grasses. On Praslin, where the reef flat is wide, the sea grasses extend to one kilometer in width. Because they need sunlight for photosynthesis, sea grasses are usually found in shallow water.
The outer islands also host sea grass meadows. The marine vegetation of the low coral islands is characterised by dominant communities of Thalassodendron. This sea grass plays an important role in the stabilization of unconsolidated sediments in the atolls. Surprisingly, on African Banks and Providence island, sea grasses have been found at depths of up to 25 meters.
Seychelles has a huge opportunity to trade in “blue carbon”. But first we need to know more about our sea grass meadows – their size, distribution and how much carbon they can capture.
Adapted from the author’s column in The People newspaper, 9/11/2012
Reference: Fourqurean, J. W., Duarte, C.M. Kennedy, H. et al. (2012). Seagrass ecosystems as a globally significant carbon stock. Nature Geoscience. 5(7): 505–509.
Click for the paper: http://goo.gl/kOhRZ