Assessment of Blue Carbon Stock of Mangroves at Malwathu Oya estuary, Sri Lanka

Mangroves occupy tropical and subtropical inter-tidal areas and provide numerous ecological functions and services. One such function is sequestration of atmospheric carbon, especially in their anoxic soils. This study is an attempt to quantify the total blue carbon stocks in vegetation and in soils of mangrove areas in Malwathu Oya estuary located in the dry/arid climatic zone in Sri Lanka. Plant biomass and the total organic carbon (TOC) content were estimated using allometric relationships available between biomass/TOC and stem diameter of constituent mangrove species. Published data on soil TOC of the same locality was used to estimate the total blue carbon stock associated with this mangrove area. Total biomass of the mangrove plants in Malwathu Oya estuary was 377 Mg ha-1 which contained 191 Mg C ha-1 of organic carbon sequestered by the plants. Above ground plant components contained 5 times more biomass and 5.5 times more carbon than those below ground. TOC in the mangrove soils was 346 Mg C ha-1 which was 1.8 times more than that in the vegetation. Total blue carbon stock of the mangrove ecosystem in Malwathu Oya estuary therefore was 537 Mg C ha-1, out of which 64% was sequestered in soils. Despite being in low rainfall coast this mangrove area retains more carbon than most other areas which may be possibly due to its pristineness and vegetation structure characterized by unique dominance of Sonneratia alba, which is considered a rare species among Sri Lankan mangroves.