Atmospheric carbon removal capacity of a mangrove ecosystem in a micro-tidal basin estuary in Sri Lanka

Characterization of the micro-tidal mangrove forests in Negombo estuary, located on the west coast of Sri Lanka, with respect to its capacity to remove atmospheric carbon and sequestration in above and below ground plant components is the objective of this study. These mangroves constitute both natural stands (e.g. Kadolkele) and woodlots planted, protected and managed by fishermen (e.g. Wedikanda) to extract twigs and branches to construct “brush parks”, a traditional method of fishing in this estuary. Both types of mangrove stands support high species richness and structural diversity, indicating planted mangrove areas have reached a semi-natural state. Allometric relationships were used to calculate biomass increment. Differences in litterfall, above and below ground biomass increment and net primary productivity (NPP) of the two types of mangrove areas were statistically insignificant, thus indicating that they are structurally and functionally comparable. Average rate of mangrove litterfall in Negombo estuary was 802 ± 25 g m⁻²y⁻¹. The average above ground biomass increment was 1213 ± 95 gm⁻²y⁻¹ and below ground increment was 267 ± 18 gm⁻²y⁻¹, thus the average NPP of these mangroves was 2282 ± 125 g m⁻²y⁻¹. NPP showed a decreasing trend from water towards land, in line with plant density and leaf area index. A statistically significant relationship was found between vegetation structure (represented by complexity index), NPP and rate of organic carbon accumulation in mangroves. Potential carbon accumulation capacity of mangroves of Negombo estuary was estimated to be approximately 12 t ha⁻¹y⁻¹, which is equivalent to the amount of carbon emitted as CO2 through combustion of 19,357 L of diesel or 22,212 L of gasoline in motor vehicles. Potential atmospheric carbon removal capacity of Negombo estuarine mangroves (350 ha in extent) therefore was estimated to be 4143 t y⁻¹, which is equivalent to removal of CO2 emitted through combustion of 6,779,000 L of diesel or 7,779,000 L of gasoline within a year.