Changes of the mangrove cover of Rekawa lagoon, Sri Lanka, over a 38 year period: mitigating the significance of a surface increase

Abstract

From an economic and ecological point of view, Rekawa lagoon is one of the most important brackish water bodies on the southern coast of Sri Lanka. The change of its floristically rich mangrove belt, over a 38-year period from 1956 to 1994, was studied using airborne remote sensing and ground surveys. The mangrove area of the lagoon has increased by 29.3% over the period concerned. Apparently this may suggest that mangrove conservation has been successful and perhaps one might claim that necessary steps should be taken to control the invasion of the land by mangroves. Paradoxically in depth analysis contrasts this view. The cause for the increase in mangrove area appears to be an increased freshwater inflow to the lagoon by recent irrigation works that have caused the water level to rise and the inundation of the low lying grassy or muddy plains by brackish water to occur more frequently, mainly at the western end of the lagoon. Actual increase of the mangrove cover is about 49.1%, but during the same period 21.2% of the previous mangrove cover has also been destroyed, leaving 29.5% increase as the net change. The disappeared mangrove area was dominated mainly by species with higher economic and ecological value, viz. Rhizophora mucronata and Lumnitzera racemosa, with closed canopies, whilst newly grown area is dominated mainly by less valuable species, viz. Aegiceras corniculatum, Avicennia officinalis and Excoecaria agallocha, with open canopies in low densities. Moreover, as revealed from the remote sensing study, the changes of crown sizes of mangrove species over the 38-year period indicate that the whole forest has been subjected to overexploitation. All these facts together led to the conclusion that ecological and economic value of mangrove vegetation of Rekawa has not increased commensurately with the spatial extent of the mangrove area over the period concerned. The need for active conservation and management measures is therefore still high.