Mangrove litter production and organic carbon pools in the Mngazana Estuary, South Africa

Abstract

Wood harvesting is reducing the density of adult mangrove trees in the Mngazana Estuary. This is expected to decrease the amount of litter produced as well as the availability of organic carbon to the estuary and the nearshore environment. Pools of organic carbon were identified and measured in non-harvested areas of the forest so that the effect of future (mangrove) harvesting on organic carbon production could be determined. Litter production was higher in summer (2.4 ± 0.2gm^–2 d^–1) than in winter (0.3 ± 0.1gm^–2 d^–1). Leaf litter on the forest floor was minimal, as a result of tidal flushing and consumption by crabs. Using previous studies it was hypothesised that the fate of litter produced was as follows: 43% is consumed by crabs, 10% decomposes on the forest floor and is taken into the sediment by bacteria, forming a sink of dissolved organic carbon, and the remaining 47% is exported out of the creeks and the estuary to the sea. Particulate organic carbon (POC) that is available for export was derived from crab faeces and the mechanical breakdown of leaves by crabs and tidal action. Significantly higher concentrations of POC were found during summer than winter and significantly more during a spring tide (5.3 ± 0.9mg l^–1) than a neap tide (3 ± 0.4mg l^–1). Dissolved organic carbon (DOC) concentrations were mostly below detectable limits (<0.2mg l^–1). The estuary is probably a sink for DOC, as very little to no DOC was exported out of the estuary. Organic matter comprised 12% (0.3g C) and 9% (0.46g C) of the sediment in the mangrove forests in Creeks 1 and 2, respectively, these concentrations being higher at Mngazana than at other estuaries in South Africa. If mangrove harvesting continues at the current rate of 1ha per year, and if there is no re-growth, no mangrove vegetation will be left after 118 years, and as a result no POC would be exported. But if harvesting increases to 3ha per year, the mangrove forest would disappear after just 40 years. Mngazana Estuary is an important source of mangrove litter and POC for the adjacent marine environment, possibly sustaining nearshore food webs.

Keywords: Dissolved organic carbon, harvesting, litter production, mangroves, particulate organic carbon, Rhizophora mucronata, South Africa

African Journal of Aquatic Science 2007, 32(1): 17–25