An investigation on the effect of sewage on leaf litter decomposition rate in a mangrove forest at Kunduchi bay – Dar es Salaam, Tanzania

The main objective of this study was to investigate the impact of sewage inundation on leaf litter decomposition rates in a mangroves forest. Dynamics of weight, carbon (C), nitrogen (N) and C:N ratios in decomposing R. mucromata and A. marina mangroves leaf litter were investigated for 8 consecutive weeks using litterbag method in the constructed mangrove wetlands located at Kunduchi bay. Split split-plot design was used, whereby leaf litter formed the main plot, incubation period of 0 to 8 weeks were allocated to sub plot and sewage inundation level (0% amd 100%) formed the sub-plot. Under both bare and vegetated sediments, results showed that sewage and the interaction among sewage, leaf litter species and duration of incubation had significant effect on leaf litter decomposition rates, as well as nitrogen, carbon and C:N ratios. Decomposition rate was low in sewage inundated sediment at kd-1 OF 0.4% for R muronata and kd-1 was 0.14% for A. marina. Bare sediments showed the lowest decay rates than the vegetated sediments. For example, under no sewage inundation, kd-1 for R mucronata was 1.3% and for A. marina kd-1 was 0.2% while in vegetated sediment with non sewage inundation R. mucronata had kd-1 of 1.46% and A. marina kd-1 was 0.86%. in conclusion, sewage seems to retard mangrove leaf litter decomposition rates and litter mineralization. Since sewage was seen to inhibit the rate of leaf litter decomposition, thus the use of mangroves for phytoremediation is unsustainable. It is therefore recommended that mangroves should not be used as bio-filters for treatment of sewage.