Are Peri-Urban Mangroves Vulnerable? An Assessment Through Litter Fall Studies

The productivity of an over-exploited and sewage polluted peri-urban mangrove was assessed through litter fall studies to establish vulnerability to human actions and climate change. Litter from three common mangrove species, Rhizophora mucronata Lam. (Rhizophoraceae), Sonneratia alba Sm. (Sonneratiaceae), and Avicennia marina (Forssk.) Vierh. (Avicenniaceae) were monitored over a period of two years. The mean annual litter fall was estimated at 12.16 ± 2.89 t ha⁻¹yr⁻¹. Litter fall was seasonal in both content and quantity, with high rates occurring in the dry North Easterly Monsoon (NEM) season, January-April (ca. 5.10 ± 1.36 g DW m⁻² day⁻¹) and lower rates in the cool and wet South Easterly Monsoon (SEM) season, June-October (ca. 2.53 ± 0.47 g DW m⁻² day⁻¹). Litter fall varied significantly between species, R. mucronata recording the highest annual rate (15.34 ± 3.34 t ha⁻¹yr⁻¹), with no significant difference between A. marina and S. alba, (11.44 ± 2.90 and 9.69 ± 5.26 t ha⁻¹yr⁻¹ respectively). Sewage exposure did not affect litter fall rates for all species, but affected leaf nutrient content as expressed by the leaf δ¹⁵N signature. A strong correlation between leaf C:N ratio and leaf δ¹⁵N signature was observed, indicating a more open N cycle, favouring δ¹⁵N accumulation. Sewage exposure therefore does not necessarily translate into elevated productivity in mangroves, but causes alteration of leaf nutrient content depending on species. Prevailing climatic conditions however, may influence litter fall and thus phenology and health of the system. The vulnerability of the Tudor Creek mangroves to climate change may be ranked high due to low production (20%<) of reproductive materials and a die back of seaward fringe S. alba stand.