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Productivity in a peri-urban mangrove: does sewage exposure have an impact?
Mohamed, M.O.S.; Mangion, P.; Mwangi, S.; Kairo, J.; Dahdouh-Guebas, F.; Koedam, N. (2008). Productivity in a peri-urban mangrove: does sewage exposure have an impact?, in: Mohamed, M.O.S. Are peri-urban mangrove forests viable? Effects of sewage pollution and wood exploitation on the structure and development of the mangroves of Mombasa (Kenya). pp. 82-101
In: Mohamed, M.O.S. (2008). Are peri-urban mangrove forests viable? Effects of sewage pollution and wood exploitation on the structure and development of the mangroves of Mombasa (Kenya). PhD Thesis. Free University Brussels. Laboratory of Plant Science and Nature Management: Brussel. 180 pp., more

Available in  Authors 
    VLIZ: Open Repository 143012 [ OMA ]

Keywords
    Mangroves; Nutrients (mineral); Sewage disposal; Marine

Authors  Top 
  • Mohamed, M.O.S., more
  • Mangion, P.
  • Mwangi, S.

Abstract
    This study presents insights on the productivity of an under-valued, over-exploited and sewage polluted peri-urban mangrove through litter fall studies on three common mangrove species over a period of two years. These species R. mucronata, A. marina and S. alba. The study site has been exposed to raw sewage for decades, dozed with sewage every tidal cycle, with the loading exponentially reducing with distance from the source. We observed a strong correlation between the leaf C:N ratio and leaf δ15N signature. Higher C:N ratio for R. mucronata corresponding with lower leaf δ15N (3.88 ± 0.64‰) signature, and lower C:N ration for A. marina and S. alb (6.48 ± 0.03‰ and 6.76 ± 0.24‰ respectively) corresponding with higher δ15N signature. This reflects species specific response to raw sewage exposure. This shows that the forest has a more open N cycle, favouring δ15N accumulation within the system. However, the level of sewage exposure did not appear to impact on litterfall rates. The mean annual litter fall was estimated at 12.16 ± 2.89 t ha-1yr-1 for the whole stand. Litter fall, in both content and quantity was highly seasonal, 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). Productivity varied significantly between species, R. mucronata recording the highest annual rate of 15.34 ± 3.34 t ha-yr-1. No significant differences in litter fall was observed between A. marina and S. alba, (11.44 ± 2.90 and 9.69 ± 5.26 t ha-1yr-1 respectively). This study shows that sewage exposure does not necessarily translate into elevated productivity in mangroves, but may alter leaf nitrate content depending on species, possibly altering the decay of litter, affecting nutrient cycling within the system.

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