Growth Response of Selected Mangrove Species to Domestic Sewage and Abiotic Stress

Abstract

The sewage system of Dar es Salaam City, Tanzania, serves only 15% of the population, making sewage one of the leading sources of marine pollution. This study was initiated to assess the potential of peri-urban mangrove forests as filters and phyto-remediators of sewage and the growth of two mangrove species under sewage-inundated conditions.  One-year old Avicennia marina and Rhizophora mucronata wildlings were planted in 10 l buckets and treated with 0, 25, 50, 75 and 100% domestic sewage in triplicates for 12 h, 24 h or 1 week under a split-split plot design. Growth and physico-chemical characteristics of the system were recorded monthly for four months. Treated mangroves grew better than untreated plants and A.marina outperformed R. mucronata. Inundation with 75-100% sewage was more effective than <50% sewage but no significant differences were observed due to the relatively brief experimental period. Irrespective of the species, pH and electric conductivity (EC) increased by the end of each inundation. P and N removal by plants was highest in 50 and 75% sewage. Additionally, three sulfide levels of 0, 0.75 x 10^-3, 1.5 x 10^-3 M NH₂S affected mangrove growth when applied in seawater concentrations of 0, 50 and 100%.