Isotopic and microbial indicators of sewage pollution from Stone Town, Zanzibar, Tanzania

In Stone Town, Zanzibar, sewage treatment is minimal, with a biological oxygen demand reduction to 60% and no removal of bacteria or nutrients. Here, Stone Town's sewage pollution was studied by measuring Enterococci and NH4+ concentrations in seawater and δ15N of benthic organisms; samples were collected along the Stone Town shoreline and from offshore coral reefs. Public perceptions of sewage pollution were investigated via interviews. Enterococci from the Stone Town shoreline exceeded USEPA guidelines for recreational use. Benthic organisms from two of the four reefs were relatively enriched (δ15N > 10‰), indicative of sewage derived N. δ15N values of organisms from Stone Town exceeded 16‰. A strong correlation was found between Enterococci and δ15N across sites, while step-wise regression indicated rainfall and tidal stage as important predictors for bacterial concentrations. These data provide an important impact assessment from which the efficacy of future policy and management change can be assessed.

► We examine the impact of sewage pollution on Stone Town's water quality and reef health. ► Precipitation and tide appear to control pollution distribution. ► δ15N results indicate the presence of sewage-derived nitrogen pollution in nearshore reefs. ► High bacterial concentrations suggest that Stone Town's waters are unsafe for recreational use.