Effects of Urban Wastewater Loading on Macro- and Meio-infauna Assemblages in Subtropical and Equatorial East African Mangroves

Abstract

Benthic infaunal community studies are a useful tool to determine the health of an ecosystem. In this study, an unbalanced ACI (After Control/Impact) design was used to compare the infaunal community structure in a contaminated peri-urban mangrove swamp with two nearby pristine mangroves of similar ecological traits in Kenya and Mozambique. Densities of Oligochaeta decreased significantly in the macro-infaunal communities of contaminated mangroves in both countries. While crustaceans increased and molluscs decreased in the Mozambican peri-urban mangrove, a clear decrease of polychaetes was observed in the Kenyan equivalent. Meiofauna similarly responded in opposite ways, increasing in the Mozambican peri-urban mangrove and decreasing in the Kenyan equivalent. Diversity indices calculated using major meiofaunal groups were lower at contaminated sites. Differences observed between the peri-urban and pristine mangroves may be attributable to domestic sewage-derived contaminants in the former, but differences in mangrove structure and abiotic factors might also play an important role. Differences between the countries may relate to the fact that the peri-urban mangroves are contaminated differently. While in Mozambique, the sewage is discharged diffusely from the peri-urban zone and spreads into the mangrove through the creek, in Kenya it is discharged directly from the landward zone, making it more localized and concentrated.

Keywords: infauna, mangroves, ecological indicators, pollution, Kenya, Mozambique.