Long-Term Impacts of Sewage Effluent Disposal on a Tropical Wetland

Abstract

Waigani Lake, near Port Moresby, Papua New Guinea, is part of an extensive wetland dominated by the Laloki and Brown Rivers. The wetland has received sewage effluent from stabilisation ponds for over 25 years. Water quality of the sewage, Waigani Lake and its outflow assessed in 1985 indicated that the wetland was significantly reducing suspended solid loads and the concentrations of dissolved nitrogen and phosphorus. Aerial photographs of Waigani Lake taken between 1966 and 1991 document the complete loss of submerged and floating-leaved plants and the decline in the extent of the littoral, emergent vegetation. These changes are related to sewage effluent disposal and, possibly, alterations in the magnitude of annual water level fluctuations. This study of a tropical wetland to which very large quantities of sewage effluent have been added demonstrates that the capacity of wetlands for water purification is limited and that further study on tropical wetlands is imperative before management strategies developed for temperate wetlands are applied directly to them.