From Swamp to Lake: Recent Changes in a Lowland Tropical Swamp

(1) Aerial photographs show freshwater vegetation changes since the 1950s in Waigani Lake close to Port Moresby, Papua New Guinea. (2) Records of diatom assemblages and plant seeds in sediment cores confirm that the lake was at one time dominated by a mixed emergent swamp, that this was replaced by floating-leaved plants and that the latter were replaced by the present open water. This is characterized as a partial reversal of a hydrosere. (3) Marked variations in organic and moisture content, and phaeopigments, total nitrogen and total phosphorus concentrations with depth in the sediment cores indicate substantial changes in the aquatic environment. Increases in both nitrogen and phosphorus concentrations closely pre-date the major vegetation changes. (4) Water-level change is an obvious cause but cannot explain all features of the vegetational changes observed. Evidence is presented to show that nutrient enrichment could have been influential. These two influences cannot be separated easily in the present record because the same cause--increase in sewage and urban run-off with human development in the catchment-has led to both.