Abstract

The Sepik-Ramu floodplain is one of the major wetland areas in south-eastem Asia. This paper reports preliminary limnological data on 26 lakes on the lower and middle reaches of the floodplain. Conductivity, pH and alkalinity increase from upstream lakes to downstream lakes. A first attempt is made to classify the lakes according to water colour and sediment load, as indicators of local hydrology. Humic-stained black waters can be distinguished from sediment-loaded white waters as well as from a number of transitional mixed waters. All the lakes studied had very shallow optical depths (Secchi depths between 0.25 and 1.84 m), owing to either high gilvin concentrations or turbidity. Extinction coefficients for downward irradiance, measured in three lakes, were higher in surface water layers (K_d = 3.4 - 19.5 m^-1) than in deeper layers (K_d = 2.5 - 11.0 m^-1). The data suggest that the lakes represent a range of stratification regimes, from nonstratified lakes to lakes stratified over prolonged periods. Local hydrology, basin morphometry and local topography seem to be the major factors controlling stratification regime. Phytoplankton biomass was lowest in black-water lakes (2.3 × 10⁵ µm³ mL^-1), and high values (1.7 × 10⁸ µm³ mL^-1) were recorded in white-water and stratified mixed-water lakes. The preliminary data suggest that there may be considerable seasonal fluctuation of phytoplankton biomass.