LIMNOLOGICAL FEATURES OF LAKES ON THE SEPIK-RAMU FLOODPLAIN, PAPUA-NEW-GUINEA

The Sepik-Ramu floodplain is one of the major wetland areas in south-e astern Asia. This paper reports preliminary limnological data on 26 la kes on the lower and middle reaches of the floodplain. Conductivity, p H and alkalinity increase from upstream lakes to downstream lakes. A f irst attempt is made to classify the lakes according to water colour a nd sediment load, as indicators of local hydrology. Humic-stained blac k waters can be distinguished from sediment-loaded white waters as wel l 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. Extinctio n 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 repr esent a range of stratification regimes, from nonstratified lakes to l akes stratified over prolonged periods. Local hydrology, basin morphom etry and local topography seem to be the major factors controlling str atification regime. Phytoplankton biomass was lowest in black-water la kes (2.3 X 10(5) mu m(3) mL(-1)), and high values (1.7 X 10(8) mu m(3) 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.