First Limnological Characterization of Crater Lake Billy Mitchell (Bougainville Island, Papua New Guinea)

In this study we present a first limnological characterization of Lake Billy Mitchell [1,013 m above sea level (a.s.l.), 88.3 m depth, 3 km2 surface area] in central Bougainville Island, Papua New Guinea. Physicochemical depth profiles indicated mixis of the entire water body with oxygen saturation reaching 55% in the deepest layers. A shallow thermocline was eroded at night, indicating atelomixis.HCO 3 - , Cl - , SO 4 2 -and Na + , Ca 2+ , Mg 2+ were the dominant anions and cations, respectively, leading to a conductivity of around 1,230 μS cm -1 . The pH was close to neutral throughout the water column, and no accumulation of CO 2 was observed at greater depths. With a total phosphorus concentration of around 25 μg liter -1 the lake can be considered as meso-to eutrophic. The phytoplankton community consisted of 18 taxa. The dinophyte Peridiniopsis cf. penardii and the filamentous green alga Planctonema lauterbornii dominated in the uppermost layer and reached a total biovolume around 16 mm 3 liter -1 . Six macrophyte taxa were found (three Spermatophyta/three Bryophyta), with the water chestnut Eleocharis dulcis covering the shoreline and Ceratophyllum demersum spreading to at least 3 m depth. Seven ciliate species were detected (<5 individuals ml -1 ) with bacterivorous scuticociliates and the prostomatid Coleps hirtus hirtus dominating the assemblage. The micrometazoan plankton community comprised the rotifer Anuraeopsis fissa, the copepod Mesocyclops cf. affinis, and a cladoceran species with-in the Ceriodaphnia cornuta group all concentrating in the upper water column. The only fish species found in the lake was the eel Anguilla megastoma , whereas in the effluent river this species occurred together with Anguilla marmorata .