Limnological Characterization of the Largest Freshwater Lake in Remote Oceania (Lake Letas, Gaua Island, Vanuatu)

Abstract: With a total area of 19.7 km2, Lake Letas (119 m maximum depth, 399 m above sea level [a.s.l.]) on Gaua Island, Vanuatu, is one of the largest caldera lakes in Oceania. The crescent-shaped lake encircles the active volcano Mount Garet (754 m a.s.l.) whose last major eruption occurred in 2009. In 2012 and 2013, we investigated the lake and its effluent river Mbe Solomul. Physicochemical depth profiles revealed a thermocline between 10 and 15 m depth. Oxygen dropped gradually from 7.67 mg liter-1 at the surface to 0.28 mg liter-1 at 95 m depth, indicating occasional holomixis. Inflow of ion-rich water from volcanic springs (913 µS cm-1) led to relatively high concentrations of (100 mg liter-1) and Cl- (60 mg liter-1) throughout the water column. Conductivity was highest at the surface (610 µS cm-1), decreased to a minimum of 571 µS cm-1 at 60 m depth, and increased slightly to 576 µS cm-1 above the sediment, probably due to solution of ions out of volcanic ashes. Surface pH at 7.8 was remarkably l...