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

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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 lower than measured in 2004 (8.9), possibly as a result of the increased volcanic activity. A Secchi depth of 1.3 m indicated meso- to eutrophic conditions. In contrast, total phosphorus concentration at the surface was relatively low at 11.8 µg liter-1. In total 47 phytoplankton taxa were identified, of which most occurred in the littoral area. Open water was dominated by the filamentous green alga Planctonema lauterbornii and the coccoid desmid Staurastrum depressiceps var. gracilis. Total phytoplankton biomass of 1.06 mg liter-1 in the epilimnion reflected the mesotrophic character of the lake. The ciliate assemblage included euplanktonic taxa (Halteria, Mesodinium, Coleps, and Stentor) that are commonly detected in temperate and tropical lakes. The zooplankton community was dominated by the copepod Thermocyclops crassus macrolasius, which reached a maximum abundance of 138 ind. liter-1 at 5 m depth. The littoral was characterized by a dense macrophyte belt consisting predominantly of Chara australis and Ceratophyllum demersum. The vegetation limit was found at 4.8 m mean depth. Two freshwater eel species, Anguilla megastoma and A. marmorata, were the only fishes found in the lake, feeding exclusively on macrozoobenthos. In the effluent river the largest Anguilla obscura hitherto reported in literature was caught (129 cm), showing that these ecosystems are a habitat of primary importance to Pacific eels.