Limnological Characterization of Three Tropical Crater Lakes in the Archipelago of Samoa (Lanoto‘o, Olomaga, Mataulano)1

Abstract: We present a first limnological study of three humic crater lakes in the archipelago of Samoa. The basins of Lakes Lanoto‘o (17.5 m deep) and Olomaga (12.2 m) on Upolu Island and of Lake Mataulano (5.6 m) on Savai‘i Island developed during consecutive periods of volcanic activity ranging from the middle Pleistocene to the late Holocene. Lake Olomaga may be a permanently stratified meromictic lake, while stratified lakes Lanoto‘o and Mataulano contained oxygen down to the bottom. Forty-seven phytoplankton and 8 zooplankton taxa were identified in the pelagic zones of the three lakes. The Samoan endemic Diaphanosoma samoaensis, the cyclopoid Mesocyclops roberti originally described from neighboring archipelagos, and a hitherto undescribed species of Microcyclops were recorded. Lake Lanoto‘o is the only lake with introduced fish: goldfish (Carassius auratus) and tilapias (Oreochromis niloticus) were stocked and it harbored more than four times more phytobiomass (8 mg L-1) than the smaller, unstocked lakes (1.2–1.6 mg L-1). Fish introduction poses a threat to unstocked lakes in Samoa. Measures should be taken to protect them from any alterations.