Species composition and vertical distribution of planktonic organisms in Lakes Lalolalo, Lanutavake, and Lano on the Pacific island of Uvea were investigated in relation to physicochemical water column profiles of temperature, conductivity, dissolved oxygen, and pH. The meromictic lakes Lalolalo (maximum depth 88.5 m) and Lanutavake (23.6 m) exhibited a strong chemocline with anoxic conditions below 10 m depth. Mixis was inhibited by sheltered topography, thermal stratification, and in Lalolalo a strong halocline. Crustaceans and rotifers were limited to the oxygenated epilimnion, but diversity and density of ciliates were highest within and below the chemocline. In Lalolalo, euryhaline rotifers dominated the community, reflecting the brackish condition of the lake. Species richness and densities were highest within the shallow lake Lano (3.5 m). On a calm day, hypoxic conditions occurred near the sediment-water interface, but higher oxygen concentrations were observed after heavy winds, indicating occasional mixis. In total, 32 phytoplankton species, 23 ciliate taxa, 18 rotiferan, 1 cladoceran, 1 copepod, and 1 gastrotrich species were identified in the pelagic zones of all three lakes.
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Vol. 68 • No. 3