Diurnal residence of the larger stages of the calanoid copepod Acartia tonsa in the anoxic monimolimnion of a tropical meromictic lake in New Guinea

Abstract

We studied diel vertical migration of the calanoid copepod Acartia tonsa in a coastal meromictic lake (Lake Nagada, Papua New Guinea). During the day, the animals migrate several metres below the oxycline to just above or into the sediments, and remain in anoxic water. Vertical migration patterns differ between ontogenetic stages; adults and larger copepodites do not occur in appreciable numbers in the water samples during the day (deepest sample taken at ~0.5 m above the sediment). Around sunset, the larger copepodites gradually appear in the water column, reaching the near-surface food layer (0-2 m) only well after sunset. Our results suggest that the evening ascent of the copepods in this tropical lake is related to relative changes in light intensity, and that there are age and sex-related differences in the responses to these stimuli.