DIURNAL RESIDENCE OF THE LARGER STAGES OF THE CALANOID COPEPOD ACARTIA-TONSA IN THE ANOXIC MONIMOLIMNION OF A TROPICAL MEROMICTIC LAKE IN NEW-GUINEA

We studied diel vertical migration of the calanoid copepod Acartia ton sa in a coastal meromictic lake (Lake Nagada, Papua New Guinea). Durin g the day, the animals migrate several metres below the oxycline to ju st above or into the sediments, and remain in anoxic water. Vertical m igration 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 similar to 0.5 m above the sed iment). Around sunset, the larger copepodites gradually appear in the water column, reaching the near-surface food layer (0-2 m) only well a fter sunset. Our results suggest that the evening ascent of the copepo ds in this tropical lake is related to relative changes in light inten sity, and that there are age- and sex-related differences in the respo nses to these stimuli.