The diel cycle of near-surface zooplankton abundance in Tudor Creek, Mombasa, Kenya during the southwest monsoon.

Diel changes in the near-surface abundance of Chaetognatha, Copepoda, molluscan larvae, decapod crustacean larvae and fish eggs and larvae were studied at a fixed station at English Point, Tudor Creek, Mombasa, Kenya on 24-25 June 1985 and 23-25 September 1985. The two dates were selected because they occurred during the same monsoon season but with tidal cycles about 7 hours out of phase with each other, thus with virtually reversed tide curves with respect to the time of day. Surface water temperature, salinity, and pH were monitored at the same time. Physical-chemical parameters showed only small variations between day and night, as follows: temperature varied from 25.0 degree C to 27.0 degree C in June and from 25.5 degree C to 27.0 degree C in September; salinity varied from 34 to 35 % in both months; pH varied from 8.1 to 8.3 in June, and 8.3 to 8.6 in September. In spite of the reversed tidal cycles the total abundance of zooplankton and that of Chaetognatha, Copepoda, Crustacea decapod larvae (excluding Brachyura), and molluscan larvae showed strikingly similar diel cycles among themselves and at both times. A major peak for these taxa occurred at 2130 or 2330 h. Fish larvae also had a major peak at 2330 h in June, but at 0730 h in September. Fish eggs did not conform to the general pattern, showing a major peak at 0130 h in June and 1930 h in September. There were no significant differences in overall abundance in either 24-h series between high and low tide, or between the ebbing and flooding tide, or between abundance and tidal elevation was detected. The observed patterns could be accounted for by cycles of vertical migration. The results suggest that is the major environment cue determining the diel cycle of near-surface abundance and that the phase of the tide is not involved, except possibly for fish eggs.

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