Tidal and diurnal influence on estuarine zooplankton

Abstract

Observations made at the mouth of Cochin backwaters, Kerala, India show that tidal variations are not much pronounced for most groups and species of estuarine zooplankton. The zooplankton seem to be able to maintain their position within the estuary during tidal exchanges. It is also possible that population of common species which extends over a large area during the period of their abundance oscillates so that the total picture of distribution remains largely the same, although individuals get shifted with tidal motions. Gradual oblique hauls from bottom to surface give a fairly accurate picture of the total zooplankton population irrespective of the time of collection in the deeper parts of the estuary. Vertical migration pattern shows that only a part of the population moves towards the surface during the night. Presence of low saline water in the surface layers inhibits vertical migration of high saline species.