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The ebb and flow of the tides in an estuary affect not only the hydrographical conditions but also to a marked extent the abundance of zooplankton populations. Observations were made during one particular cycle studied during February 1961. Water samples and plankton were collected at regular intervals of two hours at a fixed station in the mid-estuarine region of the river Godavari. The temperature of the waters seems to follow the course of the day and the atmospheric conditions, rather than the tides, whereas the salinity variations during 24-hour period closely followed the tides. The incoming tides contribute to an increase in the salinity of the estuarine waters. The volume of plankton collected appears to have some relationship with the nature of the tide, strength of the current and the direction of the flow of water. The distribution and abundance of different groups of zooplankton in the estuary were found not only to follow the tides, but more so the diurnal rhythm. The abundance of zooplankton during night was many times higher than during the day and particularly copepods and veligers showed greater densities during the night. |
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