Quantitative studies were made on zooplankton collected along with hydrographic data from ten fixed stations in the backwaters of Cochin during the post and pre-monsoon period and collections were taken both by day and night.

During the post-monsoon period because of the previous heavy monsoonal rainfall and consequent influx of freshwater the estuarine salinity structure shoed a steep gradient (0.17%₀-31.3%_o ) and zooplankton biomass was high in those stations towards the mouth of the estuary (upto 562 mg/m³).

In the pre-monsoon period because of the lower rainfall and the instruction of sea water into the estuary, the whole system became highly saline with a weaker gradient (28.0%_o-34.4%_o) and the biomass and the faunal composition at the different stations showed remarkable variations (92-26782 mg/m³).

The temperature did not vary as much as salinity in either season, but slightly from day to night.

The overall changes in temperature-salinity and the associated changes in the biomass inside the system are closely associated with the monsoons and the changes effected by the adjacent coastal waters.