e - poz 2 - Central Institute of Brackishwater Aquaculture

The Shannon-Weaver diversity index (HI of the phytoplankton varied from 1.89 atStation 5 to 2.75 at Station 2. A mean diversity index valued 2.43 was recdfor the whole lake. The Station 2 showed the maximum diversity value andrecorded 35 species of phytoplankton whereas station 5 had only 29 speciesrecording lower diversity values. This may be attributed to the comparativelys:aMe environment and a lower fluctuation of salinity at this place. The Stationwas also charmerid by the dominance d the green algae.The zooplankton population in the 12 stations was categorised into 12 groups,namely, Foraminifera, Tintinnida, Protozoa, Rdifera, Nematoda, Polychaetelarvae, Cladocera, Copepoda, Mysidaceae, Ostracoda, lnsecta and Insect larvae,and Fish larvae. The foraminiferans were present only in the marine zone of thelake. The post-monsoon period recorded the highest numerical density ofplankton whereas the monsoon period showed the lowest values. The widevariations in the zobplankton distribution were influenced mainly by the incidenceof phytoplankton, pH and salinity. The present investigation is the firstcomprehensive report on zooplankton from this sector of the Vembanad lake.The premonsoon period, maked by a higher mean salinity, exhibited highernumerical abundance of the zooplankton fauna. As mentioned above, themonsoon and the post-monsoon periods with low saline conditions had restrictedabundance of the zooplankton. Apart from salinity, the strong currentsdownstream and the turbidity also might have rendered an unfriendly environmentto many organisms during the monsoon.Primary produrtivityThe gross and net productivity at stations 1 and 3 in the Vembanad Lake werestudied. The gross productivity values showed an average value of 0.99 g Um'ldayand the net productivity of 0.51 g Um'lday during the period 199697. Qasim eta/. (1969) reported a gross productivity value ranging from 272 to 293 g Um'lyearand the net productivity from 184 to 202 g Umllyear in the Vembanad lake duringthe prebanage phase. Nair et a/. (1975) reported an average primary productivityof 200 mg Um'lday from this sector of the lake. The average productivity values