e - poz 2 - Central Institute of Brackishwater Aquaculture

The mean biomass varied from 6.6 mVm3 to 0.3 mVml The values were generallyhigh in stations 1,3 and 5. These stations are located nearer to the areas of humansettlement on ttte western fringe. The average highest value was recorded atStation 5 near the denser settlements. At this station, the average biomass was 11mg/L during October to February reflecting the inflow of the nutrients during themonsoon flow. At Station 1, near to the opening end of the city canals to the Lake,the biomass was as high as 13.5 mum'during this period. The canals receive theeffluents from the urban settlements and are known as the worst polluted segmentof the entire Kuttanad region. At the extreme south of the lake, this station had theleast influence of whatever feeble salinty incursion occurred to the Sector. Thisportion remained stagnant through out the summer season. All the three Stat~onsthat depicted high biomass concentration were on the western fringes of the lake,least influenced by the river.flow even during the monsoon season.PhytoplanktonThe net phytoplankton population in the 12 stations of the Vembanad wasinvestigated for a period of two years during 1994-96. At various sampling sltes,phytoplankton had a density variation between 17797 and 60613 unitlm' w~th anaverage value of 30682 unitdmJ. The overall density for the sector was higher lorthe green algae (42.6 %). The blue greens were concentrated at Station-1 w~th 'Ipercentgae contribution upto 70.6 %. Anacystis and Tr~chodesm~urn were ,~lsoabundant at this place, indicating a eutrophic condition. The overall contr~hut~onby the greens and the desmids were prevalent during the monsoon and postmonsoonmonths indicating an improvement in the water condition due to thrmonsoon inflow. The green algae represented by 24 species contributed to thrhigh phytoplankton abundance in the 12 stations of the Vembanad lake. Thr I)rcbmonsoon period (34796 ndmJ) showed a maximum numer~cal abundanctafollowed by the monsoon (32773 no/ml) and the post monsoon (24934 nolm')period (Table 6).Studies conducted during the prebarrage phase by Gopinathan et a/. (1974) In theCochln estuary, a part of the Vembanad Lake, observed a maximum phytoplanktonabundance in the pre-monsoon and monsoon periods. Compared to the prcbarrage phase, the diversity has been found on a higher s~de In the presentinvestigation, ~nfluenced by the prevalence of freshwater condition during the