e - poz 2 - Central Institute of Brackishwater Aquaculture

ECOLOGICAL INVESTffiATtONS WRING 1994-97Observations on the water quality parameters indicated a definite pattern ofvariation inflwnced by a combination of saline water intrusion through thebarrage, and the extent of discharge by the four rivers into the lake south of thebarrage CTable-4). The salinity remained the mast important parameter influencingthe general envinmment of the lake. Prior to the construction of the barrage, thetides of a mixed nature were felt during summer upto Pulikkezhu, 80 km south ofCochin connection to the sea, where the maximum tidal range was upto 5 cm, andat the Punnamada, the amplitude was to the extent of 22 cm. Uosanto, 1971).Contrary to this, during the post barrage phase, even in summer, the tidal rangewas restricted till Thaneermukkom Barrage at a distance of only 40 km. Anyenhancement in the salinity further south of the barrage during the summer wasdue to the percolation through the damaged sluices of the barrage. The averagepremonsoon salinity at south of the barrage was 18.47 ppt during the pre-barrgaephase Uosanto, 1971), but reduced to an average of 2.8 ppt during the post-barragephase. Comparatively higher salinity was observed at the western s~de of the lakecaused by the coriolis force operating in the estuary and also due to the absence ofany major freshwater inflow at this bank. The average salin~ty was 0.16 ppt durlngmonsoon and 0.49 ppt during post monsoon season under the present~nvestigation.Temperature was 29.2-32.8 OC ~ndicating a high heat budget for the system. Theshallow nature of the sector (average: depth,1.95 m) coupled with moderate secchidlsc transparency (average:1.26 m) brought almost the entire water column underthe euphotic zone. The average pH of water was near neutral (6.8). During thepre-monsoon stagnation period, the pH was acidic (pH: 6.4; range: 4.5-6.88)part~cularly at stations 2, 4, 7 and 11 (Table 5 & Fig. 3).The d~ssolved oxygen levels were moderate (with a monthly variation rangcng from6.0 to 7.2, average: 6.25 m%L) at all the stations except at Station-1 wheremun~c~pal d~scharge waters brought down the dissolved oxygen to 3.16 mg1Lduring the pre-monsoon period. The Chemical oxygen demand was alsomoderate to h~gh (4.0-28.0 mg/L), again the exceptionally high value recorded atstation-1 during the stagnant pre-monsoon period. The lake in general wascharacterised by low alkalinity (range : 4.0-156.0; average: 22.2 mgfL) except forstatlon 1 and 2.