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Interannual and seasonal variability of the hydrographic parameters and distribution of Villorita sp. in the Cochin estuary was studied to assess the impact due to the construction of Thannermukkom bund. Results showed that periodical closing and opening of the bund had significant effect on different hydrographic parameters. Water temperature showed an increase during January to March and July to September; values showed a decrease from April to June and also during October to December. The study zones had a freshwater dominated environment with measurable salinity occurring only during pre-monsoon and zone south of bund (B) having significantly higher salinity than other. Salinity showed a significant negative correlation with dissolved oxygen (P<0.05) at zone B. All the zones were predominantly acidic. The maximum dissolved oxygen concentration observed was during July. Dissolved oxygen concentration at zone north of bund (A) was higher in magnitude than zone south of bund (B) and at both zones (A and B) dissolved oxygen is highly correlated with most of the parameters (P<0.05). Net productivity at all zones was highly fluctuating and significant differences were observed between zones (P<0.05). Nitrate concentration was maximum during post-monsoon and minimum during pre-monsoon and significant difference between zone A and B (P<0.05) were noted. Nitrite concentration decreased gradually from December to June and increased from July to November at both zones. The maximum phosphate concentration at all the zones was during May and there was no significant difference between zones at 5% level. At zone A the predominant nature of substratum was silty-sand and at zone B it was sand. In the case of sand, silt and clay there were significant difference between zones; P<0.01 for sand and silt; P<0.05 for clay. Nearly 85 genera of phytoplanktons could be identified with 20-30 non-identifiable types. Bray-Curtis similarity index for hydrographic parameters of non transformed and fourth root transformed data showed that transformation has only homogenized the data, which resulted in higher similarity between parameters than for non transformed data, but has not changed the linkage pattern. Similar analysis for different months resulted in the formation of three clusters with very high similarity within each cluster. These were: the first cluster representing the period of closure of the bund (December to May), second cluster of opening of the bund (June to November) and the third one of the months in between. This gives a clear-cut difference between closed and opened stages of the bund and leads to the conclusion that periodical opening and closing of bund affects all the hydrographic parameters in similar fashion in each month in every year. It can be concluded that construction, existence and periodical opening and closing of bund at Thannermukkom has been seriously deteriorating the ecology of the cochin estuary especially in the southern part of the bund with respect to clam beds. Keywords. Thannermukkom bund, hydrographic parameters, water temperature, dissolved oxygen, nitrate concentration, phosphate concentration

IMPACT OF ARTIFICIAL STRU CTURES ON BIODIVERSITY OF ESTUARIES: A CASE STUDY FROM COCHIN ESTUARY WITH EMPHASIS ON CLAM BEDS