Bottom Fauna of Dredging and Dredge Spoil Disposal Sites of a Tropical Estuary

摘要

Sediment characteristics and bottom fauna were investigated extensively in and around Cochin harbour at 38 sampling stations grouped into seven areas, covering an area of 130 km~2, during premonsoon and monsoon periods. The seven areas were: (1) Ernakulam channel (4-10 m deep). (2) Mattanchery channel (2.5-5.5 m deep). (3) Vallarpadam (2.1-4.5 m deep). (4) Dredging channel (8.5-17.0 m deep). (5) North of dredging channel (6.6-14.0 m deep). (6) South of dredging channel (6.6-14.0 m deep) and (7) Disposal site (12.8-17.5 m deep). The silty clay and sandy clay substratum predominated the study area. In the dredging and disposal areas the values for organic matter were 3.40 and 4.51% during pre-monsoon and 3.51 and 4.00% during monsoon respectively. The standing stock in terms of total biomass (wet weight) in the study area varied from 37.47 to 297.30 g m~(-2) during premonsoon and 12.63 to 215.97 g m~(-2) during monsoon. The distribution of different faunal groups/species indicated a significant reduction in the dredging and disposal areas. Polychaetes dominated the fauna. Bray-Curtis' Coefficient of Similarity between areas was higher during monsoon than during premonsoon in areas 4-6 with sediment characteristics controlling the occurrence of common species. Species which are highly abundant and more homogeneously distributed have low niche breadth preferably in sand dominated areas, particularly in areas 1-4. In area 5, a similar trend was observed with high niche breadth associated with organic matter content, whereas in areas 6 and 7 a reverse in the trend was observed with finer sediment dwelling organisms showing high niche breadth. In general a decreasing trend in the species niche breadth was observed from area 1 to 7 with higher values during monsoon than during premonsoon for species niche breadth. Grouping of species obtained by group average linkage clustering and using R-mode analysis are compared.