Macrobenthos of Kakinada Bay in the Godavari delta, East coast of India: comparing decadal changes

Abstract

A one-year study (1995–1996) on the macrobenthos over a spread of (37) hydrographically differing GPS-fixed sites in Kakinada Bay (mean salinity 27.16±0.4) and adjacent mangrove channels (14.78±0.55) in the Godavari delta, one of India's largest estuarine systems, revealed a great preponderance of benthic life. There were 11 diverse taxa represented by 95 species collected through several (303) grab and dredge hauls. Based on Bray-Curtis similarity through hierarchical clustering implemented in PRIMER, it was possible to distinguish the benthos into four assemblages each of which represented sites in the Central and North bay ( Paphia textrix – Typhlocarcinus sp. Assemblage), South-East bay ( Protankyra similis – Paphia malabarica Assemblage), Mangrove Outlets ( Cerithidea cingulata Assemblage) and Mangrove channels ( Diopatra neapolitana Assemblage). Benthos densities (mean nos. dredge haul −1) were highest (299 individuals) at sites close to mangrove outlets. Species diversity (Margalef, d; Shannon-Wiener, H′) was low in general ( d 1.244 to 2.251 and H′ 0.038 to 1.502). Sediments were mostly clayey-silt in nature except in southeast bay where they are silt-sand. Organic matter (mean) was at or near 1.3%. The observations have revealed marked changes in benthic community structure relative to an earlier investigation held in 1958–1963 in this area. Over the years, species such as Turritella duplicata, Tonna dolium and Placuna placenta found in considerable numbers previously have dwindled. Anomia, Bursa and Atrina and echinoderms, Astropecten indica, Echinodiscus auritus and Temnopleurus toreumaticus and the brachiopod, Lingula sp. of common occurrence in early 60s are absent altogether attributable to long-term natural trends during the intervening years and/or events accompanying human impingement (e.g. industrial and urban growth, port expansion measures, aquaculture, mangrove denudation etc.). Despite such large-scale alterations in benthos community structure, there were no significant changes in “biodiversity” measured through Average Taxonomic Distinctness (AvTD) and Variation in Taxonomic Distinctness (VarTD). The investigation revealed that AvTD (Δ +) and VarTD (Λ +) for previous years and the present study remained well within the 95% confidence funnel implying taxonomic stability. It was found that species, which had replaced the earlier forms, were drawn from comparable (Linnean) hierarchy i.e. same phylum/class.