Ecological habitat quality assessment of a tropical estuary using macrobenthic functional characteristics and biotic indices

Seasonal variation in the macrobenthic functional characteristics, such as trophic structure and secondary production, was studied, along with their structural characteristics such as density, biomass, and community distribution in Cochin estuary (CE), a tropical monsoonal estuary along the Southwest coast of India during 2018-2019. The biotic indices, AMBI (AZTI's Marine Biotic Index) and M-AMBI (Multivariate-AMBI) were used to assess the ecological quality of the study area by using the sensitivity of macrobenthic fauna to disturbances. A distinct temporal variation was evident in the macrobenthic structural characteristics, wherein high density, biomass, and species richness were observed during the post-monsoon. Polycheate species were dominant during pre-monsoon (Prionospio cirrifera) and monsoon (Mediomastus sp.), while gastropod species, Stenothyra perpumila, was dominant during post-monsoon. The trophic structure analysis revealed the dominance of deposit-feeding polychaetes during the pre-monsoon in association with the fine sediments laden with high organic carbon. In contrast, during post-monsoon, the polychaete trophic groups were more or less evenly distributed in the sandy substratum. The higher density of suspension feeders and herbivorous-grazers observed during the post-monsoon indicated improved habitat quality compared to the pre-monsoon dominated by the detritivores. The macrobenthic secondary production was also high during the post-monsoon, contributed by a diverse assemblage of molluscs, crustaceans, and polychaetes having size > 0.7 mm. The biotic indices also showed an evident increase in the habitat quality of the estuary from pre-monsoon to post-monsoon (AMBI, moderately disturbed to undisturbed; M-AMBI, poor-moderate-good to good-high conditions), similar to the results of benthic structural and functional characteristics. The study highlights the significance of the utility of benthic functional characteristics while assessing the habitat quality of an ecosystem.