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.

Diversity and seasonal distribution of ammonia-oxidizing archaea in the water column of a tropical estuary along the southeast Arabian Sea.

Diversity and distribution pattern of ammonia-oxidizing archaea (AOA) were studied across a salinity gradient in the water column of Cochin Estuary (CE), a tropical monsoonal estuary along the southeast Arabian Sea. The water column of CE was found to be nutrient rich with high bacterial (3.7-6.7 × 108 cells L-1) and archaeal abundance (1.9-4.5 × 108 cells L-1). Diversity and seasonal variation in the distribution pattern of AOA were studied using clone library analysis and Denaturing gradient gel electrophoresis (DGGE). Clone library analysis of both the amoA and 16S rRNA gene sequences showed similar diversity pattern, however the diversity was more clear when the 16S rRNA gene sequences were analyzed. More than 70% of the sequences retrieved were clustered under uncultured Thaumarchaeota group 1 lineage and the major fractions of the remaining sequences were grouped into the Nitrosopumilus lineage and Nitrosopelagicus lineage. The AOA community in the CE was less adaptable to changing environmental conditions and its distribution showed seasonal variations within the DGGE banding pattern with higher diversity during the pre-monsoon period. The distribution of AOA also showed its preference to intermediate salinity for their higher diversity. Summer monsoon associated runoff and flushing played a critical role in regulating the seasonality of AOA distribution.