Environmental filtering of benthic communities along the sediment trophic gradient in Ashtamudi Ramsar wetland of Kerala

Ashtamudi Lake, a Ramsar-designated tropical estuarine wetland in Kerala, exhibits marked environmental gradients from marine to freshwater influence that strongly shape benthic community structure. To assess the role of sediment physico-chemical characteristics in regulating benthos, sediment and faunal samples were collected from five different locations during the post-monsoon season of the year 2023. Sediment analysis revealed marked differences, with pH values indicating overall acidic conditions (4.8-6.3), and organic carbon, nitrogen, and potassium increasing gradually toward inner stations such as Kidapram and Cherikadavu, reflecting terrestrial run-off and depositional enrichment. A total of 67 benthic taxa were recorded, with Neendakara showing the highest richness (47 species), dominated by marine families, while inner sites supported estuarine - freshwater tolerant families such as Veneridae and Corbiculidae. Richness indices (Margalef and Menhinick) showed progressive decline towards inland, and Pearson correlations indicated strong positive relationships of richness with nitrogen (r = 0.76), organic carbon (r = 0.67), and potassium (r = 0.61), as well as a moderate negative association with pH (r = –0.46). Cluster and ordination analyses (Jaccard similarity and CCA) also highlighted ecological layering from marine to freshwater regions in sediment quality status. This study highlights sediment trophic gradients and organic enrichment as key environmental filters structuring benthic diversity, underscoring the need for sediment management to maintain the ecological integrity of Ashtamudi Lake.