Revisiting the copepod diversity of Indian Sundarbans through estimation of carcasses

Debarati Sengupta, Danilo Calliari, Sourav Paul

Diversity assessment of plankton traditionally considers species richness and abundance, overlooking the vital state of individuals (i.e., alive or dead). This study aimed to integrate the vital state of zooplankters into diversity indices, using the copepods of the Indian Sundarbans (IS) as an example community. It was hypothesized that when carcasses estimation (CE) is incorporated diversity estimates deviate from those derived from the traditional diversity assessment (TDA) method. Seasonal sampling was conducted between 2022 and 2024 from seven sampling stations of the IS. The monsoon had the highest variability in temperature, salinity and pH, followed by the pre- and post-monsoon periods, and the overall highest abundance of copepod carcasses (51%), followed by the post-monsoon (18%) and pre-monsoon (11%).Variations were also noted in the relative abundances of species calculated through the TDA and CE methods, which even affected the dominance hierarchy of the community, in particularly during the monsoon. Species richness deviated more than the Shannon index, which deviated more than the Simpson and Pielou’s indices when the TDA and CE methods were compared. The maximum deviations of diversity indices were observed in the monsoon compared to pre- and post-monsoon. The CE corrected method for copepod diversity estimation indicates the need to revisit zooplankton diversity estimates, especially for volatile environments like estuaries, coastal areas. Current results together with novel approaches to characterize biological diversity (e.g., environmental DNA) suggest the need to discriminate ‘living diversity’ and ‘holistic diversity’ for a better understanding of the structure and functions of plankton, including microscopic life.