Depth-stratified zooplankton diversity in the central Indian Ocean Basin: An integrative pre-mining survey using morphology and DNA metabarcoding.

Zooplankton play a crucial role in marine food webs and biogeochemical cycles, and their rapid responses to environmental changes make them effective indicators of ecosystem health. However, limited data on zooplankton diversity within the polymetallic nodule (PMN) contract area of the Central Indian Ocean Basin (CIOB) restrict the reference information available for future monitoring. Mesozooplankton communities were characterised through morphological identification and 18S rRNA metabarcoding. Samples were obtained using a depth-stratified Multilayer Plankton Net (MPN; 0-5000 m) and mesopelagic Bongo net tows (∼250 m) at three locations: the Impact Reference Zone (IRZ), the Preservation Reference Zone (PRZ), and BC20, located outside the proposed mining area. In total, 115 taxa comprising 81 genera were identified using both morphological and molecular analyses. Most taxa were shared across sites, with 13, 10, and 12 taxa unique to IRZ, PRZ, and BC20, respectively. Both methods revealed vertical structuring of zooplankton communities. Taxonomic richness and Shannon-Wiener diversity were highest in the upper water column and declined with increasing depth, whereas community composition varied along the depth gradient. Overall, diversity was greater at IRZ compared to the other two sites. Pearson correlation revealed depth associations; most of the genera decreased with increasing depth. These results indicate that depth is the primary factor influencing zooplankton community organisation in the CIOB. The study also highlights that integrating molecular and morphological methods can provide a more comprehensive assessment. This study establishes a pre-disturbance reference for monitoring mesozooplankton diversity in potential regions targeted for deep-sea mineral exploration and future mining.