Environmental controls on zooplankton community composition across the Bangladesh coast: integrating hydrochemistry, multivariate ordination, and threshold indicator analysis
Abstract
Understanding the impact of hydrochemical variability on zooplankton communities is fundamental for assessing the ecological status of coastal ecosystems. This study investigated how seasonal and spatial hydrochemical conditions shaped zooplankton communities across three contrasting coastal ecosystems of Bangladesh using multivariate approaches and threshold indicator taxa analysis (TITAN). Hydrochemical variables exhibited strong significant seasonal and spatial variations (p < 0.05). Salinity, electrical conductivity, transparency, and pH were higher in pre-monsoon, whereas phosphate, nitrate, chlorophyll-a, and dissolved oxygen were higher in post-monsoon. A total of 42 zooplankton genera belonging to four major taxonomic groups were identified, where a subclass of small crustaceans within the phylum Arthropoda such as Copepoda was the dominating group (42.4–52.7%) of zooplankton throughout the study periods. Zooplankton abundance, species richness, and diversity were significantly higher in the western and central coastal zones than in the eastern coastal zone, with abundance reaching 320±24.6 individuals L⁻¹ during the post-monsoon. Redundancy analysis (RDA) identified TDS, chlorophyll-a, COD, temperature, DO, and salinity as significant environmental correlates of zooplankton community composition, with the first two canonical axes accounting for 94.83% of the constrained variation. Comparative TITAN analyses identified community thresholds along salinity, TDS, EC, and pH gradients, with the salinity response showing a distinct ecological transition between approximately 8.9 and 13.9 ppt. These findings highlight the association of salinity with zooplankton distribution across the Bangladesh coast and provide a quantitative ecological threshold that can support long-term biomonitoring, biodiversity conservation, and adaptive coastal ecosystem management.