Spatial patterns and key drivers of zooplankton in the north central Indian Ocean

Zooplankton is a vital component in the pelagic marine ecosystems, linking lower to higher trophic levels. A survey was conducted with R/V Dr. Fridtjof Nansen in Sri Lankan waters in summer 2018 to explore zooplankton dynamics in relation to abiotic and biotic variables. A distinct pattern in physical and biological properties characterized Sri Lankan waters into two ecosystems: (1) low production, high temperature, and low salinity in the east, and (2) high production, low temperature, and high salinity in the west. The highest mean abundance (1931 ind. m⁻³) and biomass (1.79 g dry wt. m⁻²) of zooplankton were significantly associated with cooler, high saline, and more productive waters in the North West and South West. In general, zooplankton were significantly more abundant in the west (1841 ind. m⁻³) than the east (707 ind. m⁻³). The most abundant copepod families were Paracalanidae (20.4%) and Tachidiidae (10.2%). The copepod Paracalanus parvus was the most dominant species. Our study reveals that temperature is a key driver explaining 67% of the variance in zooplankton biomass in this region. This study provides novel baseline results on spatial patterns of zooplankton abundance, biomass, and species composition from an understudied region of the north central Indian Ocean.