Understanding upper ocean zooplankton variability in the Indian Ocean using observations and CMIP6 models

Changes in zooplankton biomass and community composition in a warming Indian Ocean (IO) may have cascading ecosystem impacts, affecting nutrient cycling, water quality, and fish stocks. This study investigates the spatio-temporal variability of upper ocean zooplankton concentration across the IO. Zooplankton exhibits pronounced spatial variability and a strong seasonal cycle, with concentrations nearly 20% higher in the Arabian Sea (AS) than in the Bay of Bengal (BoB). In the AS, zooplankton biomass peaks twice annually: a dominant peak during the summer monsoon (July–September), driven by enhanced upwelling and vertical mixing, particularly in the western AS, and a secondary peak in winter (December-February), associated with monsoon-induced cooling and nutrient enrichment in the northern AS. Concentrations are lowest during spring (April–May). Future projections based on CMIP6 climate model simulations indicate a basin-wide decline in upper ocean zooplankton under warming scenarios, with regional reductions ranging from 5 to 40%. This decline is driven by enhanced upper-ocean stratification, a shallower mixed layer, and weaker upwelling, which reduce the supply of nutrient-rich deep water to the euphotic zone and consequently lower zooplankton concentrations in a warming climate. ENSO events are negatively correlated with zooplankton concentration in the AS, but this relationship significantly weakens (almost disappears) under strong warming, suggesting a shift toward greater influence of long-term climate trends over interannual variability. In contrast, ENSO and Indian Ocean Dipole (IOD) events are projected to enhance zooplankton concentrations in the BoB and equatorial IO, potentially increasing ecosystem volatility through amplified interannual fluctuations in zooplankton and associated marine food webs.