ROSSMIZE expedition: zooplankton community dynamics across the Antarctic spring–summer transition

Phytoplankton and zooplankton dynamics in the western Ross Sea are strongly shaped by interactions among sea-ice retreat, water-column structure, and spatial environmental gradients. During the ROSSMIZE expedition (November 1994–January 1995), a multidisciplinary survey across four contrasting regions of the western Ross Sea captured the transition from early-season ice influence to summer stratification. Mesozooplankton were sampled using a sensor-equipped BIONESS system, enabling high-resolution coupling of biological patterns with temperature, salinity, fluorescence, and depth. The region emerged as a mosaic of subsystems structured by latitude, ice history, and hydrographic conditions. Depth was the dominant driver of community composition and diversity: generalized additive models indicated that diversity peaked at intermediate depths, reflecting a balance between surface-driven production and deeper, more stable water masses. Temperature, salinity, and fluorescence further modulated these patterns, underscoring the sensitivity of pelagic communities to fine-scale physical gradients. Together, these results demonstrate that spatial structuring of zooplankton in the western Ross Sea is governed not only by seasonal ice dynamics but also by depth-dependent habitat features and latitudinal environmental transitions during the spring–summer period.