Major shift in mesozooplankton community structure after spring bloom in the Ross Sea Region Marine Protected Area

The Ross Sea Region Marine Protected Area (MPA) is the largest marine protected area globally, recognized internationally for its exceptional biodiversity and as a key reference site for Antarctic marine ecosystems. However, studies on mesozooplankton dynamics within this critical region remain notably scarce, particularly regarding community responses after the phytoplankton bloom. Here, we present the direct observational evidence of a shift in the mesozooplankton community structure on the Ross Sea continental shelf, comparing the bloom and immediate post-bloom periods. Initially dominated by krill larvae—vital intermediaries linking primary producers and higher predators—the mesozooplankton community rapidly transitioned post-bloom. Krill larvae markedly declined, while the pteropod Limacina rangii and small copepods ( Oithona spp. and Triconia antarctica ) became increasingly dominant. Spatial distributions of these taxa reflected distinct ecological drivers: L. rangii was concentrated in the eastern Ross Ice Shelf region, closely tracking high phytoplankton availability, whereas Oithona spp. and T. antarctica expanded broadly across the northwestern Ross Sea, associated primarily with inflows of modified Circumpolar Deep Water. This observed shift in community composition provided valuable insight into trophic interactions, energy transfer, and overall ecosystem function within the MPA. Our findings contribute to establishing an essential ecological baseline that supports future monitoring and adaptive management in the Ross Sea Region MPA. • First evidence of significant seasonal changes in mesozooplankton community structure within the Ross Sea Region MPA. • Krill larvae dominance declined sharply, transitioning to pteropods and small copepods after the phytoplankton bloom. • Limacina rangii distribution closely matched phytoplankton availability near the eastern Ross Ice Shelf. • Small copepods ( Oithona spp., Triconia antarctica ) expanded broadly, driven by modified Circumpolar Deep Water inflows. • These shifts alter trophic pathways and energy transfer, influencing ecosystem stability and management in Ross Sea MPA.