Polar Seabed Observation Systems reveal winter diel rhythms and environmental drivers of euphausiids in Great Wall Bay, Antarctic Peninsula

The adaptive diel rhythm strategies of euphausiids during wintertime in the Southern Ocean surrounding the Antarctic continent remain poorly documented. To elucidate this knowledge gap, an in situ benthic observation campaign was conducted via the Polar Seabed Observation Systems (PSOS) in June 2023, targeting the Great Wall Bay (Antarctic Peninsula) to quantify winter diel activity patterns of euphausiids. High-resolution temporal analysis of 72-h continuous video footages captured by the camera system revealed six distinct behavioral phases in the number of visible organisms: (I) Quiescent period (09:00–14:30, local time), characterized by minimal activity; (II) Rapid escalation period (14:30–01:00) characterized by a marked, steady increase in local counts; (III) Abrupt decline period (01:00–04:30) with precipitous population reduction; (IV) Resting-state period (04:30–05:00) showing behavioral stasis; (V) Re-escalation period (05:00–06:00) displaying renewed activity; and (VI) Re-decline period (06:00–09:00) culminating population reduction. Concerning biotic‒abiotic interactions, multivariate analysis identified dissolved oxygen as the predominant abiotic driver modulating euphausiid population dynamics. This preliminary in situ study demonstrates the utility of the PSOS for continuous benthic monitoring of euphausiids under extreme winter conditions, providing foundational empirical data for future high-resolution investigations integrating acoustic and optical technologies. Ultimately, these findings establish a mechanistic framework for understanding overwintering survival strategies in euphausiids, providing empirical foundations for evidence-based conservation policy formulation in Southern Ocean marine protected areas.