An unprecedented autumn Antarctic expedition found abundant plankton in deep ocean layers, challenging seasonal norms. Scientists link this to deep convection in the Ross Sea's polynya, shaping marine life and revealing a crucial "dark ecosystem" previously unknown in colder months.

An international scientific expedition to the Antarctic during autumn has documented an increased presence of small planktonic organisms in the deep and bottom ocean layers. This observation contrasts with the typical spring and summer distribution of these organisms, which are usually found in the upper ocean.

The expedition noted a balanced distribution of water properties and planktonic life across vertical ocean layers, particularly in the Ross Sea's polynya area. These findings suggest the significant influence of deep convection, driven by ice formation processes in the polynya during the colder months, on seawater properties and biological distribution.

Observations were collected during the first human expedition to Antarctica in autumn, which yielded over 3,000 chemical analysis samples and 2,500 biological analysis samples. Scientists from Shanghai Jiao Tong University presented these results.

The expedition, conducted between March and April, coincided with a period of declining Antarctic productivity. Approximately 50 researchers from nine countries—China, the United States, the United Kingdom, Australia, New Zealand, Norway, South Korea, Malaysia, and Thailand—operated in the autumnal Ross Sea, with temperatures ranging from -20 °C to -28 °C, conducting continuous observations for 20 days.

This research was part of China's 41st Antarctic expedition and represents the first multinational collaborative study focused on the autumnal ecosystem of the Antarctic marginal seas. The initiative was led by the School of Oceanography at Shanghai Jiao Tong University and the Polar Research Institute of China.

Zhou Meng, dean of the School of Oceanography, highlighted the limited prior research in Antarctica beyond the summer season due to climatic and marine challenges. The scarcity of autumn and winter observational data has restricted understanding of natural processes during these periods.

The Ross Sea is significant for Antarctic studies, as it is a key location for the formation of Antarctic Bottom Water, the densest water mass in the Southern Ocean. This region contributes to global heat and salt circulation and climate patterns. Zhou also noted the Ross Sea's high biological productivity and its role in the burial of organic carbon in the deep sea.

Zhang Zhaoru, assistant chief scientist and professor at the school, stated that deep convection processes in the polynya during autumn facilitate the transport of surface ocean particles to deeper layers. This process may contribute to carbon burial.

The research team also identified signals of deep water intrusion and ice shelf meltwater in the polynya, processes relevant to the generation of Antarctic Bottom Water through deep ocean convection. Additionally, the team observed variations in nutrient concentrations in the polynya, indicating biological activity prior to the expedition.

Zhou commented that these ecological processes in the Ross Sea polynya provide essential data for understanding biogeochemical cycles in the Antarctic marginal sea.

Scientists propose the existence of an "active dark ecosystem" in the Antarctic marginal sea during the autumn and winter, potentially lasting eight to nine months. This ecosystem includes organisms such as krill, fish, birds, and mammals.

The research aims to address questions regarding the origin of organic matter and energy in this dark ecosystem, the impact of processes like ice formation and deep convection on organism transport and carbon flux, and how winter processes influence planktonic population structures and subsequent spring productivity.

The research contributes to the international community's knowledge of biological adaptation and survival strategies in the Southern Ocean's challenging autumn environment and offers experience for future winter Antarctic expeditions.

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