Optical properties of ice in a shallow Chinese lake (Hanzhang) with consequent impacts on primary production

Primary production is a key factor in assessing aquatic ecosystems and the global carbon cycle. Despite the ice-cover period lasting several months in many lakes, less attention has been paid to primary production in winter under ice compared to the open water period. In particular, the relationship between light conditions under ice and associated primary production remain not fully understood. This study, conducted in Lake Hanzhang, China, during 2022-2023, investigated the impact of under-ice light on primary production to understand how it varies during the ice-cover period and how the optical properties of lake ice affect it. The ice structure and its optical properties were analyzed, and primary production was calculated using the Vertically Generalized Production Model. During ice growth, the maximum ice thickness reached 31.6 cm, and approximately 56 % of photosynthetically active irradiance was absorbed by the ice, with only around 10 % reaching the water beneath. The optical properties of the shallow lake ice were mainly related to the bubble volume within the ice, with a positive correlation between the extinction coefficient and bubble volume. Throughout the ice-cover period, the diurnal primary production in Lake Hanzhang varied substantially, with the average primary production under ice amounting to 148 mg C·m^-2·d^-1, indicating that photosynthesis can remain active under ice in winter. Our study revealed that the photosynthesis of phytoplankton is not restricted by ice-cover and that the growth and melting of the ice sheet are vital for primary production in the water beneath. These findings highlight the importance of studying underwater biological processes about the shortening ice period caused by global warming.