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The diel vertical migration of zooplankton includes downward migration to deep water during the day and upward migration to surface water at night. Diurnal downward migration has been well explained; zooplankton migration to surface water at night remains unclear. This study aimed to explore the reason of the nocturnal upward migration of the calanoid copepod Arctodiaptomus sp. In Lake Heihai, an alpine lake (elevation 4,090 m above sea level) without predators.Herein, we present the vertical profiles of the lake, including water temperature, chlorophyll a (Chl‐a), fatty acids, and astaxanthin synthesis precursors (ASPs) of seston in the water column. The distributions and concentrations of fatty acids and astaxanthin in Arctodiaptomus sp. were characterised through field investigations during the day and at night. Meanwhile, the effect of ultraviolet radiation (UVR) on ASPs in seston was explored through indoor experiments.Lake Heihai had the characteristics of low temperature, low food availability, high transparency, and high UVR intensity. Arctodiaptomus sp. was distributed in water layers below 6 m during the day and throughout the water column at night. Arctodiaptomus sp. accumulated large amounts of fatty acids, especially polyunsaturated fatty acids (PUFAs), and inhibitors of fatty acid oxidation–astaxanthin in the cold alpine lake. The concentrations of lutein, a kind of ASP, and PUFAs in seston were highest in surface waters. The indoor experiment verified that UVR can enhance the synthesis of ASP.The calanoids migrated to surface waters at night to obtain ASPs and PUFAs in seston, which were at high concentrations in surface waters. PUFAs and ASP may be of high food quality for zooplankton in alpine lakes. The findings indicated that the upward migration of Arctodiaptomus sp. at night was driven by the high quality of food sources (PUFAs and ASP) in surface waters. Arctodiaptomus sp. adapts behaviour and physiology to the environment of the alpine lake.We explored why zooplankton migrated to surface waters at night from the perspective of their physiological characteristics and adaptability to the environment. Our findings provide a new way to investigate the diel vertical migration of zooplankton, especially in alpine lakes. [ABSTRACT FROM AUTHOR]

Astaxanthin is ubiquitous in calanoid copepods in high latitude or altitude regions and is well studied. However, the dynamics of astaxanthin in calanoids at low latitudes are less studied. We collected samples during the ice-free season from Lake Mubanghai, an alpine lake located in a low-latitude and high-altitude region, and analyzed astaxanthin content changes among different developmental stages in Arctodiaptomus walterianus. The total astaxanthin content in A. walterianus varied from 1.66 to 4.49 μg/mg dry weight and was highest in October, the month with the lowest temperature among the three sampling dates. Both free astaxanthin and astaxanthin esters content per biomass and the ratio of free astaxanthin increased from the nauplius to adult stage, and astaxanthin esters dominated in total astaxanthin in all developmental stages. The concentrations of polyunsaturated fatty acids were low in seston but were the main fatty acids in the calanoid. The total fatty acid concentration was positively correlated with the concentrations of total astaxanthin, free astaxanthin and astaxanthin esters in the calanoid. These results suggested that astaxanthin may be beneficial to the accumulation of fatty acids. This strategy may benefit calanoids in adaptation to high mountain environments at low latitudes. [ABSTRACT FROM AUTHOR]