Modelling experiments on chlorophyll-a dynamics in a shallow arid-region lake during ice-covered period

Lake Ulansuhai, a largest shallow lake in the cold and arid region of northern China. This study investigates the role of freezing-exclusion and lake ice and snow processes in under-ice chlorophyll-a (Chl-a) dynamics during the whole ice-covered period in a shallow arid-region lake, and explores their sensitivity to key climatic and environmental drivers. Through modification of the lake ice module and incorporation of freezing-exclusion, lake model was modified and better reproduced the variations in ice/snow thickness, water temperature, and Chl-a concentration. During the ice growth period, the decline in Chl-a concentration was mainly driven by a rapid decrease in growth rate, which is caused by reduced transmitted radiation. During the melting period, both growth and loss rates went up but growth rate was dominant, promoting Chl-a prosperity. Freezing-exclusion increased Chl-a growth rate, while ice melting decreased it. Under-ice light availability drive the seasonal and inter-winter variations of under-ice Chl-a and manifests the critical role of snow and ice processes. Modelling experiments indicated that increases in air temperature and solar radiation can lead to higher Chl-a concentration, due to the thinner ice-cover and more transmitted radiation. However, TP-input has a minor impact on the Chl-a dynamics, likely attributed to the sufficient P-nutrients in this lake. • Freezing-exclusion was schemed and incorporated to a lake model. • Chl-a declines under ice growth due to reduced growth rate and the increasing growth rate promotes Chl-a during ice melting. • Freezing-exclusion increases Chl-a concentration, while ice melting decreases it. • Snow and ice processes shape under-ice light, temperature, and nutrients, and transmitted light mainly drive under-ice Chl-a. • Increases in air temperature and solar radiation can lead to thinner ice and more Chl-a.