Field Observation of Different Wind-Induced Basin-Scale Current Field Dynamics in a Large, Polymictic, Eutrophic Lake

Plain Language Summary Basin-scale current fields are fundamental to determining the environmental characteristics and ecological attributes of large polymictic lakes. However, little is known concerning the effects of different winds on the currents and further on ecological systems of these lakes. Therefore, synchronous meteorological and hydrological data were collected from Lake Taihu (China) during summer 2015. Time series analysis and stress calculations conducted to elucidate the basin-scale current field revealed frequent occurrence of a bilayer current mode, comprising a wind-driven surface current and bottom compensation current, under low or medium wind conditions. The main forcings of the lake currents are seasonal winds related to the East Asian monsoon, and the lake-land thermal contrast wind with 24-hr periodicity causes marked diurnal fluctuation. Inflows, outflows, the Coriolis force, and bottom frictional stress also make important contributions to the bilayer current mode, and weak temperature stratification can stabilize this mode. However, as wind speed increases, wind-driven surface currents can develop downward to form uniform currents throughout the water depth, and water temperature stratification is destroyed during typhoon-affected periods. During such short-term strong wind events, the current structure mainly adopts a monolayer current mode and the incidence of the bilayer current mode decreases substantially. The monolayer current mode causes a clockwise circulation and a basin-scale temporary oscillation with similar to 80-hr periodicity. The influences of the monsoon and lake-land thermal contrast wind on lake currents were masked by the pulse-like strong winds. Both modes can contribute to persistent cyanobacterial blooms in the northwest of Lake Taihu. Large lake currents profoundly influence the lake environmental characteristics and ecological attributes. However, little is known concerning about the current distribution in response to the different wind fields. Therefore, synchronous meteorological and hydrological data were observed in Lake Taihu during the summer of 2015. It was found that a bilayer current mode was frequently observed under condition of low wind speed. This current mode comprised a wind-driven surface current and bottom opposite compensation current. The main driving forces were the seasonal winds related to the East Asian monsoon and land-lake thermal contrast wind, inflow-outflow, the Coriolis force, and bottom frictional stress. A weak temperature stratification can stabilize this mode. However, wind-driven surface currents could develop downward trends with greater wind speeds to form uniform currents throughout the entire water column and the water temperature stratification was destroyed during typhoon-affected periods. During the monolayer current period, a clockwise circulation and a basin-scale temporary oscillation with a periodicity of similar to 80hr were observed. Both modes can contribute the persistent cyanobacterial blooms in the northwest region of Lake Taihu.