Near-surface turbulence mixing in a large channel-type reservoir: Potential impacts on phytoplankton distribution

Abstract

In lakes, the vertical distribution of phytoplankton is strongly influenced by turbulence mixing in the near-surface layer, driven by atmospheric forces. In large channel-type reservoirs, it additionally influenced by inflows and reservoir regulation. However, the surface turbulent mixing and its impact on phytoplankton dynamics in these channel-type reservoirs have rarely been investigated, despite its potential to significantly influence the vertical distribution of phytoplankton, which are crucial for understanding the health and productivity of large regulated water systems. In this study, high-frequency meteorological data, water temperature, flow velocity, and phytoplankton parameters were continuously measured for one week in Xiangjiaba Reservoir, located in the upper Yangtze River, during early spring, divided into two periods: a 5-day period of normal conditions and 2 days experiencing cold fronts. During the initial phase, diurnal variations in thermal structure, mixing scales, and chlorophyll-a were observed, with an average bulk Richardson number (Ri_bulk) of 3.6 and relatively low turbulence dissipation rates (ε) ranging from 10⁻⁹ ∼ 10⁻⁵ W/kg. In the subsequent cold front events, stronger atmospheric forces and intensified hydrodynamics resulted in a decrease in Ri_bulk. During wind and convection co-occurrence, Ri_bulk dropped to ∼1.84, and the co-effects between these forces increased the mixed depth;while Ri_bulk decreased further to 0.71 with only convection, deepening the mixed layer even more. Cold fronts markedly increased near-surface turbulence with ε escalating to 10⁻⁴ ∼ 10⁻³ W/kg during wind-convection events and 10⁻⁵ ∼ 10⁻⁴ W/kg during the convective-only period. Thermal stratification and active mixing regulated the phytoplankton vertical distribution. When stratified, diurnal vertical migrations of Eudorina elegans determine the Chl-a distribution, while the thermocline can act as a physical barrier, confining phytoplankton near its depth. In contrast, actively wind-induced mixing redistributed phytoplankton more evenly throughout the water column, while convective mixing led to the formation of a deep chlorophyll-a peak, indicative of phytoplankton downward migration.