Turbulent mixing and its effects on algal blooms in stratified channels: a river study through direct numerical simulations

Abstract

Cyanobacteria blooms have historically been a natural occurrence in aquatic ecosystems, but their rising frequency and severity in recent decades pose significant public health concerns. These blooms can lead to anoxic conditions, resulting in mass fish kills. Factors such as light availability, nutrient levels, calm weather, and thermal stratification have contributed to the increase in high-biomass cyanobacteria.

This thesis explores the relationship between particle dispersion in stably stratified flows, utilising direct numerical simulation (DNS) and Lagrangian particle tracking (LPT) to model particle transport in stratified open channel flow under diurnal surface heat and wind conditions. An extensive set of simulations is performed, examining how vertical thermal stratification influences particle dynamics.

Key findings include the identification of laminar layer depth (LLD) and stratified layer depth (SLD) as crucial determinants of flow regimes and mixing behaviors. The study classifies LLD and SLD responses into three categories: persistent, diurnal, and absent. Stratification significantly affects the vertical velocity and dispersion of particles, particularly neutrally buoyant ones, while buoyant and negatively buoyant particles are less influenced.

The LLD serves as a turbulence suppression zone, affecting particle dispersion. Neutrally buoyant particles are particularly impacted, with their movement determined by the flow regime.

Ultimately, this research reveals that turbulent mixing plays a critical role in algal bloom formation in stratified waters. The presence of an LLD increases the likelihood of blooms by confining particles within the euphotic zone, where light supports growth. Monitoring parameters such as the modified bulk stability parameter and diurnal timescales can help predict bloom conditions by assessing the duration of the LLD.