Modelling the vertical migration of different-sized Microcystis colonies: coupling turbulent mixing and buoyancy regulation

Abstract

Exceptional vertical migration ability provides the cyanobacterium Microcystis with competitive advantages in bloom formation and dominance establishment. Studies have been conducted on the vertical migration patterns of Microcystis since the 1970s; however, bloom simulations remain limited. Here, we used a simple model based on the viscous drag force of turbulence and analysed the motion characteristics of Microcystis colonies. The vertical distribution of turbulent kinetic energy (K), cell concentration and colony size profiles in Lake Taihu (Meiliang Bay and Gonghu Bay) and the critical vertical turbulent kinetic energy of colonies (TK, i.e. the anti-turbulence ability of colonies) were tested. The results showed that, under steady K profiles, colonies had relative rest positions (RRPs) where K ≈ TK and colonies of the same size gathered together. The vertical migration patterns were affected more by turbulence than by density (colony mass density) if the average K of the water column (MK) was not equal to TK. If MK ≈ TK, the colonies could exhibit a diurnal pattern of sinking at noon and floating upwards before dawn without steady RRPs. Our results suggest that studies on RRPs may offer optimizations for bloom forecast and control in the future due to easier simulation of K profiles than that of flow fields.

Abiotic Drivers of a Deep Cyanobacteria Layer in a Stratified and Eutrophic Lake

“…Although direct measurements of turbulent kinetic energy were not recorded, these results corroborate the findings of Zhu et al. (2018), who demonstrated that Microcystis colonies tend to aggregate at depths where the turbulent kinetic energy of the surrounding water is approximately equal to the critical vertical turbulent kinetic energy of the colony under steady conditions and only exhibited diurnal migration under similar conditions. With real world applications in mind, however, measuring water temperature profiles is more practical than measuring turbulence levels of the water column.…”

“…Does the subsurface peak disperse and form a uniform algae profile, or does it form a surface bloom? Although vertical heterogeneity has been qualitatively correlated to subsequent surface bloom formation, there is a lack of quantitative modeling to support this observation (Liu et al., 2019; Xiao et al., 2018; Zhu et al., 2018). While it is beyond the scope of this paper to make definitive claims on the topic, some observations from this data set could orient future work in the right direction.…”

“…As a result, it has been suggested the relationship between Microcystis vertical motility and timing of HAB onset should be explored more in depth (Liu et al., 2019; Xiao et al., 2018; Zhu et al., 2018). There have been several models formulated to simulate Microcystis vertical motility as a function of abiotic factors (Medrano et al., 2013; Wallace et al., 2000; Zhu et al., 2018), and Yao et al. (2017) connected their simulations of Microcystis motility to a hypothesis of necessary conditions for bloom formation, but none have had a long‐term, high‐frequency, in situ temporal data set for validation.…”

“…(2006) suggested vertical distributions of different phytoplankton species, especially Microcystis , were largely correlated with wind events (and had no correlation with nutrients) in a field study of Lake Taihu, China. As a result, it has been suggested the relationship between Microcystis vertical motility and timing of HAB onset should be explored more in depth (Liu et al., 2019; Xiao et al., 2018; Zhu et al., 2018). There have been several models formulated to simulate Microcystis vertical motility as a function of abiotic factors (Medrano et al., 2013; Wallace et al., 2000; Zhu et al., 2018), and Yao et al.…”

A theoretical modeling framework for motile and colonial harmful algae

“…The buoyant velocity of cells is then calculated through a modified Stokes settling velocity that is governed by the difference between algal cell density and the surrounding water density (Wallace et al, 2000 ). Turbulent transport has since been incorporated into these models (Medrano et al, 2013 ; Zhu et al, 2018 ). By combining their model with principal component analysis, Feng et al ( 2018 ) demonstrated that turbulence‐induced mixing explained over half of the variability of early surface bloom formation, and that buoyancy regulation was more important for bloom maintenance and formation of late‐season blooms.…”