Movement of cyanobacterial colonies in a large, shallow and eutrophic lake: A review

Abstract

Cyanobacterial blooms caused by lake eutrophication result in serious damage to lake ecosystem services, and cyanobacterial movement is the key to bloom formation. Cyanobacteria exist as unicellular organisms when under laboratory culture for long periods of time, and their movement is considered to be controlled by gas vesicles. However, recent studies have shown that colony formation has a decisive influence on cyanobacterial movement, and that buoyant cyanobacteria in natural waterbodies mostly exist as colony. Cyanobacterial colonies are formed by wrapping multiple cells in extracellular polysaccharides, which is primarily a result of fission proliferation or collision adhesion. When compared with unicellular cyanobacteria, active and passive movements of cyanobacterial colonies with larger size and buoyancy changed deeply, making it easier for colonies to float upwards toward the water surface, resulting in further formation of surface blooms. Active movement of a cyanobacterial colony refers to colony movement that is driven by effective buoyancy in a weak hydrodynamic environment. Active movement depends on three dynamic parameters of cyanobacterial colonies (density, shape and size) that are intrinsically linked, with colony size being most important. Early studies suggested that changes in cyanobacterial density are the result of cell metabolism and established formulas on the relationship between light and cell density based on laboratory experiments. However, these formulas cannot be used to describe the density of cyanobacterial colonies during severe blooms. Colony density is primarily affected by colony size and decreases with increasing colony size. Moreover, colony size is an important parameter for characterizing colony morphology. Therefore, colony size has an important influence on the effective buoyancy of a colony. Specifically, the floating rate of large colonies of cyanobacteria (100–425 μm) was 1475 times that of small colonies ( in Lake Taihu. Passive movement of a cyanobacterial colony refers to hydrodynamically dominant movement. In Lake Taihu, the passive movement of cyanobacterial colonies is mainly controlled by hydrodynamic vertical disturbances, which are dominated by wind waves, and hydrodynamic horizontal transport, which is dominated by lake currents. Wind waves are conducive to acceleration of the recovery of wintering cyanobacteria, alleviation of nutrient limitations during severe blooms and improvement of the absorption efficiency of cyanobacteria, while also directly affecting colony morphology. Studies have shown that appropriate disturbance of wind waves can enlarge colonies, while disturbances that are too strong can change the colony shape or disintegrate the colony. Moreover, strong turbulence dominated by wind waves can entrain buoyant colonies and disperse surface blooms. The ratio of turbulence intensity to upward-floating rate is a direct indicator of the occurrence of turbulence entrainment of buoyant colonies. Lake currents transport nutrients from inflows or sediments to cyanobacteria and cause horizontal redistribution of cyanobacterial colonies in lakes. It is generally believed that the prevailing southeast wind-induced currents cause colonies to accumulate and form severe cyanobacterial blooms in the northwestern area of Lake Taihu. However, recent studies have suggested that the migration of buoyant colonies with free-movement is primarily determined by wind fields rather than surface lake currents. Accordingly, it is debatable whether cyanobacterial migration models based on lake current models can be used to simulate algal patchiness drift and large-scale bloom formation. Therefore, further studies are required to conduct long-term field monitoring of colony dynamics parameters along with experiments and field observations investigating cyanobacterial colony movements to explore the temporal and spatial dynamics of field cyanobacterial colonies and their influencing mechanisms. It is also necessary to investigate movements of cyanobacterial colonies caused by wind drift, turbulence entrainment, wind waves-induced mass transfer, Langmuir circulation system transportation and other hydrodynamic processes in shallow lakes. Accordingly, the hydraulic removal and numerical models of cyanobacterial blooms can be developed to control cyanobacterial blooms in shallow lakes.