Artificial mixing to control cyanobacterial blooms: a review

Abstract

Artificial mixing has been used as a measure to prevent the growth of cyanobacteria in eutrophic lakes and reservoirs for many years. In this paper, we give an overview of studies that report on the results of this remedy. Generally, artificial mixing causes an increase in the oxygen content of the water, an increase in the temperature in the deep layers but a decrease in the upper layers, while the standing crop of phytoplankton (i.e. the chlorophyll content per m 2 ) often increases partly due to an increase in nutrients entrained from the hypolimnion or resuspended from the sediments. A change in composition from cyanobacterial dominance to green algae and diatoms can be observed if the imposed mixing is strong enough to keep the cyanobacteria entrained in the turbulent flow, the mixing is deep enough to limit light availability and the mixing devices are well distributed horizontally over the lake. Both models and experimental studies show that if phytoplankton is entrained in the turbulent flow and redistributed vertically over the entire depth, green algae and diatoms win the competition over (colonial) cyanobacteria due to a higher growth rate and reduced sedimentation losses. The advantage of buoyant cyanobacteria to float up to the illuminated upper layers is eradicated in a wellmixed system.

Evaluation of several end-of-pipe measures proposed to control cyanobacteria

“…Whereas the use of artificial mixers in stratifying water has been dealt extensively elsewhere in this issue (Visser et al 2016), we would like to devote a few words to the use of artificial mixers in non-stratifying waters. In a review on the application of aeration in American reservoirs, Pastorok et al (1980) show that in only 50 % of the cases, artificial mixing was successful.…”

“…Hence, there is no music in fighting cyanobacteria with ultrasound. Finally, mixing can have positive effects in deeper, stratifying waters (see Visser et al 2016), but mixing of shallower waters should be avoided as sediment release of phosphorus may fuel cyanobacterial blooms rather than mitigate nuisance by cyanobacteria. Therefore, none of the above seem the wide applicable solution to cyanobacterial nuisance; they should not be considered Columbus's egg.…”

“…Several of these curative measures, such as the application of cyanocides and algicides (Jancula et al 2016), and manipulations of the food web through macrophytes (Bakker and Hilt 2016) and artificial mixers (Visser et al 2016) have been dealt with elsewhere in this special issue. Nonetheless, many products remain of which quite a number often have been proposed to water authorities as end-all solutions in controlling cyanobacteria, especially following upon the typical heat waves of 2003 and 2006 with numerous cyanobacterial bloom events in northwestern and central Europe.…”

How to combat cyanobacterial blooms: strategy toward preventive lake restoration and reactive control measures

“…This will limit light availability for cyanobacteria and decrease the sedimentation losses of other phytoplankton, which will in effect decrease cyanobacterial dominance and dilute the remnants of the cyanobacterial population (Visser et al 1996;Huisman et al 1999Huisman et al , 2004. Visser et al (2016) show in a review that vertical mixing is only effective in lakes deeper than a mean of [16 m or a maximum of [30 m for Microcystis, or less for other species (Table 1 of Visser et al 2016). This implies that is not suitable for small lakes.…”

200 kHz Sonication of Mixed-Algae Suspension from a Eutrophic Lake: The Effect on the Caution vs. Outbreak Bloom Alert Levels

“…This has already worried ecologists in South Korea, where reservoirs and regulated rivers are the major sources of freshwater. The control of eutrophication involves nutrient source reduction measures, such as wastewater treatment, as well as interception of non-point sources, and in-water methods, such as chemical treatment, bio-manipulation, artificial circulation, aeration, and dredging [7][8][9][10][11][12]. The source reduction measures are not always feasible, and they are basically effective only after a long time [13,14].…”