Artificial mixing to control cyanobacterial blooms: a review

Petra M Visser,Myriam Bormans,Jef Huisman,Bas W Ibelings

doi:10.1007/s10452-015-9537-0

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 m2) 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 well-mixed system.

Highlights

Many eutrophic systems worldwide experience cyanobacterial blooms that have a suite of negative impacts on ecosystem functioning including a loss of aquatic biodiversity, low transparency and the occurrence of surface blooms or scums
High algal or cyanobacterial biomass may lead to an anaerobic hypolimnetic layer in deep lakes where reduced substances such as Fe2?, Mn2? and H2S may accumulate leading to deterioration of the lake as a source of drinking water
We present an overview of studies in which artificial mixing was applied, successfully or not, to control cyanobacteria in lakes and reservoirs

Summary

Introduction

Many eutrophic systems worldwide experience cyanobacterial blooms that have a suite of negative impacts on ecosystem functioning including a loss of aquatic biodiversity, low transparency and the occurrence of surface blooms or scums. The population dynamics of a number of n phytoplankton species depends on the growth rates and vertical velocities of the different species and on turbulent mixing of the water column and the underwater light field. The model predictions are shown, for different combinations of the total water-column depth zM and the turbulent diffusion coefficient D Buoyant cyanobacteria such as Microcystis tend to have a relatively low growth rate in comparison with most other phytoplankton species. A period of low irradiance caused by deep mixing in Lake Windermere induced over buoyancy of the Anabaena colonies, where the cells produced so many gas vesicles that the normal buoyancy regulating mechanisms no longer operated (Walsby et al 1991) This enhanced the risk of surface bloom formation, where the high irradiance and temperature in the scum, combined with total depletion of carbon greatly increased the risk of photodamage (Ibelings 1996; Ibelings and Maberly 1998).

Water flow

Findings

Conclusions and recommendations