To minimise the intake of cyanobacteria during direct raw water abstraction, the selection of the off-take points and depth should consider the spatial and temporal variability in the occurrence of cyanobacteria, especially the different distribution patterns of the potentially toxic cyanobacteria.
Cyanobacteria including Microcystis, Anabaena and Aphanizomenon often form scums in stratified lakes during calm weather conditions, which may be driven to shorelines by slight wind and accumulate there as thick scums. Their horizontal distribution is largely determined by wind direction and the morphology of the shore lines (e.g. more or less wind-exposed bays), with observed variations of cyanobacterial and cyanotoxin concentrations potentially ranging over more than 4 orders of magnitude within a water body (see example at occurrence/microcystins). Water abstraction close to the water surface should thus be avoided during the mass occurrence of these cyanobacteria, and cyanobacterial abundance should be monitored. In the case of increased wind speed, however, the cyanobacterial scums and accumulations may be entrained in deeper water layers and enter the raw water abstraction. The above mentioned cyanobacteria proliferate usually only in summer in temperate climate zones, especially in mid to late summer.
Planktothrix agardhii, and also Aphanizomenon, tend to occur in eu- to hypertrophic, shallow and thus frequently mixed lakes. Mixing causes a homogeneous distribution of the cells within the water column and thus prevents the formation of streaks and scums at the surface. Even depth-variable water abstraction can thus not avoid their intake. In this scenario, the drinking water treatment process should be optimised for the elimination of cyanotoxins, if these cyanobacteria occur in higher biomass in the raw water. This may be necessary during large parts of the year, particularly during the presence of the occasionally perennial Planktothrix agardhii.
In contrast, Planktothrix rubescens tends to occur in deep, stratified, meso – to eutrophic water bodies, where it inhabits the metalimnion (i.e. between the warm surface and the cold deeper water) in summer, while this species is only evenly distributed over the whole water column after the autumn turnover. Then surface scums may also appear. In some water bodies, Planktothrix rubescens appears during the whole year, even under ice. The intake of the metalimnetic populations can be avoided by shifting abstraction point to deeper water layers, unless other substances (e.g. iron, manganese) do not cause water quality problems. A shift of the abstraction depth to upper water layers may eventually lead to an increased intake of other plankton, though this is usually a minor problem both with respect to DOC (i.e. dissolved organic carbon) and especially to the occurrence of cyanotoxins.
Dissolved cyanotoxins usually distribute evenly throughout the water column, are diluted thereby and often biologically degraded with half –lives of e.g. 2-15 days for microcystins. An exception is cylindrospermopsin, for which biodegradation has been observed, although this toxin has been found to be very persistent in some water bodies. With sufficiently high initial concentration, this may lead to cylindrospermopsin concentrations of > 1 µg/l persisting in winter when the toxin-producing cyanobacteria have died off some months earlier (see example at occurrence/cylindrospermopsin). A surveillance programme based on cyanobacterial biomass alone is thus not sufficient to manage the risks due to cylindrospermopsin. This furthermore illustrates the need for knowledge on cyanobacterial composition and the abundance of potential toxin-producers in a water body. If Aphanizomenon (probably one of the most important cylindrospermopsin-producers in temperate zones) is building up biovolumes of > 1 mm³/L, the presence of cylindrospermopsin should be monitored and the reliability of the drinking water treatment process to remove cylindrospermopsin should be evaluated.