Water retention, respectively flow velocity
Planktonic, potential bloom-forming cyanobacteria (species freely suspended in the water body ) hardly occur in running waters with medium to high flow velocities, but mass developments may appear quickly if flowing water bodies with sufficient nutrients are impounded (water retention > 30d). Also in rivers with stretches of reduced flow (e.g. river lakes with water retention of 3 – 30 d) the hydro-physical conditions may be sufficiently stable to enable cyanobacteria such as Planktothrix agardhii, which typically inhabit mixed water bodies, to develop. Even high biomass may be reached in the case of sufficient nutrient concentrations and other favourable water body conditions, although extremely high biomass concentrations, as observed in Microcystis scums, are unlikely.
Turbidity, secchi depth
Water bodies with mass developments of cyanobacteria typically have a pronounced greenish turbidity with secchi depths of < 0.5 m. At the same time, the increased turbidity can support the cyanobacterial dominance as many cyanobacteria grow faster than other phytoplankton at low irradiances. Also with secchi depths between 0.5 and 2 m in summer a higher share of cyanobacteria on the phytoplankton is likely. High biomass and toxin concentration are possible.
At secchi depths of > 2m, high cell densities of cyanobacteria are rather unlikely. However, in deeper, less eutrophic water bodies, floating cells of Microcystis or Anabaena, recruited from the whole volume of the water, may form locally and produce restricted “scums”. This does not tend to occur in shallow, well-mixed water bodies.
Thermal stratification
In standing bodies of water, thermal stratification has an important influence on the growth of cyanobacteria. In stratified lakes, colony-forming cyanobacteria including Microcystis, Aphanizomenon and Anabaena may float to the surface during calm weather conditions due to their production of buoyancy-conferring gas vesicles, and form scums. Slight wind may drive them to the shore, where they can accumulate to very high biomass. Though the above mentioned cyanobacteria also inhabit mixed water bodies, stagnation of the water body allows them to optimally meeting their light and nutrient requirements by buoyancy control.
A characteristic cyanobacterium in deep, stable stratified and less eutrophic lakes is Planktothrix rubescens which grows in the metalimnion in summer and is entrained in the whole water column only during the autumn turnover. Mass developments with surface scums may then appear under nutrient-rich conditions.
Most cyanobacteria can tolerate and grow in relatively low light conditions and thus can also survive in strongly mixed water bodies. Some species (e.g. Planktothrix agardhii) with a high adaptation to low irradiances preferably occur in shallow, frequently mixed lakes or slowly flowing waters. The constant mixing of the water column prevents surface scums as observed for species in stagnant water bodies (e.g. Microcystis). However, sometimes other phytoplankton (especially diatoms) may outcompete cyanobacteria under mixed water body conditions.
Temperature
The temperature growth optima for most cyanobacteria in middle European waters are between 20 – 30 °C with a high variability both within and between species. An increased water temperature is, also in the context of global warming, regarded as a driving force for mass developments of cyanobacteria. Higher temperature, however, probably acts more indirectly by causing a stable thermal stratification of the water column which in turn favours the growth of certain cyanobacteria (e.g. Microcystis). The basic requirements for cyanobacterial mass developments are, however, still sufficiently high nutrient concentrations.
pH
The growth of cyanobacteria is often related to elevated pH (up to 10). This is, however, not the cause but the consequence of increased cyanobacterial proliferation, as the pH of the water rises due to the uptake of bicarbonate from the water by the high biomass. Cyanobacteria may also occur at pH between 5 and 7, but high biomasses sufficient to cause a cyanotoxin risk in this pH region, are rather unlikely. At pH < 5 higher biomasses of cyanobacteria have been observed only very rarely.