Decision support tool

Background information

Catchment and water body

High nutrient concentrations, especially phosphorus in water bodies are the prerequisite for mass developments of cyanobacteria. Mixing conditions and water retention time (or flow velocities) then influence whether the available nutrients can be used for mass developments.

The nutrient concentrations in the water body depend primarily on the nutrient loads from the catchment area in relation to the outputs and sedimentation. Though mixing conditions and water retention or flow velocities basically are determined by nature, to date they are often largely influenced by water resource management. Any decisions from water management should thus consider the conditions favouring cyanobacterial mass developments.

The following background information on hydrodynamic conditions in the water body and nutrient loads from the catchment are important to assess the risk of cyanobacterial mass developments and for measures to control the latter.

Nutrients from the catchment - sources, pathways, loads

Nutrients within the water body

Physico-chemical water body conditions: water retention, turbidity, stratification, temperature, pH

[1] Phosphorus

Phytoplankton is often able to store sufficient phosphorus for 3–4 cell divisions. Thus, analysis of dissolved phosphorus only in the water body provides only limited information on the potential of cyanobacterial growth as also the phosphorus stored in the cells is readily available after cell destruction. To estimate a water bodies` capacity for cyanobacterial mass developments concentration of both dissolved as well as cell-bound phosphorus, i.e. total phosphorus (TP) is needed.

[2] Stratification - Epilimnion, metalimnion and hypolimnion

Thermal stratification of lakes is the change in temperature with depth caused by the change in water’s density with temperature. One differs between the epilimnion (the warmer top layer), metalimnion (or thermocline, which is the small layer with a strong decrease in temperature ) and hypolimnion (the colder bottom layer).