Report No FR0248

W Parr and S Clarke

March 1992



To review chemical, physical and biological methods of controlling cyanobacterial and total phytoplankton standing crop in water supply reservoirs in terms of practicability, cost and best environmental option.

To provide water quality monitoring guidelines for reservoirs to enable correct reservoir management decisions to be made, to determine whether any phytoplankton control measures employed are having the desired effect, and to enable the prediction of when phytoplankton-related problems are most likely to occur.


The recent mild weather conditions, notably the long, warm summers of 1989 and 1990, resulted in cyanobacteria dominating the phytoplankton community in many reservoirs where they had previously been of only minor importance. In reservoirs where cyanobacteria usually dominate, blooms have often been more intense than normal. Increasing public awareness, stimulated by animal mortalities and examples of human illness, has highlighted the toxic nature of some cyanobacteria and emphasised the need for a review of the various control options available. The review is needed to assess which of the largely untried methods potentially offer the most practicable and cost-effective cyanobacterial control measures, so indicating in which areas funds for method development would best be placed.


  1. Reducing nutrient loading to reservoirs remains the most certain and well-tested method of controlling phytoplankton standing crop and reducing the proportion of cyanobacteria. Of the options available for doing this, nutrient stripping major feeder streams and/or piped inlets appears to offer the most advantages, especially for reservoirs fed via abstraction from nutrient-rich lowland rivers.
  2. Manipulation of the fish community appears to offer the most promising long-term biological method for phytoplankton control; however, the published literature on this subject is somewhat contradictory with regard to what extent the level of cyanobacteria is likely to be reduced.
  3. Further experimentation with air-curtain destratification systems is required to determine their pattern of use for optimal phytoplankton control.
  4. A cautious welcome is given to the use of barley straw as a method of controlling phytoplankton in small waterbodies and reservoirs, but further work is required before a recommendation can be given.
  5. Chlorophyll-a determination remains the most useful and practicable method of measuring algal standing crop.
  6. Further work is required to investigate the use of phycocyanin content as a measure of cyanobacterial abundance.
  7. The concentration of total phosphorus, rather than that of the soluble reactive phosphorus sub-fraction, is more important in determining phytoplankton standing crop. Despite the greater expense, total phosphorus should be measured as the standard phosphorus determinand.
  8. The sampling advice for cyanobacteria (NRA 1990) should be followed when sampling water supply reservoirs used for recreational purposes.
  9. For small and medium-sized, well-mixed reservoirs a single sub-surface sample may be considered representative.
  10. Samples taken from the inlet for a treatment workers may not be representative of the water quality of the reservoir as a whole and may significantly underestimate the concentration of algae present.


  1. Water utilities owning reservoirs with artificial mixing systems are encouraged to experiment with intermittent destratification from spring through to autumn.
  2. A long-term experiment of fish population manipulation is required, initially in ponds and subsequently in a small (hyper)eutrophic waterbody in which phytoplankton-related problems have occurred and for which a recorded history of phytoplankton density, plankton species composition and nutrient levels is known.
  3. The use of barley straw in a bankside aerobic fermenter through which reservoir water is passed should be addressed. The legal situation with regard to using rotting barley straw liquor as an algistat in potable water supply reservoirs should be made clear.
  4. Further work should be carried out to investigate the possible use of phycocyanin content as a measure of cyanobacterial standing crop.
  5. The following sampling programm for reservoirs should be regarded as a minimum for monitoring phytoplankton levels and predicting when related problems may occur:
  6. For a more complete understanding of phytoplankton dynamics and effects the following parameters should also be measured:


Chemical, physical and biological methods of controlling phytoplankton in water supply reservoirs are reviewed, with particular emphasis on reducing cyanobacterial standing crops. Reducing the nutrient input to waterbodies remains the most effective method of controlling phytoplankton, but depending on where the nutrients (phosphorus) are derived from and the number of streams feeding a reservoir, numerous options are available. Those methods which prevent nutrients from entering reservoirs in the first place are to be favoured, rather than having to rely on ameliorative measures in the reservoirs themselves. Though not yet used in the UK, phosphate stripping of major feeder streams or of water abstracted from lowland rivers for supplying reservoirs offers many advantages.

Of the biological methods, those involving control by manipulating fish populations or by introducing exotic species appear to offer the greatest scope for reducing 'background' phytoplankton levels, whereas microbiological control techniques are more likely to curtail peak levels, probably at a higher economic cost and more successfully in smaller reservoirs.

Physical control methods have been shown to reduce phytoplankton standing crop in some reservoirs, but not in others, the greatest success being in deep reservoirs; however, in reservoirs with dominant Oscillatoria populations, such systems may increase the size of the standing crop. Water utilities are encouraged to experiment with intermittent destratification using the mixing systems already installed in reservoirs, since this is thought likely to offer a more effective method of phytoplankton control. Guidelines for where, when and how to sample, together with what to measure, are given for an understanding of how phytoplankton interfere with reservoir water quality and for predicting the extent of phytoplankton growth/extent of likely problems caused by this in the future.

Copies of the Report are available from FWR, price 35 less 20% to FWR Members.