Algal blooms occur when nutrients entering into surface waters cause rapid growth in the algae present in the water. Algal blooms have caused sporadic problems in water treatment processes for decades. When water abstracted for drinking water treatment contains algal blooms, blockages can occur in filters and odours may develop in the treated water.

Algal growth is influenced mainly by water composition, temperature and light intensity. Lowland water storage reservoirs, where the abstracted river water carries treated sewage effluent and agricultural run-off, contain relatively high levels of nitrogen and phosphorus compounds which promote the growth of algae. Climate change is expected to further exacerbate algal bloom formation in the future.

A characteristic of algal blooms, which contributes to their nuisance value, is the tendency to form surface scum. Cyanobacteria have a buoyancy control mechanism which maintains their vertical position in relation to wind-induced circulation in the water column. Under calm conditions, when natural circulation within the water ceases, algae float and produce scum, which accumulates at the water surface. This presents a possible health risk for recreational activities which involve water contact and for animals which drink from scum-covered water.

Besides potentially producing toxins, some algae and cyanobacteria also produce substances which cause taste and odour problems. Amongst the best known odorants are geosmin and methylisoborneol (MIB). These substances affect taste and odour at very low concentration levels (just a few nanograms per litre). Typically, the substances are contained within the cells, but are released when algal cells die or are ruptured during water treatment.

Blue-green algae or cyanobacteria become nuisance algae at high densities

Blue-green algae or cyanobacteria become nuisance algae at high densities

"Cyanobacteria evolved at least 2,500 million years ago from photosynthetic bacteria. These organisms are the evolutionary ancestors of all modern plants, using chlorophyll-a and two photosystems to convert sunlight into organic biomass by means of photosynthesis. Thus, cyanobacteria were the first organisms in evolutionary history to produce oxygen."

Corina Carpentier (The Carpet of the Sun, 2014)

Another significant adverse effect of algal blooms is their propensity to cause de-oxygenation. During the daytime, algae use carbon dioxide and transfer oxygen into the water. At night, this process is reversed, with the carbon dioxide respiration causing localised oxygen depletion and, consequently, fish mortality.

Available Sensor Technologies

Cyanobacteria as well as algae contain pigments to capture light for their photosynthesis. It is these pigments which can be used to quantify the algae and monitor the development of algal blooms. As different classes of algae contain unique combinations of pigments, monitoring of separate algal classes on the basis of their pigment fingerprint is also possible.

There are two main ways to determine the pigment composition of a water sample, to derive its algal composition. The first is by detecting the wavelengths at which light absorption reaches peak values using a light intensity and spectrum sensor, a hyperspectral radiometer. This method is often used in remote sensing applications. The second methods involves measuring the amount of fluorescence light around 680 nm as emitted by algae and cyanobacteria. Submersible field monitors often make use of this technique. Both methods have their advantages and drawbacks, as described in more detail in the Sensileau Sensor Database, under the parameters Algae and Chlorophyll-a.

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During this webinar, Louise Vanysacker sheds light on how De Watergroep (Belgium) monitors blue-green algae in their intake reservoir. Arco Wagenvoort of AqWa Ecologisch Advies explains how to perform a valid fluorescence measurement. The third speaker, Ron van der Oost of Waternet Water Utility (Amsterdam, NL), discusses the guidelines for the Dutch Bathing Water Protocol. This webinar illustrates the various perspectives of different experts in this field with regard to the monitoring of blue-green algae.

This webinar is in Dutch.

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In this podcast, Arco Wagenvoort of AqWa Ecologisch Advies and Corina Carpentier of Sensileau discuss how the quality of fluorescence measurements for determining the density of (blue) algae can be ensured. Topics covered during this conversation include the functioning of online fluorescence meters, tips for checking the quality of field meters, and the implications of using live algae for calibration. This podcast is in Dutch.

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For the past two years, Sensileau and its partner AQUON Laboratory (NL) have organised a yearly Calibration Day for algal fluorescence sensors used in the national monitoring programme for natural bathing waters in the Netherlands. In the coming years, this service will be expanded to include a larger variety of field monitoring instruments to monitor algal densities. Sensileau also offers this service for contractors of the Dutch Ministry of Infrastructure and Water Management, who have been tasked with algal monitoring in large rivers and coastal zones under the direct responsibility of the Ministry.

Interested in participating in one of our calibration events? Let us know what type of field monitoring equipment you have to monitor algal densities, and we will contact you to discuss how we can include you in one of our annual calibration events.

Invite me to the next calibration event

Further Reading

Want to dive in deeper? We recommend the following publications:

  • Almuhtaram, H., F.A. Kibuye, S. Ajjampur, C.M. Glover, R. Hofmann, V. Gaget, C. Owen, E.C. Wert, and A. Zamyadi. 2021. State of knowledge on early warning tools for cyanobacteria detection, Ecological Indicators 133: 108442.
  • Carpentier, C. (2014). The Carpet of the Sun: On the quantification of Algal Biomass. Dissertation Thesis. Masaryk University, Faculty of Science, RECETOX – Research Centre for Environmental Chemistry and Ecotoxicoligy, Brno, Czech Republic. 219 pp.
  • Kaylor, M.J., A. Argerich, S.M. White, B.J. VerWey and I. Arismendi. 2018. A cautionary tale for in situ fluorometric measurement of stream chlorophyll a: influences of light and periphyton biomass, Freshwater Science 37: 287-95.
  • Zoboli, O., K. Schilling, A. L. Ludwig, N. Kreuzinger, and M. Zessner. 2018. Primary productivity and climate change in Austrian lowland rivers, Water Sci Technol 77: 417-25.