For best experience please turn on javascript and use a modern browser!

Focus on research: aquatic microbiologist Jolanda Verspagen

Blue-green algae were a top news item in July, thanks to the high temperatures. When the weather is hot and there is little wind, blue-green algae are a real nuisance in open water and form a health hazard for humans and animals alike. The aquatic microbiology research group at the UvA has been investigating blue-green algae for years and looking for possible measures which can be taken in order to reduce the inconvenience caused by the algae. The researchers could be found this summer on and around about twenty lakes between Amsterdam and Utrecht.

Although the name commonly used to refer to them would suggest otherwise, the blue-green algae do not belong to the algae species. They are a so-called cyanobacteria that, under the influence of light, convert CO2 in the air into sugars and oxygen by means of photosynthesis. Blue-green algae have played an important role in the evolution of the earth: their oxygen production greatly improved circumstances necessary for life.

The bacteria thanks its name to the blue-green pigments involved in the photosynthesis, and which occur especially when the bacteria decays.

Blooming blue-green algae

Jolanda Verspagen from the Aquatic Microbiology research group explains how blue-green algae blooms. “One of the properties of blue-green algae is their gas blisters, a kind of balloon, which make them rise to the surface of the water. There they are exposed to a lot of light and begin to photosynthesize, producing sugars in the process. At a certain moment they produce so much sugar that this weight compensates for the air blister, and so they sink down again. Deeper down in the water column they don’t get sufficient sunlight, so they need to burn the sugar to obtain energy. This makes them lighter, and they rise to the surface again. This rising and sinking can occur a couple of times a day.”

(plaatje kringloop)

The higher the concentration of blue-green algae at the surface of the water, the less light that is able to penetrate the water. The result is that blue-green algae burn their sugar more rapidly, and then rise to the surface, where they form a layer which makes the water below even darker, so that even more algae rise to the surface, so that the layer keeps growing. “From the point of view of the blue-green algae this is a clever tactic,” says Verspagen. “Especially since this means that other algae grow more slowly. Blue-green algae are present from about June, and they take to massive blooming in the summer. Towards the end of September this begins to slack off, and they sink to the sediment to hibernate.”

Risks for humans and animals

Verspagen finds it difficult to predict whether blue-green algae will form a problem this summer. “It depends on whether the weather remains warm for a longer period, since that is when they grow fastest. If August is both warm and windless, we may have a big problem,” she warns. “The floating top layer forms when there is no wind. If a gentle wind blows from one direction for a longer period of time, certain shores will be completely covered with the algae.”

In the spring a two-layered system develops in many lakes, whereby the top layer of water is warmed up by the sun, while the lower layer remains cold. The depth of the warm layer depends on the wind, and also on the depth and the shape of the lake and on the amount of sun. “The algae remain in the top warm layer. The boundary between the warm and cold water forms a barrier. The deeper the warm layer, the higher the concentration of algae formed. In theory, lakes with such a two-layer system should have problems with blue-green algae sooner than other lakes.”

Not all blue-green algae are poisonous, and it’s not a problem to swim in water where it is present. The damaging nature is, to a large degree, determined by the concentration. “The toxins in blue-green algae are poisonous for your liver. If you ingest too much, your liver will disintegrate. This very rarely happens since people prefer not to swim in the green water and practically never swallow any of it. However, cows regularly die of blue-green algae poisoning and there have been a couple of instances of mass mortality in bird populations around lakes infected with blue-green algae in the Netherlands. And although it was never scientifically proved that this was due to blue-green algae, it is plausible that the birds were affected by it. For fish the problem is not so much the toxins but oxygen deficiency beneath the algae.”

Green soup

Verspagen had no feelings for blue-green algae until she carried out research on the subject for her PhD thesis. “The more I studied the algae, the more interesting I began to find them,” according to Verspagen. For the Department of Waterways and Public Works she investigated what could be done about the blue-green algae problem in the Volkerak Zoom lake. This large lake in Zeeland came into existence in 1987 when dams were constructed to separate the lake from the sea. It had previously been an estuary, a transition area between saline and fresh water until it was cut off from sea water. “At first it seemed to be a great success: rare water plants grew there, and the lake attracted a lot of birds. But eventually blue-green algae began to bloom, and things went from bad to worse. A sort of backup stock of the algae developed in the sediment, and the larger this stock became, the greater the amount of algae that rose to the surface in the spring. After a few years the lake resembled a kind of green soup each summer.”

Over a period of two years Verspagen took fortnightly measurements in the area in order to determine the behaviour of the blue-green algae. On the basis of the results, she developed a mathematical model to predict what could be done against the algae. The model showed that, in order to dispel the blue-green algae, the lake would need to be flushed with a large amount of either fresh or saline water. The Department now uses Verspagen’s model for further calculations.


Verspagen is a member of the Aquatic Microbiology group under Professor Jef Huisman. This summer the group is taking samples from twenty lakes between Amsterdam and Utrecht. “We are developing models in which we can combine the properties of the lake and the blue-green algae,” says Huisman. “We examine the shape and the depth of the lake, the amount of wind that has free play on it, the sunshine and the temperature of the air. On the basis of these factors we then try to predict the risk of running into problems with blue-green algae. We also investigate what effect climate change can have on the growth of blue-green algae.”

The boat in which the scientists go out on the lakes is full of apparatus, including a SCAMP, a very accurate thermometer which is put into the water and then sinks to the bottom at a constant rate, whereby the instrument measures the water temperature every millimetre. Furthermore, temperature fluctuations are measured in order to determine the degree of blending of the colder and warmer layers of water, a fluorometer indicates the amount of algae at various depths and a photometer measures the light intensity under water. Samples are taken at various depths, and these are subsequently examined under a microscope in the lab, where the number and various different sorts of blue-green algae in the samples are determined, and the toxins produced by the algae are measured. The amount of nutrients in each water sample are also measured.

Bubble bath

The Nieuwe Meer is a good example of previous successes booked by the ‘blue-green algae team’ in the war on algae. There was a problem with blue-green algae in this lake, which has a two-layer system, and the UvA researchers thought that a solution might be found by forcing a breakthrough between the warm and cold layers of water. A sort of bubble-bath installation was set up, with long pipes on the bottom of the lake through which air was forced. This kept the water in motion so that the blue-green algae were also mixed in much deeper water. “They are now much deeper than they used to be, they receive less light and therefore grow more slowly. Other algae are able to cope with the reduction in light better, and now grow faster - these are, however, harmless,” according to Verspagen. “The water in the lake still rather resembles green soup, but at least the blue-green algae no longer have the upper hand.”

Similar bubble-installations have now been installed in various places throughout Europe. In 2003 it became evident how well the machines work. The group wanted to take measurements in the Nieuwe Meer and had asked permission to turn off the system temporarily. “We had planned the experiment months before and had no idea that it was going to be the hottest summer in 500 years,” says Huisman. “As agreed, the installation was turned off in the middle of August. But the water was then at its warmest temperature and within a week there was a mass growth of blue-green algae. We had to discontinue the experiment, just in time to prevent formation of a floating top layer; recreation in the water was, however, threatened. Huisman: “We began to suspect that climate change could have an enormous effect on the growth of blue-green algae, which is the reason that we are now carrying out further investigations.”