Ancient blue-green algae bloom more and more

Every summer there they are again, blue-green algae. Why are they a problem, are they getting worse with climate change and what can we do about them? These are questions that over the past few years the Netherlands Institute of Ecology (NIOO-KNAW) has been and still is seeking answers to. The research has provided new insights into fundamental processes, from the cellular level to large-scale changes in lakes and ponds. This allows better assessment of risks and possible development of new control methods.

Blue-green algae are an essential component of many aquatic ecosystems, including streams, rivers, pools and ponds, large lakes, seas and oceans. They have good survival strategies and can grow rapidly under favourable conditions - plenty of nutrients and high temperatures. Sometimes this can lead to problems, such as in swimming areas during the summer. The problems are mainly caused by some blue-green algae species making toxic molecules for humans and animals. Although the amount of poison in a cell (3 to 6 μm in size) is extremely small, it can cause problems when blue-green algae are abundant. Many blue-green algae possess the ability to float, allowing them to form a thin, highly visible and highly toxic floating layer. The likelihood of floating layer formation is especially prevalent in warm weather when there is little wind and limited water flow: this ensures stable surface water conditions. All factors combined mean that blue-green algae cause disturbances especially in summer and during windless and warm weather. In recent years, we have also seen them increasingly flourish in major rivers during prolonged droughts, when the flow there has been greatly reduced. It is still unclear what risk this poses to birds and mammals in the natural areas around the rivers.

Blue bacteria

Blue-green algae are bacteria: officially cyanobacteria, named after their blue-green colour. Together with true algae and aquatic plants, they are at the base of the food web of aquatic ecosystems. As single-celled organisms, they differ from algae in that they are prokaryotic, meaning they have no cell nucleus. Algae are eukaryotic, so with a cell nucleus, and can be thought of as microscopic plants. Blue-green algae have existed for more than three billion years and were the first organisms able to make oxygen through photosynthesis. Like plants, in photosynthesis they use sunlight, carbon dioxide (CO2) and nutrients such as nitrogen and phosphorus to grow. They thus provided the first oxygen in the atmosphere and are still contributing to it. Some blue-green algae species produce toxins. If there are many of them in the water, the amount of toxin can increase to proportions that pose a risk to humans and animals. This risk depends very much on the species and on the amount of toxins they produce. Even within a species, the amount of toxins can vary. This is because species consist of different strains, which are very similar to each other but genetically slightly different. Some strains within the same species lack the gene to make toxins. So even with lots of blue-green algae, the water is not always poisoned.

Nitrogen increases toxicity

In recent years, NIOO has combined laboratory experiments with measurements in the field for research into blue-green algae. This research has shown that with more nitrogen compounds in the water, blue-green algae produce more toxins and thus become more toxic. The increase in toxicity is because the toxins consist of amino acids, nitrogen-rich molecules. The field measurements also show that there can be a temporary shortage of nitrogen compounds in the water when blue-green algae flourish, as they have absorbed all the nitrogen. In such cases, toxic species may also be present in the water that can absorb nitrogen gas (N2) from the air. Even with nitrogen deficiencies, they can continue to grow.

Currently, research at NIOO focuses on the differences between toxic and non-toxic strains. Analyses are being done to test how the genetic composition changes over a summer and whether toxic strains benefit more from higher concentrations of nitrogen compounds in the water. If so, toxicity is expected to be high at the beginning of the summer because there are still many nitrogen compounds in the water that favour the growth of toxic strains. During the summer, toxicity will then decrease as the concentration of nitrogen compounds decreases due to blue-green algae growth. But if there are species in the water that absorb nitrogen gas, even with few nitrogen compounds, toxicity may remain high during summer. Experiments should further reveal who will win the competition during climate change, the role of genetic composition and the consequences for toxicity.

Nutrient restriction

The most sustainable way to limit disturbances by blue-green algae is to reduce the amount of nitrogen and phosphate in surface water. In recent decades, many nutrients have accumulated in the beds of aquatic areas (the sediment), which can be released back into the water. Dredging and disposal of the sediment is an effective measure, but very expensive.

There have, however, been many measures taken to reduce the inflow of wastewater from sewage treatment plants, industry and agriculture into surface waters after it became clear in the 1970s and 1980s that nutrients were the cause of blue-green algae nuisance. The concentrations of nutrients and (blue) algae decreased significantly since then, but the decrease in nutrients - especially nitrogen - now seems to be stagnating. The problems will therefore persist, mainly due to increasingly frequent warmer summers.

Monitoring with satellites

NIOO has a long history in blue-green algae research. In the 1990s, for example, research focused on the blue pigment phycocyanin, from which the bacteria get their name. The pigment allows the blue-green algae to capture a wider spectrum of light, especially in the green range. This is valuable for the bacteria, because algae (just like plants) cannot capture green light, and during strong algal growth, the green light that is ‘left over’ can be used by the blue-green algae through their blue pigment for photosynthesis and thus growth.

Because the colour of the water can be measured with satellites and that can indicate the presence of blue-green algae, NIOO, among others, has linked satellite images to detect blue-green algae from space. This makes it possible to measure their distribution over large areas, for example in the IJsselmeer where blue-green algae are susceptible to wind due to their buoyancy and distribution can vary greatly. By monitoring with satellites, the spread and thus the risk in the entire IJsselmeer can be estimated. 

For a long time, in the 1990s and 00s, NIOO conducted research in the IJsselmeer on blue-green algae species because they were common there. The research resumed in 2022. The IJsselmeer is the Netherlands' largest freshwater lake and an important nature and recreation area, but we know little about its biological water quality. Measurements from Rijkswaterstaat over the past 40 years show that concentrations of nitrogen and phosphorus have been greatly reduced. This has caused a sharp decrease in the total amount of algae: an indication of improved water quality, although blue-green algae are still the dominant group. Even though they are mostly non-toxic species, they seem to be a poor base for the food web, as they are not readily edible by animal plankton, which in turn are eaten by fish. Further research should show the extent to which the blue-green algae are a limitation for fish and therefore birds living in this important nature reserve.

Swimming water

In summer, the total amount of blue-green algae at swimming sites is determined by sampling every week to two weeks. In some cases, a distinction is made between toxic and non-toxic species. If the amount (of toxic species) is too high, a sign is posted with a warning or negative swimming advice. This is tracked on the website Zwemwater.nl. In recent years, during some months more than half of the swimming water locations had a sign like this. Recent research by NIOO shows that the number of outdoor swimming locations with a warning or negative swimming advice increases as temperatures rise. It also showed that Google searches for blue-green algae are more common in warmer summers. So it seems that more people want to be aware of the presence of the bacteria when summers are warmer. There could be several reasons for this, for example because warmer summers see more warning signs at ponds and lakes, blue-green algae is more often in the media then, and the weather is better suited for swimming then. Regardless of the underlying reasons, the searches provide insight into the extent of the problem at that time. It is expected that the amount of blue-green algae in the water and the disturbances because of them will further increase due to climate change.

Methods of control

There are several control methods that have been extensively tested. Some methods have been around for a long time, such as introducing movemet into the water column with air bubbles. As a result, blue-green algae no longer benefit from their buoyancy and other non-toxic algae win the competition. In addition, nutrients already in the water can be removed or nutrients can be prevented from entering back into the water from the sediment. For instance, phosphate can be bound to modified clay, removing an essential nutrient. We can also directly tackle blue-green algae present in the water. In recent years, tests have been done with hydrogen peroxide. As blue-green algae are particularly sensitive to this, they die off, while other algae often survive the treatment. Previously, NIOO investigated the use of zebra mussels, after discovering that in places in the IJsselmeer where zebra mussels occur, blue-green algae did not prevail. Lab tests showed that the zebra mussels that filter the water thereby also remove blue-green algae. But as these mussels are not native, they cannot be used anymore. So there are different methods, each with advantages and disadvantages, which have proved very effective in some lakes but not in others. For each lake, therefore, the most appropriate control method must be considered.

Detection methods

As long as there are too many nutrients in the water and there is no good method for dealing with blue-green algae, it is important to properly assess the risk. To this end, NIOO together with colleagues researched different detection techniques. Traditional methods to determine the amount of blue-green algae were compared with new methods that can potentially distinguish toxic species (DNA analyses) and that can measure the actual toxins (chemical analyses). Traditional methods often overestimate the risk to public health, because in many cases the non-toxic species and/or strains are dominant and therefore the risk is not as high. These findings will be taken into account when drafting protocols that give a more realistic picture of the risks. This is done in close consultation with the Platform Blauwalgen: a network of policy makers, water managers and researchers. Together with the Royal Netherlands Meteorological Institute (KNMI) and Deltares, NIOO has conducted tests with measuring buoys that measure the concentration of blue-green algae every 10 minutes and thus allow risks to be assessed and adjusted more quickly. Models have also been developed that predict the risks and produce a ‘lake forecast’ to clarify in advance whether or not it is possible to swim there. The measurements from the buoy give a good indication of the amount of blue-green algae in the water, but the uncertainty of the models is still too high. The practical applicability of the method is now being further investigated.

Blue-green algae in nature

In water-rich nature reserves, blue-green algae can pose a risk to birds and mammals, although little is known about this. In nutrient-rich waters with a lot of algae, oxygen deficiency may occur for a short time: algae consume a lot of oxygen at night, so that even though algae produce oxygen, oxygen consumption may increase sharply, leaving too little oxygen in the water. This can lead to fish death and botulism. Botulism is caused by toxins from the bacterium Clostridium botulinum, which can mainly kill waterfowl and fish. There are also regular reports of animal deaths in summer that have been linked to blue-green algae - including waterfowl such as ducks, herons and seagulls - but the exact cause is often unknown. Puddles and pools in nature reserves where mammals come to drink can also develop blue-green algae, which is a particular risk if the water becomes very turbid and especially if floating layers form. Little is known about this risk and how it affects animal health in nature.

This is the third article in a series on 70 years of ecological research at the Netherlands Institute of Ecology (NIOO-KNAW). Every edition features another line of research. Find out more about 70 years of ecology here.

This article also appeared in the March issue of Vakblad Natuur Bos Landschap.