What is "algal bloom"?
When phytoplankton, microscopically small algae, occurs in large amounts in the water it is called algal bloom. The algae make the water turbid, coloured or, under certain circumstances, covered by obvious accumulations of floating masses at the surface. Most algal blooms are very important and constitute food for small, freely buoyant animals (zooplankton), which in turn are eaten by bigger animals, for example the Baltic herring. However, some algal blooms consist of species that may produce toxins and which may constitute health hazards for both humans and animals.
What kinds of different algae are there?
There are freely buoyant (planktonic) so-called phytoplankton and algae that are attached to bottom substrata. Phytoplankton is divided into several groups, just like the attached algae. The latter are often belt-forming and common types are green algae, brown algae and red algae. When using the term algal bloom, we mean the microscopically small algae (the phytoplankton).
Blue-green algae, or cyanobacteria as they ought to be called, are not algae in the true sense, but a kind of bacteria that has certain similarities with groups of phytoplankton. Due to their similarities with phytoplankton they are popularly categorized under the group, algae. They are very common in the summer algal blooms. To this group belongs the species, which in the summertime normally accounts for most of the reports about troublesome "algal blooms" in the Baltic Sea, Nodularia spumigena. It is also the only species in the open Baltic Sea, which is known to be toxic at times. The other species may be toxic and are therefore to be treated with caution.
How do I recognize an algal bloom?
Algal blooms may look very different. The water may for instance become green, blue-green, yellowish or red to red-brownish, distinctly turbid and/or filled with small needle-like or point-like phytoplankton. A bloom of one of the commoner Baltic species, the cyanobacteria Nodularia spumigena, looks like a greenish-yellow to yellowish-white rhubarb soup, and sometimes like a thick pea soup. Under normal, non-harmful blooms, the water is often only distinctly turbid. Algal blooms may sometimes occur in clumps, "flocks", and sometimes in dense masses at the surface.
Certain blooms are very limited locally, sometimes only a few metre-wide spots. Others may cover entire bays, lakes, and huge areas of the open sea. The latter types often display obvious differences in concentration. The blooms may be clearly gathered in bands or "wind roses" in areas where phytoplankton is concentrated by wind and waves. But they may also occur evenly distributed in the water column. Blooms are often concentrated at shores and close to capes/points of islands or the mainland.
Not everything colouring the water is algae. In early summer (often around the beginning of June) pine trees, among others, release their pollen. It is common that pollen accumulates on the sea surface in accumulations that may look like algal blooms, but is non-harmful. Accumulations of pollen are often yellowish. Sediment mixed into the water column might also colour the water. Accumulations of filamentous (thready) algae may also float at the surface in big lumps and look like algal blooms. However, these normally stick nearly 100 % in a sieve, while most phytoplankton pass through a sieve. Very thick surface accumulations of the Baltic species Nodularia spumigena might form lumps that stick in a sieve.
When can algal blooms occur?
Algal blooms may turn up more or less throughout the whole year, but the potentially harmful blooms are most common at the height of summer (July-August) and during parts of the autumn. During the last couple of years local accumulations (some documented as toxic) have been recorded in the Baltic Sea, for example in the Stockholm archipelago, even as late as in November.
How come there are algal blooms?
The blooms are dependent upon favourable physical, chemical and biological conditions giving the algae possibilities to reproduce in mass quantities. Some of the factors that govern the growth of algae are, among others, light, temperature and nutrient conditions.
Phosphorus and nitrogen are among the most important nutrients for algae. Algae needs more nitrogen than phosphorus for its growth. If any of the necessary substances is available only in a limited amount, that substance determines the extent of growth.
In winter, nutrients are brought up to the surface through the mixing of the water column. During the spring, the spring bloom uses much of the available nitrogen and phosphorus for growth. Usually nitrogen is depleted before the phosphorus. It is said that the nitrogen level is the limiting factor for the spring bloom.
In the summertime, it is normal that there are insufficient amounts of nitrogen of the right type in the water. Due to this, many species cannot grow during this period. Cyanobacteria, on the other hand, have the ability to use nitrogen from the air (molecular nitrogen) dissolved in the water, and can grow as long as there is phosphorus available. During the summer 2004-2005 more phosphorus than normal was found in the water, causing large blooms of cyanobacteria. In oxygenated conditions in the deep waters of Baltic Proper, phosphorus is bound to the sediments. However, due to low oxygen levels, large amounts of phosphorus were instead released from the sediments. This phosphorus was mixed into the surface water through the autumn/winter circulation of the water column, and could thereby be utilized by the algae for growth.
Where may algal blooms show up? (fresh, marine and brackish waters)
Algal blooms may turn up in all kinds of waters. They occur both offshore, at coasts and in lakes and watercourses all over the world. Harmful blooms may occur in all of these environments.
Blooms of cyanobacteria occur both in fresh and brackish waters. Dinoflagellate and flagellate blooms occur in brackish (in the Baltic Sea) and purely marine waters.
How dangerous is it?
Some phytoplankton species contain or excrete toxins that may cause feelings of light sickness, skin reactions, conjunctivitis (eye irritation), swollen lips, pain in the ears, sore throats, hay fever, fever, head ache and/or stomach/intestinal problems with feeling of illness, vomiting and diarrhoea, but may also have a lethal effect (the latter is most common in domestic and wild animals).
Please, note that this information is predominantly concerned with the Baltic Sea area. There are other areas in the world where the problems are greater.
For adult humans the risk is normally small of being affected. The prerequisite for being affected with really serious problems (disregarding the skin problems) is that one has swallowed larger amounts of water containing algal toxins. For adult humans such poisoning is a minor problem, as the water normally does not look pleasant, and may smell so bad that no one would think of drinking the water or taking a swim in it. Children, on the other hand, who much more easily swallow involuntary gulps of water, run a greater risk. In the Kattegat - Skagerrak area there is a certain risk for being poisoned by algal toxins through the consumption of mussels and oysters.
Domestic animals and wild animals are those groups that run the greatest risk of being affected in connection with algal blooms. Animals that drink water rich in algae may become seriously ill, and may also die of poisoning.
Who runs the greatest risk of being affected?
Children (especially small children) are a risk group, as they often swallow involuntary gulps of water. They do not have the same ability to keep their mouths shut as adults do, and therefore they may swallow rather large quantities of water involuntarily. Contact with algae on the beach may also cause skin rashes. Therefore, small children ought to be kept away from waters with blooms, and be kept under good supervision.
The dominating risk groups are domestic animals (for example dogs, sheep, cattle and horses) and wild animals which do not hesitate to drink lake or seawater, and which do not care about the presence of algae in the water. Besides, dogs prefer water with a slightly bad smell, and the majority of domestic animals do not hesitate to drink saline water. In the summertime they even prefer seawater to fresh water as it contains salt that they want. Dogs (primarily, but also other domestic animals) which walk into water rich in phytoplankton and which then lick themselves, run a very great risk of being affected by poisoning symptoms. It may be enough for a dog to lick its paws to be a victim of serious problems.
Is it possible to see if a bloom is harmful?
No, it is not possible to visually determine if a bloom is harmful or not. To decide that, sampling for toxicity must be done. However, some species are often found toxic, and their blooms should be avoided: for example the cyanobacteria Nodularia spumigena, a very common summer species in the Baltic Proper, which very much resemble yellow/green/white rhubarb soup when blooming. This species also often forms thick surface accumulations. Cyanobacteria of the genus Microcystis, common in lakes and coastal areas in the Baltic Sea, may look like small green round grains at the surface.
Can I take a swim?
It may go well to swim in waters with a bloom. But there is always a risk for being affected, by for example, skin irritations or other problems such as ear pain, sore throat and headache. It is not possible to judge by looking at the water, whether a bloom causes skin irritations or not. To be on the safe side, it is better not to take a swim in water rich in algae.
When do I have to avoid taking a swim?
Avoid swimming when the water looks unpleasant, i.e. when the water is evidently turbid or discoloured by accumulations of algae. Similarly it is safer to avoid swimming also for a period after the algal bloom has disappeared, if the weather has been very calm and the water exchange is poor in the area. Under such circumstances one should rather look for more exposed places to swim - places with good water exchange and without signs of algal blooms. If possible, choose such places where you know that blooms have not been present lately.
Could it be dangerous even if the water looks normal?
Yes, but this applies mostly to animals, as they readily drink the water. Under extreme conditions toxins may be present up to a month, although the algae have been decomposed. This may happen during calm weather and in very protected shores/places (places not exposed to good water exchange). See also the previous question.
Does the toxin disappear when boiling the water?
No. Some toxins are considered to be thermo-stable, i.e. they can stand very high temperatures, and are not destroyed by boiling the water. Therefore, water from areas with algal blooms should not be used for cooking.
What should I do I have been stricken with illness caused by algal blooms?
People who experience skin or eye problems, or who in other ways feel sick, ought to get in touch with a doctor. Adults may wait and see if the symptoms/problems are light, while children need to be examined, as it is difficult to judge how much water they have swallowed.
In cases of eye irritation, rinse the eyes carefully.
Skin that has been in contact with water rich in algae should be carefully rinsed and, in cases of irritation, should be washed with soap and water.
What should I do if I think my domestic animals have been poisoned by algae?
Contact a veterinarian as soon as possible.
For how long does the bloom last?
The length of a bloom period depends on, among other things, on weather development, nutrient availability and the type of species involved. Warm and calm summer weather favours blooms of cyanobacteria while windy weather favours the break up the blooms and makes them disappear.
Blooms occurring at sea during the summer high-pressure periods are often of large scale. At the most they last about a month. During autumn and early winter there are often small-scale blooms or accumulations at shores in coastal areas. They consist partly of the same species as the large-scale blooms of summer, but the proportions are often different. Still, they may be harmful.
Are algal blooms natural?
An algal bloom is in fact a phenomenon that is very normal, very important and regularly occurring. The term algal bloom means that phytoplankton strongly increase in numbers during a limited period of time thanks to a good supply of nutrients.
Additional supplies of nutrients from sewage, land run-off from agricultural land, as well as phosphorus released from the deep bottoms with oxygen-depleted conditions may, however, lead to extra large blooms.
Have algal blooms increased in recent years?
Blooms of cyanobacteria seem to have increased in recent years. But there are also signs indicating a periodic occurrence of blooms. One reason for an increased presence of blooms could be the increased amounts of nutrients in our waters. Generally speaking, the Baltic seems to have been subject to a strong eutrophication during recent decades. Long periods with oxygen depleted conditions in the Baltic deep bottom waters, leading to an increased release of phosphorus (accumulated through years of nutrient discharges from land) which is periodically released from the bottoms, can thereby fuel future cyanobacterial blooms.
