Aarhus University Seal / Aarhus Universitets segl

Algae blooms

Algae blooms are caused by microscopic algae occurring in very high or unusual concentrations.

When algae blooms with very high concentrations of algae occur, the water may become discoloured. The basic reason is usually that there are – or have been – large amounts of nutrients present, which the algae have been able to exploit to build up the large quantities of algae. For this reason, algae blooms are mostly seen in coastal nutrient-rich areas.

In addition to the growth of algae (in-situ growth), the physical accumulation of algae, which otherwise occur in low concentrations, can also contribute to algae blooms. The most important physical mechanism that may lead to the accumulation of algae is horizontal advection, which, coupled with the ability of the algae to move up and down in the water column, e.g. may be the cause of local blooms in connection with "down welling" in fronts and along the coast. Large blooms of, among other things, cyanobacteria may also occur in situations where algae from the entire body of water rise to the surface and are concentrated in/near the surface of the sea.

Harmful algae blooms may also be caused by relatively low concentrations of algae that produce toxins. A good example of this type of algae blooms are those, which produce toxins that accumulate in filter feeder animals, such as mussels, oysters and fish. These algae can cause a number of different types of poisoning (PSP, DSP, ASP, AZP and CFP) in humans, who eat mussels and oysters, as well as, in the case of CFPs, fish. In these cases, water is not discoloured or unclear.

If you want to learn more

Illustration: Martin Søndergaard ©
Large amounts of blue-green algae can lie as a paint-like layer in the water surface.
Illustration: Martin Søndergaard ©

Algae blooms may be harmful because they have negative effects on:

1. Ecosystems

  • The algae can lead to increased oxygen consumption and thereby cause oxygen depletion
  • The algae can shade the underwater vegetation
  • The algae can be poisonous and cause fish and bottom-dwelling animals to die as well as affect the benthic vegetation and ecosystem function in the free water masses by inhibiting/killing their grazers and other algae

2. Recreational interests

  • The algae can be poisonous to bathers, who get the algae on their skin or consume water contaminated by algae and/or visitors to the coast, who inhale aerosols with algal toxins.
  • The algae may cause discoloration of the water and the water may smell – especially when the algae decay
  • The algae may cause the water to be unclear
  • The algae may produce toxins that may accumulate in mussels and oysters, causing them to become harmful to eat.
  • Algae can "scare" fish away from fishing grounds

3. Business interests

  • The algae may produce toxins that can accumulate in mussels and oysters so that they become harmful to eat – which means that commercial harvesting (wild stocks/aquaculture) of mussels and oysters is prevented
  • Algae may delay the release of fish in aquaculture, thereby shortening the production season
  • Algae may kill aquaculture fish

Mesodinium rubrum in Skive Fjord, May/June 2019

To the right, you can see an animation of drone recordings from the southern part of Skive Fjord – close to the outlet of Skive Å (4-6-2019) made by Michael Bo Rasmussen. The animation shows "clouds" of discoloured red water caused by the mixotrophic ciliate Mesodinium rubrum. NOVANA monitoring from the station in Skive Fjord (Station Vib3727-00001), located further to the north of the fjord, also shows a marked flowering (> 400 μg C/L) in May/June 2019.

"Cloud" formation/very significant local blooms of Mesodinium rubrum is a well-known phenomenon for mesodinium and is due to the fact that Mesodinium is a highly skilled swimmer, which can actively move to the part of the water column with attractive conditions. In windless periods, where the surface water is not regularly stirred, the swimming behaviour of  Mesodinium can therefore result in local accumulations, which are seen as red “clouds”.

NB: Mesodinium rubrum is not toxic, but in connection with very large blooms and the accumulation of high biomasses, oxygen depletion may occur when the blooms break down and degrade when consuming the oxygen.

NBB: It is possible to bathe during blooming of Mesodinium rubrum, but it is not recommended to bathe in the red water if the blooms are about to decay/smell bad.

Animation from Skive Fjord. Images are composed of drone fottage from May/Jue 2019 by Michael Bo Rasmussen.