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.

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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 i Skive Fjord, maj/juni 2019

Her til højre ses animation af droneoptagelser fra den sydlige del af Skive Fjord - tæt på udløbet fra Skive Å (4-6-2019) foretaget af Michael Bo Rasmussen. Animationen viser ”skyer” af misfarvet rødt vand forårsaget af den mixotrofe ciliat Mesodinium rubrum. NOVANA moniteringen fra stationen i Skive Fjord (Station Vib3727-00001), som ligger længere mod nord i fjorden, viser også tilstedeværelse af en markant opblomstring (> 400 µg C/L) i maj/juni 2019.

”Sky”dannelse/meget markante lokale opblomstringer af Mesodinium rubrum er et velkendt fænomen for Mesodinium og skyldes, at Mesodinium er en meget dygtig svømmer, som aktivt kan bevæge sig hen i den del af vandsøjlen hvor forholdene er attraktive. I vindstille perioder, hvor overfladevandet ikke løbede opblandes, kan Mesodiniums svømmeadfærd derfor medføre lokale akkumuleringer, som ses som røde ”skyer”.

NB: Mesodinium rubrum er ikke giftig, men i forbindelse med meget store opblomstringer og akkumulering af høje biomasser kan der opstå iltsvind, når opblomstringerne bryder sammen og nedbrydes ved forbrug af ilt.

NBB: Man kan godt bade i en opblomstring af Mesodinium rubrum, men det anbefales ikke at bade i det røde vand, hvis opblomstringen er ved at gå i forrådnelse/lugter grimt.