Microalgae are generally harmless to other marine life forms, and they are a vital part of the food chain. They are the ocean’s “grass”, which provides the basis for marine food chains.

“However, a small number of microalgae species are described as harmful, and some of them can lead to fish and other marine organisms dying”, explains Lars-Johan Naustvoll, a marine scientist and expert on algae.

In Norway, marine life has chiefly been killed by algal blooms involving the species Chrysochromulina leadbeaterii, Prymnesium polylepis (previously known as Chrysochromulina polylepis), Prymnesium parvum, Pseudochattonella farcimen and Karenia mikimotoi (Gyrodinium aureolum).

It has now been confirmed that the first of those algae has been responsible for the recent death of farmed fish held in cages in Nordland and Troms. (Find out more about the situation on the Directorate of Fisheries’ website.)

Common along our coast

Chrysochromulina leadbeateri is a type of phytoplankton that floats freely in the water. The species, which is relatively common, can often be observed in spring, but normally only in small numbers. In that case, it is just part of the great diversity of microalgae found in our coastal waters. Very occasionally, the alga occurs in quantities described as blooms, which can result in the fish dying.

“The species then goes from being a natural part of the phytoplankton to being an algal bloom”, says Naustvoll.

It isn’t the first time this species has been linked to salmon deaths in that part of Norway. There was a large bloom in May and June of 1991, and a smaller one in May 2008.

Damage the gills of farmed fish

“Wild fish swim away from the fjord or head to deeper waters when these blooms occur, but it is impossible for farmed salmon to escape. That’s why the fish die”, says Naustvoll.

Studies suggest that this species of alga excretes haemolytic compounds. When they come into contact with the gills of the fish, they cause damage that may result in the fish not absorbing enough oxygen and eventually dying.

At moderate concentrations, below fatal levels, the algae may cause changes in the behaviour of the fish. They swim up to the surface and gasp for air.

Spreads with the currents

By observing behavioural changes and fish deaths, scientists can get an idea of where the blooms are occurring.

“We are monitoring the situation carefully by collecting data from the affected areas. By combining this information with models of ocean currents, we are trying to keep fish farmers informed about where the algal bloom will spread to”, says Naustvoll.

He stresses that it is difficult to say with any certainty how the situation will develop.

“We cannot guarantee that the algae will spread to the places indicated by our forecasts. There are many factors involved.

In addition, it’s difficult to say how long the problem will persist in the affected areas.

May run out of nutrients

“Algae consume inorganic nutrients. Consequently, the increase in the amount of algae will subside as the nutrients get used up”, says Naustvoll.

Algal blooms that “get stuck” in fjords tend to use up the inorganic nutrients and then die out. On the other hand, if the algae are transported to new areas where there are still nutrient, they may continue to bloom.

Past evidence suggests that changes in the weather such as stronger winds and water turbulence help to diminish blooms.

Update 24.05.19

More on the cause of algal blooms

Incease in nutriens leads to a higher production of phytoplankton. For a harmful algal bloom to take place, the harmful alga first need to be present in the area. The availability of nutrients, their composition, fjord- and weather conditions, foraging from other animals, available sunlight and competition between algae species are all factors. Different species have different preferences.

Of the nutrients, nitrogen and phosphate in the "correct" mix is most important for the algae.

There are several sources of nutriens in the coastal water that may contribute to plankton production, such as runoff, sewage, agriculture, industry and aquaculture.

The effect of extra nutriens from human activity will be location dependent. In shallow areas with reduced water circulation, added nutriens would contribute more than in exposed areas with more water and current.

For the Chrysochromulina alga to grow, it needs to be close to the water surface. The amount of sunlight that is available there, enables the photosynthesis. In Norwegian coastal areas, freswhater makes the water columt stable. Sun and low winds also contribute to reduced turbulence. Under such conditions, the algae are left undisturbed.

Chance and many factors are required for a harmful algal bloom to take place.