Harmful algal blooms: What are the consequences of climate change for our coastal areas?

Algae in the ocean can pose a significant risk to humans, marine life, and the seafood industry. Under favourable conditions for algae growth, certain algae species can multiply rapidly, a phenomenon known as algal blooms. Although algae always release small amounts of toxins, a massive increase in algae numbers leads to a high concentration of toxins in the water. These toxins can accumulate in marine organisms, such as mussels. Researchers at the Nansen Center in Bergen have now taken a closer look at how future changes in sea temperature and salinity may affect the frequency of blooms of the two most harmful algae species along the Norwegian coast.

In Norway, two types of algae in particular – Dinophysis acuta and the Alexandrium tamarense complex – represent a risk. Both species can make mussels dangerous to eat, and over the past 30 years they have led to cases of food poisoning. The species D. acuta produces a toxin that can cause serious stomach problems if shells that have absorbed this toxin are consumed. The A. tamarense complex produces another type of toxin that can lead to paralysis. When high levels of toxins are detected in mussel farms, the sale of mussels must be temporarily stopped – until the levels are safe again. Such situations lead to financial losses for the farmers. Harmful algal blooms are therefore an important reason why mussel production in Norway has seen limited growth in recent decades.

As the global temperature rises, conditions in the ocean also change. Researchers expect that harmful algae will be able to spread further north in the northern hemisphere, and further south in the southern hemisphere, because the marine environment in these areas becomes more favourable for them. But how much this will actually affect the frequency of harmful algal blooms has so far been unclear.

A new study published in Nature Communications Earth & Environment has investigated how climate change affects the occurrence of the two most toxic algae species along the Norwegian coast. The researchers used climate models, observations of algae over 14 years and probabilistic models for harmful algal blooms. The period 1995-2014 was used as a reference for today’s conditions and compared with a simulated future where the atmospheric temperature is 3°C higher. Increased precipitation and freshwater input from rivers – which affect ocean salinity – were also included in the analyses.

The study, led by Edson Silva at the Nansen Center and the Bjerknes Center, shows that warmer waters will lead to more harmful algal blooms in spring and autumn. In the summer, however, fewer such events are expected because the analysed algae do not thrive when the water gets too warm. They grow rapidly at moderate temperatures, but their growth is inhibited when it gets too warm.

Furthermore, the two algae species react differently to changes in salinity:

D. acuta is hardly affected by changes in the salt content of the water. For this species, increased temperatures alone will lead to around 50% more blooms annually.
A. tamarense, on the other hand, prefers salty water and thrives less when the sea becomes less salty. The calculations show that annual blooms of this species could be reduced by around 40% in a world that is 3°C warmer.
The findings provide important insights that will benefit both authorities and industry. For the seafood industry, it means that the risk picture associated with different types of harmful algae will change, affecting operations, food safety and monitoring. The results of the study can be used for both monitoring and warning, to identify where the risk is increasing the most, and where measures should be implemented first.