Concern over 'red tide' in Dutch coastal waters

In een bootje op de Ouwerkerkse kreek
© Karen Brandenburg/NIOO-KNAW

Concern over 'red tide' in Dutch coastal waters

More toxic dinoflagellates due to climate change?
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Climate change could lead to an increased occurrence of harmful algal blooms in Dutch coastal waters in the future. NIOO-researchers Karen Brandenburg and Dedmer van de Waal have been studying the dense blooms formed by tiny organisms known as dinoflagellates. They warn that without timely monitoring, these blooms will cause more and more problems.

Blue-green algae have long been a nuisance in Dutch recreational waters. But since 2012, another type of harmful bloom has been found in a small creek in the province of Zeeland each year, caused by Alexandrium ostenfeldii - a highly toxic dinoflagellate.

These Alexandrium blooms are a serious threat to human health, says Karen Brandenburg, who defended her PhD-thesis on the subject at Utrecht University recently. "Alexandrium ostenfeldii is known to produce a number of potent neurotoxins, which accumulate in shellfish and may lead to paralytic shellfish poisoning syndrome in humans."

Red tide rising

But how do dinoflagellates form these massive blooms, and what will be the impact of more CO2 and a warmer climate? Should we be worried about the future of our Dutch coastal waters? Those are some of the questions Brandenburg and research leader Dedmer van de Waal have been looking into.

Since the discovery of the blooms in the Ouwerkerkse Kreek, the Water Board Scheldestromen has collected a large amount of data, and its dataset has been complemented with additional field surveys by the two researchers.

Even in the very near future, warns Van de Waal, the danger these blooms pose to human health is very real. "Our recommendation is to monitor them thoroughly, so that we at least know how they are developing."

(Text continues below pictures)

  • Karen Brandenburg/NIOO-KNAW
  • Karen Brandenburg/NIOO-KNAW
  • Karen Brandenburg/NIOO-KNAW
  • Karen Brandenburg/NIOO-KNAW

Temperate zones at risk

Brandenburg says the effects of climate change are indeed likely to add to the impact of the problem, not just in the Netherlands but worldwide. More CO2 in the atmosphere and higher temperatures can both cause a further increase in the occurrence of harmful algal blooms.

"Our results show that more CO2 accelerates the growth of harmful algae, and of toxic dinoflagellates in particular. Higher temperatures also have an impact, but only in temperate climates such as that of the Netherlands."

Other factors are the weather and the amount of nutrients in the water. Brandenburg: "The abundance of phosphate and nitrate in the creek contributes to the occurrence of large blooms when the water temperature rises above 15°C in summer. Under those conditions, you'll find ten thousand algal cells in a single millilitre of water."

High winds, on the other hand, and strong precipitation - which lowers the salinity of the water - were found to be responsible for a sharp drop in the number of Alexandrium-cells in Zeeland. So wind and rain can be major steering factors when it comes to the duration of the blooms.

Why Alexandrium?

Of course, there are many different species of algae. Why, then, is this particular one proving to be so successful? The researchers found that there is a wide variety of functional traits, not just across but also within Alexandrium-populations.

"We have shown that strains from the same population can have different growth rates and produce different amounts of toxins", explains Brandenburg. The algae also differ in how fast they can take up nutrients. Brandenburg and Van de Waal: "We expect this to be a major contributing factor to the success and distribution of these algae. Especially in an environment that is changing due to eutrophication and climate change".

Brandenburg's PhD research has shown that toxic algae are occurring in Dutch waters in concentrations high enough to be dangerous. Further research should establish if other creeks and coastal waters are also at risk of being colonised. Brandenburg: “Our recommendation is to make this a focus of the monitoring programme."

Further reading

  • Brandenburg, K. M., Velthuis, M., & Van de Waal, D. B. (2019). Meta-analysis reveals enhanced growth of marine harmful algae from temperate regions with warming and elevated CO2 levels. Global Change Biology. doi:10.1111/gcb.14678
  • Brandenburg, K. M., de Senerpont Domis, L. N., Wohlrab, S., Krock, B., John, U., van Scheppingen, Y., E. van Donk, & Van de Waal, D. B. (2017). Combined physical, chemical and biological factors shape Alexandrium ostenfeldii blooms in the Netherlands. Harmful Algae, 63, 146-153. doi:10.1016/j.hal.2017.02.004

The PhD research has been funded by the Gieskes-Strijbis Fonds.

With more than 300 staff members and students, the Netherlands Institute of Ecology (NIOO-KNAW) is one of the largest research institutes of the Royal Netherlands Academy of Arts and Sciences (KNAW). The institute specialises in water and land ecology. As of 2011, the institute is located in an innovative and sustainable research building in Wageningen, the Netherlands. NIOO has an impressive research history that stretches back 60 years and spans the entire country, and beyond.

More information:

PhD Thesis: ‘Harmful algal traits and bloom dynamics under climate change’, June 2019.