Fatal accident Scheveningen linked to unusual plankton bloom

Metres-high seafoam during a fatal accident on 11 May in Scheveningen, in which five surfers drowned, is likely to have been caused by an exceptional combination of algal remains and strong winds. That's according to a new report compiled by researchers from a number of organisations including NIOO.

The report, coordinated by the Royal Netherlands Institute for Sea Research (NIOZ), reconstructs the last four days before the accident, which made international news headlines last month. The authors limited themselves to conclusions that currently seem the most reasonable, and the available data will be analysed further.

The data reveal that weather conditions from the end of April led to the large quantity of foam in the corner of the Northern Harbour Head on the beach of Scheveningen. It is likely that sunny weather in the preceding period first led to the growth of an exceptionally large quantity of seafoam algae.

Unseasonal winds

Around 10 May, the algal bloom was decreasing, partly due to reduced light as a result of cloud cover and more wind-driven mixing of the seawater. As a result of this, the algal remains were released into the sea.

On Monday 11 May, a north-northeasterly wind blew more or less parallel to the coast and at the start of the afternoon went up to force 7 on the Beaufort Scale. The wind subsequently drove the foam that had been formed towards the south, where it accumulated against obstacles running across the beach straight into the sea, such as the Northern Harbour Head of Scheveningen.

At the start of the evening, the wind turned slightly more to the north and the currents changed, as a result of which the accumulated foam started to move in the area between the Northern Harbour Head and the beach of Scheveningen. This is when the five experienced surfers drowned.

Colony-forming algae

Katja Philippart from the Royal Netherlands Institute for Sea Research (NIOZ) explains how there could be so many algae present in the seawater in early May. ‘The scientific name of this algal species is Phaeocystis globosa, and in the sea it occurs in two forms: solitary cells or in colonies. In colonies, the cells are held together by a mucous-like matrix, which protects them against viral infections and being consumed by sea animals."

To  form colonies, the algae need a lot of solar irradiance (sunlight) and a high availability of nutrients such as nitrogen and phosphate. In early May, the conditions for this were perfect. 

Philippart: ‘The cloudy weather of Sunday 10 May could have triggered the disintegration of the colony cells into an unparalleled quantity of individual solitary cells, as a result of which the sugar-like remains of the matrix ended up in the sea. Due to viral infections, the solitary algal cells quickly burst open and the proteins were also released from the cells into the water."

Monitoring programme

Measurements in the Marsdiep suggest that, after this plankton bloom, the quantity of algal cells in the seawater was four times as high as during the average peak over the past ten years. That's important information, says NIOO algae expert Dedmer van de Waal.

"We analysed monitoring data from the Department of Waterways and Public Works to see what circumstances are ideal for foam algae to grow, and to map which areas meet the criteria. This has given us a clear spatial overview, with the analyses showing how over the past two decades foam algae have declined in open sea but become more abundant in coastal waters."

Van de Waal says the monitoring data are "of essential importance" for understanding these algal blooms and what causes them. "Understanding them can help us predict future blooms." That's why he is calling for the department's current monitoring programme to be extended.

Raising awareness

Van der Waal also stresses that the report required "a lot of teamwork, and the bringing together of complemtary forms of expertise." This combination of people and data allowed the authors of the report to describe the most likely scenario on the day of the accident.

However, it will be difficult to set up an automatic warning system for water sports practitioners, says Katja Philippart. "For that, younot only have to keep on accurately monitoring the quantity of seafoam algae and foam, but you must also be able to predict the current wind strength and direction in real-time very locally and in considerable detail."

This is why, in the short term, the report is calling for lifeguards and water sports schools and practicioners to be given more information, so that they will be capable of making a good estimate of the possible accumulation of seafoam for themselves.

Organisations that contributed to the report are BuWa, CNR, Deltares, Highland Statistics, KBIN, NIOO, NIOZ, Rijkswaterstaat, TUD, UU, UvA and Water Insight. For NIOO, Dedmer van de Waal and Karen Brandenburg took part in the research.