Foam in Scheveningen due to a large plankton bloom and strong northerly winds

Published: 5 June 2020

The metres high seafoam during the fatal accident involving five surfers on 11 May in Scheveningen most probably arose due to an exceptional combination of many algal remains and, for this time of the year, an unusually strong north-northeasterly wind. That is the conclusion of researchers from various organisations in the report about the cause of the foam formation. The authors limited themselves to conclusions that currently seem the most reasonable, and the available data will be further analysed. At present, the scientists advise providing information and training to lifeguards and people who practise water sports, because the development of a sufficiently automated warning system will take more time.

Sea foam accumulated in the corner between the Noordelijk Havenhoofd and Scheveningen beach on 11 May 2020 (Photo: Haaglanden Safety Region).

Reconstruction of the last four days

The reconstruction from the available data reveals that a conjunction of 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 most likely that in the preceding period, a lot of sun first led to the growth of an exceptionally large quantity of seafoam algae in the sea. Around 10 May, the 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.

In their report, the scientists describe the most likely scenario on the day of the accident. On Monday 11 May, the north-northeasterly wind blew more or less parallel to the coast and at the start of the afternoon was a force 7 wind on the Beaufort Scale. The wind subsequently drove the foam that had been formed towards the south where it accumulated against obstacles that run 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.
Colony-forming algae

Plankton scientist Katja Philippart from the Royal Netherlands Institute for Sea Research (NIOZ) coordinated the research and 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 the cells against viral infections and from being consumed by sea animals. The algal cells are then not subject to the normal ecological system of “eat or be eaten” and the seafoam algae can therefore rapidly increase their biomass.’

To  form colonies, algae need  a lot of solar irradiance (sunlight) and a high availability of nutrients such as nitrogen and phosphate. In early May, the conditions were such that the algal cells grew in colonies and so a much greater biomass was achieved than would have been possible in the case of the solitary form. 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.

However, if there is a lack of light (less sun, stronger mixing) then the colony matrix falls apart. Philippart: ‘The cloudy weather of Sunday 10 May could have triggered the disintegration of the exceptional quantity of 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.’

Calculated concentrations of marine algae along the Dutch coast of April 21, 2020 (left) and May 10, 2020 (right) based on satellite images (Sentinel-3 OLCI). Copyright: Water Insight BV, Wageningen

Researchers advise more awareness

Although the scientists on the basis of the measurement data have been able to determine the most likely scenario for the development of the exceptional quantity of algal foam on 11 May, it will be difficult to set up an automatic warning system for people who practise water sports, states Philippart. ‘Then you must not only be able 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. Therefore, in the short term, we are calling for lifeguards, water sports schools and people who practise water sports to be given more information so that they are capable of making a good estimate of the possible accumulation of seafoam.’

The following researchers contributed to the report:

Katja Philippart (NIOZ en UU), Anouk Blauw (Deltares), Henk Bolhuis (NIOZ), Karen Brandenburg (NIOO en UU), Corina Brussaard (NIOZ en UvA), Theo Gerkema (NIOZ), Peter Herman (Deltares en TUD), Annelies Hommersom (Water Insight), Pascalle Jacobs (NIOZ), Marnix Laanen (Water Insight), Maria van Leeuwe (RUG), Anneke van den Oever (BuWa), Steef Peters (Water Insight), Marc Philippart (Rijkswaterstaat), Jaime Pitarch (CNR), Theo Prins (Deltares), Kevin Ruddick (KBIN), Lazaros Spaias (Water Insight), Dedmer van de Waal (NIOO), Dimitry Van der Zande (KBIN) and Alain Zuur (Highland Statistics).