Feeling the pulse on the intertidal Wadden Sea

‘Thanks to this long time series and the complete coverage of the Wadden Sea, SIBES offers a wealth of information that will provide new insights for science and management,’ says SIBES scientific coordinator Allert Bijleveld from NIOZ.

Four months of fieldwork

The Synoptic Intertidal Benthic Survey (SIBES) was partly the result of bird research on Griend. What do the birds eat there and where do they find that food? In addition, the Dutch Petroleum Company (NAM) was legally obliged to monitor the possible effects of gas extraction around the Wadden Sea. That is why NAM was also a co-financier of SIBES from the outset. In 2008, the first integral sampling of the tidal flats was carried out. Over a period of four months, a group of field workers walked (and sailed) across (and over) all the tidal flats of the Dutch Wadden Sea. They sampled every 500 metres of tidal flats with the addition of a number of randomly selected sampling stations.

Bottom animals and particles

They collected standardised samples of all organisms larger than 1 millimeter, the so-called macrozoobenthos. These were identified, measured and weighed in the laboratory. They also determined sediment characteristics, such as mud fraction and median grain size.

This sampling campaign has been repeated annually, at more than four thousand points, spread over 1200 square kilometres of the Dutch mud flats. In the first fourteen years of the project, more than fifty thousand points were sampled in this way, and more than three million individuals of 177 different animal species were collected.

Not a uniform grey mass

Many conclusions can already be drawn from the collected data and in more than thirty scientific articles, emphasises Bijleveld. ‘It is crystal clear that the intertidal Wadden Sea is not the monotonous grey mass that you might think you see when taking a quick glance over the dike. The differences in the grain size of the sediment alone show a very diverse picture, from firm and sandy to soft and silty.’

Variety of life

With this variation in soil composition, the biodiversity, density, and community structure of macrozoobenthos also vary. An extremely high density of species and individuals may be found in some places, while in other places the soil appears much ‘poorer’ with only one or two individuals. The total biomass and biodiversity show a largely variable picture across the tidal flats.

‘Biomass does not necessary follow the density of individual benthic animals,’ Bijleveld warns. ‘In almost all years, the common cockle was the most abundant when it came to biomass. In terms of numbers of individuals, the much smaller mudsnails are the most dominant,’ says Bijleveld. ‘Both measurements are therefore important.’

Valuable time series

Even more important than the rankings, according to Bijleveld, is the continuation of measurements by the SIBES team. ‘The longer a sampling program like SIBES continues, the greater the insights gained, for instance with the changing climate, and the larger statistical power of this programme to show differences.’ 

Effects of gas extraction

One of the dossiers where the enormous amount of data has yielded important information, is the gas extraction under Oost-Ameland and Moddergat, Lauwersmeer and Vierhuizen, the so-called MLV area. Over the years, the grain size on the mud flats that are affected by the (extremely deep) subsidence caused by gas extraction has decreased statistically significant to the rest of the Dutch tidal flats. ‘This means that the sand there has become a little finer. So, it seems that a subsiding seabed caused by gas extraction is being filled in at the surface with a different type of sediment than the original material. This has proven to be reliably measurable, regardless of other influences in other places, such as the dredging of shipping channels.’

Effect on benthic life

The influence of the subtly changing sediment is directly related to changes in the composition of the benthic life, Bijleveld states. ‘Benthic animals that prefer to live a little deeper are increasing in number in the area affected by the deep subsidence, while the more superficially living species are actually decreasing there, compared to similar places.’

Climate change is becoming visible

The influence of the changing climate is also becoming visible with SIBES. In their new publication, Bijleveld and colleagues show that the most important benthic animals are shifting. First, the cockle was the most important species in terms of biomass. In the years immediately after a large-scale spatfall of young cockles there is typically a peak in biomass. But in recent years, we have seen a steady decline in cockle biomass. There is no longer a large peak in growth and there have been several years with extremely warm summers in which the clams died en masse. This shows that the biomass of cockles is declining significantly. ‘SIBES suggests that in some years the Wadden Sea becomes too warm for the cockle,’ Bijleveld concludes.

More soil structure

Climate change is not bad news for all organisms. Bijleveld: ‘After harsh winters, we usually see a decline in sand mason worms. Now we see that, in the absence of harsh winters, these worms are increasing in abundance over the years. In many places, the tubes of these animals are the beginning of a subtle ‘reef formation’ on the mudflat. They offer structure to, for example, young shellfish to settle on the tidal flats. We can therefore see that biodiversity increases in places where sand mason worms settle.’

Funding

SIBES is funded by the NIOZ, NAM and, since 2019, also by Rijkswaterstaat. Funding is guaranteed until 2025. All data from the first fourteen years of the SIBES monitoring programme has been made available with this publication for anyone who wants to conduct further research. With a three-year embargo, a new year is added each time. Bijleveld: ‘This three-year embargo is necessary to enable our scientific publications based on SIBES data.’

Open database

The SIBES researchers are not only inviting the scientific community to make use of this incredible collection of data and to collaborate on scientific publications. ‘The government can also benefit from it,’ emphasises Bijleveld. ‘Just look at the patchwork of areas with different soil compositions and animals. If you extract gas or lay cables through it in the future, the original composition may be disrupted with negative effects on the natural value of the area. With SIBES, we can monitor such changes and effects. If the measurements are maintained, we can also see if the mud flats are recovering from human intervention and how long that will take.’

Bird food

According to Bijleveld, the information from the measurement programme can also be combined with other data sets. ‘Bottom-dwelling animals are, somewhat disrespectfully, referred to by some as “bird food”. And indeed, the presence of shellfish such as Baltic tellins and mudsnails show a strong correlation with the numbers and distribution of waders such as the red knot. Mud shrimp and bar-tailed godwits are also correlated, as are shrimps and sanderlings, and ragworms and dunlins.’

Another field season

This year, the SIBES field staff will be out sampling between June and September for the eighteenth field season. Bijleveld emphasizes the strength and importance of the continuation of the programme, both with his colleagues and with the financiers. ‘The insights that we have gained in more than thirty scientific publications, and even more reports, are important and significant. And every year the value of this dataset increases even more, especially in light of all the human interventions in and around the Wadden Sea.’

More information

• The publication on the SIBES project, including the full dataset 2008 to 2021, is available via the website of Scientific Data.

Text: NIOZ
Photo's: Jan Veen; Dennis Mosk