The Wadden Sea, spanning along the coast of the Netherlands, Germany, and Denmark, is a remarkable region in many ways. It is the world’s largest intertidal area, stretching nearly 500 km in length. The shallow coast undergoes massive transformations twice a day with the rise and fall of the tide. When the tide goes out, thousands of hectares of mudflats are exposed, making abundant invertebrates such as shellfish, snails, and worms available as food for millions of birds. When the tide comes in, the area is submerged again, and fish and seals invade the shallows for their daily meals.
In 2009, the Wadden Sea was granted UNESCO World Heritage status due to its exceptional universal value. This recognition elevates it to the same level of global significance as the Great Barrier Reef, Grand Canyon, and the Amazon Rainforest. With this prestigious status comes the responsibility of ensuring proper monitoring and protection.
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How do you monitor such a dynamic ecosystem?
Beneath the surface of the Wadden Sea’s mudflats lies a hidden world teeming with benthic life – organisms that play a crucial role in the ecosystem. While the mudflats might seem simple at first glance, they support a large community of creatures that are adapted to the dynamic intertidal live. This benthic life shifts over time, both within and between years, making regular monitoring essential to understanding the ecosystem’s health. And what’s more, benthic life is affected by human interventions. These can be large scale in relation to climate change. But also regional scale in relation to activities such as fisheries, gas extraction and dredging.
The NIOZ (Royal Netherlands Institute for Sea Research) has long recognized the importance of these organisms and has a long-standing tradition of studying the Wadden Sea’s benthic communities. However, logistical and financial constraints meant that monitoring efforts were always limited to certain areas.
That changed in 2008. In that year, NIOZ, NAM (The Dutch petroleum company), joined forces and designed a large-scale annual sampling programme across the entire Dutch Wadden Sea. Since 2019 also Rijkswaterstaat is an equal financial partner with NAM and NIOZ. NAM was a co-financer of SIBES from the outset because they were legally obliged to monitor the possible effects of gas extraction under the Wadden Sea. The project quickly found its name: SIBES – Synoptic Intertidal BEnthic Survey.
How does SIBES work?
The sampling design involves systematically collecting samples every 500 meters across the exposed mudflats, with an additional 20% of locations/sampling stations chosen at random on grid lines to allow estimating spatial patterns. This approach allows researchers to track ecosystem changes with confidence, helping to distinguish between natural fluctuations and human impacts such as gas extraction, salt mining, dredging activities, or other human interventions.
In total, this sampling design results in approximately 4,500 sampling points each year —an enormous logistical challenge to collect and process the samples. The NIOZ research vessel RV Navicula (now replaced by the RV Wim Wolff) and her crew play a vital role in making this ambitious collection effort possible.
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Since 2008, the NIOZ research vessel has sailed the Dutch Wadden Sea each spring and summer, serving as the moving base station for SIBES. Like a mother duck, it releases four to six small rubber boats around high tide. Researchers in these boats use core samplers to extract sediment from the sea floor, sieve the material, and collect living organisms and shell fragments into sample jars. The sediment itself is also sampled, as sediment composition often influences benthic life and vice versa. When high tide ends, the dinghies return to the ship, their home base.
The method of sampling from boats was chosen for its speed and efficiency, allowing researchers to cover large areas quickly. However, some very shallow areas cannot be accessed by boat, so they are sampled on foot during low tide.
Adventures in the field
We could fill entire books with the adventures we’ve had during sampling. Like the time we spotted a sunfish casually swimming in the Wadden Sea or when we got stuck in the mud and had to wait an entire tidal cycle for the water to return. Many of us have mixed feelings about those exhausting days in the Dollard region, where we always return to the boat completely covered in mud. And there are the countless times we have had to abort sampling due to approaching thunderstorms—because, of course, safety comes first.
Spending countless hours in the Wadden Sea, rain or shine, is both physically demanding and unpredictable. But through all of this effort, we have also witnessed long-term changes in the ecosystem. We’ve seen first-hand the effects of climate change, like the mass die-offs of cockles after heatwaves – stark reminders of the shifting conditions in this system.
Despite the challenges, there are also moments of excitement and discovery. The thrill of discovering a new shell species for the Wadden Sea, the satisfaction of a job well done alongside your colleagues, providing the data for scientific publications, and endless beautiful scenery make it all worthwhile. No two sampling seasons are ever the same, and that is also thanks to the many citizen scientist who join us each year, making this work possible.
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From sampling to high-quality ecological data
Once the samples are collected, they are preserved in formalin aboard the research vessel. But that’s just the beginning— the biggest job starts in the laboratory. Lab assistants spend countless hours analyzing grain size composition, identifying organisms, measuring and weighing them, and incinerating and weighing again. Since July 2011, the process of weighing and incinerating has been automated using a robot developed at NIOZ, making the work more efficient. Yet, the lab work still takes 7 times the effort of taking the field samples.
Organizing and managing the vast amounts of data generated is another daunting part of the entire operation. It’s no small feat, but thanks to the incredible effort of our database manager, we’re able to turn this information into high-quality ecological data that can provide valuable ecological insights.
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The impact of SIBES
SIBES data has already proven its value in over 30 scientific publications and even more reports. It has helped us to better understand the spatial distribution and habitat us of benthic species, shifts in community composition, facilitation between species, dynamics of sediment composition, habitat use of bird species in the Wadden Sea. Also, it has been insightful for understanding human impact. For instance, with SIBES it has been demonstrated that sediment composition and benthic life was different in the gas extraction area with deep land subsidence. These insights are just the beginning—many more studies are currently in progress, for instance, on effects of the climate crisis on benthic species and biodiversity.
SIBES has grown into a recognized brand, not just within the Wadden Sea but across the globe. The same methods and sampling design have been applied to intertidal areas in Portugal, Germany, Australia, Guinea-Bissau, Mauritania, Oman, and China.
Recently, the SIBES dataset was made publicly available. After an embargo period of three years, all the data can now be downloaded from the NIOZ Data Archival System. In sharing these data, we hope to enhance collaborations and understanding of the impact of various pressures on macrozoobenthic invertebrates, sediment composition, food webs, the ecosystem, and biodiversity in the Wadden Sea and other intertidal habitats.