Wadden Mosaic crew on the mudflats
While still waiting for the final euro’s, professor Tjisse van der Heide and colleagues already started an enormously laborious task in 2019: taking no less than 1394 samples from the Wadden Sea, to map the subtidal Wadden Sea and to investigate conservation and restoration measures for their potential to strengthen the ecosystem of this World Heritage Nature Reserve.
NIOZ already has a history of more than ten years of sampling every 500 meters of intertidal mudflat for sediments and organisms, big and small, living in and on these sediments. In 2019, the institute started an equally laborious task: sampling 1394 gridpoints in the Wadden Sea that are constantly under water. This is part of the project called Wadden Mosaic. It is a joint effort of NIOZ, Natuurmonumenten (the Dutch Society for Nature Conservation) and the University of Groningen and it is funded by Waddenfonds and the provinces of Groningen, Friesland and Noord-Holland.
Project leader for NIOZ is professor Tjisse van der Heide. ‘The important first step we took in 2019 was the first round of sampling of those 1394 points. For the majority we could use a box core, taking chunks of 20 by 20 by 30 cm out of the bottom, and subsequently analyzing the physical structure, as well as the organisms living in it’, Van der Heide explains.
The project started before the funding was 100% guaranteed. Van der Heide: ‘Co-financing by the provinces was a bit unsure; they were hesitant because the mussel- and shrimp fisheries community was initially “not very enthusiastic” about our plans. Eventually, however, all of the involved provinces co-funded the budget that was granted to us by the Waddenfonds. This meant we could continue with the important second part of the project: trying to identify potential measures that may strengthen or even rebuild the subtidal ecosystem of the Wadden Sea.’
Boxcore on the deck of RV Navicula
The Wadden Mosaic project will focus on investigating the effectiveness of four different potential conservation and restoration measures: reintroducing hard substrates on the bottom of the Wadden Sea, such as rocks, stones or shells, restoring live shellfish reefs, stimulating recovery of seagrass beds and, finally, closing areas for bottom dredging activities that might disrupt the sediment and the species living in and on it. ‘That last measure in particular was met with quite a bit of scepticism from professional fishermen, but I feel that it is necessary to investigate what the natural gains would be when leaving the sediment completely at peace, especially given all the discussion surrounding this topic’, Van der Heide says.
Many small steps, and a giant leap
Amidst all these field work efforts, Van der Heide was awarded a prestigious VIDI-award by the National Research Council NWO, for which he started the fieldwork in 2019. ‘With the awarded grant I could hire two PhD students – Clea van de Ven and Carlijn Lammers. And with some funds I had left from NIOZ, I appointed a postdoc to create a team of 3 researchers. They are looking into the specific patterns that plants use to clonally spread themselves across dunes and mudflats, and how this affects the environment and, in turn, the plants again.’
Sorting out organisms from subtidal Wadden Sea bottom
Field experiments on Texel and Rottumerplaat
At his previous position at the Radboud University Nijmegen, Van der Heide and his former PhD-student Valérie Reijers, who is now the postdoc in the team, discovered that marram grass (Ammophila arenaria) spreads itself following the same, so-called ‘Levy walk’ that many other organisms use to colonize new territories: with many small steps, combined with the occasional giant leap. Van der Heide: ‘The small steps are intended to build a dense network of plants, that is particularly good at trapping a lot of sand and therefore, quickly lifting itself above sea level. The large leaps are intended to quickly seek new territory and extend the dune. Valérie found out that in dunes in the US, this grass takes relatively more large steps and relatively fewer smaller steps, resulting in broader but lower dunes than the ones that are made by Dutch grasses. Now, as a postdoc scientist, she will continue that research, and hopefully find out whether this difference is because marram grass and other coastal plant species are genetically hardwired for this behaviour, or if they adapt to the local ecological circumstances.’
In lab experiments, where they added different amounts of sand to growing marram grasses, Reijers and colleagues already noticed that the number of ‘small steps’ is dependent on the amount of sand that is offered: less sand means more small steps. Now they hope to look into cordgrasses (Spartina spp.) as well, to see if they have tailored their expansion strategy to optimize the trapping of sediment particles under water, much like marram grass does on the beach. Van der Heide: ‘Like with the Wadden Mosaic project, this knowledge may eventually help us to understand how we can improve conservation and restoration measures in the Wadden Sea ecosystem at places where it needs our support.’