The 'Big Five' of the Wadden Sea

Life of the seabed of the Wadden Sea is mapped annually on behalf of Rijkswaterstaat. The programme within which this work is carried out is called “Monitoring Waterstaatkundige Toestand des Lands” (MWTL), and aims to monitor the condition of Dutch waters. There are also monitoring programmes that cover the North Sea, delta and inland waters.
The information collected is used for policy purposes. Trends can be identified and the results can be assessed against standards and targets. As part of this monitoring, we sampled benthic life in the Wadden Sea in 2020.

In August and September 2020, samples were taken with a core sampler (shallow waters) or with a boxcorer (deeper waters) in five sub-areas of the Wadden Sea, including the Ems-Dollard (see maps 1 and 2 below).
The collected samples were processed by specialists at Waardenburg Ecology and the fauna identified. After analysing all the samples we were intrigued as to the five most numerous species. These 'Big Five' are shown in the bar chart below.

It immediately became clear that it was not possible to simply compile the Big Five for the entire Wadden Sea as each sub-area has its own unique community of species. In total, twelve species/species-groups appear in the areas' top 5 lists, including eight worms, two snails and two amphipods.

This area is characterized by intertidal mud flats. Here, the five most numerous species/species-groups are all worms, with four belonging to the bristleworms (Polychaeta), and one to Oligochaetes (Oligochaeta).

This area is a part of the Wadden Sea that remains covered by the sea. In the samples from the western Wadden Sea, the laver spire shell is by far the most numerous, with more than 38,500 individuals per square meter! The other four most numerous species are all worms.

This is a large, intertidal mudflat in the eastern Wadden Sea. Although this is a similar ecotope to Balgzand, the top 5 most abundant species are very different. The bristle worm (Tharyx) is the most numerous species on the Piet Scheveplaat. Also in the top 5 is a commensal amphipod (Urothoe poseidonis), which is more abundant here than in any of the other sub-areas. The other common species are the laver spire shell and the worm (Tubificoides).

Just like the western Wadden Sea, this area is characterized by some deeper parts that are continually covered. Yet the differences between the two areas are enormous: in the western Wadden Sea we find the laver spire shell in its thousands, but here it does not even make the top 5. The low density of benthic animals here is striking, with only a few hundred per square meter. The top 5 is formed by the commensal amphipod (Urothoe poseidonis) followed by four bristle worms.

Just like Balgzand and Piet Scheveplaat, the Heringsplaat is an intertidal mudflat. However, the benthic fauna is strikingly different here. In the other two mudflats, worms are the most abundant, but on the Heringsplaat an amphipod (Corophium volutator) is by far the most numerous, followed by the spire snail. The top 5 is further completed by two worms and the laver spire shell.

From this research we can conclude:

1. The Big Five of benthic animals in the Wadden Sea is different in each sub-area. Even in the same habitat-types we see major differences: in two of the three intertidal mudflats, worms are the most numerous, but in a third area this is an amphipod.
2. The numbers of animals vary widely, even in similar habitats. For example, the western Wadden Sea has more than 40,000 animals per m2, but the eastern Wadden Sea only has a few hundred.