Ontwikkeling van de bodemdiergemeenschap in de geulen van referentiegebied Rottum: Tussenrapportage 18 jaar na sluiting (situatie tot en met het jaar 2023)

Since November 2005 a small part of the Dutch Wadden Sea has been closed to bottom disturbing anthropogenic activities. The area is situated just to the south of Rottumerplaat and Rottumeroog islands and covers about 7,400 hectares. The purpose of closing this area is to monitor the development of the sublittoral marine fauna in the Wadden Sea when they are not disturbed by human activities. The monitoring programme focuses on the development of the benthic communities present in the gullies. The benthic fauna in two gullies in the closed area are being monitored and compared with the fauna in two control gullies outside the closed area, which are under the influence of human bottom disturbing activities. This report describes the interim results eighteen years after the area was closed. Of the investigated gullies, the closed Boschwad gully is the smallest in surface area and has a lower water depth and finer sediment grain sizes than the other gullies. In line with what was previously observed, differences between samples, taken in the same gully, are large and differences between averages of samples in different gullies are small. Also in the new survey years 2022 and 2023, the small gully differences are not statistically different for the biodiversity parameters species richness, number of individuals, diversity. Analysis of the entire study period shows that these parameters did not develop differently in the closed gullies than in the open gullies so that no effect of closure was detectible for these parameters. In addition, the development of species composition over the years is also roughly the same for the different gullies. The last period (2017 to 2023), especially the open channel Zuidoost Lauwers deviates somewhat more from the other gullies in composition. For example, the worm composition is somewhat different and fewer shellfish are found. Furthermore, cockles are found somewhat more often in the closed gully Boschwad and American razor clams somewhat more often in the closed gully Schild, as was also observed earlier. Differences in composition are likely best explained by differences in location, water depth and bottom composition and distance to the sea inlet, instead of closure for human disturbance that is investigated here. Recommendations for the experimental design were made to reduce variability between samples, to better investigate differences and to respond to the new area closure for shrimp fisheries.