Cyclic water level dynamics determine wetland functioning : Restoring or introducing wetland dynamics to improve habitat heterogeneity and food availability for wetland birds

Dutch; Flemish. Clay marshes are rare but highly productive ecosystems in Europe, supporting large populations of waterbirds and marsh birds. However, sustaining these high numbers of species and individuals proves to be a challenge, even though it is of high priority for the national and Natura 2000 management goals. Clay marshes typically exhibit boom-and-bust cycles, where bird numbers peak after marsh creation, following drainage or after re-inundation, and then decline. The exact mechanisms behind this boom-bust phase remain unknown but appear to be driven by water level, with a significant role for vegetation succession and the associated habitat and food availability. This research takes place in the Oostvaardersplassen, a eutrophic clay marsh, undergoing a multi-year drawdown to enhance heterogeneity and biodiversity, particularly for marsh birds. We studied habitat and food availability, in relation to water level and key species (red deer, geese and large carp) to understand the boom-bust cycles. Results show that water level drives vegetation succession and determines the accessibility and availability of food for birds. During drying periods, the previous lakebed is quickly colonized by pioneer vegetation, followed over time by perennial vegetation, such as reed. Grazing, by geese during inundated periods (e.g., winter and wet spring 2024) and by red deer during dry periods, strongly influences the vegetation composition, vegetation height, and specifically reed expansion. However, reed continues to expand even in the presence of herbivores. The presence of fish, in this case large common carp, reduces food availability for wetland birds, especially benthos biomass. Water level interacts by creating spatial and temporal variation in fish presence during low water levels, allowing benthos biomass to increase and making it possible for benthivorous fish and benthivorous birds to coexist. Furthermore, our research shows that repeatedly implementing a multi-year drawdown will not always lead to the same outcome, dependent on seed bank composition and environmental filters (e.g., water level, nutrients, soil elevation) that play a role in the vegetation community that will eventually be established from this seed bank. In conclusion, there is ample room for natural processes, such as large-scale water level fluctuations, in productive (managed) clay marshes. Natural water level dynamics steer boom-bust cycles in bird populations by determining habitat heterogeneity and food availability, also leading to varying community assemblages across other species groups. The implementation of a water level drawdown will lead to alternative successional trajectories driven by water level and seed bank composition, presumably without losing productivity in the long term (depending on drawdown frequency). In this era of global change and global loss of biodiversity, restoration of wetland systems can aid in halting the deterioration of nature areas and increase biodiversity. Asynchronous cycles in water level and vegetation over space and time are likely to result in the highest biodiversity. The results from this thesis can support the restoration of both natural and managed wetlands and aid in decision making. Therefore, we recommend the implementation of a water level drawdown in eutrophic clay wetlands to generate more biodiverse and resilient ecosystems.