Casper van Leeuwen

Land-water transition areas play an important role in the functioning of both aquatic and terrestrial ecosystems. Enhancing habitat complexity and heterogeneity by restoring or adding land–water transition areas to degraded aquatic ecosystems can be effective management to stimulate productivity by lower trophic levels – and hence increase food availability for biota of conservation interest, including fish and birds. Here, we studied whether hydrological connectivity can be used as an environmental indicator (connected or disconnected) for the development trajectories of newly constructed land–water transition areas in shallow lakes. We capitalized on a large-scale restoration project called “Marker Wadden”, which created new land–water transition areas with and without hydrological connectivity in a degraded shallow lake in the Netherlands (Lake Markermeer). We compared how the new areas with and without hydrological connectivity developed with respect to abiotic parameters and biomasses of benthic, pelagic, and emergent macroinvertebrates. In sites disconnected from the open water, water depths became significantly lower than in hydrologically connected sites during summer, likely due to evaporation. In these shallower waters, daytime temperatures and organic matter content of the sediment were higher, while dissolved oxygen concentrations remained sufficient. Therefore, biomasses of benthic macroinvertebrates and emergent insects became higher in the disconnected sites. These lower trophic levels could provide higher food availability for benthivorous and insectivorous birds, while remaining inaccessible to fish. This puts forward that hydrological connectivity (connected or disconnected) can be used as an environmental indicator for aquatic food web development trajectories, and that it regulates relative food availability for fish and birds. Restoring land–water transition areas without hydrological connectivity provides higher biomasses of lower trophic levels, which are only accessible to birds. Restoring areas with hydrological connectivity results in relatively lower biomasses of invertebrates, but these provide food to birds feeding on invertebrates, and fish and fish-eating birds. Creating areas including both types of land–water transition zones, connected and disconnected to open water can benefit fish and birds of both feeding guilds.

Soft shoreline engineering is increasingly used to combine shoreline fortification with the enhancement of biodiversity and biological production of land–water transitions. From 2016 to 2021, the large-scale ecosystem restoration project Marker Wadden has created new multiple wetland islands from local sediments in the highly modified Lake Markermeer, the Netherlands. Instead of replacing steep rip-rap shorelines with soft shorelines, new islands with soft land–water transitions were engineered to offset the marked declines in bird and fish populations in this Natura 2000 area, protected under the European Union Birds and Habitats Directives. This new approach was evaluated by assessing the added value of the newly created wetland islands with soft shorelines to the existing steep rip-rap fortified shores of the lake, for the enhancement of fish spawning and nursery habitat. Young-of-the-year fish densities at the Marker Wadden islands were highest in sheltered bays and wetlands with nutrient-rich silt sediments, and lowest at wind-exposed sandy beaches. Both newly engineered soft shorelines and existing rip-rap shorelines contributed to habitat diversity, although fish densities declined considerably with increasing exposure to wind-induced wave power. Building soft shorelines as a new archipelago instead of replacing existing shoreline habitats increased the total length of the land–water transitions in the lake. The 800 ha Marker Wadden archipelago covers only 1% of the 70,000 ha lake surface area, but provides a 16% increase in shoreline habitat and a fivefold increase in soft shoreline for the lake. We conclude that designing wetland islands as a means of lake restoration can contribute effectively to sheltered habitat enhancement for fish spawning and nurseries, and thereby to the potential conservation of fish communities. The approach of building islands achieves this without compromising the complementary functionality of the original, more mature, shorelines of the lake.

Land-water transition areas play a significant role in the functioning of aquatic ecosystems. However, anthropogenic pressures are posing severe threats on land-water transition areas, which leads to degradation of the ecological integrity of many lakes worldwide. Enhancing habitat complexity and heterogeneity by restoring land-water transition areas in lake systems is deemed a suitable method to restore lakes bottom-up by stimulating lower trophic levels. Stimulating productivity of lower trophic levels (phytoplankton, zooplankton) generates important food sources for declining higher trophic levels (fish, birds). Here, we study ecosystem restoration project Marker Wadden in Lake Markermeer, The Netherlands. This project involved the construction of a 700-ha archipelago of five islands in a degrading shallow lake, aiming to create additional sheltered land-water transition areas to stimulate food web development from its base by improving phytoplankton quantity and quality. We found that phytoplankton quantity (chlorophyll-a concentration) and quality (inversed carbon:nutrient ratio) in the shallow waters inside the Marker Wadden archipelago were significantly improved, likely due to higher nutrient availabilities, while light availability remained sufficient, compared to the surrounding lake. Higher phytoplankton quantity and quality was positively correlated with zooplankton biomass, which was higher inside the archipelago than in the surrounding lake due to improved trophic transfer efficiency between phytoplankton and zooplankton. We conclude that creating new land-water transition areas can be used to increase light and nutrient availabilities and thereby enhancing primary productivity, which in turn can stimulate higher trophic levels in degrading aquatic ecosystems.

Wetlands provide vital services on which human societies depend. As they have been rapidly degrading due to anthropogenic impacts worldwide, wetland restoration is increasingly applied. When a return to the original state of a wetland is constrained, forward-looking restoration can provide a new way to enhance an ecosystem's ecological integrity. However, the direction in which new ecosystems will develop is strongly coupled to the initial environmental conditions and may benefit from active decisions on (future) management. To improve the natural values of a degrading freshwater lake in the Netherlands, a forward-looking restoration project was initiated in lake Markermeer in 2016, involving the construction of a 700-ha archipelago called the “Marker Wadden”. This archipelago should provide new habitat to higher trophic levels in the lake's food web through the development of currently missing Common reed (Phragmites australis) dominated marshlands with gradual land-water transitions. However, the restoration project faces strong grazing pressure by Greylag geese (Anser anser) that possibly inhibit reed establishment. Here, we aimed to unravel the effect of herbivory by Greylag geese (using exclosures) and the introduction of reed rhizomes on early vegetation development and carbon dynamics on the bare soils of this new ecosystem in a manipulative field experiment. Our results showed that excluding herbivores strongly increased reed-vegetation cover, density and maximum height, but only when reed rhizomes were actively introduced. Spontaneous vegetation development on bare soils was limited, and colonization by Broadleaf cattail (Typha latifolia) dominated over reed. Net ecosystem exchange of carbon and ecosystem respiration were strongly linked to vegetation development, with highest methane emissions in the most densely vegetated plots. We conclude that the establishment of reed marshes can strongly benefit from excluding herbivores and the introduction of reed, and that otherwise other vegetation types may establish more slowly in newly created wetlands. This illustrates how active management of vegetation development has the potential to benefit novel ecosystems.

Omnivorous waterbirds play an important role in aquatic ecosystems as dispersal vectors via direct ingestion, transportation, and egestion of plant and invertebrate propagules (i.e. endozoochory). Predatory birds also have the potential to disperse plants and invertebrates that were first carried internally or externally by their prey animals. However, the potential contribution of predatory waterbird species to propagule dispersal in aquatic ecosystems remains understudied. We chose the grey heron Ardea cinerea (Ardeidae) to study the potential of predatory waterbirds to disperse propagules within and among aquatic ecosystems. We hypothesised that: (1) herons disperse a wide variety of plant and invertebrate propagules, from different habitats, with different morphologies (i.e. dispersal syndromes), and including both native and alien species; (2) propagules are ingested with prey species that are primary dispersal vectors (i.e., herons are secondary dispersers); (3) heron pellets show a similar abundance and richness of propagules across their widespread range. We collected 73 regurgitated heron pellets containing undigestible remains from 12 locations across the U.K. and The Netherlands, and examined the taxonomic diversity of plant seeds, invertebrates and prey remains. Pellets were dominated by mammal hairs (99% by volume), and bones confirmed the ingestion of small mammals (prevalence of 38%, e.g. water voles Arvicola amphibius), fish (14%), and birds or amphibians (6%). A total of 266 intact plant seeds were recovered from 71% of the pellets, representing 50 taxa from 17 plant families, including the alien Cotula coronopifolia. The cumulative number of plant species dispersed was lower at higher latitudes. Eight plant species recorded had not previously been recorded as dispersed via waterbirds, and only three species have an endozoochorous dispersal syndrome. Plant taxa were dominated by Caryophyllaceae, Cyperaceae, Juncaceae, and Poaceae, with 24 species from the littoral zone (Ellenberg moisture values of 7–12) and 21 terrestrial species (Ellenberg moisture values of 4–6). Intact invertebrate propagules were found in 30% of the pellets, dominated by Cladocera (Daphniidae) and Bryozoa (including the alien Plumatella casmiana). Our results demonstrate that grey herons disperse plant seeds and aquatic invertebrates widely in north-western Europe. Herons regurgitate pellets that contain plant and invertebrate propagules from both aquatic or terrestrial habitats, for which secondary dispersal via ingestion along with prey is the likely underlying mechanism (i.e. propagules either attached to or in the digestive systems of the various prey). Our findings showcase the potential of predatory waterbirds as vectors of plants and invertebrates, and how they may facilitate connectivity between freshwater and terrestrial habitats.

Wind-induced turbulence can strongly impact ecological processes in shallow lake ecosystems. The creation of shelter against wind can be expected to affect both primary producers and herbivores in aquatic food webs. Shelter may benefit particular primary producers more than others by changing relative resource availabilities for different primary producers. Herbivore community compositions may be affected either directly or indirectly as a consequence of changes in their food quantity and quality that, in turn, may affect the transfer efficiency between primary producers and herbivores. A reduction in trophic transfer efficiency resulting from wind-induced turbulence potentially can lead to declines of higher trophic levels, but is generally understudied. Here, we focus on the impact of wind on aquatic primary producers and trophic transfer efficiency. We hypothesised that reducing wind-induced turbulence will stimulate higher trophic production in shallow lakes. However, the multitude of impacts of wind-induced turbulence on aquatic food webs make it challenging to predict the direction of change when creating sheltered conditions. We tested our hypothesis in the shallow waters of a newly constructed archipelago named Marker Wadden in lake Markermeer in the Netherlands. Lake Markermeer has experienced declining numbers of birds and fish. These declines have been related to wind-induced sediment resuspension that potentially limits primary production and trophic transfer efficiency. Marker Wadden is a large-scale restoration project that aims to add sheltered and heterogeneous habitat to the otherwise mostly homogeneous lake, thus targeting the potential problems associated with wind-induced turbulence. We executed a 2-month manipulative field mesocosm experiment in the shallow waters of Marker Wadden to study the effect of reduced wind-induced turbulence (i.e., shelter) on aquatic food webs. Specifically, we studied the effects on primary producers, trophic transfer efficiency between phytoplankton and zooplankton (using zooplankton biomass divided by phytoplankton Chl a as a proxy), and benthic fauna. The experiment consisted of three treatments: no shelter, shelter without macrophytes and shelter with submerged macrophytes (Myriophyllum spicatum) present at the start of the experiment. Our results clearly showed that under unsheltered conditions phytoplankton was the dominant primary producer, whereas in sheltered conditions submerged macrophytes became dominant. Interestingly, submerged macrophytes appeared rapidly in the sheltered treatment where first no macrophytes were visibly present; hence, at the end of the experiment, there was little difference among the sheltered treatments with and without initial presence of submerged macrophytes. Despite that phytoplankton concentrations were 23-fold higher under the unsheltered conditions, this did not result in higher zooplankton biomass. This can be explained by a five-fold greater trophic transfer efficiency between phytoplankton and zooplankton under the sheltered conditions. Furthermore, under the sheltered conditions the Gastropoda density reached 746 individuals m⁻², whereas no Gastropoda were found under the no shelter treatment. These findings indicate that for shallow lakes that are negatively affected by wind-induced turbulence, measures aimed at ameliorating this stressor can be effective in facilitating submerged macrophyte recovery, increasing Gastropoda densities and restoring trophic transfer efficiency between phytoplankton and zooplankton. Ultimately, this may support higher trophic levels such as fish and water birds by increasing their food availability in shallow lake ecosystems.