Enhancing ecological integrity while preserving ecosystem services: Constructing soft-sediment islands in a shallow lake

1. Ecosystems are increasingly managed to provide multiple

present the underlying ecological framework and first scientific results of this innovative ongoing project. 5. Within 4 years, the Marker Wadden project shows how forward-looking sustainable development of lake ecosystems using a rewilding approach can enhance natural processes and attract birds and fish, without conflicting with existing ecosystem services. This inspires new directions for halting and reversing the degradation of other vital ecosystems worldwide.

K E Y W O R D S
ecosystem multifunctionality, forward-looking restoration, land-water connections, littoral zone, Marker Wadden, nature-based solution, novel ecosystems, rewilding

INTRODUCTION
Human society strongly depends on vital functions and services of natural ecosystems (IPBES, 2019;Zedler & Kercher, 2005). These include food production, fisheries, recreation, drinking water supply, energy generation and carbon storage (Clarkson et al., 2013;Zedler & Kercher, 2005). Consequently, ecosystems do not only have intrinsic natural and cultural values but are increasingly requested to provide multiple functions simultaneously, including economic returns. This can lead to ecosystem degradation by overexploitation, pollution and habitat loss (Davidson, 2014;WWF, 2020). Many of the world's freshwater lakes, estuaries and wetlands have important functions for humans, but are also increasingly scarce, used and competed for (Reynaud & Lanzanova, 2017;Schallenberg et al., 2013). Worldwide, many aquatic ecosystems are influenced by engineering for a specific, single service (e.g. damming a river to generate hydropower) that can strongly impair its ecological integrity. Engineering often homogenizes abiotic and biotic conditions, inhibits natural dynamics such as water-level fluctuations and leads to reductions in biodiversity, trophic complexity and functional diversity at each trophic level (Gibbs, 2000;Millennium Ecosystem Assessment, 2005). Consequently, many aquatic ecosystems are currently in need of sustainable nature development strategies, because their ecological integrity has eroded as a result of a wide range of simultaneous demands and associated modifications (Davidson, 2014).
Classical ecological restoration of freshwater ecosystems, which to a large degree has focused on returning degraded ecosystems to their pre-human-use conditions, is often challenging (Higgs et al., 2014;Higgs et al., 2018). Worldwide, many lakes are modified for flood protection, drinking water reservoirs or to fulfil new, desired, ecosystem functions such as energy generation or recreation (Reynaud & Lanzanova, 2017;Schallenberg et al., 2013). With classical restoration, where past modifications are undone, at least some of these new ecosystem functions to humans are unavoidably lost (Higgs et al., 2014). Furthermore, classical restoration projects that affect (part of) these desired functions mostly face complicated, long-term negotiations (Perring et al., 2015;Suding et al., 2015) -including about what should be the historical reference state of the ecosystem to return to (Higgs et al., 2014). To overcome such challenges, novel strategies for degrading ecosystems are needed that enhance their ecological integrity, while preserving their ecosystem services and socioeconomic benefits (Corlett, 2016;Gulati et al., 2008;Higgs et al., 2014;Higgs et al., 2018;Martin, 2017).
Here we present a forward-looking approach to enhance the ecological integrity of the shallow lake Markermeer in the centre of the Netherlands, which is -like many freshwater lakes in the world -a human-created lake (Hogeboom et al., 2018). The large lake Markermeer was formed by the construction of two dikes and multiple land reclamations in a marine estuary, named the Zuiderzee (Figure 1a and 1b). First, a 32-km long dike (named the Afsluitdijk) was completed in 1932, which turned the main tidal estuary of the river IJssel into a 357,000-ha freshwater lake over time. The new lake IJsselmeer, still fed with fresh water by the river IJssel, experienced a drastic change in salinity, food web composition and hydrology (Cremer et al., 2009 Heino et al., 2021). Both lakes developed distinct ecological values and started to provide many ecosystem services to humans, including recreation, drinking water supply, fishing and water supply for agriculture (Gulati & van Donk, 2002). However, the engineering activities did not automatically introduce elements such as gradual land-water transitions, heterogeneity in water depths or water-level dynamics into these lakes that one would expect in well-functioning large lowland freshwater lakes formed by natural processes (Schindler & Scheuerell, 2002).
Both lakes initially developed towards important ecosystems for piscivorous and benthic feeding waterbirds. Benthivorous waterbirds like the common pochard Aythya ferina, tufted duck A. fuligula and greater scaup A. marila profited from establishing populations of the non-native freshwater mussels zebra mussel Dreissena polymorpha and quagga mussel D. rostriformis. Piscivorous waterbirds like the great F I G U R E 1 Overview of the location and structure of the Marker Wadden project. (a) The central marine estuary in the Netherlands in 1850 with its main inflow from the river IJssel. (b) After a series of catastrophic floods in the late nineteenth and early twentieth centuries, several dikes were built to prevent flooding. The estuary was closed by a 32-km long dike (Afsluitdijk) in 1932, followed by several phases of land reclamation (polders), and finally the construction of a 27-km dike (Houtribdijk) in 1975. This created the 70,000-ha freshwater lake Markermeer in the southwest with limited riverine input and the 113,000-ha lake IJsselmeer in the northeast still fed by the river IJssel. (c) To enhance the ecological integrity of lake Markermeer without the loss of existing ecosystem services, from 2016 to 2020 the soft-sediment islands of the 'Marker Wadden' were built close to the Houtribdijk. Five islands were constructed -on the leeward side of stone dikes and sand dunes -by creating ring dikes from local deep Pleistocene sands (extracted from between 8 and 35 m deep in the lake's sediment) that were subsequently mostly filled with fine clays and silts from the top 5-8 m of the lake's sediment. In 2017, a harbour was constructed with stone dikes, and in 2018 a long-stretching sand dike was built in a southwest direction. This allowed the creation of more islands in the sheltered areas in subsequent years via sand dikes filled with fine clays and silts. Subsidence of the clays and silts to just below the water-level resulted in marshlands with water levels <1 m, which were reconnected to the lake water in late 2020. Only the main island is accessible to visitors via a small harbour with a visitor centre.  (Noordhuis, 2014). However, over the last decades, the ecological integrity of particularly lake Markermeer has been strongly declining (Lammens et al., 2008;Noordhuis, 2014). Compared to the 1980s, numbers of many benthivorous and ). The goal was to improve the lake's ecological integrity by adding the structure and dynamics that are more typical for a natural freshwater lake at low altitudes (Heino et al., 2021), but without compromising the many ecosystem services it currently provides (Lammens et al., 2008).
Classical restoration of lake Markermeer is ecologically impossible, because the historical reference of the lake is marine, and after damming, the freshwater lake developed into a different ecosystem that has no true historical reference condition. Furthermore, classical restoration by, for example, removing the dikes is socio-economically undesired due to the current economic and societal functions (flood protection, freshwater storage, water recreation, freshwater fisheries) (Gulati & van Donk, 2002). Therefore, the Dutch Society for Nature Conservation ('Natuurmonumenten'), in cooperation with provincial and national authorities, proposed an innovative large-scale nature-based solution. It follows a rewilding approach by restoring natural processes as much as possible, whereas it uses engineering to achieve this. This approach encompasses the building of a 700-ha archipelago consisting of five islands in the lake -named 'Marker Wadden' -between 2016 and 2020. Here we present the concept and first results of the Marker Wadden project, an ambitious, large-scale project in the Netherlands aimed at improving the ecological integrity of lake Markermeer while maintaining the lake's current ecosystem functions and services.

THE MARKER WADDEN PROJECT
To reverse the decline of lake Markermeer's ecological integrity, the Marker Wadden project aims to enhance the food web bottom-up via nature-based solutions. The project, therefore, targets three factors that are currently missing in comparison to more natural lakes and are thought to limit primary production ( Figure 2). First, historically the marine estuary was a highly productive, nutrient-rich coastal system due to marine and riverine inputs ( The Marker Wadden project aims to mitigate the negative effects of these aforementioned factors by constructing an archipelago of islands that add land-water connections, shelter, shallow and deeper waters to the lake (Figures 1 and 2c). These constructions aim to add previously missing habitat types and dynamics more typical of lowland nat-ural freshwater lakes (Heino et al., 2021;Schindler & Scheuerell, 2002) to this human-created lake. The construction of the archipelago started in 2016 by building stone dikes and sand dunes on the windward side (west) to provide shelter for subsequent constructions. On the leeward side of these strong structures, islands were constructed from the sediment of lake Markermeer itself. These islands were constructed as ring dikes made of deeper Pleistocene sands, extracted from the lake bottom at depths ranging between 8 and over 35 m deep. The areas within these ring dikes were subsequently predominantly filled -to levels above lake water level -with the fine clays and silts from the top 5-8 m of the lake bottom (Troelstra et al., 2018). Subsidence of the clays and silts to below the lake's water level   The construction of the islands also created a series of landwater connections, which would be typical for more natural freshwater lakes but were previously lacking in lake Markermeer. These landwater connections were hypothesized to stimulate primary production by increasing runoff of nutrients from land to water. The shallow waters that develop between the islands would be more productive due to higher nutrient availability, quick warming in spring, and shelter from the wind, which reduces resuspension of the fine clays and silts. Hence, primary production was thought to increase, as this is no longer hampered by nutrients and light limitation (Schallenberg et al., 2013). The establishment of submerged macrophyte-and shorelinevegetation could further help in trapping suspended solids (Barko & James, 1998). These processes combined were hypothesized to positively affect the lake's food web via stimulation of primary production, providing habitat structure and increasing the efficiency of energy transfer to higher trophic levels -leading to higher functional diversity at all levels of the food web. The vision behind Marker Wadden is that it could induce highly productive conditions providing foraging and spawning and breeding habitat for higher trophic levels such as fish and waterbirds.
The Marker Wadden project also aimed to enhance the recreational function of the lake. The largest of the five newly constructed islands was therefore made accessible to the general public, whereas the other islands remain closed for the public (Natuurmonumenten, 2013). On the largest island, the Dutch Society for Nature Conservation constructed a small settlement (Figure 1c). This has been built off-grid using exclusively sustainable materials and includes a small harbour, five holiday houses and a visitor centre. Moreover, they constructed a group accommodation and a field station for educational and research purposes, run a ferry to enable recreationists to visit the island, offer guided tours, and educate about nature. This is combined with dedicated regular communication about the project and the involvement of volunteers during all aspects of the project to ensure societal acceptance. The project therefore deliberately integrates F I G U R E 2 Schematic overview of the changes in the study area over the last century. (a) Before the 1930s, a highly productive tidal estuary was present with gradual land-water transitions. (b) In the twentieth century, basalt dike constructions created the homogeneous shallow freshwater lake Markermeer with stabilized water levels, which trapped high amounts of fine sediments (called 'floating mud') and lead to high turbidity. A littoral zone was absent, and primary production decreased. (c) Between 2016 and 2020, the Marker Wadden archipelago was constructed to add more heterogeneous habitat to the freshwater lake. The islands include soft shores with gradual land-water transitions that provide nutrients for primary production and create sheltered areas where turbidity decreases due to a reduction of resuspension and accumulation of easily resuspended sediments in deeper areas recreation, nature education, innovation and research (Natuurmonumenten, 2013).

FIRST OBSERVATIONS ON ECOLOGICAL INTEGRITY
Our first observations on ecological implications of the Marker Wadden project -even though it is still under development -suggest that nature is able to quickly profit from the newly created habitats (for a timeline, see Figure 3). The expected effects of the islands on primary production via reduction of suspended sediment concentrations followed by an increase in light levels in the water column have proven difficult to assess at this early phase of the project, in part because the building activities initially created sediment resuspension themselves.
However, the construction of the archipelago included the creation of three basins of each 3-4 ha at an early building phase (late 2016; Figure 1c). Each basin was a part of the lake that became surrounded by dikes -sheltering the water and separating it from the rest of the lake -but otherwise leaving it identical to the open lake water. Before the project started, these species rarely occurred in the eastern part of the lake (Vonk et al., 2019), likely because the fine clay soil type combined with strong winds made this part of the lake less suitable for macrophyte establishment (Van Zuidam & Peeters, 2015).
In the marshlands, vegetation developed on the land-water transition zones within one growing season. In the first year, marsh fleawort Tephroseris palustris was the most dominant species, probably because it is a wind-dispersed, early pioneering plant that can easily establish in shallow water. Willows Salix sp. colonized and dominated the drier marshland zones. The aim of the project was to develop helophyte marshes rather than wet forests. Therefore, willows were actively removed and their germination prevented by water management until 2020, and rhizomes of common reed Phragmites australis and broadleaf cattail Typha latifolia were actively sown and protected against herbivores -as grazing pressure in aquatic systems can be high (Bakker et al., 2016). Protection against avian herbivores (notably grey- atilis and Eurasian ruffe Gymnocephalus cernua, but also four typically pioneering non-native Gobidae can be found (Emmerik, 2018(Emmerik, , 2019. Numerous bird species use the islands, which are extensively monitored. Over 20,000 sand martins Riparia riparia, 3000 northern shovelers Spatula clypeata, 1000 pied avocets Recurvirostra avosetta, 1000 black terns Chlidonias niger and hundreds of little gulls Hydrocoloeus minutus colonized the islands within 3 years. For northern shovelers, black terns and common terns Sterna hirundo the islands harboured, respectively, > 6%, 2% and 2% of the flyway population at a given moment in 2019 (Van der Winden, 2019). For common ringed plover Charadrius hiaticula and common terns, more than 10% of the national population was attracted to the new habitat. Many of the observed bird species had been present in much lower numbers and/or did not breed in such numbers for decades in the Netherlands. Rarer species such as greater flamingos Phoenicopterus roseus and Eurasian spoonbills Platalea leucorodia were also encountered, and nests were found of gull-billed terns Gelochelidon nilotica and long-tailed ducks Clangula hyemalis -two bird species that had not been breeding in the Netherlands for decades.
Overall, functional diversity seems to locally increase in many trophic layers and a shift may be on its way from a simplistic food web to a structurally more complex food web. This allows more coexistence of different trophic levels in the food web under the wider range of abi-otic conditions. Natural processes are quickly taking advantage of the increased heterogeneity, land-water transitions and gradual shorelines typical of more natural temperate freshwater lakes. Even though the former estuarine conditions in this ecosystem did not return, the ecosystem seems to be developing towards higher ecological integrity that might increase resilience to future perturbations (Carpenter & Cottingham, 1997;Scheffer et al., 2001). Whether or not the Marker Wadden project is sufficient to change the downward trend in the lake, completely remains to be determined.

THE FUTURE OF NATURE DEVELOPMENT
The twenty-first century requires nature-based solutions and thus new views on nature, sustainability, resilience, ecological restoration, rewilding and other forms of nature development. Initiatives for improving the ecological integrity of many human-impacted areas are often slowed down or stopped by societal resistance to give up existing benefits derived from ecosystems, or by a lack of vision on how a system should be developed. Here, we present a new perspective on nature development and rewilding ecosystems, in a situation where a return to a former ecosystem state was impossible due to the lack of historic reference, and strong societal adherence to existing (novel) benefits.
In the first 4 years since the start of the Marker Wadden project, the trends are positive. There are no signs that important functions of the lake such as flood protection, freshwater storage or fisheries are negatively affected. With respect to recreation, the largest of the islands was opened for the general public in September 2018 -and welcomed over 20 000 visitors, 180 recreational charter vessels and 2 000 recreational ships for an overnight stay in the harbour in the year 2019 (Natuurmonumenten, 2019). In 2019, the islands featured over 150 times in the regional and national news, which increased to over 250 times in 2020. It is too early to assess how the project likely affects the many ecosystem services that the lake currently provides, but the first observations and responses from society are very positive.
The concept of a forward-looking approach to enhancing ecological integrity that we outline here can hopefully inspire other scientists and practitioners to design and initiate innovative solutions that do not collide with ecosystem multifunctionality. The Marker Wadden is designed with a dual function, aiming to facilitate human activities as well as increase ecological integrity and natural values of a degrading ecosystem. Although illustrated with an aquatic case study, this new way of thinking may pave ways to enhance natural values in many types of human-made systems and counteract the loss of vital ecosystems globally. If given a chance, nature has great capacity to maintain ecological integrity while providing ecosystem functions to human societies.