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Aquatic ecosystems provide vital services, and macrophytes play a critical role in their functioning. Conceptual models indicate that in shallow lakes, plants with different growth strategies are expected to inhabit contrasting habitats. For shallow peat lakes, characterized by incohesive sediments, roles of growth forms, life-history strategies and environmental factors in determining the occurrence of aquatic vegetation remain unknown. In a field survey, we sampled 64 points in a peat lake complex and related macrophyte occurrence to growth forms (floating-leaved rooted and submerged), life-history strategies for overwintering (turions, seeds, rhizomes) and environmental factors (water depth, fetch, and porewater nutrients). Our survey showed that macrophyte occurrence relates to water depth, wind-fetch, and nutrients, and depends on growth form and life-history strategies. Specifically, rooted floating-leaved macrophytes occur at lower wind-fetch/shallower waters. Submerged macrophytes occur from low to greater wind-fetch/water depth, depending on life-history strategies; macrophytes with rhizomes occur at greater wind-fetch/depth relative to species that overwinter with seeds or turions. We conclude that growth form and life-history strategies for overwintering predict macrophytes occurrence regarding environmental factors in peat lakes. Therefore, we propose an adapted model for macrophyte occurrence for such lakes. Altogether, these results may aid in species-selection to revegetate peat lakes depending on its environment.

The European flat oyster (Ostrea edulis) is a biogenic reef former, internationally recognised as threatened and declining in the NE Atlantic by OSPAR and one of the focal species in nature inclusive designs in offshore windfarms in The Netherlands. European flat oyster reefs have disappeared from the Dutch North Sea in the early 1900s due to overfishing and diseases but are now subject of nature restoration. Since 2018, pilot projects have started in the Dutch North Sea to restore European flat oysters at suitable locations, such as offshore windfarms or natural reefs, which are protected from bottom trawling. We compared European flat oyster performance in five pilot projects, using translocated adult oysters sourced from Ireland, Norway, and the Netherlands. The aim of this research was to assess the performance of translocated oysters between pilots, to assess the installation and monitoring techniques, and to come forward with recommendations for future pilot projects. We found that translocation of both foreign sourced flat oyster populations (Ireland and Norway in nearshore and offshore areas) and local oysters (in nearshore areas) result in good oyster performance. Oysters were able to grow (max 3.67 mm/month) and reproduce (larvae present) in their new environment. We found that growth rate was explained by origin and average water temperature, to a lesser extent by number of months, location and salinity and not to other environmental factors such as pH and O2. Correlations between growth and environmental conditions need to be considered with caution, since not all pilots were sampled just before and after the growing season. Oysters were Bonamia-negative at the start and end of the pilots, indicating that the offshore Dutch North Sea is still Bonamia-free. By the year 2050 more than ten new offshore farms will be constructed in the Dutch North Sea and some sites will be suitable for oyster restoration. We conclude that local and foreign sourced oysters performed well at all locations. Based on the success and failure of the different outplacement and monitoring techniques, we provide recommendations on good practice for the future, including developing standardized monitoring protocols.

Extensive subtidal eelgrass ( Zostera marina ) meadows (~150 km 2 ) once grew in the Dutch Wadden Sea, supporting diverse species communities, but disappeared in the 1930s and have been absent ever since. Identifying the most critical bottlenecks for eelgrass survival is a crucial first step for reintroduction through active restoration measures. Seagrasses are ecosystem engineers, inducing self-facilitating feedbacks that ameliorate stressful conditions. Consequently, once seagrass, including its self-facilitating feedbacks, is lost, reintroduction can be challenging. Therefore, we aimed to test whether 1) sediment stabilization and 2) hydrodynamic stress relief would facilitate eelgrass survival in a field experiment replicated at two sites in the Dutch Wadden Sea. We induced feedbacks using biodegradable root-mimicking structures (BESE-elements) and sandbag barriers. Root mimics had a significant positive effect, increasing the chances of short-term survival by +67% compared to controls. Contrary to our expectations, barriers decreased short-term survival probabilities by -26%, likely due to hydrodynamic turbulence created by the barrier edges, leading to high erosion rates (-14 cm). Site selection proved crucial as short-term survival was entirely negated on one of the two study sites after five weeks due to high floating and epiphytic macroalgae loads. No long-term survival occurred, as plants died at the other site two weeks later. Overall, we found that sediment stabilization by root-mimicking structures was promising, whereas manipulating hydrodynamic forces using sandbag barriers had adverse effects. A mechanistic understanding of transplant failures is required before attempting large-scale restoration. Our study indicates that for seagrass restoration in the Wadden Sea, one should carefully consider 1) the reintroduction of positive feedbacks through restoration tools, 2) donor population choice and transplantation timing, and 3) site selection based on local biotic and abiotic conditions. Optimizing these restoration facets might lower additive stress to a degree that allows long-term survival.

In degraded landscapes, recolonization by pioneer vegetation is often halted by the presence of persistent environmental stress. When natural expansion does occur, it is commonly due to the momentary alleviation of a key environmental variable previously limiting new growth. Thus, studying the circumstances in which expansion occurs can inspire new restoration techniques, wherein vegetation establishment is provoked by emulating natural events through artificial means. Using the salt-marsh pioneer zone on tidal flats as a biogeomorphic model system, we explore how locally raised sediment bed forms, which are the result of natural (bio)geomorphic processes, enhance seedling establishment in an observational study. We then conduct a manipulative experiment designed to emulate these facilitative conditions in order to enable establishment on an uncolonized tidal flat. Here, we attempt to generate raised growth-promoting sediment bed forms using porous artificial structures. Flume experiments demonstrate how these structures produce a sheltered hydrodynamic environment in which suspended sediment and seeds preferentially settle. The application of these structures in the field led to the formation of stable, raised sediment platforms and the spontaneous recruitment of salt-marsh pioneers in the following growing season. These recruits were composed primarily of the annual pioneering Salicornia genus, with densities of up to 140 individuals/m² within the structures, a 60-fold increase over ambient densities. Lower abundances of five other perennial species were found within structures that did not appear elsewhere in the pioneer zone. Furthermore, recruits grew to be on average three times greater in mass inside of the structures than in the neighboring ambient environment. The success of this restoration design may be attributed to the combination of three factors: (1) enhanced seed retention, (2) suppressed mortality, and (3) accelerated growth rates on the elevated surfaces generated by the artificial structures. We argue that restoration approaches similar to the one shown here, wherein the conditions for natural establishment are actively mimicked to promote vegetation development, may serve as promising tools in many biogeomorphic ecosystems, ranging from coastal to arid ecosystems.

Globally, peatlands have been affected by drainage and peat extraction, with adverse effects on their functioning and services. To restore peat-forming vegetation, drained bogs are being rewetted on a large scale. Although this practice results in higher groundwater levels, unfortunately it often creates deep lakes in parts where peat was extracted to greater depths than the surroundings. Revegetation of these deeper waters by peat mosses appears to be challenging due to strong abiotic feedbacks that keep these systems in an undesired bare state. In this study, we theoretically explore if a floating peat mat and an open human-made bog lake can be considered two alternative stable states using a simple model, and experimentally test in the field whether stable states are present, and whether a state shift can be accomplished using floating biodegradable structures that mimic buoyant peat. We transplanted two peat moss species into these structures (pioneer sp. Sphagnum cuspidatum and later-successional sp. S. palustre) with and without additional organic substrate. Our model suggests that these open human-made bog lakes and floating peat mats can indeed be regarded as alternative stable states. Natural recovery by spontaneous peat moss growth, i.e., a state shift from open water to floating mats, is only possible when the water table is sufficiently shallow to avoid light limitation (< 0.29 m at our site). Our experiment revealed that alternative stable states are present and that the floating structures facilitated the growth of pioneer S. cuspidatum and vascular plants. Organic substrate addition particularly facilitated vascular plant growth, which correlated to higher moss height. The structures remained too wet for the late-successional species S. palustre. We conclude that open water and floating peat mats in human-made bog lakes can be considered two alternative stable states, and that temporary floating establishment structures can induce a state shift from the open water state to peat-forming vegetation state. These findings imply that for successful restoration, there is a clear water depth threshold to enable peat moss growth and there is no need for addition of large amounts of donor-peat substrate. Correct species selection for restoration is crucial for success.

In this study we recorded sponge-dwelling colonies of four Synalpheus species from several locations around the island of Sulawesi, Indonesia. We measured characteristics of the colony organization like colony size, sex ratio, within-colony morphological variation and size frequencies, and recorded the infection with abdominal and thoracic bopyrid isopod parasites. In one particular colony of Synalpheus neptunus neptunus, we found one large ovigerous female or 'queen' together with many male individuals, indicating a eusocial colony organization. A percentage of males in four of the five colonies of this species lack a rostrum. This morphological variation within the colony could be an indication of a highly developed social structure, where behavioural variety (e.g., labour division) is expected. Illustrated diagnoses and restricted synonymies of the species recorded are provided. /// Dans cette étude ont été étudiées des colonies de quatre espèces de Synalpheus habitant des éponges de plusieurs sites autour l'ile de Sulawesi, Indonésie. Les caractéristiques de l'organisation de la colonie comme la taille de la colonie, le sex ratio, les variations morphologiques et les fréquences de taille à l'intérieur de la colonie ont été mesurées, et l'infection par des isopodes bopyridés parasites abdominaux ou thoraciques a été notée. Dans l'une de ces colonies de Synalpheus neptunus neptunus, une grande femelle ovigère ou \"reine\" a été trouvée parmi de nombreux individus mâles, indiquant une organisation de la colonie de type eu-social. Un certain pourcentage de mâles dans quatre des cinq colonies présentent une absence de rostre. Cette variation morphologique à l'intérieur de la colonie pourrait être une indication d'une structure sociale hautement développée, dans laquelle des comportements différents (p.e. division du travail) sont à attendre. Des diagnoses illustrées et des synonymies réduites sont données pour quatre espèces.