Artificial reefs (ARs), a form of anthropogenic intervention in marine habitats, have a long history of deployment and continue to proliferate worldwide. Based on a comprehensive literature review and meta-analysis, we show that (i) ARs have evolved from socioeconomic-oriented tools into often-used components of active marine restoration, yet this transition conflicts with the continued use of eco-unfriendly materials, hindering upscaling; (ii) ARs positively impact marine organisms at community, population, and individual levels, but their contributions to organismal fitness remain limited compared to natural reefs. To address these limitations, we advocate a paradigm shift toward "rewilding ARs"─temporary structures designed to create opportunities for natural reef formation, enhance habitat quality, and gradually degrade to minimize human impact. These features support the transition from active intervention to spontaneous recovery, promoting sustainable biodiversity recovery by improving organism fitness and facilitating upscaling. Integrating insights from ecologists, engineers, legal experts, environmental consultants, and NGOs, we introduce six guiding principles for "rewilding ARs" to ensure effective, durable, no-regret, scalable, permit-friendly, and outcome-optimized implementation. Lastly, we present pioneering examples of innovative ARs progressing toward these principles, serving as references for future endeavors.

Artificial hard structures, such as offshore wind turbine foundations and oil/gas platforms, create novel habitats that are colonised by marine epifauna. Although changes in epifaunal community composition over time are well-documented, the resulting implications for food web structure remain poorly known. We therefore compared the trophic structure of benthic communities associated with an 8-year-old wind turbine foundation and a 40-year-old gas platform in the southern North Sea. We used carbon and nitrogen stable isotope analysis in both consumers and food sources across five depth zones characterised by different species assemblages. Consumer stable isotope ratios were standardised to local source baselines to enable cross-site comparison of Bayesian isotopic niche dimensions. Consumer isotopic niche dimensions varied with depth and structure age, with the highest nitrogen ranges generally observed in deeper zones, while the older structure consistently exhibited broader nitrogen and carbon range values. Differences in carbon ranges between the two locations likely reflect differences in realised source exploitation between communities. Differences in nitrogen ranges were primarily driven by taxa with low δ15N values at the gas platform, potentially indicating localised biogeochemical processes or sources. Our results indicate that the ecological succession of artificial hard substrate communities is accompanied by expanded trophic niche dimensions, pointing towards broader trophic diversity around older structures. This alteration in trophic structure with ecological succession on offshore artificial hard structures has potential implications for carbon flow and ecosystem properties and underlines the need for understanding the long-term functioning of these structures to improve placement, management, and decommissioning strategies.

Reef-building bivalves such as mussels and oysters are ecosystem engineers that strongly modify sediment properties by attenuating water flow and accumulating fine biodeposits. Yet, despite advances in understanding reef-sediment dynamics, it remains partially resolved how reefs and their environment interact to control sediment characteristics within and beyond reef boundaries. We explored how the extent and spatial scale of the sediment modification depend on the prevailing environmental conditions (e.g., bathymetry, orbital and flow velocity) and reef characteristics (e.g. size, density) in the intertidal Dutch Wadden Sea. We sampled sediment silt and total organic matter content at three sediment depths (0-1 cm, 9-11 cm, 25-28 cm) on twelve reefs from their centre up to 400m beyond their borders, and used linear models to assess relationships. We found that within reefs, organic matter and silt content were higher compared to the surroundings, and that denser reefs had a greater impact, with only a marginal influence of environmental conditions. Effects on sediment properties beyond reef boundaries were found to extend up to 100m at the surface, but were detected up to 200m in deeper layers, with total organic matter and silt content decreasing nonlinearly with distance. These effects were more pronounced around larger reefs and correlated negatively to hydrodynamic conditions that increase sediment resuspension, such as orbital and flow velocity. Overall, our findings highlight that intertidal bivalve reefs modify their environment well beyond their physical borders. However, the scale of the extended-engineering footprint is context dependent as it varies with reef size and density, and current and wave action.

Sediment transport and seabed composition can both be influenced by bioturbation and hydrodynamically driven sediment mixing and deposition. In a dynamic intertidal environment, it is challenging to distinguish the relative contribution of both processes. We aim to unravel their relative importance by combining several tracers, each having its own specific timescale and target particle size. We combined (1) 210Pb that quantifies long-term (years-decades) mixing of fine sediment fractions with (2) Chlorophyll a and (3) luminophores that both quantify short-term mixing of fine sediment fractions (days-weeks), and (4) multi-grain quartz and single-grain feldspar luminescence dating, which use the bleaching of sand grains' inherent luminescence signal by light to assess mixing of sand and thereby quantifies long-term mixing. Single grain feldspar luminescence is here for the first time applied in the intertidal environment. We compare results for a sandy and a muddy intertidal flat at the island of Texel (Wadden Sea, the Netherlands), each with their own characteristic benthic community. Recent bioturbation became apparent from Chlorophyll a and luminophore profiles: particles were rapidly reworked to a depth of decimetres. 210Pb also suggested mixing and non-local exchange of particles by bioturbation. The combination of luminescence signals suggested that after deposition, not all sand grains did resurface repeatedly and for longer time periods through bioturbation. Coarse- and fine-grained tracer profiles show the differential behaviour and reworking of the mud and sand fraction within the sediment matrix: as expected with particle-selective bioturbation, mud is preferentially bioturbated and infiltrates passively, while sand grains have a higher ability to conserve layering. Single-grain feldspar luminescence is a promising technique to demonstrate the long-term reworking of sand grains, however, in young and dynamic environments, a combination of tracers remains necessary to inform on the origin of mixing.

Biogenic reefs, shaped by organisms like corals, tubeworms, and bivalves, have faced extensive degradation worldwide, resulting in a vast reduction in their extent. The remaining reefs are essential habitat for marine vertebrates, providing food, shelter, and nursery grounds. However, the role of natural shellfish reefs as essential fish habitats is poorly understood, particularly in temperate regions. We assessed the effect of reef presence on marine vertebrate diversity and composition and compared traits for habitat and feeding strategy at 2 shellfish reefs in the North Sea: a mixed shellfish reef and a horse mussel reef. We also assessed the complementarity of 2 non-invasive techniques: eDNA metabarcoding and baited camera analysis (baited remote underwater video systems [BRUVs]). Stations inside and outside reefs were compared. Vertebrate diversity was consistently higher within both reefs than in their surroundings, declining with increasing distance from the reefs up to 400-1500 m from the reef edge. Community composition varied, with several species, such as gobies, gunnels, clingfish, and sculpins, occurring more frequently within the reefs. Commercial species like cod, pollock, and mullet also occurred more often in reef areas, with reefs likely serving as juvenile habitats for species like ling. Functional diversity showed that reefs attracted mostly invertivores and pelagic species adapting their diet to food availability. eDNA identified more species overall, while BRUVs detected both adult and juvenile stages, highlighting the value of complementary methods in biodiversity assessments. These results emphasize the importance of temperate reefs as essential fish habitats and inform more effective conservation and fisheries management strategies.

Sea level rise, increased storminess, and changes in sediment supply due to nourishments are all expected to drive coarsening (i.e., ‘sandification’) of muddy coastal sediments in the decades to come. Since the composition of soft-bottom benthic communities is associated with the sediment grain-size and mud content, this may result in habitats becoming less suitable for some species, leading to species shifts. Species-sediment relations can help to predict how this foreseen sandification may affect benthic fauna. We explore and quantify the sandification-sensitivity of benthic communities, with a tidal basin in the Dutch Wadden Sea as a model system. We identify the species' sediment optima and tolerance ranges using non-linear quantile regression models, summarise preference and sensitivity at the community level, and determine the difference between optimal and realised sediment habitat. We find that sediment optima are taxon-specific and that most species in this area are sediment generalists. On community level, there is a difference between the preferred and realised sediment habitat. In many areas, the actual inhabited sediment is coarser and sandier than expected based on the preferences of the resident species. Future sandification of the area would further decrease sediment habitat suitability for benthic communities in these places. This detailed knowledge of area-specific sensitivity of benthos can be used to inform coastal management decisions.

Small fish, including species with small adult sizes and juveniles of larger species, play a central role in marine food webs as prey for top predators, such as seabirds, marine mammals, and piscivorous fish. However, reliable data on small fish are lacking as conventional fisheries surveys primarily focus on larger, commercially important species and underestimate small fish. Consequently, little is known about absolute biomasses, and fine-scale distribution patterns of this important trophic group. Based on 1307 quantitative Triple-D samples from the Dutch Exclusive Economic Zone and the UK sector of the Dogger Bank, biomass densities were estimated for pooled small demersal fish and for the most abundant species individually. Our estimates suggest that small demersal fish biomass is at least twice as high as reported in trawl-based studies. Uniformly distributed species such as dab (Limanda limanda) and plaice (Pleuronectes platessa) contributed most to the overall small fish biomass, while sandeels (Ammodytidae) showed particularly high local densities. These new prey biomass estimates might change our understanding of the North Sea ecosystem's carrying capacity, and establishes a baseline for monitoring changes in small fish communities driven by climate change and human impacts.

The early to middle Eocene (56 – 41 Ma) is characterized by high atmospheric CO2 concentrations between 1,000 and 1,500 ppm, making it the warmest interval of the Cenozoic [1,2]. The future atmospheric CO2 concentration could reach similar levels around 2100, based on the high CO2 emissions scenario SSP5-8.5 [3]. By studying the Eocene climate, we gain understanding of how our climate system could operate under these extreme conditions.An important aspect of climate is the seasonal temperature variability: the differences between summer and winter temperatures. Past seasonality can be reconstructed from sub-annually resolved climate archives such as the incremental growth bands of mollusk shells. We performed clumped isotope analysis on micro-samples of 11 fossil shells of early to middle Eocene age from shallow marine settings in northwestern Europe: 8 bivalves (species Venericor planicosta) and 3 gastropods (Haustator solanderi).We obtained seasonal shell chronologies from the variability in the oxygen isotope records of the micro-samples, and we used the corresponding clumped isotope records to reconstruct the seasonal temperature variability of the seawater independent of its isotopic composition [4].Our results suggest a moderate seasonal temperature variability of approximately 6 – 7 °C during both the early (56 – 48 Ma) and middle (48 – 41 Ma) Eocene. A comparison with Eocene climate model simulations suggests that models overestimate the observed seasonality due to colder winter temperatures in the model simulations compared to the reconstructions. This temperature record sheds light on the role of seasonality in mid-latitude shallow marine environments in hothouse climates and can aid our understanding of regional and seasonal scale model-data discrepancies. [1] Rae, J. W. B., Zhang, Y. G., Liu, X. et al. (2021). Atmospheric CO2 over the past 66 million years from marine archives. Annual Review of Earth and Planetary Sciences, 49(1). https://doi.org/10.1146/annurev-earth-082420-063026[2] The Cenozoic CO2 Proxy Integration Project Consortium (2023). Toward a Cenozoic history of atmospheric CO2. Science, 382(6675). https://doi.org/10.1126/science.adi5177[3] Chen, D., Rojas, M., Samset, B. H. et al. (2021). Framing, context, and methods. In V. Masson-Delmotte, et al. (Eds.), Climate change 2021: The physical science basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change (pp. 147–286). Cambridge University Press.[4] de Winter, N.J., Agterhuis, T., & Ziegler, M. (2021). Optimizing sampling strategies in high-resolution paleoclimate records. Climate of the Past, 17(3). https://doi.org/10.5194/cp-17-1315-202

Bivalve mollusc shells have proven to be promising recorders of environmental variability on short time-scales: incremental growth over their lifetimes (~ 1 – 100 years) allows for high resolution temporal sampling in their carbonate shells1. Seasonal and even daily environmental variability have successfully been reconstructed using fossil shells, e.g. 2–4.However, these shells are not made up of pure carbonates but also contain organic matter and internal fluids5. Understanding the formation pathways and associated isotopic and trace elemental fractionation of these components of the shell carbonate system is important to deconvolute the bulk carbonate chemical signal. Furthermore, measurements of oxygen isotopes (δ¹⁸O) of internal fluids and carbonate coupled with clumped isotope (Δ₄₇, Δ₄₈) measurements of the carbonate can constrain disequilibrium precipitation and diagenetic alteration processes6,7. Accounting for these processes allows for improved δ¹⁸O-based temperature reconstructions. As it is yet not well-constrained where internal fluids are present in biogenic carbonates, their significance for shell formation and as an environmental indicator is currently largely unknown8. Utilizing bivalve molluscs cultivated under closely monitored environmental conditions, we develop a method to quantify the different components of the shell carbonate system, analyse their respective isotopic and elemental signatures and correlate these with conditions experienced during growth. Modern bivalve shells collected from a wide range of present-day climate zones allow us to assess the performance of mollusc shells as archives for environmental conditions. This approach aims to provide a robust framework for improved future mollusc-based climate reconstructions and more accurate interpretation of chemical and isotope proxies in carbonate archives from past climates and environments. 1. Ivany, L. C. Reconstructing paleoseasonality from accretionary skeletal carbonates - challenges and opportunities. Paleontol. Soc. Pap. 18, (2012).2. de Winter, N. J. et al. Amplified seasonality in western Europe in a warmer world. Sci. Adv. 10, eadl6717 (2024).3. Kniest, J. F. et al. Dual clumped isotopes from Mid-Eocene bivalve shell reveal a hot and summer wet climate of the Paris Basin. Commun. Earth Environ. 5, 1–10 (2024).4. Arndt, I. et al. 20,000 days in the life of a giant clam reveal late Miocene tropical climate variability. Palaeogeogr. Palaeoclimatol. Palaeoecol. 112711 (2025) doi:10.1016/j.palaeo.2024.112711.5. Lécuyer, C. & O’Neil, J. R. Stable isotope compositions of fluid inclusions in biogenic carbonates. Geochim. Cosmochim. Acta 58, 353–363 (1994).6. Nooitgedacht, C. W., van der Lubbe, H. J. L., Ziegler, M. & Staudigel, P. T. Internal water facilitates thermal resetting of clumped isotopes in biogenic aragonite. Geochem. Geophys. Geosystems 22, e2021GC009730 (2021).7. Staudigel, P. et al. Fingerprinting kinetic isotope effects and diagenetic exchange reactions using fluid inclusion and dual-clumped isotope analysis. Geochem. Geophys. Geosystems 24, e2022GC010766 (2023).8. de Graaf, S. et al. Analytical artefacts preclude reliable isotope ratio measurement of internal water in coral skeletons. Geostand. Geoanalytical Res. 46, 563–577 (2022).

Abstract. Sediment transport and seabed composition can both be influenced by bioturbation and hydrodynamically driven sediment mixing and deposition. In a dynamic intertidal environment, it is challenging to distinguish the relative contribution of both processes. We aim to unravel their relative importance by combining several tracers, each having its own specific timescale and target particle size. We combined (1) 210Pb that quantifies long-term (years–decades) mixing of fine sediment fractions with (2) Chlorophyll-a and (3) luminophores that both quantify short-term mixing of fine sediment fractions (days–weeks), and (4) multi-grain quartz and single-grain feldspar luminescence dating, which use the bleaching of sand grains’ inherent luminescence signal by light to assess mixing of sand and thereby quantifies long-term mixing. Single grain feldspar luminescence is here for the first time applied in the intertidal environment. We compare results for a sandy and a muddy intertidal flat at the island of Texel (Wadden Sea, the Netherlands), each with their own characteristic benthic community. Recent bioturbation became apparent from Chlorophyll-a and luminophore profiles: particles were rapidly reworked to a depth of decimetres. 210Pb also suggested mixing and non-local exchange of particles by bioturbation. Luminescence age distributions suggest that quartz luminescence signals were fully reset upon recent deposition, while bioturbation enhanced resetting of feldspar luminescence signals. Coarse- and fine-grained tracer profiles show the differential behaviour and reworking of the mud and sand fraction within the sediment matrix: as expected with particle-selective bioturbation, mud is preferentially bioturbated and infiltrates passively, while sand grains have a higher ability to conserve layering. Single-grain feldspar luminescence is a promising technique to demonstrate the long-term reworking of sand grains, however, in young and dynamic environments, a combination of tracers remains necessary to inform on the origin of mixing.

Marine reefs are habitats that underpin key ecosystem services, including supporting diverse faunal communities. However, the ecological functioning of reefs in subtidal temperate waters has remained underexplored. Simultaneously, interest in restoring lost reefs is increasing due to the establishment of marine protected areas and the development of offshore wind farms. To explore the ecological importance of these reefs, we investigated how different subtidal temperate reef types affect faunal communities and their food web structures in the North Sea as an example of a temperate sea. We focused on the shallow nearshore Voordelta and the deeper offshore Borkum Reef Grounds in the Netherlands. Both sites hosted either geogenic (rocks) and/or biogenic ( Lanice conchilega or bivalve) reefs, which we compared to a sandy non-reef reference. We sampled benthic macrofauna using box corers or grabs and mobile communities with baited traps and reconstructed food webs using stable isotope analysis and the literature. Results showed that reef presence enhanced benthic taxa richness, evenness, and abundance, with biogenic reefs creating intermediate communities between sand and rocks. Certain species of commercial interest ( Cancer pagurus and Trisopterus luscus ) were positively associated with the reefs. Reefs were found to increase food web complexity and connectivity, which are linked to the network’s stability and resilience to disturbances and influence the proportion of intermediate consumers. We conclude that temperate subtidal reefs enrich ecosystems and stabilise food webs, suggesting that their restoration and conservation could help mitigate anthropogenic impacts and enhance the overall health of marine ecosystems.

This paper presents the results of a study into the ecological long-term effects of medium-deep sand extraction along the Dutch coast. A comparison was made of the benthic fauna in and outside medium deep extraction pits of various ages, ranging between 2 and 12 years. The pits and reference areas were also compared in terms of water depth, median grain size, silt content and percentage of organic matter. The water depth of all sand extraction pits was on average 4 m larger than the surrounding reference areas. The average silt content in the extraction pits had increased by a factor 4 compared to the surrounding reference areas. The average silt content in the pits was 9.5 %. This increase differed between pits (0 and 46 %). The percentage of organic matter had almost doubled from 0.87 % in the reference areas to 1.50 % in the pits. The medium-deep sand extraction pits are faunistically characterized by a higher average macro-benthic biomass and density but the number of species shows no clear difference with the reference areas. Faunistic heterogeneity in the pits is lower than in the surrounding reference areas. The macro-fauna in the pits is characterized by the greater abundance of deposit feeders and interface feeders while the benthic fauna in the reference areas is characterized by bivalves i.e. filter feeders. Small demersal fish did not show clear trends although some species, such as sandeel, showed a difference in abundance between pit and reference area. The effect of age of the mining pit on faunistic composition could not be separated from differences related to geographical position. The results of this study show that recolonization of a newly mined medium deep sand extraction pit is fast but that the complete recovery and return of the original benthic fauna in these pits can be a very long process. The sedimentary differences between the pits suggests that recovery depends on the abiotic environment. The data suggest that by moving from shallow mining pits to medium deep pits a critical depth is passed beyond which hydrographical changes alter the settlement of fines and determine the composition of the macrofaunal community.

Long-term measurements from a mooring off the Dutch coast, placed in the turbidity maximum zone (TMZ), are analyzed to study the temporal variability of the alongshore transport of SPM (Suspended Particulate Matter), covering a two-year period. In particular, the effect of wind speed and wind direction is investigated, as it is dominant over the tidal effect. The data reveal that the general net transport at the mooring location is northward; annually, the mean northward transport is 1 kton per meter in the cross-shore direction. For calm conditions, when wind speeds are less than 5 m/s, the overall effect of winds on the net transport diminishes to zero. Energetic conditions (wind speed higher than 5 m/s) are studied for a sequence of storms from opposing wind directions. Here, the gross tidal transports (i.e., transport during ebb or flood separately) show a delay with respect to the onset of the wind event, ranging from half a day to a day. The correlation between the net transport per tidal cycle and the wind characteristics during that cycle, is used to extrapolate the results to a 10-year period, using wind data from a nearby weather station. It is shown that annual transports of SPM can vary by a factor of two between years, due to different wind and wave conditions. Within a year the seasonal variability is larger, a factor of five exists between summer and winter months.

The number of offshore artificial structures in the North Sea is continuously increasing. Apart from the structures that have been added to the marine environment accidentally (e.g., shipwrecks), structures are also deliberately developed to meet the increasing needs for renewable energy. These structures provide habitat for fouling organisms. The fouling communities vary in abundance and composition based on location, depth, and structure age. Most fouling species filter particles from the water column, changing phytoplankton production and affecting larval settlement success, while releasing ammonium that can fuel phytoplankton growth as well as (pseudo)faeces that enriches the seabed, changing local biogeochemical cycles. Our study used in-situ incubation chambers to investigate oxygen, nitrogen, and phosphate fluxes associated with fouling organisms to improve understanding of these changes in biogeochemical cycles. Divers used incubation chambers (domes) on shipwrecks in the southern North Sea where over 55 years mature fouling communities have established. A series of water samples was collected from each dome during deployment to measure the change in concentration of ammonium, nitrite, nitrate, and phosphate. All fauna enclosed in the domes was collected after each measurement for further analysis. The full macrofauna dataset contained 65 unique species on 4 shipwrecks (25 to 50 species per sample). Abundance ranged from 2187 to 59,427 individuals per sample (683 cm2). 2 ). On average, a decrease in oxygen concentration of 126 mu mol/g ash free dry weight/h was found. The sequential water samples also showed clear changes in nutrient concentration with time in all incubations. The largest changes were observed with high fouling community abundances and biomass. Ammonium, nitrite, and phosphate always increased, with 1.5-to- 5-fold increases from start to end of the incubation, while for nitrate both an efflux and influx were measured. Oxygen decreased in all incubations. Mean fluxes (all in mu mol per m2 2 per hour with standard error) were significant for ammonium (945 f 300), nitrite (80 f 30), phosphate (61 f 8), and oxygen (-11,794 f 3289), but not for nitrate (-206 f 122). Per gram AFDW, only ammonium (12.7 f 3.5) and oxygen (-126 f 48) had fluxes that differed significantly from zero. Compared to average seabed (sandy bottom) oxygen demand and community fluxes from previous studies, the observed fluxes were high. Our findings resembled those from temperate biogenic reef studies. Further data collection across a larger spatial and temporal scale is needed to fully understand offshore structure effects on marine environments.

This study explores the impact of a wind storm on sediment resuspension and marine biogeochemical dynamics. Additionally, the storm took place during an expedition researching bottom trawling, enabling the direct comparison of certain natural and fisheries-related disturbances. The storm was initiated by a decline in atmospheric pressure and a 2 h period of gale force winds, which was followed by over 40 h of elevated bottom currents. Storm induced turbidity, potentially a cumulative post-fishing impact, was remarkably higher compared to what was observed in a recent trawling event. Storm-induced mixing and movement of water masses led to decreased silicate and increased phosphate concentrations in the water column, accompanied by lower salinity and higher fluorescence. The erosion depth of the seabed averaged around 0.3 cm during the peak turbidity period. Trawl-induced erosion in the area has been measured at over twice that depth, and has been linked to intermittent reductions in near-bed oxygen levels. In contrast, storm-induced turbidity coincided with increased oxygen due to wave mixing, suggesting inherent differences in how trawling and storms can oxidize reduced substances. These findings suggest that storms have a greater regional impact, whereas the local impacts of bottom trawling on biogeochemistry can be more significant.

Seawater temperature is an essential quantity for paleoclimatological and paleoecological studies. A potential archive that can provide century-long, temporally well-constrained and high-resolution temperature proxy data is available in the form of bivalve shells. However, the number of well-accepted and robust temperature proxies contained in shells is limited to stable oxygen isotopes and carbonate clumped isotopes. Many studies have therefore investigated the possibility to reconstruct temperature from element/Ca properties, specifically Sr/Ca ratios in case of aragonitic shells. As demonstrated here, in agreement with thermodynamic expectations and the lattice strain model, shell Sr/Ca of laboratory-grown Arctica islandica specimens is strongly positively coupled to water temperature. If ultrastructure-related bias is mathematically eliminated, up to 75% of the variability in shell Sr/Ca data can be explained by water temperature. However, in field-grown specimens, this relationship is superimposed by other environmental variables that can hardly be quantified and mathematically eliminated. The explained variability of Sr/Ca is reduced to merely 26% and the prediction uncertainty too large for reliable temperature estimates. Most likely, the equable, less biased conditions in the laboratory resulted in the production of a more uniform shell ultrastructure (with larger and more elongated biomineral units) which in turn was associated with less variable Sr/Ca values and a stronger link to water temperature. Without a detailed understanding and quantification of the factors controlling ultrastructural variations in field-grown bivalves, it remains impossible to employ shell Sr/Ca of wild A. islandica specimens for precise temperature estimates, merely a qualitative temperature reconstruction seems feasible.

Abstract Noisy human activities at sea are changing the acoustic environment, which has been shown to affect marine mammals and fishes. Invertebrates, such as bivalves, have so far received limited attention despite their important role in the marine ecosystem. Several studies have examined the impact of sound on anti-predator behavior using simulated predators, but studies using live predators are scarce. In the current study, we examined the separate and combined effects of boat sound playback and predator cues of shore crabs (Carcinus maenas) on the behavior of mussels (Mytilus spp.). We examined the behavior of the mussels using a valve gape monitor and scored the behavior from the crabs in one of two types of predator test conditions from video footage to control for effects from potential, sound-induced variation in crab behavior. We found that mussels closed their valve gape during boat noise and with a crab in their tank, but also that the stimulus combination did not add up to an even smaller valve gape. The sound treatment did not affect the stimulus crabs, but the behavior of the crabs did affect the valve gape of the mussels. Future research is needed to examine whether these results stand in situ and whether valve closure due to sound has fitness consequences for mussels. The effects on the well-being of individual mussels from anthropogenic noise may be relevant for population dynamics in the context of pressure from other stressors, their role as an ecosystem engineer, and in the context of aquaculture.

Innovative techniques are needed to assess oyster performance in flat oyster reef restoration projects. A valve gape monitor, a device that continuously measures opening and closing of live bivalves, can potentially be used as an effective method to determine survival and behaviour of the European flat oyster Ostrea edulis. The method has been successfully used in combination with a number of bivalve species to investigate valve gape activity in response to environmental factors. In this study, eight O. edulis were equipped with valve gape sensors in order to relate gape to environmental conditions such as food availability. Valve gape activity was monitored under controlled laboratory conditions, with and without food, in a concrete basin in the Oosterschelde and in the field (Voordelta, Dutch North Sea). Under controlled laboratory conditions, oysters clearly responded to changes in food availability. Starved oysters closed their valves significantly longer than oysters that received food, and the relative gape width in fed oysters was larger. In the concrete basin (Oosterschelde), a positive correlation between valve opening and Chlorophyll-a was found. Additionally, valve gape activity and tidal movement appeared to be linked. When exposed to a full tidal cycle (Voordelta), a negative correlation between valve opening and Chlorophyll-a was found. However, there was no correlation between valve gape and current velocity. In autumn, longer periods of inactivity were seen, but when valves opened, the valve gape was larger. These data indicate that valve gape can provide valuable information on behaviour (gape frequency and gape width), but also show that it is not necessarily a good proxy for feeding rate. Nevertheless, these results show that the gape monitor can be used to determine the natural behaviour of flat oysters under field conditions, and that gape opening provides information on behaviour and the stress response of bivalves to environmental conditions.

This repository contains all data generated for the publication "Sr/Ca in shells of laboratory-grown bivalves (Arctica islandica) serves as a proxy for water temperature – Perspectives for (paleo)environmental research?" currently under review.