To support the successful application of sponges for water purification and collagen production, we evaluated the effect of depth on sponge morphology, growth, physiology, and functioning. Specimens of Eastern Mediterranean populations of the sponge Chondrosia reniformis (Nardo, 1847) (Demospongiae, Chondrosiida, Chondrosiidae) were reciprocally transplanted between 5 and 20 m depth within the Kaş-Kekova Marine Reserve Area. Control sponges at 5 m had fewer but larger oscula than their conspecifics at 20 m, and a significant inverse relationship between the osculum density and size was found in C. reniformis specimens growing along a natural depth gradient. Sponges transplanted from 20 to 5 m altered their morphology to match the 5 m control sponges, producing fewer but larger oscula, whereas explants transplanted from 5 to 20 m did not show a reciprocal morphological plasticity. Despite the changes in morphology, the clearance, respiration, and growth rates were comparable among all the experimental groups. This indicates that depth-induced morphological changes do not affect the overall performance of the sponges. Hence, the potential for the growth and bioremediation of C. reniformis in mariculture is not likely to change with varying culture depth. The collagen content, however, was higher in shallow water C. reniformis compared to deeper-growing sponges, which requires further study to optimize collagen production.

Global ocean warming and acidification will alter the physicochemical conditions in the deep North-Atlantic Ocean. Here, extensive sponge grounds, often dominated by the demosponge species Geodia barretti , provide three-dimensional structure, habitat and significantly contribute to benthic-pelagic coupling and nutrient cycling processes in the deep sea. It is unknown if G. barretti remains physiologically functional under the future physicochemical properties of an Anthropocene ocean. In this study, individuals of G. barretti collected from 300 m water depth in the Barents Sea, were exposed to four treatments resembling future ocean conditions (no treatment, 4 °C increase in seawater temperature, decrease of seawater pH by 0.3, and a combination of the high temperature, low pH). Over the course of 39 weeks, oxygen consumption, dissolved inorganic nutrient fluxes, and bacterioplankton clearance rates were measured as indicators of metabolic activity. We found that all indicators within each sponge individual and per treatment were highly variable over time and no effect of manipulated seawater treatments on these parameters could be demonstrated. Oxygen consumption rates in all groups closely followed a seasonal pattern, potentially caused by (a)biotic cues in the seawater flowing through the experimental aquaria. While similar metabolic rates across all treatments suggest that G. barretti physiologically coped with simulated future ocean conditions, observed tissue necrosis in experimental animals might indicate that the response of the complex, high microbial G. barretti sponge (i.e., sponge host and microbial symbionts) to future ocean conditions may not be reflected in basic physiological processes.

The seabed rarely consists solely of bare sand: often other materials, such as shells are present. They can influence sand transport by armoring the bed and modifying its roughness. Biogenic shells come in different shapes and sizes, depending on the mollusc species that produce them. To understand how changes in bivalve species composition affect sediment transport, we need a mechanistic understanding of how shell content and shell shape influence the near‐bed flow and sand transport. We performed experiments in a racetrack flume, testing the effect of elongated ( Ensis leei ) versus rounded ( Spisula subtruncata ) shells on unidirectional current‐driven sand transport. For both types of shells, a higher depth‐averaged flow velocity was needed for initiation of motion and a decrease in bedload transport of sand was found. At a shell content of 20%, the threshold of motion of sand increased up to 75%, and bedload transport was reduced by up to 50%. These effects are explained by a balance between roughness‐induced turbulence and bed armoring. Compared to a bare bed, shells decreased bed roughness by reducing ripple formation; rounded shells lowered roughness more than elongated shells, which formed roughness elements themselves, but also covered a larger fraction of the bed. However, there was no clear difference between round versus elongated shells on the overall sand transport; only shell content was key for the overall effect. Our results imply that sediment transport is likely overpredicted when a high number of shells is present in the seabed.

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.

Quantifying sediment transport is crucial for thoroughly understanding coastal systems and accurately designing coastal management interventions (e.g., sand nourishments). Lagrangian particle tracking models are valuable tools for investigating sediment transport, as modelling in a Lagrangian framework provides complete records of particle transport sources, sinks, and the pathways between them. Here we present two examples of application of Lagrangian sediment tracking modelling in coastal settings. Both studies are conducted using SedTRAILS [1], a Lagrangian particle tracking model that derives particles position from flow velocity fields generated from hydrodynamic models.In the first application we simulate the dispersal of a nourishment on the ebb-tidal delta of Ameland Inlet (Wadden Sea, Netherlands). The flow velocity fields employed by the SedTRAILS simulation are generated from a Delft3D simulation of Ameland Inlet. The nourishment is modelled as a sample of representative sand parcels randomly sourced within the nourishment area and its gradual erosion is modelled by continuously releasing the parcels at regular intervals for the duration of the simulation. The accuracy of the Lagrangian simulation results are validated by comparing maps of particles position generated at different time steps of the SedTRAILS simulation with maps of sand spatial distribution derived from the Delft3D simulation at the same time steps. Ultimately, we are able to model the pathways of individual nourishment particles up to six months after its displacement.In the second application we couple SedTRAILS with measurements of sand grains luminescence (i.e., the ability of a mineral grain to store energy when buried and release it upon exposure to sunlight) to reconstruct sand transport history in coastal settings. In order to do so, we combine SedTRAILS with a model that quantifies sunlight exposure of a given sand particle as a function of turbidity and its position in the water column [2], allowing to compute the cumulative sunlight exposure of such particle during its transport history. Since luminescence signals produce evidence of how long a sand particle was buried for, we are able to infer and simulate the forcings that the particle was exposed to before burial (i.e., during its transport history). As luminescence signals also yield information on how much sunlight the sand particle was exposed to before being buried, we can eventually combine the modelled cumulative sunlight exposure with evidence on resetting of luminescence signals as a function of light exposure [3] to infer, for the first time, coastal sand transport history from luminescence measurements.Overall, the two studies provide an overview of how Lagrangian particle tracking modelling can (on its own and in combination with other techniques) provide unique insights on where, when and how sand is transported across coastal systems, which can advance our understanding of coastal systems and be exploited for accurately designing coastal management interventions.References[1] Pearson S.G. et al. (2023). Proceedings of the Coastal Sediments 2023, 1212-1221.[2] Storlazzi C.D. et al. (2015). Coral Reefs, 34 (3), 967-975.[3] de Boer A.-M. et al. (2024). Netherlands Journal of Geosciences, 103, 22.