Quantifying luminescence signal resetting of sand grains in turbid waters is essential for both sediment dating and tracing, yet direct measurement under natural subaqueous conditions remain scarce. Here, we present the first depth-resolved experiment that combines in-situ luminescence resetting, subaqueous light spectra and suspended sediment concentration in a tidal inlet. Sand-sized quartz and feldspar grains were exposed to daylight at multiple depths during a one-day deployment, while optical and sediment conditions were continuously monitored. Single-grain luminescence measurements reveal depth-dependent resetting with a bleaching front below which no significant signal resetting occurs within a day. The position of this bleaching front depends on signal bleachability and agrees with predictions based on spectral irradiance and mineral-specific photo-ionization cross sections. By directly linking subaqueous light conditions, sediment concentration, and mineral-specific bleaching behaviour, our findings provide empirical quantification that can inform luminescence dating, provenance studies, and tracing of sediment transport in dynamic coastal systems. ... Read more

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 nonlocal 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. ... Read more

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. ... Read more

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. ... Read more