Large-scale trends in some mass physical properties of Danish Wadden Sea sediments, and implications for organism-sediment interactions

Abstract

Mass physical properties (wet/dry bulk densities, mud/sand contents and concentrations, water content, shear strength) were determined on 132 surficial sediment samples from intertidal flats in Ho Bugt and north of Langli Island between the Hjerting Løb tidal channel and the Skallingen Peninsula, Danish Wadden Sea. Except for shear strength, all parameters were found to be highly intercorrelated (r<0.97), and the 12 equations thus generated can be used conveniently for regional modelling purposes. Investigations which rely on small sample numbers can be optimised by prior pilot studies in which average conditions are defined along environmental gradients and, once suitable in-situ calibrations have been performed, the method can be expanded to other parameters. Water content is a particularly useful predictor because it is a universal master variable by which differences in compaction between different environments can be normalised. Shear strength values show large variations at low mud contents, which decrease sharply towards higher mud contents. Nevertheless, shear strength appears to decrease with increasing mud content, a trend observed in spatial plots of comparative data from the German Wadden Sea. This suggests that the data comprise both spatially coherent and incoherent values. Similarly to the German Wadden Sea, counter-intuitive convex mud concentration curves were recorded along sand-mud gradients, peaking at intermediary mud contents in mixed sediments. Based on a growing body of published literature, interpretations of these novel trends suggest far-reaching implications for organism-sediment interactions.