Effects of benthic diatoms, fluff layer, and sediment conditions on critical shear stress in a non-tidal coastal environment

Abstract

Sixteen sediment samples were collected from a square grid (4×4) with a horizontal distance of about 150 m between positions in Århus Bay in the south-west Kattegat (14 to 15 m water depth). Critical shear stress (τc) was measured in all samples and related to sediment parameters: grain-sizes, organic matter, water content, porosity, and chlorophyll-a (chl-a) content, in upper layers. Samples were divided into a low (A) τc group and a high (B) τc group in relation to an erosion rate. A significant (P<0·001) difference in median τc was found between groups A (0·0284 N m−2) and B (0·0380 N m−2). Average chl-a concentrations in groups A (1·4 μg g−1) and B (1·8 μg g−1) were not significantly different (P=0·47) but there was a significant and positive correlation (r2: 0·7, P<0·001) between τc and diatom film abundance. Sediment organic matter and water content were significantly higher in group B compared with A, which contradicts that watery and organic rich sediments generally exhibit low τc. This was explained by the presence of a diatom film cover on the fluff layer that inhibits the action of erosive forces. A fluff layer is characterized by a high water and organic content. The fluff layer was present in the majority of the samples but the highest average chl-a content and a significant (P=0·020) higher abundance of diatom film was observed in group B (high τc). Benthic diatoms were dominated by Haslea crucigeroides, Pleurosigma strigosum, and Bacillaris paxillifer. Spatial variability of sediment parameters was high and variability of a stability/erodibility parameter even exceeded those recorded for highly heterogeneous tidal flats. The occurrence of benthic diatoms at 14–15 m of water depth in the eutrophic Århus Bay was supposedly related to a measured increase in Secci depth in the bay and thereby increased light penetration depth.