Variations in sediment properties, Skeffling mudflat, Humber Estuary, UK

The generic importance of biogenic mediation of sediment erosion and transport is a matter of debate and a multidisciplinary approach is required to investigate biologically mediated mechanisms of sediment stability. Biogenic influence on sediment behaviour can be inferred from a variety of correlative parameters that act as proxies for biological effects. These include pigment content, organic content and biomass. These biological “indicators” are routinely measured by biologists on a number of differing scales and depth resolutions. Few attempts have been made to examine the importance of an appropriate “match” between the erosion process, the measured physical response and the scale/resolution of the measured biological parameter. This scale dependency was examined along an extensive shore normal transect on the Skeffling mudflat (Humber Estuary, UK). Measurements of physical sediment properties, macrobenthos and selected biogeochemical properties (extracellular polymeric substances) were made. Biogeochemical properties were measured on a “traditional” cm scale and at a depth resolution of 5 mm but also on a microspatial scale, at a 0.2 mm depth resolution. Sediment stability was measured using a cohesive strength meter (CSM). Correlation analysis was used to determine the interactions between variables. A complementary investigation of the sediment micro-fabric (low-temperature scanning electron microscopy) was also conducted. Results demonstrate that the depth resolution of biogeochemical measurements is an influential factor in the interpretation of the biogenic stabilisation of intertidal cohesive sediments. Sediment stability varied with time and with bed feature. Stability increased with time except where influenced by other factors such as rain which markedly reduce surface stability. Critical erosion threshold increased towards the shore whilst suspension index (erosion rate) decreased, and crests were generally more stable than troughs. The study emphasises the temporal and spatial variability of mudflat stability and the importance of biological processes on the erosional behaviour of cohesive sediments.