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Small-scale temporal and spatial variability in the erosion threshold and properties of cohesive intertidal sediments

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T.J. Tolhurst, E.C. Defew, J.F.C. de Brouwer, K. Wolfstein, L. Stal, David Maxwell Paterson

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Intertidal mudflats can be highly patchy due to both biotic and abiotic variability. The problems caused by such patchiness for the measurement of the erosion threshold of sediments has only received limited attention, primarily because of the limitations of the devices used to measure erosion threshold. The cohesive strength meter (CSM) erosion device partly overcomes these problems because it can collect numerous Measurements in a short time. This paper assesses the effects of site selection, small-scale spatial patchiness in a diatom biofilm and small-scale temporal variability over a tidal emersion on file measurement of erosion thresholds of cohesive intertidal sediments. Measurements were made with three CSM devices over a complete tidal emersion and at the beginning of the next emersion in the Westerschelde estuary. the Netherlands, in April 2001. Three different criteria were used in site selection: random sampling, random sampling with a user-bias towards visible diatom biofilms and sampling of only visible diatom biofilm areas. This resulted in different patterns of the measured sediment erodability. Contact core samples were collected to measure bio-dependant properties of the sediment known to influence erosion characteristics (water content and carbohydrates). In addition, variations in algal biomass (F-0(15)) over time were measured at the sediment surface with a fluorescence monitoring system (FMS). Three different operational extractions of carbohydrates were analysed to assess the roles of different carbohydrates in stabilising the sediment (colloidal-S-, EDTA- extracted and ETOH-precipitated).

Different responses in the erosion threshold to the tidal emersion were found depending upon the presence or absence of a diatom biofilm and on the sampling strategy used. Areas with a visible diatom biofilm showed a rapid increase in erosion threshold at the beginning of the emersion, whilst areas without a visible biofilm tended to show an increase at the end of emersion. Both colloidal-S and ETOH carbohydrate correlated with erosion threshold. F-0(15) Correlated with erosion threshold at the beginning of the emersion period, but this relationship became progressively weaker during the course of emersion. The implications of patchy biota, small-scale variability and temporal changes in erosion threshold for measuring the stability of sediments over a tidal cycle are discussed. (c) 2005 Elsevier Ltd. All rights reserved.



Original languageEnglish
Pages (from-to)351-362
Number of pages12
JournalContinental Shelf Research
Issue number3
Publication statusPublished - Mar 2006

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