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

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Abstract

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.

Introduction

The stability of intertidal cohesive sediments is influenced by a large number of interacting physical and biological parameters (Paterson, 1997; Burt et al., 1997; Black et al., 1998b; Black and Paterson, 1998a). In many natural intertidal systems, biological activity can be the primary control on sediment stability with variation in stability directly linked to spatial and temporal variation in benthic biota (Kornman and de Deckere, 1998; Widdows et al., 1998; Paterson, 1989; Sutherland et al., 1998; Tolhurst et al., 1999). The natural variation or “patchiness” of benthic systems can vary at several spatial scales (Reise, 1985; Thrush, 1991) and recognition of the potential importance of small-scale variation led to the development of techniques and devices capable of measuring the variation of properties (Black & Paterson, 1997; Harper et al., 1997; Wiltshire et al., 1997; Taylor and Paterson, 1998; Black et al., 1999). Cone penetrometry and other surrogate methods (Hansbo, 1957; Kravitz, 1970) have been used to determine sediment stability while some in situ erosion devices, such as the Cohesive Strength Meter (CSM) system (Paterson, 1989; Tolhurst et al., 1999) are capable of measuring small-scale spatial and temporal variation in sediment stability. These techniques can now be combined to relate physical and biochemical parameters at similar scales for the first time. It has been shown that micro-scale analysis of biochemical and sediment properties reveals information missed by coarse sampling methods (Davison et al., 1997; Wheatcroft and Butman, 1997) and that related correlations can be improved (Paterson et al., 1998). This paper examines the relationship between biological and physical parameters and sediment stability over an intertidal transect on the Humber Estuary and includes examination of the bed forms, crest/trough formation characteristic of some stations (Black and Paterson, 1998a; Dyer, 1998). Measurements of biochemical sediment properties were made on two spatial scales (μm and mm) to determine which was most relevant to sediment stability.

Section snippets

Materials and methods

The Skeffling mudflat on the Humber estuary is about 5 km wide with a 1 : 1000 slope with a tidal range of about 6 m. Details of the site and the stations used can be found in Black and Paterson (1998a) and Dyer (1998). Measurements of sediment properties were made at four stations A, B, C and D located 200, 580, 1550 and 2180 m from the shore, respectively. A crest/trough system of bedforms develops seawards of station A, covering a large proportion of the mudflats, only the extreme upper shore

Macrobenthos

Two species, Macoma balthica and Streblospio shrubsolii were found at all stations (Table 1). High densities of Oligochaeta were found at stations A and B. Nereis diversicolor and Retusa alba were the dominant predators at these stations, while Nephtys hombergii was dominant at stations C and D. The total density and number of species decreased along the transect from station A to D. The biomass also decreased towards the sea. Biomass was dominated by three species: Macoma balthica, Nereis

Correlation with erosion threshold

Only cryolander Chl a was significantly correlated with the erosion threshold (Table 2). This suggests that the photosynthetic biomass (diatoms) was significant in controlling sediment stability. From the literature (Holland et al., 1974; Yallop et al., 1994; Paterson, 1997), the secretion of EPS would be the expected mechanism for this effect. The total EPS content of the sediments was not measured but only the colloidal form that has been shown to represent diatom biomass most strongly (

Conclusion

This study has shown that the temporal and spatial variability of sediment properties (stability, erosion rate) on a macro- and micro-scale are clearly significant. This variation is related to sediment bedform and is influenced by the organisms inhabiting the bed. In addition, there is a clear temporal signal that can be affected by local climatic conditions (e.g. rain). Rain reduces sediment stability and therefore the variation between bedform features. However, this effect was short-lived

Acknowledgements

This work was funded by the European Commission Mast 3 programme under the “INTRMUD” award (MAS3-CT95-0022). The participation of T.J. Tolhurst was under the auspices of a studentship award from the University of St Andrews. The work of Prof. K. Dyer in co-ordination of the INTRMUD project was much appreciated and the valuable comments of Dr. C. Amos and two anonymous reviewers on the manuscript.

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