Benthic controls of resuspension in UK shelf seas: Implications for resuspension frequency

C.E.L Thompson,M.E Williams,L Amoudry,T Hull,S Reynolds,A Panton,G.R Fones

doi:10.1016/j.csr.2017.12.005

Abstract

In situ measurements and ship-based resuspension experiments using annular flumes are used to determine sediment stability and critical erosion thresholds for four sites with significantly different sediment characteristics, located in the Celtic Sea at water depths of 100 m. Seasonal and spatial variability of sediment characteristics and erodability is examined, and found to be the result of changes in percentage of organic carbon in the surface sediments (R2 = 0.82) and bulk density (R2 = 0.73) respectively when individual characteristic bed parameters are considered. Principal component analysis and linear regression analysis are used to determine a predictive model for erosion threshold in the Celtic Sea (R2 = 0.99), based on grain size, sorting, kurtosis, bulk density, porosity, percentage fines, organic carbon content and chlorophyll a concentration. Physical sediment characteristics were found to be more significant controls of bed stability than biological factors. Local hydrodynamic conditions are used to determine the likelihood and frequency of resuspension given these critical erosion thresholds. Resuspension is driven by tidal currents, and is common year-round, leading to a constant re-working of bed sediments in particular at the muddier sites. This is confirmed by in situ measurements of suspended sediment concentration.

Highlights

The importance of sediment resuspension in shelf seas is well established (Harris, 2014; Nittrouer and Wright, 1994; Solan et al, 2017)
Tidal velocities vary across the western shelf region (0.2–1.6 ms−1, Uncles and Stephens, 2007), they tend to be low in the Celtic sea, with bed shear stresses typically < 0.5 Pa in central regions
Bed stability and critical erosion thresholds were determined for four sites in the Celtic Sea over a single annual seasonal cycle

Summary

Introduction

The importance of sediment resuspension in shelf seas is well established (Harris, 2014; Nittrouer and Wright, 1994; Solan et al, 2017). The relative importance of these resuspension events on many sedimentwater column exchange processes remains unclear; their location, duration and magnitude is difficult to predict; and few in situ investigations of bed stability have been undertaken at typical shelf depths. One of the key parameters that determine when and where resuspension events take place is the critical erosion threshold (τcrit), which is used to represent the stability (erodability) of the bed. This is often difficult to measure in situ, especially at depth, and often more measured bed parameters are used as a proxy for bed stability. Beyond the physical sediment properties, biological processes influence sediment stability through biostabilisation (Black et al, 2002), bioturbation (Widdows et al, 2000) or the addition of organic matter; certain faunal behaviours may directly induce resuspension (Cross et al, 2013; Davis, 1993)

Methods

Results

Discussion

Conclusion