Measuring and understanding biogenic influences upon cohesive sediment stability in intertidal systems

Abstract

Intertidal cohesive sediment systems are found throughout the world in areas of low hydrodynamic energy. These systems are ecologically and economically important but are under pressure from global warming, sea level rise and other anthropogenic influences. To protect and conserve these systems it is important to understand the sediment dynamics, especially the erosional properties of the sediment. The study of sediment erosion and transport is complex, encompassing biological, chemical and physical properties of the ecosystem. This thesis contributes towards this area of research, firstly in regard to the methods used to measure sediment erosion on exposed and submerged sediments and secondly with respect to assessing influences upon sediment stability through changes in the ecosystem, comprising of both the sediment environment and the macrofaunal community. Chapter 3: In partnership with Sediment Service a thorough re-evaluation of the Cohesive Strength Meter (CSM), a commercially available device used to measure surface sediment strength, was performed. New components, deployment method and calibration protocol were devised and tested. The new design was not effective, but the deployment and calibration have improved the ease of use and interpretation CSM data. Chapter 4: The study of intertidal sediment stability was conducted during the submerged period of the tidal cycle. Protocols and methods were devised or modified to sample submerged sediments with the aim to determine how sediment properties are affected by submersion and the resulting effect on sediment stability. Sediment stability increased with submersion. The existence of a fine layer of sediment on the surface, similar to the fluff layer found in submerged sediments, is given as a suggested explanation as it may be removed by the incoming tide. However, no other changes in sediment properties were detected. This may be due to flaws in the methods used in detecting fine scale changes in the sediment surface. In situ and laboratory experiments revealed contrasting effects of submersion on sediment stability with disturbance from the sampling and movement of sediment from the field to the laboratory given as an explanation for this. Chapter 5: The influence of the ecosystem engineering polychaete Arenicola marina on sediment properties was examined with an exclusion experiment. A. marina was excluded from five 20m² plots on an intertidal mudflat on the German island of Sylt. A holistic approach was used to measure the ecosystem, including a range of biotic and abiotic sediment properties as well as the macrofauna community. It was hypothesised that A. marina’s exclusion would alter the macrofaunal community and increase sediment stability. However, there was no consistent change in the macrofauna community or sediment environment with the exclusion of A. marina and subsequently no change in sediment stability. Chapter 6: The impact of bait digging for A. marina was examined with six 5m² plots dug up and A. marina removed, the plots were then monitored over a three month period. Bait digging disturbance was expected to have an impact upon the sediment environment and macrofauna community, resulting in a reduction in both sediment stability and microphytobenthic abundance. However, bait digging had minimal impact on the macrofauna community and caused no change in the sediment environment, despite the removal of a large proportion of the A. marina population. No change was recorded in the sediment stability or biomass of the microphytobenthos, indicating that with the exception of removing A. marina, bait digging of this nature was not detrimental to the sediment ecosystem. However, the consequences of larger, longer term digging operations can not be determined from this work and further studies are suggested. The study of intertidal sediment stability was progressed with advances made in methods and protocols. The work highlighted the importance of studying sediment stability as an ecosystem function through a holistic ecosystem approach rather than isolating individual variables.