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.
Type
Thesis, PhD Doctor of Philosophy
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