A comprehensive investigation into the relationship between physical forcing and sus-
pended particles in the shallow shelf region of the Western English Channel has been
conducted, in order to evaluate the temporal dynamics of suspended particle populations.
Measurements were taken across tidal cycles and seasons at station L4, part of the Western
Channel Observatory (WCO), using the combination of a free-fall microstructure profiler
and holographic imaging. Confirmation that L4 is weakly stratified is given, and that
the formation of the seasonal thermocline is substantially altered by the spring-neap cy-
cle. Stratification is variable and prone to periodic and partial erosion from atmospheric
forcing during any point in any season. L4 undergoes moderate turbulent dissipation,
principally as a result of tidal forcing. Typically, values of ε do not exceed 10−4 W kg−1 .
L4 also exhibits tidal asymmetry, chiefly in response to stratification which, albeit weak,
is frequently able to suppress turbulence when generated from the sea bed. The potential
energy anomaly is small at L4, as expected for a weakly-stratified environment. Maxi-
mum values in summer were shown to not exceed 50 J m−3 . Values of bed stress, τ0 , are
rarely greater than around 0.18 N m−2 . Nonetheless, the critical erosion threshold falls
below this, and is therefore smaller than that observed in similar locations around the
UK. Seasonality in the amount of material resuspended from the seabed is important at
L4. The presence of certain biological particles strongly influence particle size and may
also determine if a given particle is lifted from the bed. Particles ≥ 200 μm are relatively
rare, the site is dominated by particles smaller than this value in line with many other UK
sites. Under certain conditions the theoretical maximum limit of particle size, the Kolmogorov length scale, does not hold and many examples of occasions when this threshold
is exceeded are shown. This may generate important consequences in subsequent work
undertaken at this site and other temperate shelf locations globally, particularly as these
results indicate that maximum particle size appears to be governed less by the size of the
local turbulent eddies and more by the presence of biological particles. This is another
key seasonal component to particle dynamics in the Western English Channel. Phytoplankton populations are readily advected into and out of the L4 site, calling into question
the current sampling strategy of the WCO to rely exclusively upon point measurements.
Small increases in atmospheric forcing have the ability to rapidly disperse patches of
phytoplankton, possibly to the point of cell mortality. Traditional sampling techniques for
assessing zooplankton density have been shown to radically underestimate the number of
animals present at L4, which will increase error estimates on current ecosystem models.
Date of Award | 2013 |
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Original language | English |
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Awarding Institution | |
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Supervisor | Alex Nimmo Smith (Other Supervisor) |
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- Turbulence
- Holography
- SPM
- L4
- Western English Channel
- Kolmogorov length scale
- Resuspension
- Phytoplankton
- Zooplankton
- Advection
- Shelf sea
THE DYNAMICS OF SUSPENDED PARTICLES IN A SEASONALLY STRATIFIED COASTAL SEA
Cross, J. (Author). 2013
Student thesis: PhD