Continental shelf seas play a key role in the global carbon cycle, linking the
terrestrial, oceanic and atmospheric carbon pools. Present day CO2
conditions and underlying processes were investigated at a seasonally
stratified station (L4) in the western English Channel and a region of
freshwater influence, Liverpool Bay, from 2007 - 2009. The sites were
compared via in situ observations and 1-D coupled physical-ecological
model simulations. This research tests three questions regarding CO2
dynamics at these contrasting shelf sea regimes on the NW European Shelf.
1. Seawater pCO2 in seasonally stratified sites is biologically controlled
over the spring and summer.
Weekly sampling in the L4 site found that metabolic activity (production)
was the dominant process affecting seawater pC02 variations. Seasonal
changes in both sea surface temperature and advection had a significant
impact on seawater pCO2 dynamics. Model simulations suggest that L4 is a
weak annual source of CO2 to the atmosphere (0.17 ± 0.15 mol C m-2 y(1)
and agreed with observations that L4 is an atmospheric CO2 sink over the
biologically productive season.
2. Regions Of Freshwater Influence are sources of CO2 to the atmosphere
over the seasonal cycle.
In Liverpool Bay, sampling revealed a seasonal seawater pCO2 cycle largely
consistent between the two years, with a strong sink of atmospheric CO2
during spring and summer which contributed to an overall annual sink (-3.8
mol CO2 m-2 y(1 ). A detailed study, at one station, showed that photosynthesis, predominately determines the seawater pCO2 in 2007-08, whereas salinity fluctuations dominated in 2008-09.
3. Daily variability of seawater pCO2 is likely to skew up-scaling estimates
of regional fluxes taken from point measurements.
Daily variations in seawater pCO2 arise from metabolic, advective and
solubility factors. Measurements during neap tides, proximal to the coast
and at times of intense biological production had the greatest daily
variability, up to 136 μatm, equivalent to 80% of the seasonal range. The
influence of diurnal variability on flux integrations for Liverpool Bay, gave
an uncertainty of ~50% for the annual net sea-air CO2 flux during each year.
The thesis highlights the shortcomings of using a single year or low
resolution sampling and demonstrates why there is a need for extensive
monitoring between seasons and over several years to gain a robust
'synoptic' picture, minimising both temporal and spatial ambiguity in global
coastal carbon flux estimates.