Mireille Consalvey PhD Thesis - University of St Andrews

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day. The rate of change remained largely positive but did become negative at
some points; the reasons for these switches were unclear. The increase in
minimum fluorescence, taken as a general increase in biomass over the course of
the day, appeared to be largely irrespective of light or tidal state at site. It is
possible that some changes were occurring in the sediments (e. g. compaction)
which may have led to an increase in chlorophyll a in the surface layer, or drying
out which may serve to hold and concentrate cells in a surface layer (Perkins et
aL submitted). However, this would require further substantiation and
infonnation relating to the water contents and biofilm structure (LTSEM). The
maximum light utilisation efficiency showed the same pattern as seen at the
Hythe (day 2). The reasons for this were discussed above and also negatively
correlated with PPFD.
Whilst every attempt was made with the laboratory studies to simulate in
situ conditions, it is not possible to determine if experimental artefacts occur as a
result of transporting the cells to the laboratory and cells not being exposed to a
natural high tide etc. Therefore, for the Eden study the expenment was conducted
at the site on 3 days, with different tidal/light states. In all instances the cells
moved to the sediment surface as soon as (days I and 3) and not long after (day
2) the site became exposed, irrespective of increasing/decreasing irradiances. On
day 2 (dawn) F,, 15 started to increase when light was barely detectable. Whilst the
increase occurred in line with the increase in PPFD associated with sunrise, there
was a decrease towards the end of the exposure penod, suggesting either an
endogenous tidal rhythm or a physical cue (e. g. subsurface water; see Hopkins
1966). An obvious decline in F, 15 was also seen on day 3 (dusk) which occurred
just after low tide and co-incided with decreasing light levels, indicating either an
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