Mireille Consalvey PhD Thesis - University of St Andrews

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sediment surface, occupying the spaces in between euglenoid cells leading to a
maximisation of all available space by phototrophic organisms. Point Clear was
the only site whereby the change in chlorophyll a seemed to be complemented by
an increase in cells at the sediment surface (as viewed under LTSEM). Whilst
there was a sharp decline in chlorophyll a in the surface layers of the Hythe
(2000), a biofilm was still visible both to the naked eye and under LTSEM.
Whilst the biofilm appeared green towards the end of the day, it was seen to
contain primarily diatoms, therefore the decline in biomass might reflect the shift
in species. However, the green colour would suggest that the euglenoid cells
were still within the photic zone and would therefore hypothetically still be
sampled at 200 gm. Cyanobacteria were recorded below the diatom layer,
supporting the findings of Ploug et al. (1993) and Yallop et al. (1994). This
vertical positioning as well as the capacity for cycling of position has clear
implications on primary productivity.
LTSEM is a valuable tool for characterising the structure and taxonomic
composition of microphytobenthic biofilms but it is essential to appreciate that it
is a qualitative technique. The matrix of EPS that characterises
microphytobenthic biofilms often prevents taxonomic assessment apart from
basic classifications (e. g. naviculoid or by size). Very often LTSEM analysis
concentrates on the surface layer, but in this study, and other studies, the biomass
may be concentrated in the top 400 ýtrn and cells are photosynthetically viable
below the sediment surface. Therefore, LTSEM images do not represent all of
the photosynthetic community. Whilst the fracture face can provide some
resolution of sub-surface cells, the number seen clearly do not correlate with
what would be expected from chlorophyll a content.
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