Mireille Consalvey PhD Thesis - University of St Andrews

increasing PPFD; first an apparent decrease in the cross-sectional area of the
thylakoid membranes and then a downwards migration at PPFD > 600 ýImol M-2
s-1. The differential responses of taxa to high light intensities may lead to
structured biofilm communities, and may help maintain high primary
productivity rates (for the whole biofilm), despite potentially damaging light
intensities.
An increase in the surface maxima relative to the downwelling PPFD has
been described by many authors (Lassen et al. 1992; Kiffil et al. 1994,1996) but
was not seen in this study. This may be attributed to the dense biofilms, sediment
characteristics (see section 6.1) or the sampling technique. Determining the
sediment surface is a key aspect of microscale analysis and one that is seldom
discussed in the literature. The LTSEM images demonstrated the highly
heterogeneous nature of the sediment/biofilm. Figure 6.23 shows a Gyrosigma
cell emerging through a naviculoid layer and opens the question where does the
surface start? This question is particularly relevant when considering cells such
as GyrosigmalPleurosigma sp. that are as large as the sampling interval (100
ýjm). In reality sampling will start somewhere between the two extremes and is
not possible to determine without correlative microscopic examination.
Figure 6.23: Gyrosigma Sp. emerging (Arlesford Creek 1999). Scale bar = 50
pm. Arrows indicate possible biofilm. surface.
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