Mireille Consalvey PhD Thesis - University of St Andrews

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The advent of fibre-optic microsensors (Jorgensen and Des Marais 1986;
Dodds 1992; KtihI and Jorgensen 1992; Lassen et al. 1992; Ploug et al. 1993) has
enabled the light climate within sediment/biofilms to be non-destructively
resolved at the 100 gm level. The most useful sensors are those that detect scalar
irradiance (spherically integrated radiance from all directions) as this represents
the light climate to which the cells are exposed at a similar scale, measures of
downwelling radiance can inaccurately estimate the total quantum flux.
Biofilms are a matrix of cells, exopolymers, sediments and water. The
upper layers of microphytobenthic communities can attenuate a large proportion
of incoming light (Dodds 1992 and references therein). The cuphotic zone depth
(I% of incident light) is usually less than 2 mm (Paterson et al. 1998) (Table 6.1
Light attenuation co-efficients). Microphytobenthic biofilm communities are
highly light absorbing and scattering media with the backscattered component
increasing with depth (Lassen et al. 1992). The photic zone may vary from ýtm
(fine muds) to cm (sands) (Jorgensen and Des Marais 1986; Lassen et al. 1992).
Lassen et al. (1992) and KUM et al. (1994; 1996) demonstrated the phenomenon
whereby the surface PAR of the sediment was higher than could be expected
from the incident illumination due to back scattering.
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