Mireille Consalvey PhD Thesis - University of St Andrews

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kihorator wo Inicrol)[1vtobcrithi
fill, U) atioll LIS11
Motility was shown to only occur at times co-inciding with low tide with the
strength/direction of the phototaxy determining the migration response. Palmer
and Round (1965) and Round and Palmer (1966) developed a similar model.
They suggested diatoms to have a free running 24 h biological clock rather than a
24.8 h tidal one. It was hypothesised that vertical migration was driven by a
permanent positive geotaxy, which was overridden by a rhythmic positive
phototaxy, however, there must be some cue that prevents cells migrating
downwards indefinitely e. g. anoxic layer (Kingston 1999).
Wbilst such models provide a suitable framework to describe the most
general patterns of migration, they are clearly over simplified. Tenris must be
introduced to account for differential responses to nutrients, dissolved C02, light
history/conditions over the exposure period, grazing pressure and disturbance. It
would seem that some form of biological clock controls the migratory rhythms of
many microphytobenthic species but that the parameters that entrain this clock
vary. It has been suggested that these migratory rhythms are the main cause of
short-ten-n variability in intertidal microphytobenthic primary productivity and
therefore must be considered in any model of microphytobenthic primary
productivity (Pinckney and Zingmark 1991; 1993). Considering the high level of
variability described above, it is essential to characterise these migratory rhythms
for as many sites and microphytobenthic assemblages as possible in order to
determine if there are generic patterns and to ultimately discern the cues that
drive migration.
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