Diel patterns of microphytobenthic primary production in intertidal sediments: the role of photoperiod on the vertical migration circadian rhythm

Diel primary production patterns of
intertidal microphytobenthos (MPB) have been attributed to short-term
physiological changes in the photosynthetic apparatus or to diel changes in the
photoautrophic biomass in the sediment photic layer due to vertical migration.
Diel changes in primary production and vertical migration are entrained by
external factors like photoperiod and tides. However, the role of photoperiod
and tides has not been experimentally separated to date. Here, we performed laboratory
experiments with sediment cores kept in immersion, in the absence of tides, with
photoperiod or under continuous light. Measurements of net production, made with
O2 microsensors, and of spectral reflectance at the sediment surface
showed that, in intertidal sediments, the photoperiod signal can control diel
patterns of net primary production and sediment oxygen availability through the
vertical migration of the MPB photoautotrophic biomass. This vertical migration
is controlled by an endogenous circadian rhythm exclusively entrained by
photoperiod in the absence of tides, which in turn determines the diel
oscillation in primary production. Fourier spectral analysis of temporal patterns
confirmed that the photosynthetic circadian rhythm was mainly characterized by
light/dark migratory cycles without any relevant contribution of the subjective
in situ tidal period.