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.