Abstract
In 1981 two large (1 200 1) seawater samples from the St. Lawrence Estuary were kept under constant temperature and light conditions for periods of 50 and 68 h, respectively. In both tank experiments, semidiurnal variations in NH4 were observed that could be related to cyclical NH4 uptake by the phytoplankton. Semidiurnal cycles in photosynthetic efficiency (αB) and intracellular chlorophyll a in the tank, phased on tides at sea, were also evidenced in both experiments. These results support the hypothesis that variations in phytoplankton photosynthetic activity, which are possibly endogenous, can be phased on semidiurnal variations in vertical tidal mixing (variations of the mean light conditions in the mixed layer). In addition, observed variations in intracellular chlorophyll a suggest the possibility of endogenous cycles of phytoplankton light and shade adaptation.
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