Hierarchical control of phytoplankton succession by physical factors

The influence of physical factors on phytoplankton succession was assessed during an annual cycle in the St. Lawrence Estuary (Canada), where nutrients remain abundant throughout the whole year. Typically, the phytoplankton production period is short (Jun to Sep) and characterized by the occurrence of 3 distinct peaks. The July bloom was dominated by the 2 diatoms Thalassiosira nordenskioldii and Chaetoceros debilis, while Leptocylindrus minimus and Nitzschia seriata were dominant at the beginning and at the end of September, respectively. The occurrence of the 3 bloom periods is related to an increase in mean light intensity in the mixed layer, caused by strong density stratification which decreases the depth of the mixed layer. During these bloom periods, the succession of diatom species is mainly controlled by variations in temperature. Flagellates were observed all yearround, although they were more abundant during the diatom bloom periods. Their abundance is related to variations in surface temperature. In the Estuary, where nutrients are usually non-limiting, the observed succession of taxa is restricted to the first stage of the Margalef (1958) succession model (small cells with high growth rates, which are typical species for frequently destabilized environment). Our results demonstrate that the frequency of destabilization of the water column selects the growth rates of the cells, through nutrient conditions. Mean light intensity in the mixed layer determines the occurrence of non-motile forms such as diatoms, and temperature sets the conditions for optimal metabolic activity (flagellate numbers and succession of diatom species). This results in a conceptual model where stability conditions, mean light in the mixed layer and temperature hierarchically control phytoplankton succession.