Hierarchical control of phytoplankton succession by physical factors

Abstract

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.

Hydrological and biological processes modulate carbon, nitrogen and phosphorus flux from the St. Lawrence River to its estuary (Quebec, Canada)

“…This argument is justifiable given the high rate of mineralization taking place within the estuary (Tremblay and Gagné 2009) and the inclusion of organic N and P fractions highly valuable as substrate for microbial heterotrophs whose respiration further consumes O 2 . Primary production within the turbid transition zone of the upper estuary is light-limited (Levasseur et al 1984) and supports a rich heterotrophic microbiota (Frenette et al 1995). Calculation of the % contribution of SLR to total estuarine flux using TP (14%) and TN (47%) markedly Assuming that all TP exported to the estuary is mineralized within a year c Assuming that all TN exported to the estuary is mineralized within a year increased the importance or river enrichment to estuarine microbial production, making them all the more significant.…”

Marine mammals and the community structure of the Estuary and Gulf of St Lawrence, Canada: evidence from stable isotope analysis

“…In aquatic systems in particular, predator-prey relationships are often determined largely by size (Werner & Gilliam 1984). Phytoplankton in the Lower Estuary is dominated by small-sized diatoms (< 25 µm; Levasseur et al 1984) and zooplankton collected in this study were generally >160 µm. Planktonic ciliated protozoa (25 to 75 µm), intermediate in size between phytoplankton and the sampled mesozooplankton, are regarded as an important trophic link between these 2 groups in the Lower Estuary (Sime-Ngando et al 1995).…”

Turbulent nitrate fluxes in the Lower St. Lawrence Estuary, Canada

“…About 75% of the surface nutrients found in the LSLE may therefore originate from the CIL because of this upwelling/mixing mechanism [Greisman and Ingram, 1977;Savenkoff et al, 2001], the remaining part being attributed to the St. Lawrence and Saguenay rivers discharge. These vertical nutrient fluxes at the HLC sustain a high primary productivity in the LSLE, with daily production rates during the summer nearly equivalent to those during spring blooms [Levasseur et al, 1984;Therriault and Levasseur, 1985;Plourde and Runge, 1993;Plourde et al, 2001]. This high primary productivity sustains high levels of secondary production that is then exported to the Gulf, mostly via the Gaspe Current Plourde and Runge, 1993].…”