Late spring phytoplankton bloom in the Lower St. Lawrence Estuary:the flushing hypothesis revisited

Abstract

In the Lower St. Lawrence Estuary (LSLE), environrnental conditions (stratification, surface light and nutnents) are favorable for phytoplankton growth starting in May, but the spring phytoplankton bloom typically does not occur until early summer (late June-July). Possible explanations for the late onset of the phytoplankton bloom include flushing of the surface layer due to the spring freshwater runoff, loss of phytoplankton cells from the thin euphotic layer through sinking and rnixing, and ternperature limitation of phytoplankton growth rates. We use 1-and 2-D time-dependent models of phytoplankton dynamics to explore these hypotheses. In particular, we illustrate the role of (1) phytoplankton cell sinking versus vertical turbulent inixing and (2) flushing of freshwater runoff on pnmary production in the LSLE. Results of the 1-D simulations show the dramatic effect of phytoplankton cell sinking in a thin euphotic zone, while at the Same time high vertical turbulent mixing rnay act to maintain these sinking phytoplankton cells in the euphotic layer. Nevertheless, the 1-D analysis cannot account for spatio-temporal patterns in the development of the phytoplankton bloom observed during a high resolution physical, chernical and biological sampling field experiment perforrned in the summer of 1990 in the LSLE. 2-D simulations, run with seaward advective velocities in the range 0.15 to 0.3 m s-I, close to observed values, generate downstream patterns of phytoplankton biomass that resernble these observed patterns. Comparison with observations helps to specify the range of sinking and advective velocities that operate in concert to control the timing and spatial location of the bloom.

Prevalence, structure and properties of subsurface chlorophyll maxima in Canadian Arctic waters

“…The mechanism(s) controlling the vertical position, thickness and seasonal persistence of the SCM in the Arctic Ocean remain to be elucidated, but previous studies demonstrated the importance of turbulent mixing (Zakardjian et al 2000, Huisman et al 2006 and variations in the buoyancy of diatoms (physiologically induced changes in response to nutrient supply and decrease of irradiance at the bottom of the euphotic zone: Steele & Yentsch 1960, Cullen 1982. The latter is presumably important in the strongly stratified BS.…”

Spatial variability in surface-water <i>p</i>CO<sub>2</sub> and gas exchange in the world's largest semi-enclosed estuarine system: St. Lawrence Estuary (Canada)

“…The lowest surface-water pCO 2 values were found downstream of the MTZ in the lower reaches of the SLE near Pointe-des-Monts, where the channel widens into the GSL. Due to favorable environmental conditions (nutrients, light, and stratification), phytoplankton blooms typically occur in late spring or early summer in the LSLE (Zakardjian et al, 2000), with maximal biological production occurring in its downstream portion due to the mixing of cold nutrient-rich waters, which upwell at the head of the Laurentian Channel, with warmer freshwaters flowing in from the north-shore rivers (Savenkoff et al, 1994). Seaward of the estuary-gulf boundary, the pCO 2 gradually increased from 207 to 478 µatm, coinciding with rising surface-water temperatures (T = 3.9 to 13.7 • C).…”

Seasonal versus synoptic variability in planktonic production in a high‐latitude marginal sea: The Gulf of St. Lawrence (Canada)

“…Because of the lack of data for the remaining biological scalars for the same period, idealized profiles were used. Values of 1 mmol N/m 3 for ammonium [e.g., Levasseur et al, 1990;Tremblay et al, 2000;Zakardjian et al, 2000], 0.05 mmol N/m 3 for DON and 0.005 mmol N/m 3 for PON were assigned to each depth interval from the surface to the last active layer. Concentrations for mesozooplankton and microzooplankton were set to 0.4 mmol N/m 3 [e.g., Sime-Ngando et al, 1995;Roy et al, 2000;Savenkoff et al, 2000] in the upper 25 m and to 0 below this depth.…”

“…In order to improve our capability to predict these responses, we need to develop models that reproduce the spatiotemporal variability of the primary and secondary production cycles. Modeling of planktonic production in the St. Lawrence marine system has been limited to one-dimensional (1-D) models of the carbon cycle in the northwestern [Tian et al, 2000] and northeastern [Tian et al, 2001] GSL, to a 2-D modeling study of the phytoplankton production in the Lower Estuary [Zakardjian et al, 2000], and to 3-D modeling of copepods population dynamics [Zakardjian et al, 2003]. This paper aims at describing and quantifying the circulation-planktonic production coupling in the GSL using a detailed 3-D physical-biological model.…”