MEPS 192:31-48 (2000)  -  doi:10.3354/meps192031

ABSTRACT: In the Lower St. Lawrence Estuary (LSLE), environmental conditions (stratification, surface light and nutrients) 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 mixing, and temperature 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 mixing and (2) flushing of freshwater runoff on primary 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 may 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, chemical and biological sampling field experiment performed 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^-1, close to observed values, generate downstream patterns of phytoplankton biomass that resemble 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.

KEY WORDS: Phytoplankton bloom · Physical-biological coupling · Vertical turbulent mixing · Sinking · Flushing rates · St. Lawrence Estuary