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

In the Lower St. Lawrence Estuary (LSLE), environmental conditions (stratif ication, surface light and nutrients) are favorable for phytoplankton growt h starting in May, but the spring phytoplankton bloom typically does not oc cur 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 p hytoplankton growth rates. We use 1- and 2-D time-dependent models of phyto plankton dynamics to explore these hypotheses. In particular, we illustrate the role of (1) phytoplankton cell sinking versus vertical turbulent mixin g and (2) flushing of freshwater runoff on primary production in the LSLE. Results of the 1-D simulations show the dramatic effect of phytoplankton ce ll sinking in a thin euphotic zone, while at the same time high vertical tu rbulent mixing may act to maintain these sinking phytoplankton cells in the euphotic layer. Nevertheless, the 1-D analysis cannot account for spatio-t emporal patterns in the development of the phytoplankton bloom observed dur ing a high resolution physical, chemical and biological sampling field expe riment performed in the summer of 1990 in the LSLE. 2-D simulations, run wi th 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 sp ecify the range of sinking and advective velocities that operate in concert to control the timing and spatial location of the bloom.