Abstract
A plankton community (< 202 μm) from the St. Lawrence Estuary was isolatedin four outdoor mesocosms with SLOW and fast mixing regimes. Variations in the concentrationsof nutrients, chlorophyll a (Chl a), nitrogen transport rates and plankton species compositionwere monitored over a 10 day period. The vertical mixing times (Tm) for the slow andfast mixing regimes were 180 and 60 min, corresponding to a vertical eddy diffusivity (Kv) of2.34 and 7.03 cm2 s–1, respectively. The different mixing regimes had a strong effect on thephysiology of the phytoplankton and the specific structure of the plankton assemblage. TheSlow mixing regime stimulated the development of a mixed community of flagellates, smalldiatoms and proto-metazooplankton while the fast mixing regime triggered the developmentof a large diatom-dominated community with lower abundances of proto-metazooplankton. Atthe end of the 10 day experiment, the Chl a concentrations were 50 % higher in the mesocosmswith the fast mixing regime than in those with the slow mixing regime. These results indicatethat, under low nutrient conditions, higher turbulence gives a competitive advantage to diatomsand decreases the zooplankton grazing pressure, resulting in net positive growth. Extrapolationof these results to natural systems suggests that a wind-driven mixing event may increasethe net phytoplankton biomass production of a stratified water column, even if there is no externalinput of nutrients.