Effect of mixing on the structure of a natural plankton community: a mesocosm study

A plankton community (< 202 mu m) from the St. Lawrence Estuary was isolated in four outdoor mesocosms with SLOW and fast mixing regimes. Variations in the concentrations of nutrients, chlorophyll a (Chl a), nitrogen transport rates and plankton species composition were monitored over a 10 day period. The vertical mixing times (T-m) for the slow and fast mixing regimes were 180 and 60 min, corresponding to a vertical eddy diffusivity (K-v) of 2.34 and 7.03 cm(2) s(-1), respectively. The different mixing regimes had a strong effect on the physiology of the phytoplankton and the specific structure of the plankton assemblage. The Slow mixing regime stimulated the development of a mixed community of flagellates, small diatoms and proto-metazooplankton while the fast mixing regime triggered the development of a large diatom-dominated community with lower abundances of proto-metazooplankton. At the end of the 10 day experiment, the Chl a concentrations were 50 % higher in the mesocosms with the fast mixing regime than in those with the slow mixing regime. These results indicate that, under low nutrient conditions, higher turbulence gives a competitive advantage to diatoms and decreases the zooplankton grazing pressure, resulting in net positive growth. Extrapolation of these results to natural systems suggests that a wind-driven mixing event may increase the net phytoplankton biomass production of a stratified water column, even if there is no external input of nutrients.