ABSTRACT: Turbulent mixing is increasingly implicated as a key factor regulating ecological dynamics in coastal planktonic systems. Although photosynthesis is directly fueled by light energy, it has been hypothesized that the 'auxiliary' energy providedby mixing can subsidize or control ecosystem function. Unrealistic mixing has also been cited as one explanation for difficulties in reproducing natural plankton dynamics in enclosed experimental ecosystems (mesocosms). To explore the importance of mixingin shallow planktonic ecosystems, we traced changes over a 4 wk period in population, community, and ecosystem level properties in replicate 1 m³ experimental ecosystems subjected to different mixing regimes. Mixing energy was delivered byslowly rotating impellers on a cycle of 4 h on and 2 h off to match the semidiurnal pattern of tidal mixing that characterizes many temperate estuaries. Three mixing levels were generated by altering impeller rotation rates. The intermediate level wasscaled to match typical mixing intensities of waters in Chesapeake Bay, the low mixing level approximated calm oceanic surface waters, and the high mixing level approximated the environment within a tidal front. High and low mixing levels encompassed a 6xrange in turbulence intensity, a 9x range in the surface-bottom mixing time and eddy diffusivity coefficients, and a 230x range in turbulent energy dissipation rates. Mixing had a significant negative effect on copepod and gelatinous zooplanktonabundance and also altered the timing of peak copepod densities. Chlorophyll a dynamics and phytoplankton group composition, as assessed with accessory pigment concentrations, also exhibited modest differences among mixing treatments. Mixing hadnegligible effects on nutrient concentrations and on community and whole-system productivity and respiration. Important caveats in interpreting the results of this experiment are that system size excluded observation of the effects of large-scale mixingprocesses, trophic complexity was limited (e.g. no fish), and in this whole-ecosystem context it was difficult to distinguish direct from indirect effects of mixing. Nevertheless, our results imply that ecosystem-level processes in planktonic systems mayoften be less sensitive to differences in small-scale turbulence than population and community dynamics, and also that mixing effects may be strongly dependent on the specific structure of particular ecosystems.