COASTAL PLANKTON RESPONSES TO TURBULENT MIXING IN EXPERIMENTAL ECOSYSTEMS

Turbulent mixing is increasingly implicated as a key factor regulating ecological dynamics in coastal planktonic systems. Although photosynt hesis is directly fueled by light energy, it has been hypothesized tha t the 'auxiliary' energy provided by mixing can subsidize or control e cosystem function. Unrealistic mixing has also been cited as one expla nation for difficulties in reproducing natural plankton dynamics in en closed experimental ecosystems (mesocosms). To explore the importance of mixing in shallow planktonic ecosystems, we traced changes over a 4 wk period in population, community, and ecosystem level properties in replicate 1 m(3) experimental ecosystems subjected to different mixin g regimes. Mixing energy was delivered by slowly rotating impellers on a cycle of 4 h on and 2 h off to match the semidiurnal pattern of tid al mixing that characterizes many temperate estuaries. Three mixing le vels were generated by altering impeller rotation rates. The intermedi ate level was scaled 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 6x range in t urbulence intensity, a Sr range in the surface-bottom mixing time and eddy diffusivity coefficients, and a 230x range in turbulent energy di ssipation rates. Mixing had a significant negative effect on copepod a nd gelatinous zooplankton abundance and also altered the timing of pea k copepod densities. Chlorophyll a dynamics and phytoplankton group co mposition, as assessed with accessory pigment concentrations, also exh ibited modest differences among mixing treatments, Mixing had negligib le effects on nutrient concentrations and on community and whole-syste m productivity and respiration. Important caveats in interpreting the results of this experiment are that system size excluded observation o f the effects of large-scale mixing processes, trophic complexity was limited (e.g. no fish), and in this whole-ecosystem context it was dif ficult to distinguish direct from indirect effects of mixing. Neverthe less, our results imply that ecosystem-level processes in planktonic s ystems may often be less sensitive to differences in small-scale turbu lence than population and community dynamics, and also that mixing eff ects may be strongly dependent on the specific structure of particular ecosystems.