MEPS 171:23-41 (1998)  -  doi:10.3354/meps171023

Coastal plankton responses to turbulent mixing in experimental ecosystems

John E. Petersen*, Lawrence P. Sanford, W. Michael Kemp

Horn Point Laboratory, University of Maryland Center for Environmental Science, Cambridge, Maryland 21613, USA

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 provided by 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 mixing in 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 by slowly 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 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 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 zooplankton abundance 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 had negligible 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 mixing processes, 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 may often 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.

KEY WORDS: Mixing · Turbulence · Plankton · Copepod · Mesocosm · Production · Respiration · Auxiliary energy

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