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 slolvly rotating impellers on a cycle of 4 h on and 2 h off to match the sernidiurnal 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 withln 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 diffusivitv 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 densit~es. Chlorophyll a dynamlcs and phytoplankton group composition, as assessed with accessory pigment concentrations, also exhibited modest differences among mixing treatments. Mixing had negligible effects on nutnent concentrations and on community and whole-system productivity and respiration. lmportant 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 thls whole-ecosystem context it was difficult to distinguish direct from indirect effects of mixing. Nevertheless, our results imply that ecosystem-level processes in planktonic systenls 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.

Although experimental ecosystems a r r bdsic and versatile tools w~d e l y used In coastal research periphyt~c glo\vth on container walls IS an ~n t n n s l c artifact that must be considered when lnterprctlng results To bettel understand holv this a1 tifact may confound extrapoldtion of results f~o m controlled exper~ments to conditions In natural estudr~ne ecosystems w e examined ho\v wall perlphyton varied wlth container size and shape in summer and dutunin expenments Kepllcate ( n = 3) cvlind11ca1 mesocosms ot 3 volumes (0 1 1 0 10 m') werr establ~shed In both constant-depth (depth -1 m) and conbtant-shape (radiuddepth = 0 56) serles \ l e~o c o s n~s were in~tlated with unfiltered estudrine water and homogenized s e d~m e n t s Pel~phyton b~ornass and gross primary production (GPP) per unlt of wall area ~n c l e a s e d wlth incredslng r a d~u s ( I ) or decreasing ratio of \vall area (A,,,) to water volume ( V ) for mesocos~ns In both s e r~e s (A,,/V = 2/r) As a consequence per~phyton biornass and metabol~sm expressed per u n~t of water volume Increased as a quadratic functlon of increasing A,, / V ratlo Results also suggest a secondary s c a l~n g effect whereby wall perlphyton qrowth may he directly reldted to mesocosm depth although mechanisms for th15 effect I enialn uncledr Slgnlflcant correlations between perlphyton biomass (per m2 wall area) and 3 rnv~ronmental f a c t o~s (11ght a t t r n u a t~o n c o e f f~c~e n t nutnent concentration and zooplankton dbundance) suqqest that these factors may have played Important roles In r e g u l a t~n g wdll grolvth Add~tlonally, effects of wall penphyton growth on plankton community dynamics were also indicated by the signlflcant n e g a t~v e relat~ons between penphyton biomass and measures of both phytoplankton and zooplankton abundance The overall effect of periphyton on the e\perlmental ecosystems was evldent In the fact that perlphyton accounted for over 5 0 " ) of total ecosystem GPP and b~o m a s s after 2 to 4 ~v k of these expenments For mesocosm experimc.nts d e s~g n e d to examlne dyndmics of planktonic-benth~c ecosystems, our ~e s u l t s lmply that growth of wall periphvton which 1s controlled b\l fact015 s c a l~n g to the ~a d i u s of expenmental ecosystems tends to domintlte major b~otic pools and ratcs withln weeks