EFFECTS OF TURBULENCE INTERMITTENCY ON GROWTH-INHIBITION OF A RED TIDE DINOFLAGELLATE, GONYAULAX-POLYEDRA STEIN

Thomas and Gibson (1990a, b) quantified the rate of strain gamma requi red to inhibit the growth of the red tide dinoflagellate Gonyaulax pol yedra Stein in a Couette flow device that produces an effectively cons tant gamma value to represent the small-scale shear encountered by din oflagellates in turbulent flow. A continuous gamma value of about 8 ra d s(-1) was found to cause zero-growth rates, >8 caused negative growt h, and <2 caused no effect. Measurements of the same organism in the s ame device with intermittent gamma show that the daily average gamma t hreshold for zero growth <(gamma)over bar>(GI) does not require contin uous shear but decreases to Values smaller than 0.09 rad s(-1) with in creasing intermittency (decreasing fraction of the time of shear appli cation) up to a limiting minimum time period of 5-15 min. When gamma v alues of 9 rad s(-1) were applied for periods of only 2.5, 1, and 0.25 hours during a day, increasingly negative growth rates were observed (although 5 min had no effect). Thus <(gamma)over bar>(GI) was reduced by 2 orders of magnitude by intermittency of the simulated turbulence . Because turbulence near the sea surface is intermittent especially w hen waves are breaking, the effects on dinoflagellate growth inhibitio n will be larger and extend to much greater depths than might be expec ted from only the <(gamma)over bar> profile. Comparison with the lagge d correlations of wind wave spectral components versus oceanic phytopl ankton growth (Tynan, 1993), showing larger negative correlations betw een dinoflagellate concentration and waves than with winds, suggests a similar response exists between dinoflagellate growth inhibition in n atural populations and turbulence intermittency and strength. Diatoms exhibit the opposite response, with growth enhancement.