Quantified small-scale turbulence inhibits a red tide dinoflagellate, Gonyaulax polyedra Stein

Abstract

The development of marine dinoflagellate red tides off southern California requires optimal temperature and light regimes, a source of nutrients that may be supplied by wind-induced upwelling and upmixing, and vertical migration by cells to this source. Red tides occur after the winds decrease and the water becomes highly stratified with a shallow mixed layer. This implies low turbulence levels may be an additional requirement for red tide development. Because dinoflagellates with sizes about 35 μm are much smaller than the inertial-viscous, or Kolmogorov scales L κ ≡ (ν 3/ϵ) 1 4 = (ν/γ) 1 2 of oceanic turbulence, the important flow parameters are the viscous dissipation rate per unit mass ϵ (cm 2s −3 or ergs g −1s −1), the rate-of-strain γ ≡ (ϵ/ν) 1 2 ( rad s −1) , and the stress τ ≡ μγ (dyne cm −2), where ν is the kinematic viscosity and μ is the dynamic viscosity. In the present work we have cultured the red tide dinoflagellate, Gonyaulax polyedra Stein, under conditions of known ϵ, γ and τ. Growth was inhibited at ϵ values from 0.18 to 164 cm 2 s −3 (γ from 4.4 to 132 rad s −1) but not at 0.05 ( γ = 2.2) so the threshold stress τ for growth inhibition was 0.02–0.04 dyne cm −2 (0.002–0.004 Pa). This is in the expected range for light winds at the sea surface, suggesting small-scale scale turbulence is the reason higher winds inhibit red tides. Threshold turbulence levels are related to calculated surface levels at various wind speeds and to postulated subsurface euphotic zone levels under incipient red tide conditions. Motile cells in shear-inhibited cultures lost their ability to swim forward vigorously but rather spun in place due to the loss of longitudinal (trailing) flagella, observed microscopically, without loss of girdle flagella.