Small-scale turbulence affects the division rate and morphology of two red-tide dinoflagellates

Abstract

The effects of small-scale turbulence on two species of dinoflagellates were examined in cultures where the turbulent forces came randomly from all directions and were intermittent both spatially and temporally; much like small-scale turbulence in the ocean. With Lingulodinium polyedrum (Stein) Dodge (syn. Gonyaulax polyedra), division rate increased linearly (from ∼0.35 to 0.5 per day) and the mean cross-sectional area (CSA) decreased linearly (from ∼1100 to 750 μm 2) as a function of the logarithmic increase in turbulence energy dissipation rate ( ε). These effects were noted when ε values increased between ∼10 −8 and 10 −4 m 2 s −3. However, when ε increased to ∼10 −3 m 2 s −3, division rate sharply decreased and mean CSA increased. Over the same range of ε, Alexandrium catenella (Wheedon and Kofoid) Balech had its division rate decrease linearly (from ∼0.6 to 0.45 per day) and its CSA increase linearly (from ∼560 to 650 μm 2) as a function of the logarithmic increase in ε. Even at the highest ε examined (∼10 −3 m 2 s −3), which may be unrealistically high for their ambits, both L. polyedra and A. catenella still had fairly high division rates, ∼0.2 and 0.45 per day, respectively. Turbulence strongly affected chain formation in A. catenella. In non-turbulent cultures, the mode was single cells (80–90% of the population), but at ε of ∼10 −5 to 10 −4 m 2 s −3, the mode was 8 cells per chain. At the highest ε (∼10 −3 m 2 s −3), the mode decreased to 4 cells per chain. The vertical distributions of A. catenella populations in relation to hydrographic flow fields were studied in the summers of 1997 and 1998 in East Sound, Washington, USA (latitude 48°39′N, 122°53′W). In both summers, high concentrations of A. catenella were found as a subsurface bloom in a narrow depth interval (∼2 m), where both current shear and turbulence intensity were at a minimum. Other researchers have shown that A. catenella orients its swimming in shear flows, and that swimming speed increases with chain length. These responses, when combined with our observations, support a hypothesis that A. catenella actively concentrates at depths with low turbulence and shear.