Limnol. Oceanogr., 44(6), 1999, 1423–1435

While it is universally accepted that plankton continually experience a dynamic fluid environment, their sensitivity to the features of the surrounding flow field at the relevant length and time scales of the organism is poorly characterized. The present study uses bioluminescence as a tool to understand how the red tide dinoflagellate Lingulodinium polyedrum (5 Gonyaulax polyedra) responds to well-characterized hydrodynamic forces present in fully developed laminar and turbulent pipe flow. The response of L. polyedrum to hydrodynamic stimulation was best characterized by wall shear stress; at similar values of wall shear stress, the response was similar for laminar and turbulent flows. The response threshold occurred in laminar flow at a wall shear stress of approximately 0.3 N m22. At these low flow rates, video analysis of the velocity of flash trajectories revealed that responding cells were positioned only near the pipe wall, where local shear stress levels were equal to or greater than threshold. For cell concentrations ranging over four orders of magnitude, threshold values of wall shear stress were restricted to a narrow range, consistent with an antipredation function for dinoflagellate bioluminescence. For laminar flows with above-threshold wall shear stress values # 1 N m22, mean bioluminescence increased with wall shear stress according to a power (log-log) relationship, with the slope of the power function dependent on cell concentration. The increase in bioluminescence within this range was due primarily to an increasing population response rate and, to a lesser extent, an increase in maximum flash intensity per cell and the increased flux of organisms with higher flow rates. For wall shear stress levels . 1 N m22, the maximum intensity per cell remained approximately constant with increasing wall shear stress, even as the flow transitioned from laminar to turbulent, and the smallest turbulent length scales became less than the average cell size. All plankton experience a dynamic fluid environment due to the effects of wind, waves, tides, and currents. For phytoplankton, turbulence regulates the vertical transport of cells (Lewis et al. 1984; Cowles and Desiderio 1993), which affects primary production through changes in the average level of incident illumination (reviewed by Kiørboe 1993), the sedimentation rate of cells from the mixed layer (Ruiz et al. 1996), the coagulation of cells caused by shear-induced collisions between suspended phytoplankton (Jackson 1990), and the encounter rate of phytoplankton to grazers and microbes (Bowen et al. 1993). It also relaxes diffusion limitation of nutrient uptake, which can lead to enhanced growth (Pasciak and Gavis 1975; reviewed by Kiørboe 1993 and Karp-Boss et al. 1996). Turbulence can negatively affect phytoplankton growth and cell division (Thomas and Gibson 1990a, 1995; Berdalet 1992; reviewed by Estrada and Berdalet 1997) and alter cell motility (Thomas and Gibson 1990a). The present study applies bioluminescence as a tool to characterize how dinoflagellates, one of the most shear-sensitive groups of marine phytoplankton (Thomas and Gibson