Luminescent response of the red tide dinoflagellate Lingulodinium polyedrum to laminar and turbulent flow

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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 (= 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 m−2. 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 m−2, 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 bio‐luminescence 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 m−2, 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.