Breaking ocean waves play an important role in near-surface processes, including mixing, current generation, and enhanced heat, mass and momentum flux. Breaking waves also entrain bubbles that enhance air-sea gas flux, produce aerosols, generate ambient noise and scavenge biological surfactants.

Fluid turbulence is a key factor in these processes, but is difficult to measure in the transient, two-phase flow inherent to wave breaking. Although measurements of turbulence have been made with PIV and LDV techniques outside the spatial region of air entrainment, a method that allows both high spatial and temporal resolution of flow dynamics inside the breaking wave crest is not available. Turbulence measurements inside the wave crest are particularly important for understanding the processes controlling air entrainment .

Recent studies have demonstrated the use these dinoflagellates to visualize flow fields within Couette and turbulent pipe flow and naturally occurring dinoflagellate bioluminescence has been used to visualize flow separation on a gliding dolphin and the laminar to turbulent transition on a sphere. Dinoflagellate flashes can be used either as markers to illuminate fluid streak lines in complex flows (PIV) or, as we are proposing here, as quantitative indicators of supra-threshold flow agitation. The successful application of this approach to breaking wave dynamics will provide time-evolving estimates of fluid shear stress inside a breaking wave crest on O(ms) time scales and O(mm) length scales.

The IMT Laboratory has conducted experiments measuring the heterogeneous, time-varying shear stress inside a breaking wave using bioluminescent dinoflagellates, Pyrocystis fusiformis as numerous and tiny biological sensors responsive to fluid shear stress.

Dinoflagellate Bioluminescence as a Flow Visualization Tool

Bioluminescence is one of the most cosmopolitan organism behaviors in the marine environment. In coastal waters, where opportunities to observe flow-stimulated bioluminescence are typically best. Tthe primary sources of bioluminescence are unicellular plankton called dinoflagellates. It has been postulated that concentrations > 100 dinoflagellates L -1 are sufficient to highlight moving objects and that visual predators use flow-stimulated bioluminescence at night to locate their prey. Studies of coastal bioluminescence, red tides, and the movement of dolphins, fish, divers and torpedoes have recorded abundances of bioluminescent dinoflagellates.

Dinoflagellate flashes can be used individually as markers to illuminate path lines in complex flows or as point indicators of suprathreshold flow agitation. Different species of dinoflagellates offer a range of size, flow thresholds, flash brightness, and flash duration.

In order to effectively use flow-induced dinoflagellate bioluminescence as a method of flow visualization, one must know the species, concentration, luminescent response characteristics of the organisms to quantifiable levels of flow stimulation, and have suitable low background light levels and appropriate imaging capabilities.

Overall, flow-stimulated bioluminescence is consistent for three independent flow fields: Couette flow, fully developed pipe flow, and converging nozzle flow. Not only are response thresholds consistent, as previously described, but for all pipe flow experiments maximum bioluminescence, which is associated with individual flash intensity, is relatively constant for wall shear stress values greater than about 10 dyn cm -2 , regardless of whether the flow is laminar or turbulent.