The spatial patterns and patchiness exhibited by phytoplankton are important to the trophodynamics of the marine ecosystem. Concentrated patches and layers of plankton found within the coastal thermocline are thought to have particular ecological significance, as they may host elevated rates of growth, grazing, and microbial degradation. Using field observations and models, the role of turbulent mixing and shear-flow dispersion in the development of plankton aggregations was examined. First, two method studies utilizing Particle Image Velocimetry and temperature microstructure techniques were conducted to improve the characterization of in situ turbulence. The temperature microstructure method was then used to investigate the temporal and vertical structure of turbulence and mixing in phytoplankton thin layers in Monterey Bay, California, and the Gulf of Aqaba, Red Sea. In Monterey Bay, the turbulence observations coupled with advection-diffusion models show that swimming by motile dinoflagellates is a plausible mechanism for layer formation, while in the Gulf of Aqaba the observations indicate that the layers likely formed in horizontal intrusions along the adjacent coastal region. The observations also demonstrate that temporal and spatial variations in turbulent mixing around the thermocline regulate the vertical gradients of chlorophyll a in layers and control layer occurrence and persistence. Lastly, the dynamics of shear-flow dispersion in internal waves was investigated using a particle-tracking model. For idealized oscillatory, sinusoidal horizontal velocity profiles, the irreversible dispersion is maximal when the time required for particles to sample the velocity deviations via vertical diffusion is comparable to the oscillation period of the shear. For observed flows from the Gulf of Aqaba, the total dispersion and irreversible dispersion of modeled particle clouds both increase with the rate of vertical diffusion, cloud height, and cloud age. The estimates of irreversible dispersivity based on the particle-tracking approach also agree remarkably well with estimates from an existing analytical model for the dispersion of a point source in a time-varying, linear velocity profile.
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