MEPS 374:57-74 (2009)  -  DOI: https://doi.org/10.3354/meps07738

ABSTRACT: We examined the role of turbulence on the evolution of the spatial structure of a thin phytoplankton layer. The approach used the small autonomous underwater vehicle (AUV), T-REMUS (turbulence-remote environmental measuring units), which is equipped with optical and physical micro- and fine-scale sensors. As part of the Layered Organization in the Coastal Ocean (LOCO) experiment, T-REMUS was deployed in a very shallow region of Monterey Bay, California, USA, over an 8 h nighttime period in summer 2006. A thin layer of chlorophyll a (chl a) was observed throughout the entire experimental period. The center of the thin layer deepened with time, crossed isotherms, and then settled into a strong turbulence layer. This result is in sharp contrast to previous conclusions that biological thin layers only occur in regions of weak turbulence. Our observations indicated that the turbulence field itself was constrained to be in a thin layer by the surrounding strong density stratification. The chl a material, acting as a passive Lagrangian tracer, became embedded within the turbulent field. With time, both the turbulent field and the embedded chl a thin layer were observed to collapse vertically.

KEY WORDS: Thin layers · Turbulent mixing · Chlorophyll a · AUV · Monterey Bay · Sinking · Vertical dispersion and contraction