The evolution of a thin phytoplankton layer in strong turbulence

Simultaneous and collocated spatial measurements of turbulence, fine structure, and chlorophyll a fields were made from the Autonomous Underwater Vehicle T-REMUS as a part of the Layered Organization in the Coastal Ocean (LOCO) experiment. The T-REMUS was operated in a cycling 5° yo-yo mode. Deployments were of 8 h duration and consisted of a series of across isobath legs, each of 2.5 km in extent. From the suite of sensors onboard the T-REMUS vehicle we are able to measure directly the turbulent eddy velocity, w e = ε / N , and turbulent Reynolds number, Reb=(ε/vN2), in the vicinity of a thin chlorophyll a layer. Using the turbulent eddy velocity, we develop criteria for when phytoplankton will behave as passive Lagrangian tracers. The turbulent Reynolds number is used as an indicator of turbulence strength, with the criterion of Reb=200 the boundary between weak and strong turbulence. We present data for the case of a spatially extensive thin layer being advected into the T-REMUS LOCO site. Using the above criterion, we observe that thin chlorophyll a layers can exist and be embedded as passive Lagrangian tracers in both weak and strong turbulent conditions. During a time period of weak turbulence Reb<200 little diffusion occurs and the thin layer remains compact. Under strong turbulence Reb>200 the thin layer weakens and diffuses as it is advected through the experimental site.