Observations of turbulent mixing in a phytoplankton thin layer: Implications for formation, maintenance, and breakdown

Abstract

Coincident measurements of chlorophyll a (Chl a) and temperature microstructure from Monterey Bay, California, U.S.A., are used to investigate the dynamics of a phytoplankton thin layer in the context of vertical turbulent mixing. The thin layer was situated in the thermocline of a 20.5-m water column, between a strongly turbulent surface mixed layer and a weakly turbulent stratified interior. The differential mixing established an asymmetric layer with a stronger Chl a gradient at the base of the layer than at the top. Sinking by Akashiwo sanguinea could have balanced turbulent mixing above the layer, but cannot explain the mid-column convergence needed to maintain the thin layer. Swimming by A. sanguinea directed towards a mid-column nitracline could have balanced turbulent mixing on both sides of the layer and supported the observed Chl a gradients. Results from a Eulerian advection-diffusion model suggest that cell swimming may have varied over the lifetime of the layer, with stronger or more directed swimming required for layer formation and weaker or less directed swimming needed to maintain the layer. In-layer growth is unlikely to have played a primary role in supporting the observed Chl a gradients. From the one-dimensional (vertical) observations, the role of shear-induced straining cannot be evaluated, but shear instabilities appear important to forcing elevated rates of turbulent diffusion within the layer. Finally, a Lagrangian particle-tracking model is used to explore the implications of a balance of vertical velocity and turbulent diffusion on the vertical motions of individual cells that constitute the thin layer.

Turbulent, stratified flow through a suspended shellfish canopy: implications for mussel farm design

“…This is different to the thin layers process in that these peaks were transient and likely associated with individuals rather than a coherent population constrained by stratification (e.g. Steinbuck et al 2009). The fluorescence profiles look quite similar qualitatively (Fig.…”

“…This has some relevance to the thin layers phenomenon, whereby populations are concentrated into narrow vertical bands (e.g. Steinbuck et al 2009),…”

Layered organization in the coastal ocean: An introduction to planktonic thin layers and the LOCO project

“…In many cases, the processes responsible for main taining the layer could also be responsible for forming it, but the timescale for layer development may dictate that other processes or conditions must be present during formation. An example was discussed in Steinbuck et al (2009), in which it was found that a high dinoflagellate swimming speed was required during layer formation to overcome turbulent mixing and to match the observed formation time, but a reduced swimming speed was required to actually maintain the layer. Alternatively, variation in physical conditions (density, shear or turbulent mixing) may also allow layers to form more rapidly than would be expected based on conditions during the maintenance of the layer.…”

Stratification and mixing regimes in biological thin layers over the Mid‐Atlantic Bight

“…Temporal variability in these physical properties is also clearly indicated over time periods much shorter than the lifetime of the layers. Such variability is common in many studies that detail turbulent mixing at high resolution, e.g., the previous thin-layer studies presented by Wang and Goodman (2010) and Steinbuck et al (2010). Resolving both mixing and gradients at these scales is key to interpreting the details of both ''diffusing'' plankton and ''replenishing'' nutrients in layers.…”

“…Thin layers have regularly been observed in regions of enhanced stratification, which can provide a ''shelf'' upon which organisms can passively settle at their neutral density and may also act to counter shear-driven instability (Dekshenieks et al 2001;Rines et al 2002;Steinbuck et al 2009). Previous studies have indicated a more ambiguous relationship with shear in that thin layers have been observed in regions of both weak (Dekshenieks et al 2001;McManus et al 2005) and strong shear (Ryan et al 2008;Johnston et al 2009;Steinbuck et al 2009). Shear not only acts as a divergent mechanism by providing an energy source for turbulence Birch et al 2008), but can also cause thinning through straining of preexisting patches (Franks 1995;Stacey et al 2007;Birch et al 2008).…”