Turbulent mixing in fine-scale phytoplankton layers: Observations and inferences of layer dynamics

Abstract

Turbulence measurements in fine-scale phytoplankton layers (∼1 to ∼10 m) in the Gulf of Aqaba (Red Sea) were used to evaluate mechanisms of layer formation, maintenance, and breakdown. Simultaneous profiles of chlorophyll a (Chl a) fluorescence and temperature microstructure were measured in the upper 40 m of a 430 m water column over a 16-d period, using a Self Contained Autonomous MicroProfiler (SCAMP). Layers of concentrated phytoplankton were identified in 95 of the 456 profiles. The layers were situated in density stratified regions between 15 and 38 m depth and were characterized by intensities of 0.1 to 0.35 μg Chl a L⁻¹ (as much as two times background concentrations) and an average thickness of 10 m. We show that turbulent mixing and isopycnal displacements associated with internal waves modulated the thickness of the layers. Variations in mixing rates within layers were connected to the vertical structure of the stratified turbulence and the stage of layer development. The breakdown of a persistent phytoplankton layer was tied to strong turbulent mixing at the base of the surface mixed layer, which encroached on the layer from above. Hydrographic observations and scaling analysis suggest that the layers most likely formed in horizontal intrusions from the adjacent coastal region. The cross-shore propagation of phytoplankton-rich intrusions may have important implications for the trophic state of offshore planktonic communities.

Introduction

There has been a recent surge of interest in planktonic layers in the coastal ocean with vertical scales ranging from the fine-scale (∼1 to ∼10 m) to the microscale (<∼1 m). These features have anomalously high concentrations of plankton compared to vertically adjacent portions of the water column and may span kilometers in the horizontal (Dekshenieks et al., 2001). This small-scale vertical organization of plankton may have important ecological implications, including effects on behavior, reproduction, growth, predation, remineralization, and community composition (Cowles et al., 1998; McManus et al., 2003). Technological advancements in ocean optics and acoustics have permitted detailed descriptions of thin planktonic layers (Cowles et al., 1993; Holliday et al., 2003; Twardowski et al., 1999).

Several physical and biological processes have been proposed to explain the formation of fine- and microscale plankton layers (sometimes described as thin layers). Nielsen et al. (1990) documented the role of in situ growth in the development of a thin, mid-column phytoplankton layer. Sullivan et al. (2010) observed phytoplankton thin layers form via diurnal vertical migration. Kononen et al. (2003) found both migratory behavior and growth to be important in explaining relatively thin deep chlorophyll maximum (DCM) layers. Alldredge et al. (2002) observed a thin layer of marine snow form via particle settling at a depth of neutral density. Franks (1995) proposed a shear-induced straining mechanism for phytoplankton thin layer formation, further developed by Stacey et al. (2007) and Birch et al. (2008) and supported in field observations (Ryan et al., 2008). Bo Pederson (1994) linked observations of phytoplankton layers to intrusions generated by boundary mixing in shallow tidal flows.

The vertical structure and development of planktonic layers are also affected by the interplay of turbulence and density stratification. Dekshenieks et al. (2001) found phytoplankton thin layers only in regions of the water column with Richardson numbers, Ri, greater than the critical value of 1/4, suggesting that the stratification was sufficiently strong to suppress shear instabilities. McManus et al. (2003) observed a thin layer associated with buoyancy Reynolds numbers, Re_b, typically less than 15, indicating that the turbulence was dominated by stratification and likely insufficient to diffuse the layer through vertical mixing. Using scaling analysis, Stacey et al. (2007) showed that straining or particle buoyancy might have maintained thin layers of phytoplankton in the presence of turbulent diffusion in East Sound, Washington.

In this paper, we present observations of turbulence in fine-scale phytoplankton layers that occur in the upper 40 m of the Gulf of Aqaba, Red Sea. Scaling analysis and hydrographic observations suggest that the layers most likely formed in horizontal intrusions from the adjacent coastal region. The detailed physical measurements highlight the role played by stratified turbulence, advection, and internal waves in regulating layer dynamics and development.