Turbulent mixing in fine-scale phytoplankton layers: Observations and inferences of layer dynamics
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, Reb, 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.
Section snippets
Study site
The Gulf of Aqaba is a desert-bordered, semi-enclosed gulf located at the northeast end of the Red Sea. The gulf is approximately 180 km long, 6–25 km wide, and has a maximum depth of 1820 m. The sampling station (29.492°N, 34.929°E) was located approximately 1.6 km offshore of the Steinitz Marine Laboratory at the northern end of the gulf and had a local depth of approximately 430 m (Fig. 1). The temperature and salinity of summer surface waters are near 27 °C and 41 psu, respectively (Paldor and
Hydrographic setting
Over the study period, the vertical structure of temperature, salinity, irradiance, nitrate, phosphate, and Chl a were relatively stationary for the water column as a whole, as seen in profiles collected by the Israel NMP south of the study site (Fig. 2). This vertical structure is consistent with previous studies (e.g., Lindell and Post, 1995) and is similar to that observed at other NMP stations in the northern gulf over the same period. Typical of late summer conditions, the upper water
Inferences of formation and maintenance
We now examine the formation and maintenance of all observed phytoplankton layers. The scaling approach of Stacey et al. (2007) is applied to assess the role of straining, swimming, and cell buoyancy. Horizontal intrusions and in-layer growth are evaluated subsequently.
Layer formation via intrusions
A number of previous studies have examined the role of coastal intrusions in supporting phytoplankton layers. For example, Simpson et al. (1982) developed a model of intrusion formation resulting from vertical mixing in the tidal flows around islands. The resulting nutrient-rich intrusions that may result was proposed as a mechanism to explain elevated phytoplankton concentrations in the pycnocline around the Scilly Isles in the Celtic Sea. Bo Pederson (1994) developed a similar intrusion
Acknowledgments
We thank M. Ohevia, M. Stacey, L. Walter, the Israel NMP, and volunteers from the IUI. We thank the reviewers for insightful comments that greatly improved the quality of the manuscript. The authors gratefully acknowledge support from the US–Israel Binational Science Foundation (2004-264) and the NATO Science for Peace Program (SFP 98220). J.V.S. acknowledges support from a NDSEG fellowship and EPA STAR fellowship.
References (55)
- et al.
Simulation of wind-driven circulation in the Gulf of Elat (Aqaba)
J. Mar. Systems
(2000) - et al.
Thin layers of plankton: formation by shear and death by diffusion
Deep-Sea Res. I
(2008) Thin layers of phytoplankton: a model of formation by near-inertial wave shear
Deep-Sea Res. I
(1995)- et al.
Advances in defining fine- and micro-scale pattern in marine plankton
Aquat. Living Res.
(2003) - et al.
Observations of intermediate nepheloid layers on the northern California continental margin
Cont. Shelf Res.
(2004) - et al.
Seasonal variations of temperature and salinity in the Gulf of Elat
Deep-Sea Res.
(1979) - et al.
Mixing and phytoplankton growth around an island in a stratified sea
Cont. Shelf Res.
(1982) - et al.
Occurrence and mechanisms of formation of a dramatic thin layer of marine snow in a shallow Pacific fjord
Mar. Ecol. Prog. Ser.
(2002) - et al.
Seasonal dynamics of phytoplankton in the Gulf of Aqaba, Red Sea
Hydrobiologia
(2006) - et al.
Phytoplankton drives nitrite dynamics in the Gulf of Aqaba, Red Sea
Mar. Ecol. Prog. Ser.
(2002)
Small-scale variation of convected quantities like temperature in turbulent fluid
J. Fluid Mech.
The seasonality of tidal circulation in the Gulf of Elat
Israel J. Earth Sci.
Response of the microbial food web to manipulation of nutrients and grazers in the oligotrophic Gulf of Aqaba and northern Red Sea
Mar. Biol.
Elemental composition of marine Prochlorococcus and Synechococcus: implications for the ecological stoichiometry of the sea
Limnol. Oceanogr.
The oceanographic and biological tidal cycle succession in shallow sea fronts in the North Sea and the English Channel
Estuarine Coastal Mar. Sci.
Vertically rising microstructure profiler
J. Atmos. Oceanic Technol.
Small-scale planktonic structure: persistence and trophic consequences
Oceanography
In situ characterization of phytoplankton from vertical profiles of fluorescence emission spectra
Mar. Biol.
Temporal and spatial occurrence of thin phytoplankton layers in relation to physical processes
Mar. Ecol. Prog. Ser.
Mixing in Inland and Coastal Waters
Clade-specific 16S ribosomal DNA oligonucleotides reveal the predominance of a single marine Synechococcus clade throughout a stratified water column in the Red Sea
Appl. Environ. Microbiol.
Dynamics of community structure and phosphate status of picocyanobacterial populations in the Gulf of Aqaba, Red Sea
Limnol. Oceanogr.
Changes in the circulation and current spectrum near the tip of the seasonally mixed Gulf of Eilat
Israel J. Earth Sci.
Turbulence in stratified shear flows: implications for interpreting shear-induced mixing in the Ocean
J. Phys. Oceanogr.
Boundary mixing in a rotating, stratified fluid
J. Fluid Mech.
Boundary mixing in a stratified fluid
J. Fluid Mech.
On the nature of turbulence in a stratified fluid. Part I: the energetics of mixing
J. Phys. Oceanogr.
Cited by (21)
Thin layers of phytoplankton and harmful algae events in a coastal upwelling system
2020, Progress in OceanographyCitation Excerpt :Layer formation by in situ growth occurs when growth is stimulated over a narrow depth interval due to favorable light and nutrients levels (Birch et al., 2008). Finally, intrusions can generate TLP through the horizontal transport of nutrients or phytoplankton into adjacent waters (Steinbuck et al., 2010). Thin layers formation in estuarine systems, where salt water containing nutrient-limited marine phytoplankton mixes with nutrient-replete freshwater, has been explained by this mechanism (Kasai et al., 2010).
Characterizing the vertical distribution of chlorophyll a in the German Bight
2019, Continental Shelf ResearchShelf sea subsurface chlorophyll maximum thin layers have a distinct phytoplankton community structure
2019, Continental Shelf ResearchCitation Excerpt :A similar relationship was also observed in the southern Celtic Sea/Western Channel by Sharples et al. (2001), who documented a thin layer to persist over current velocities of approximately 0.2–0.6 m s−1, but increase in thickness from approximately < 1–4 m with increasing velocity. Internal waves have been documented to modulate the thickness of SCMs also (Steinbuck et al., 2010), but data available were not suitable for the assessment of effects of possible internal waves on SCM characteristics. The community within surface waters, the SCM and deep waters for four stratified sites consistently had photosynthetic functionality (Table S3) and was broadly similar in composition, with > 35% of biomass consistently contributed by the diatoms Pseudo-nitzschia spp., Chaetoceros spp. and rhizosolenids (locally), and the dinoflagellate Ceratium lineatum (Fig. 8).
Chlorophyll-a thin layers in the Magellan fjord system: The role of the water column stratification
2016, Continental Shelf Research