The Vertical Structure of Turbulent Dissipation in Shelf Seas

Simpson, John H.; Crawford, William R.; Rippeth, Tom P.; Campbell, Andrew R.; Cheok, Joseph V. S.

doi:10.1175/1520-0485(1996)026<1579:tvsotd>2.0.co;2

Cited by 207 publications

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“…2b). This result is consistent with previous investigations in stratified shelf seas (Simpson et al 1996;Sharples et al 2001) and as such is considered to be representative of the vertical distribution of turbulent mixing at each of the sites visited.…”

Section: Resultssupporting

confidence: 92%

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Distribution and chromatic adaptation of phytoplankton within a shelf sea thermocline

Hickman

Holligan

Moore

et al. 2009

Limnology & Oceanography

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Observations of vertical gradients in phytoplankton community structure were made through the water column of the seasonally stratified Celtic Sea, including within the thermocline. A deep chlorophyll maximum (DCM) was located within the thermocline at all stations, coupled to the nitracline. Vertical gradients in phytoplankton community composition were routinely observed within the thermocline. The cell abundance maxima for Synechococcus occurred in the upper part of the DCM coincident with a picoeukaryote abundance minima. Picoeukaryote abundance typically increased at or just above the peak of the DCM. Diatoms were observed occasionally at the DCM peak. Pigment compositions and phytoplankton absorption spectra indicated that the different phytoplankton communities were chromatically well adapted to the spectral composition of irradiance at the depths where they occurred in the water column. Profiles of vertical eddy diffusivity revealed that timescales for mixing between the phytoplankton layers within the thermocline were in excess of typical phytoplankton growth rates. The observed vertical gradients in community structure could therefore result from selection and niche partitioning of phytoplankton types on the light and nutrient gradient within the thermocline. The data further indicate that the pigments, light absorption characteristics, and cell size contribute to the phytoplankton selection process.The Celtic Sea, part of the temperate Northwest European shelf, is a tidally dynamic environment where water column structure is strongly influenced by the balance of solar heating and tidally generated mixing (Simpson and Hunter 1974). Much of the region becomes thermally stratified in April, initiating the spring phytoplankton bloom (Pingree et al. 1976). The water column remains stratified throughout the summer, with the thermocline forming a boundary between nutrient-depleted surface mixed layer (SML) above and the nutrient-rich bottom mixed layer (BML) below (Pingree et al. 1977). During this seasonal stratification, a deep chlorophyll maximum (DCM) is present within the thermocline (Pingree et al. 1977). The DCM is typically located toward the base of the density gradient, lies within the euphotic zone, and is strongly coupled to the nitracline (Holligan et al. 1984a,b;Sharples et al. 2001).Energy from tidally induced turbulence is dissipated at the base of the thermocline, causing upward mixing of nitrate into the thermocline from the BML and downward mixing of phytoplankton from the base of the DCM (Sharples et al. 2001). Previous observations in the Celtic Sea have revealed a vertical flux of nitrate into the thermocline of around 2 mmol N m 22 d 21 with the DCM maintained at a depth corresponding to ,5% of surface irradiance (Sharples et al. 2001). The DCM is often observed to be a biomass and photosynthesis maximum as well as a pigment maximum (Holligan et al. 1984a,b;Pemberton et al. 2004;Moore et al. 2006). Monospecific blooms with chlorophyll a (Chl a) concentrations .10 mg Chl a m 23...

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Section: Resultssupporting

confidence: 92%

“…The Celtic Sea, part of the temperate Northwest European shelf, is a tidally dynamic environment where water column structure is strongly influenced by the balance of solar heating and tidally generated mixing (Simpson and Hunter 1974). Much of the region becomes thermally stratified in April, initiating the spring phytoplankton bloom (Pingree et al 1976).…”

mentioning

confidence: 99%

Distribution and chromatic adaptation of phytoplankton within a shelf sea thermocline

Hickman

Holligan

Moore

et al. 2009

Limnology & Oceanography

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“…Maximum dissipation occurred later higher up in the water column, with maximum dissipation at the thermocline (ϳ90 m above the seabed) occurring ϳ5 h after the peak in nearbed dissipation. This phase lag of dissipation away from the bottom boundary is thought to be driven by the vertical progression of the region of maximum shear during the tidal cycle (Simpson et al 2000). Minimum values of turbulence dissipation rates were always found within the thermocline, typically between 10 Ϫ8 and 10 Ϫ9 m 2 s Ϫ3 (the noise level of the FLY measurements is equivalent to a dissipation of ϳ3 ϫ 10 Ϫ10 m 2 s Ϫ3 ).…”

Section: Resultsmentioning

confidence: 99%

Phytoplankton distribution and survival in the thermocline

Sharples

Moore

Rippeth

et al. 2001

Limnology & Oceanography

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152

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Observations of the vertical structure of density, concentrations of chlorophyll a and nitrate, and turbulent dissipation rates were made over a period of 25 h in a well-stratified shelf region in the Western English Channel, between neap and spring tides. Maximum turbulent dissipation at the base of the thermocline occurred almost 5 h after maximum tidal currents. This turbulence aids phytoplankton growth by supplying bottom-layer nutrients into the subsurface chlorophyll maximum but reduces phytoplankton concentrations in the thermocline by mixing cells from the base of the subsurface maximum into the bottom mixed layer. The turbulent dissipation observations were used to estimate an average nitrate flux into the thermocline of 2.0 (0.8-3.2, 95% confidence interval) mmol m, which is estimated to have been capable of supporting new phytoplankton growth at a rate of 160 (64-256) mg C m Ϫ2 d Ϫ1 . Turbulent entrainment of carbon from the base of the subsurface biomass maximum into the bottom mixed layer was observed to be 290 (120-480) mg C m Ϫ2 d Ϫ1 . This apparent excess export from the chlorophyll maximum is suggested to be a feature of the spring-neap cycle, with export dominating as the tidal turbulence increases toward spring tides and erodes the base of the thermocline. The observed rate of carbon export into the bottom mixed layer could account for as much as 25% of the gross annual primary production in stratifying shelf seas. Such turbulent losses, combined with grazing losses and low light levels, suggest that phytoplankton need to be highly adapted to environmental conditions within the thermocline in order to survive.The seasonal thermocline is an important physical barrier in the ocean, separating the surface mixed layer from the deeper water. The stability of the vertical temperature (density) gradient inhibits diapycnal transfer of properties (e.g., momentum, heat, nutrients, algal cells, and oxygen). As the transition region between the nutrient-poor, well-lit surface layer and the darker, nutrient-rich deeper water, the thermocline plays a role in determining the biological properties of the water column. Since the development of continuous fluorescence measurements as a technique for observing the vertical structure of algae (Lorenzen 1966), the thermocline has been observed to be a region of enhanced chlorophyll concentration (e.g., Anderson 1969;Cullen and Eppley 1981;Holligan et al. 1984). Observed concentrations of subsurface chlorophyll range between 0.5 mg mϪ3 in the open ocean up to 100 mg m Ϫ3 in shelf seas. Although in some cases this higher chlorophyll concentration has been shown to be the result of a lower carbon : chlorophyll ratio (Steele 1 Corresponding author (j.sharples@soc.soton.ac.uk). AcknowledgmentsRay Wilton (University of Wales, Bangor) provided invaluable technical support for FLY. Our thanks to the crew and RVS support staff on RRS Challenger during cruise CH145.

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“…Analyses suggest that the level 2.2 closure (with vertical diffusion, but neglecting the advection term of turbulent kinetic energy in equation (A6)), with a diagnostic master length scale limited by the Ozmidov scale, can cope rather well with tidal, wind, and density-driven mixing of the stratified flow over seasons using no restoring conditions. The level 2 model (left member of equation (A6) set to zero) strongly underestimated the mixed-layer depth [e.g., see also Simpson et al, 1996;Meier, 2000]. The ocean model is coupled to a dynamic [Flato, 1993] and thermodynamic [Semtner, 1976] sea ice model.…”

Section: Experimental Settingmentioning

confidence: 99%

Modeling the formation and circulation processes of water masses and sea ice in the Gulf of St. Lawrence, Canada

et al. 2003

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[1] The seasonal cycle of water masses and sea ice in the Gulf of St. Lawrence is examined using a three-dimensional coastal ice-ocean model with realistic tidal, atmospheric, hydrologic, and oceanic forcing. The model includes a level 2.5 turbulent kinetic energy equation. A model simulation over 1997-1998 is verified against available data on sea ice, temperature, and salinity. The results demonstrate a consistent seasonal cycle in atmosphere-ocean exchanges and the formation and circulation of water masses and sea ice. The accuracy of radiative, momentum, and sensible heat exchanges at the sea surface, and the production of turbulent kinetic energy from winds and tides, are critical to the accuracy of the modeled circulation. The analysis of the mean error on near-surface temperature and salinity in the late summer and fall using standard bulk exchange coefficients and radiation (about 1°C too cold and 1 salinity unit too fresh) shows the tradeoff between tidal mixing at the head of the Laurentian Channel, and winddriven circulation and mixing in the surface waters. The results suggest year-long stratification in the estuary and northwestern Gulf, with little mixing except near the head region, where relatively deep warmer waters are mixed to the surface during winter, and cold intermediate waters are efficiently withdrawn during summer. The results suggest that the summer cold waters found at intermediate depths in the estuary and northwestern Gulf are not formed in situ. A significant fraction of these waters enters through the Strait of Belle Isle in wintertime, eventually reaching the estuary within about 6 months.

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The Vertical Structure of Turbulent Dissipation in Shelf Seas

Cited by 207 publications

References 0 publications

Distribution and chromatic adaptation of phytoplankton within a shelf sea thermocline

Distribution and chromatic adaptation of phytoplankton within a shelf sea thermocline

Phytoplankton distribution and survival in the thermocline

Modeling the formation and circulation processes of water masses and sea ice in the Gulf of St. Lawrence, Canada

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