Chemical mixing in the bottom boundary layer of a eutrophic reservoir: The effects of internal seiching on nitrogen dynamics

Deemer, Bridget R.; Henderson, Stephen M.; Harrison, John

doi:10.1002/lno.10125

Cited by 18 publications

(30 citation statements)

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“…Conversely negative DT/D z, which indicates unstable stratification, with a mean value of 20.058C m 21 (Figure 7b) occurred at the peak of positive <V> or negative d<T>/dt (Figures 7e and 7f), i.e., during the colder up-slope flow phase. Similar Water Resources Research 10.1002/2015WR017725 synchronization between benthic <V> and DT/D z has recently been observed by Deemer et al [2015] at the depth of the thermocline of a small lake.…”

Section: 1002/2015wr017725supporting

confidence: 72%

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Movements of the thermocline lead to high variability in benthic mixing in the nearshore of a large lake

Chowdhury

Wells

Howell

2016

Water Resources Research

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The thermocline of Lake Ontario is in constant motion, and as it washes back and forth along the sloping lakebed there is a striking asymmetry in near‐bed stratification and benthic turbulence between its rise and fall. Detailed field observations of the stratification and water currents from the summers of 2012 and 2013 showed that the thermocline motions had large amplitudes (as high as 15 m) and a dominant period between 16 and 17.5 h, corresponding to a near‐inertial internal Poincaré wave. During the falling phase, the warmer down‐slope flow was strongly stratified with near‐bed water temperature gradients of 1°C m−1. In contrast during the rising phase of colder up‐slope flow, there was an unstable stratification in near‐bed water and large temperature overturns due to the differential advection of stratified waters, i.e., the shear‐driven convective mechanism. Using a Thorpe‐scale analysis of overturns, the inferred turbulent diffusivity during the up‐slope flow was Kz =5 × 10−4 m2 s−1. In striking contrast during the down‐slope flow, the strong stratification had lower turbulent diffusivities of Kz =10−6 m2 s−1. The near bottom region of Lake Ontario within the thermocline swash‐zone has intense biological activity and the highest concentrations of invasive dreissenid mussels. We discuss the potential biological implications of the striking variability in benthic mixing and near‐bed stratification for nutrient cycling in the Lake Ontario nearshore.

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Section: 1002/2015wr017725supporting

confidence: 72%

“…Similar synchronization between benthic < V> and ΔT/Δz has recently been observed by Deemer et al . [] at the depth of the thermocline of a small lake.…”

Section: Results and Observationsmentioning

confidence: 99%

Movements of the thermocline lead to high variability in benthic mixing in the nearshore of a large lake

Chowdhury

Wells

Howell

2016

Water Resources Research

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“…The resulting intermittent instability drives free convection and accelerates the fluxes at the sediment-water interface by more than 1 order of magnitude, as experimentally observed by Kirillin et al (2009), Lorke et al (2005) and Chowdhury et al (2016). In lakes with anoxic water layers, the seiche-induced oscillations can be accompanied by periodical changes in the oxygen concentration in a large internal shoreline area as described by Deemer et al (2015) for Lacamas Lake. Bernhardt et al (2014) observed similar seiche-induced oxygen fluctuations at the sediment-water interface in the shallower area of the eutrophic Lake Arendsee resulting from the formation of a distinct metalimnetic oxygen minimum during summer.…”

Section: Introductionmentioning

confidence: 72%

Oxycline oscillations induced by internal waves in deep Lake Iseo

Valerio

Pilotti

Lau

et al. 2019

Hydrol. Earth Syst. Sci.

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Abstract. Lake Iseo is undergoing a dramatic deoxygenation of the hypolimnion, representing an emblematic example among the deep lakes of the pre-alpine area that are, to a different extent, undergoing reduced deep-water mixing. In the anoxic deep waters, the release and accumulation of reduced substances and phosphorus from the sediments are a major concern. Because the hydrodynamics of this lake was shown to be dominated by internal waves, in this study we investigated, for the first time, the role of these oscillatory motions on the vertical fluctuations of the oxycline, currently situated at a depth of approximately 95 m, where a permanent chemocline inhibits deep mixing via convection. Temperature and dissolved oxygen data measured at moored stations show large and periodic oscillations of the oxycline, with an amplitude of up to 20 m and periods ranging from 1 to 4 days. Deep motions characterized by larger amplitudes at lower frequencies are favored by the excitation of second vertical modes in strongly thermally stratified periods and of first vertical modes in weakly thermally stratified periods, when the deep chemical gradient can support baroclinicity regardless. These basin-scale internal waves cause a fluctuation in the oxygen concentration between 0 and 3 mg L−1 in the water layer between 85 and 105 m in depth, changing the redox condition at the sediment surface. This forcing, involving approximately 3 % of the lake's sediment area, can have major implications for the biogeochemical processes at the sediment–water interface and for the internal matter cycle.

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“…BBLs may be responsible for much of the vertical mixing of density, nutrients, and pollutants in lakes [ Goudsmit et al ., ; Wüest and Lorke , ] and may also play an important role in the vertical mixing of some estuaries [ Inall , ]. Further interest in internal wave BBLs is motivated by their influence on sediment transport [ Pierson and Weyhenmeyer , ] and sediment‐water biogeochemical fluxes [ Lorke et al ., ; Holtappels et al ., ; Deemer et al ., ].…”

Section: Introductionmentioning

confidence: 99%

Turbulent production in an internal wave bottom boundary layer maintained by a vertically propagating seiche

Henderson

2016

JGR Oceans

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Internal seiches, which supply the energy responsible for mixing many lakes, are often modeled as vertically standing waves. However, recent observations of vertical seiche propagation in a small lake are inconsistent with the standard, vertically standing model. To examine the processes responsible for such propagation, drag and turbulent production in the bottom boundary layer of a small lake are related to the energy supplied by a propagating seiche (period 10–24 h). Despite complex and fluctuating stratification, which often inhibited mixing within 0.4 m of the bed, bottom stress was well represented by a simple drag coefficient model (drag coefficient 1.5 × 10−3). The net supply of seiche energy to the boundary layer was estimated by fitting a model for internal wave vertical propagation to velocity profiles measured above the boundary layer (1–4.5 m above lakebed). Fitted reflection coefficients ranged from 0.3 at 1 cycle/d frequency to 0.7 at 2.4 cycles/d (cf. near‐unity coefficients of classical seiche theories). The net supply of seiche energy approximately balanced boundary layer turbulent production. Three of four peaks in production and energy flux occurred 0.8–2.2 days after strong oscillating winds, a delay comparable to the time required for seiche energy to propagate to the lakebed. A model based on the estimated drag coefficient predicted the observed frequency dependence of the seiche reflection coefficient. For flat‐bed regions in narrow lakes, the model predicts that reflection is controlled by the ratio of water velocity to vertical wave propagation speed, with sufficiently large ratios leading to weak reflection, and clear vertical seiche propagation.

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Chemical mixing in the bottom boundary layer of a eutrophic reservoir: The effects of internal seiching on nitrogen dynamics

Cited by 18 publications

References 45 publications

Movements of the thermocline lead to high variability in benthic mixing in the nearshore of a large lake

Movements of the thermocline lead to high variability in benthic mixing in the nearshore of a large lake

Oxycline oscillations induced by internal waves in deep Lake Iseo

Turbulent production in an internal wave bottom boundary layer maintained by a vertically propagating seiche

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