Turbulence in a small boreal lake: Consequences for air–water gas exchange

Abstract

The hydrodynamics within small boreal lakes have rarely been studied, yet knowing whether turbulence at the air-water interface and in the water column scales with metrics developed elsewhere is essential for computing metabolism and fluxes of climate-forcing trace gases. We instrumented a humic, 4.7 ha, boreal lake with two meteorological stations, three thermistor arrays, an infrared (IR) camera to quantify surface divergence, obtained turbulence as dissipation rate of turbulent kinetic energy (ε) using an acoustic Doppler velocimeter and a temperature-gradient microstructure profiler, and conducted chamber measurements for short periods to obtain fluxes and gas transfer velocities (k). Near-surface ε varied from 10 −8 to 10 −6 m 2 s −3 for the 0-4 m s −1 winds and followed predictions from Monin-Obukhov similarity theory. The coefficient of eddy diffusivity in the mixed layer was up to 10 −3 m 2 s −1 on the windiest afternoons, an order of magnitude less other afternoons, and near molecular at deeper depths. The upper thermocline upwelled when Lake numbers (L N) dropped below four facilitating vertical and horizontal exchange. k computed from a surface renewal model using ε agreed with values from chambers and surface divergence and increased linearly with wind speed. Diurnal thermoclines formed on sunny days when winds were < 3 m s −1 , a condition that can lead to elevated near-surface ε and k. Results extend scaling approaches developed in the laboratory and for larger water bodies, illustrate turbulence and k are greater than expected in small wind-sheltered lakes, and provide new equations to quantify fluxes.

Tree line advance reduces mixing and oxygen concentrations in arctic–alpine lakes through wind sheltering and organic carbon supply

“…We scaled wind speed from anemometer height ( z ) to standard heights of 10 m above ground using law of the wall scaling, , where = 0.4 is the von Kármán constant, and C D is the drag coefficient. We computed C D for nine lakes with full meteorological observations as a function of air and surface water temperature, wind speed, humidity, and anemometer height (Smith, 1988) using an iterative procedure (Hicks, 1975) following MacIntyre et al (2020). The mean C D decreased linearly with increasing W c , indicating lower wind drag in less sheltered lakes as a result of more frequent stable atmospheric conditions (Figure S3).…”

BAWLD-CH&lt;sub&gt;4&lt;/sub&gt;: A Comprehensive Dataset of Methane Fluxes from Boreal and Arctic Ecosystems

“…Diffusive modeling approaches include an estimate of the gas transfer coefficient, k. Gas transfer velocity estimates are commonly calculated using equations established by Cole and Caraco (1998). However, more recent efforts with EC systems, chambers, and either calculation or measurement of the near-surface turbulence that enables flux across the air-water interface indicates that fluxes using Cole and Caraco's (1998) wind-based model of gas transfer velocities underestimate fluxes from non-sheltered waterbodies by a factor of two to four (Heiskanen et al 2014;Mammarella et al 2015;MacIntyre et al 2020). Sheltered waterbodies, such as small lakes surrounded by trees, are an exception and can have reduced mean lake k values (Markfort et al 2010).…”

Shoring up the foundations of production to respiration ratios in lakes

“…Pairing multiple diel O 2 curves with sediment incubation experiments (Sadro et al 2011 b ) offers a more robust characterization of a lake's P:R ratio than a single lake‐center sonde. Meanwhile, accounting for physical processes such as stratification and mixing dynamics (Antenucci et al 2013; Brothers et al 2017 a ; Andersen et al 2017 b ), as well as potential uncertainties in surface gas exchange rates, which may be especially high in small, sheltered ponds (e.g., MacIntyre et al 2020), allows researchers to better calculate and interpret resulting metabolic data. Such integrated approaches are a welcome and necessary development in the field of aquatic metabolism, and have led to many of the research advances discussed below, allowing us to re‐assess the meaning and implications of P:R ratios in lakes.…”