In a stratified lake, where an internal seiche maintained an oscillating bottom boundary layer (BBL), processes including strain-induced periodic stratification (SIPS), a downslope mean flow, and upslope-propagating internal waves controlled fluctuating BBL mixing rates. To investigate these processes, high-resolution temperature and velocity profiles were measured within 2 m of the sloping bed, through more than 100 cycles of upslope and downslope flows. The associated seiche-induced velocity reversals coincided with the arrival of upslope-propagating cold fronts. Although fronts are sometimes associated with internal wave breaking, in this lake the measured frontal slopes were small and breaking did not occur. When fronts were not propagating past instruments, near-bed stratification was consistent with SIPS, being intensified by downslope flows and reduced by upslope flows. Owing to a downslope near-bed mean flow, which was previously shown to cancel an upslope internal-wave Stokes drift, peak downslope velocities regularly exceeded peak upslope velocities. Given these strong downslope velocities, turbulent dissipation rates were most intense during downslope phases. Therefore, during downslope flow, creation of stratification by SIPS coincided with elevated dissipation, creating conditions for effective mixing. Correspondingly, two simple parameterizations for mixing efficiency suggest that most of the total near-bed buoyancy flux (≥ 82%), and most of the irreversible flux (87%), occurred during the downslope-flow phase of internal seiche cycles. The observed mechanism for dominant mixing during downslope flow has potential to act in other environments where internal waves propagate up sloping beds.
Methane (CH4) produced in anoxic sediments plays a significant role in the carbon economy of many lakes and reservoirs. CH4 released from sediments first crosses the bottom boundary layer (BBL), the layer of water overlying the lakebed where currents are slowed by friction with the sediments below. Physical and biogeochemical conditions in the BBL, which can fluctuate hourly to daily with basin‐wide internal waves (seiches), likely influence CH4 transport from sediments into the hypolimnion. In this study, we estimated CH4 fluxes across the BBL of a eutrophic lake using a novel in situ flux gradient approach adapted from marine applications. For 2–6 h periods throughout the spring and summer, we estimated CH4 fluxes across the BBL using simultaneous measurements of CH4 concentrations, turbulent mixing, and thermal stratification. Sub‐daily variation in CH4 fluxes was high, and CH4 fluxes sometimes changed several‐fold within hours. These rapid shifts in BBL fluxes were likely influenced by fluctuations in seiche‐driven variations in the intensity of BBL turbulent mixing. Fluxes increased from spring to summer, concurrent with the development of lake stratification, and fueled an accumulation of CH4 below the thermocline. Throughout the summer, CH4 flux across the BBL exceeded CH4 accumulation below the thermocline, suggesting significant methanotrophy in the hypolimnion, consistent with incubation‐based oxidation rates. Our results are the first to demonstrate sub‐daily and seasonal variability in the timing and magnitude of CH4 fluxes within a lake BBL, and highlight a need to quantify such variability in other lentic systems.
Lateral intrusions flowing from the boundaries of lakes and oceans might influence basin-scale distributions of heat, chemical solutes, sediments, and organisms. Here, observations of numerous intrusions from multiple locations in a small lake were used to examine the relation of intrusions to a lakewide internal seiche, and to estimate mean exchange between sloping bottom boundary layers (BBLs) and the lake interior. BBL waters were observed periodically separating from the bed and flowing laterally offshore. Boundary layer separation was initiated where strong downslope flows encountered upslope-propagating temperature fronts. These separation events propagated upslope, coherent with the vertically propagating internal seiche, resembling previously reported cases of upslope-propagating internal bores. Following separation, jets flowed offshore immediately above temperature fronts, transporting boundary layer water at least 61 m into the interior. Jet propagation could be traced upwards, from the bed at 9.4 m depth to the base of the surface mixed layer. Mean offshore transport was most intense within the metalimnion (4-7 m depth). Intruding intermediate-temperature thermocline water was likely supplied both by river inputs and by boundary mixing of cold and warm waters. These findings suggest that periodic jets, resulting from seiche-induced boundary layer separation, transported significant quantities of water from the boundary layer toward the stratified lake interior, with possible implications for distributions of sediments, solutes, and organisms.
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