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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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

In lakes and reservoirs, the bottom boundary layer (BBL) mediates chemical fluxes between sediments and the overlying water column. At the internal shoreline, where the thermocline contacts the lakebed, the motions of internal waves can create fluctuating redox conditions and dynamic physical forcing that may support ecologically important reactions such as denitrification. We characterized physical and chemical dynamics within the internal shoreline of a eutrophic reservoir during the spring and early summer of 2012 (18 May to 18 July). An internal seiche was found to generate quasi-periodic fluctuations (periods about 12-24 h) in BBL stratification, temperature, and redox conditions. To examine possible implications for chemical mixing and microbial processing, differences between vertically offset, simultaneous BBL measurements of velocity, temperature, N2, and N2O were made over 23 h. Vertical differences in BBL temperature, N2, and N2O formed and collapsed during the wave cycle, with the largest differences occurring following the arrival of an internal bore. Through much of the wave cycle, chemical differences were explained by physical advection and mixing. However, chemical differences measured after bore arrival were not explained by advection, possibly owing to local production of N2 and N2O. These results highlight the dynamic physical environment within the internal shoreline, and the potential of this zone to contribute to system wide denitrification and nitrous oxide production. [ABSTRACT FROM AUTHOR]

When submerged, flexible vegetation bends back and forth under waves, and stems are tilted only a small angle from vertical, simple models for stem motion and wave dissipation can be derived. Here, previous simple models for the wave-induced bending of elastic vegetation are extended to account for buoyancy. Buoyancy results in stem tension which, together with fluid drag, is incorporated in the Euler-Bernoulli problem, in which each stem is modeled as a cantilevered elastic beam. Solutions are governed by a new 'dimensionless buoyancy' β, in addition to the 'dimensionless stiffness' S identified by previous researchers. If β ≪ S 1 / 2 , buoyancy is negligible and previous results for elastic stems are recovered. Specifically, stems are nearly immobile for S ≫ 1 , but for S ≪ 1 stems move with surrounding water except in a thin 'elastic boundary layer' extending a distance S 1 / 4 l * above the bed, where l * = stem length. Conversely, if β ≫ S 1 / 2 , then elasticity is negligible along most of the length of the stem and new behaviour is found. Specifically, stems are nearly immobile for β ≫ 1 , but for β ≪ 1 stems move with surrounding water except in a thin 'buoyant boundary layer' extending a distance β 1 / 2 l * above the bed. For essentially inflexible cases (S ≫ 1 or β ≫ 1), simulated depth-integrated wave dissipation roughly equals the value D r predicted for rigid stems. For highly flexible cases (i.e. for S and β both ≪ 1), dissipation is limited to elastic or buoyant boundary layers, and therefore scales with the maximum of S 1 / 4 D r and β 1 / 2 D r . For the simple stems considered here, which have constant diameter and density, simulated dissipation for all S and β was approximated by the expression [ (S + β 2 / 4) / (4 + S + β 2 / 4) ] 1 / 4 D r . This simple formula may require modification for vegetation with complex geometry. Nevertheless, this analysis identifies β as a key parameter for inclusion in dissipation formulations, together with parameters such as S identified by previous authors. • Dissipation is predicted for waves propagating through flexible vegetation. • Force-balance model analyzed for small-tilt motion of buoyant, elastic stems. • New dimensionless buoyancy quantifies buoyant resistance to oscillatory motion. • A simple expression approximates depth-integrated wave dissipation. • Results may aid assessment of coastal protection by wetlands. [ABSTRACT FROM AUTHOR]