Oxycline oscillations induced by internal waves in deep Lake Iseo

Abstract

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.

Internal seiches in a karstic mesotrophic lake (Prošće, Plitvice Lakes, Croatia)

“…In comparison with the energies observed for the forced oscillations (i.e., for periods of 25.60 and 11.64 h), the energies associated with free baroclinic oscillations (i.e., periods of 7.11, 6.09 and 5.12 h shown in Tab. 1) were 1-2 orders of magnitude lower, where the highest among them (0.58 K 2 h) was found for a depth of 15 m and period of 6.09 h. This is in accordance with results of previous studies of Alpnacher See, Switzerland (Münnich et al, 1992), Sau reservoir, Spain (Vidal et al, 2005), Bromont Lake, Canada (Pannard et al, 2011) and Iseo Lake, Italy (Valerio et al, 2019) which show that higher vertical modes (i.e., forced baroclinic oscillations) dominate over the V1H1 mode (i.e., free baroclinic oscillations). The authors of these studies argue that the forced, higher vertical modes are more energetic due to the resonance between the periodic wind forcing and waves.…”

“…In lakes and reservoirs, the most commonly observed mode is the V1H1 mode (e.g., Vidal et al, 2007;Simpson et al, 2011); however, the presence of higher vertical modes has also been reported (e.g., Münnich et al, 1992;Vidal et al, 2005Vidal et al, , 2007Vidal et al, , 2008Pannard et al, 2011;Simpson et al, 2011;Valerio et al, 2019), and it is even possible to reach the 5 th mode V5 (Vidal et al, 2007).…”

Probability and Consequence of Postfire Erosion for Treatability of Water in an Unfiltered Supply System

“…Previous applications of ELCOM are widely referenced in limnological studies (Trolle et al, 2012), including studies of reservoir hydrodynamics and sediment transport (Dissanayake et al, 2019;Trolle et al, 2012). AEM3D features in more recent publications describing reservoir and lake modeling (e.g., Madani et al, 2020;Ryu et al, 2020;Tranmer et al, 2018;Valipour et al, 2019;Valerio et al, 2019;Zamani & Koch, 2020). Zamani and Koch (2020) demonstrate the appropriateness of AEM3D for modeling reservoirs, such as Upper Yarra reservoir, that have complex bathymetry, steep walls, and a wind-mixed surface layer.…”

Spatio-temporal differences of sediment accumulation rate in the Lake Gościąż (Central Poland) as a response of meteorological conditions and lake basin morphometry

“…Internal waves drive baroclinic shear instabilities that enhance the vertical turbulent diffusivity and reduce the rate of dissolved oxygen depletion in the hypolimnion (Bouffard et al, 2014). Deep motions characterised by larger amplitudes at lower frequencies are favoured by the excitation of internal waves of the second vertical mode in strongly thermally stratified lakes (Valerio et al, 2019). Internal waves are a common response to wind forcing in not only large but also small stratified lakes (Boegman, 2009).…”