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Hydrology and Earth System Sciences An interactive open-access journal of the European Geosciences Union
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© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 23 Oct 2018

Research article | 23 Oct 2018

Review status
This discussion paper is a preprint. It is a manuscript under review for the journal Hydrology and Earth System Sciences (HESS).

Oxycline oscillations induced by internal waves in deep Lake Iseo

Giulia Valerio1, Marco Pilotti1,4, Maximilian Peter Lau2,3, and Michael Hupfer2 Giulia Valerio et al.
  • 1DICATAM, Università degli Studi di Brescia, via Branze 43, 25123 Brescia, Italy
  • 2Leibniz-Institute of Freshwater Ecology and Inland Fisheries, Müggelseedamm 310, 12587 Berlin, Germany
  • 3Université du Quebec à Montréal (UQAM), Department of Biological Sciences, Montréal, QC H2X 3Y7, Canada
  • 4Civil & Environmental Engineering Department, Tufts University, Medford, MA 02155, USA

Abstract. Lake Iseo is undergoing a dramatic de-oxygenation of the hypolimnion, representing an emblematic example among the deep lakes of the prealpine area that are, to a different extent, suffering from reduced deep water mixing. In the anoxic deep waters, the release and accumulation of reduced substances and phosphorus from the sediments is a major concern. Since the hydrodynamics of this lake was shown to be dominated by internal waves, in this study we investigate for the first time the role of these oscillatory motions on the vertical fluctuations of the oxycline, currently situated at a depth of around 95m, where a permanent chemocline inhibits deep mixing by convection. Temperature and dissolved oxygen data measured at moored stations show large and periodic oscillations of the oxycline, with amplitude up to 20m and periods ranging from 1 to 4 days. A deep dynamics characterized by larger amplitudes at lower frequencies is shown to be favoured 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 anyhow. These basin-scale internal waves cause in the water layer between 85 and 105m depth a fluctuation of the oxygen concentration between 0 and 3mgL−1 that, due to the bathymetry of the lake, changes the redox condition at the sediment surface. This forcing, involving about 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.

Giulia Valerio et al.
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Giulia Valerio et al.
Giulia Valerio et al.
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Short summary
This paper provides experimental evidence of the occurrence of large and periodic movements induced by the wind around 100 m below the water surface in a deep Italian lake (Iseo). These movements induce vertical oscillations of the oxycline up to 20 m. Accordingly, in 3 % of the sediment area alternating redox conditions occur, which that might force unsteady sediment–water fluxes. This finding has major implications for the internal matter cycle in Lake Iseo.
This paper provides experimental evidence of the occurrence of large and periodic movements...