Hypolimnetic oxygen depletion rates in deep lakes: Effects of trophic state and organic matter accumulation

This study investigated the consumption of oxygen (O-2) in 11 European lakes ranging from 48 m to 372 m deep. In lakes less than similar to 100 m deep, the main pathways for O-2 consumption were organic matter (OM) mineralization at the sediment surface and oxidation of reduced compounds diffusing up from the sediment. In deeper lakes, mineralization of OM transported through the water column to the sediment represented a greater proportion of O-2 consumption. This process predominated in the most productive lakes but declined with decreasing total phosphorous (TP) concentrations and hence primary production, when TP concentrations fell below a threshold value of similar to 10 mg P m(-3). Oxygen uptake by the sediment and the flux of reduced compounds from the sediment in these deep lakes were 7.9-10.6 and 0.6-3.6 mmol m(-2) d(-1), respectively. These parameters did not depend on the lake's trophic state but did depend on sedimentation rates for the primarily allochthonous or already degraded OM. These results indicate that in lakes deeper than similar to 100 m, mineralization of autochthonous OM is mostly complete by the time of sedimentary burial. This explains why hypolimnetic O-2 concentrations improve more rapidly following TP load reduction in deeper lakes relative to shallower lakes, where larger sediment-based O-2 consumption by settled OM and release of reduced substances may inhibit the restoration of hypolimnetic O-2 concentrations

Effects of upstream hydropower operation and oligotrophication on the light regime of a turbid peri-alpine lake

ISSN:1015-1621ISSN:1420-905

Energetics of Radiatively Heated Ice-Covered Lakes

We derive the mechanical energy budget for shallow, ice-covered lakes energized by penetrative solar radiation. Radiation increases the available and background components of the potential energy at different rates. Available potential energy drives under-ice motion, including diurnally active turbulence in a near-surface convective mixing layer. Heat loss at the ice-water interface depletes background potential energy at a rate that depends on the available potential energy dynamics. Expressions for relative energy transfer rates show that the pathway for solar energy is sensitive to the convective mixing layer temperature through the nonlinear equation of state. Finally, we show that measurements of light penetration, temperature profiles resolving the diffusive boundary layer, and an estimate of the kinetic energy dissipation rate can be combined to estimate the forcing rate, the rate of heat loss to the ice, and efficiencies of the energy pathways for radiatively driven flows. Plain Language Summary Global observations reveal a pervasive decline in the annual ice cover duration of inland waters. This has stimulated urgent new research into cold and polar aquatic environments. Predicting thermal changes in ice-covered waters requires the extension of current fluid-dynamical theories to incorporate the physics that governs cold water near its temperature of maximum density. In this work, we present new mathematical expressions for the transformation of solar energy that penetrates the ice and show that feasible under-ice measurements can be used to estimate the fraction of this energy that is transferred to the ice as heat, contributing to its rate of melting