The oxygen-consuming processes in the hypolimnia of freshwater lakes leading to deep-water anoxia are still not well understood, thereby constraining suitable management concepts. This study presents data obtained from 11 eutrophic lakes and suggests a model describing the consumption of dissolved oxygen (O(2)) in the hypolimnia of eutrophic lakes as a result of only two fundamental processes: O(2) is consumed (i) by settled organic material at the sediment surface and (ii) by reduced substances diffusing from the sediment. Apart from a lake's productivity, its benthic O(2) consumption depends on the O(2) concentration in the water overlying the sediment and the molecular O(2) diffusion to the sediment. On the basis of observational evidence of long-term monitoring data from 11 eutrophic lakes, we found that the areal hypolimnetic mineralization rate ranging from 0.47 to 1.31 g of O(2) m(-2) d(-1) (average 0.90 ± 0.30) is a function of (i) a benthic flux of reduced substances (0.37 ± 0.12 g of O(2) m(-2) d(-1)) and (ii) an O(2) consumption which linearly increases with the mean hypolimnion thickness (z(H)) up to ~25 m. This model has important implications for predicting and interpreting the response of lakes and reservoirs to restoration measures.

Lake Ohrid in southeastern Europe is one of the few ancient, long-lived lakes of the world, and contains more than 200 endemic species. On the basis of integrated monitoring of internal and external nutrient fluxes, a progressing eutrophication was detected (,3.5-fold increase in phosphorus (P) concentration in the lake over the past century). The lake is fortunately still oligotrophic, with high concentrations of dissolved oxygen (DO) in the deep water that are requisite for the unique endemic bottom fauna. Hypolimnetic DO is not only very sensitive to changes in anthropogenic P load-via mineralization of organic material-but also to global warming via decrease of vertical mixing and less frequent complete deep convection. Moreover, these two human effects amplify each other. To keep DO from falling below currently observed minimal levels-given the predicted atmospheric warming of 0.04uC yr 21 -the P load must be decreased by 50% in coming decades. However, even with such a reduction in P load, anoxia is still expected toward the end of the century if the rate of warming follows predictions.

Lake Prespa and Lake Ohrid, located in south-eastern Europe, are two lakes of extraordinary ecological value. Although the upstream Lake Prespa has no surface outflow, its waters reach the 160 m lower Lake Ohrid through underground hydraulic connections. Substantial conservation efforts concentrate on oligotrophic downstream Lake Ohrid, which is famous for its large number of endemic and relict species. In this paper, we present a system analytical approach to assess the role of the mesotrophic upstream Lake Prespa in the ongoing eutrophication of Lake Ohrid. Almost the entire outflow from Lake Prespa is found to flow into Lake Ohrid through karst channels. However, 65% of the transported phosphorus is retained within the aquifer. Thanks to this natural filter, Lake Prespa does not pose an immediate threat to Lake Ohrid. However, a potential future four-fold increase of the current phosphorus load from Lake Prespa would lead to a 20% increase (+0.9 mg P m )3 ) in the current phosphorus content of Lake Ohrid, which could jeopardize its fragile ecosystem. While being a potential future danger to Lake Ohrid, Lake Prespa itself is substantially endangered by water losses to irrigation, which have been shown to amplify its eutrophication.

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We quantified the areal hypolimnetic mineralization rate (AHM; total areal hypolimnetic oxygen depletion including the formation of reduced substances) in two formerly eutrophic lakes based on 20 yr of water-column data collected during oligotrophication. The upward diffusion of reduced substances originating from the decomposition of organic matter in the sediment was determined from pore-water profiles and related to the time of deposition. More than 80% of AHM was due to degradation of organic matter in the water column (including sediment surface) and diffusion of reduced substances from sediment layers younger than 10 yr. Sediments older than 10 yr, including the eutrophic past, accounted for , 15% of AHM. This ''old'' contribution corresponds to a 20-43% fraction of the total sediment-based AHM. The contribution from old sediment layers to AHM is expected to be even lower in lakes with deeper hypolimnia (. 12 m). In summary, oxygen consumption in stratified hypolimnia is controlled mainly by the present lake productivity. As a result, technical lake management measures, such as oxygenation, artificial mixing, or sediment dredging, cannot efficiently decrease the flux of reduced substances from the sediment.In deep stratified lakes there is generally a clear correlation between lake productivity and areal hypolimnetic oxygen depletion rate (AHOD; g O 2 m 22 d 21 ), as first shown by Hutchinson (1938). Consequently, eutrophication leads to an increase in AHOD (Rast and Lee 1978; Chapra and Canale 1991) and potentially low dissolved oxygen (DO) levels in the hypolimnion during summer stratification. Low hypolimnetic DO can in turn have a serious effect on biological processes, either via direct toxicity on fish and bottom organisms (Kalff 2002) or indirectly via toxic by-products of anaerobic mineralization (Wang and Chapman 1999). Moreover, it has been shown that hypolimnetic anoxia can enhance the sediment release of soluble reactive phosphorus (P; Larsen et al. 1981;Hupfer and Lewandowski 2008), although the DO concentration at the sediment-water interface is not the only controlling factor (Moosmann et al. 2006).As a result, reaching or maintaining a sufficient hypolimnetic DO level is often a focus of lake management. Commonly used approaches focus on (1) the reduction of external P loading (Gä chter and Wehrli 1998), (2) increased DO supply to the hypolimnion through oxygenation or aeration (Singleton and Little 2006) or through artificial mixing in winter (Mü ller and Stadelmann 2004), or (3) the removal of accumulated deposits of organic matter through sediment dredging (Annadotter et al. 1999). However, in many lakes AHOD decreased only slowly following the reduction of external P sources and the subsequent decrease in lake productivity. Such a resilience of AHOD during oligotrophication was observed in Lake Shagawa (Larsen et al. 1981) and Lake Eire (Charlton et al. 1993). A similar effect was described by Matthews and Effler (2006) for Lake Onondaga, where an abrupt decrease in anthropogeni...