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.