TRACING RATE AND EXTENT OF HUMAN INDUCED HYPOXIA DURING THE LAST 200 YEARS IN THE MESOTROPHIC LAKE TIEFER SEE (NE GERMANY)

The global spread of lake hypoxia, [O₂] < 2 mg/l, during the last two centuries has a severe impact on ecological systems and sedimentation processes. While the occurance of hypoxia was observed in many lakes, a detailed quantification of hypoxia spread remained largely unquantified. We track the evolution of hypoxia and its controls during the past 200 yrs in lake Tiefer See (TSK; NE Germany) using 17 gravity cores, recovered between 10 and 62 m water depth in combination with lake monitoring data. Lake hypoxia was associated by the onset of varve preservation in the TSK, and has been dated by varve counting to 1918±1 at 62 m water depth and reached a lake-floor depth of 16 m at 1997±1. This indicates oxygen concentration to fell below the threshold for varve preservation at the lakefloor (>16 m). Sediment cores at 10-12 m depth do not contain varves indicating well oxygenation of the upper water column. Monitoring data show that the threshold for hypoxia at the depocenter is a period of five months of [O₂] < 5 and two months of [O₂] < 2 . Detailed TOC, and XRF core scanning analyses of the short cores indicate that the depletion in DO started several decades prior to the varve preservation. This proves a sudden change in the depositional conditions in the lake following a transition phase of several decades during which varve preservation was not accomplished. Furthermore, varve preservation does occur at seasonal stratification and not necessarily requires permanent stratification.