Oxygen is the most important element in natural waters for any higher aquatic life, such as fish. As with humans, almost all members of aquatic ecosystems need oxygen to breathe. Building on this, treatment of drinking water is easier for oxic than for anoxic water. Critical for hypoxic conditions in lakes are the deep waters during stagnation periods, typically during summer or warm/wet periods, when the water columns are density stratified. Then, the oxygen supply to the deep water is restrained and the decomposition of organic matter leads to oxygen depletion in the stratified deep layers. Especially in shallow lakes, the resulting oxygen removal can be drastic.
In this presentation, we show how oxygen depletion mechanisms in lakes take place and how depletion is related to key processes such as primary productivity (via nutrient loading), molecular diffusion of oxygen into the sediment, molecular diffusion of reduced substances out of the sediment and, finally, the deep convective mixing in winter which refills the oxygen reservoirs of the deep layers. Here, we will clarify how these different processes affect the oxygen budget: primary production drives oxygen depletion directly, whereas lake morphology and external wind forcing influence the rate of depletion. Finally, climate change can affect the replenishment of oxygen in the deep water and thereby change the oxygen budget. The examples shown in this presentation will be set into the context of the water quality in Switzerland, which has gone through a rigorous improvement in the last 40 years.