The importance of physical transport and oxygen consumption for the development of a metalimnetic oxygen minimum in a lake

Although the occurrence of metalimnetic oxygen minima (MOM) in lakes during summer stratification was described by early limnologists, not much is known about the processes leading to its formation. Generally, dissolved oxygen (DO) consumption and net transport contribute to the observed DO decrease, but the latter was rarely considered in former studies. To examine the importance of both processes to MOM development in Lake Arendsee, Germany, we measured DO concentration and temperature with high spatial and temporal resolution during a complete stratification period. Vertical turbulent diffusivities were estimated from temporal changes of the heat content of the lake. The MOM development was not caused by locally enhanced DO consumption rates, but rather by a combination of vertical gradients in DO concentration and consumption. A vertical DO mass balance revealed that net DO transport into the metalimnion was in the same order of magnitude as DO consumption in the MOM. We found that DO consumption was governed by microbial respiration and the vertical variations of DO depletion rates in the metalimnion could be explained by a minor contribution of temperature and a higher contribution of turbidity, implying that the downward flux of particulate organic carbon promoted the MOM development. The intensive metalimnetic respiration in lakes forming a MOM can be expected to accelerate nutrient cycling close to the photic zone and thus, may further stimulate primary production.