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

Abstract

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.

Assessing vertical diffusion in a stratified lake using a three‐dimensional hydrodynamic model

“…Comparing the model‐based vertical diffusion coefficients K z with the values shown in Kreling et al (), it turned out that the model reproduced the magnitude of K z in the metalimnion as well as the temporal trend of K z over the season (Figure ). Both model‐based and measurement‐derived values of K z yielded minimal values below 1.25 × 10 −6 m 2 s −1 in the middle of July and 5–10 times larger values in early summer.…”

“…In a recent study, Giling et al () pointed out that the interplay between physical drivers, chemical conditions and biological activity remained largely unresolved in this layer. The environmental conditions within the metalimnion with respect to light, temperature, or nutrients can vary from lake to lake and even within a given lake these variables change strongly within space and time and, hence, can induce complex local patterns and dynamics such as deep chlorophyll maxima (Leach et al, ), oxygen maxima (Wilkinson et al, ) or oxygen minima (Kreling et al, ). Apart from the case study level, generalized model‐based predictions of biogeochemical activity in the metalimnion are therefore difficult to achieve.…”

Mixing efficiency in the thermocline of lakes observed from eddy correlation flux measurements

“…Lake Scharmützelsee has an elongated shape with a surface area of 12.1 km 2 , a mean depth of 8.9 m and a maximum depth of 29.5 m. Lake Arendsee has an elliptic shape and about half of the surface area (5.14 km 2 ). On average it is 29 m deep with a maximum depth of 49.5 m. Both lakes are dimictic and suffer from severe oxygen depletion during summer stratification: While the hypolimnion of Lake Scharmützelsee turns anoxic [ Kreling et al ., ], Lake Arendsee develops a metalimnetic oxygen minimum as well as anoxic bottom waters [ Kreling et al ., ].…”

Three‐Dimensional Effects of Artificial Mixing in a Shallow Drinking‐Water Reservoir

“…Even with the reduction of the oxygen delivery rate, the DO level in the hypolimnion remained high (over 8 ). However, a metalimnetic minimum DO (MMDO) (Kreling et al, ; Morris et al, ; Shapiro, ) developed in the lower metalimnion during the operation of the SSS system, as seen in Figures c and d. This was caused by the weak vertical mixing induced by the SSS system with its jet nozzles pointing almost horizontally, which has limited capacity to transport DO vertically toward the metalimnion (Gerling et al, ).…”