Metalimnetic Oxygen Minima

Extracts from Wetzel’s Limnology, 3rd ed. (2001), pp159-160.B. Metalimnetic Oxygen Minima
The converse condition, a metalimnetic oxygen minimum exhibiting a negative heterograde curve (Fig. 9-5b)*, is much less frequently observed. Numerous causal mechanisms have been associated with such metalimnetic reductions, and most or all are likely to be in effect simultaneously in situations where such oxygen minima are observed. To single out one mechanism as operational to the exclusion of others (as, e.g., Czeczuga, 1959) can be misleading. Oxidizable material produced in the epilimnion or brought into the lake from outside the basin continuously sinks. Sinking rates slow when it encounters denser memlimnetic water, which allows more time for decomposition (Birge and Juday 1911; Thienemann, 1928). Organic matter introduced into reservoirs from storm water can flow. depending on water temperatures and densities, in the metalimnion, as interflow, and contribute to accelerated oxygen reduction in the memlimnion (Nix, 1981}. Moreover, decomposition rates are usually higher in the metalimnion, where temperatures arc greater than in the colder hypolimnion. As a result, more readily oxidizable organic matter is decomposed al this level, with concomitant consumption of oxygen by bacterial respiration; more resistant organic matter settles slowly into the hypolimnion (Kuznetsov and Karsinken, 1931; Vinberg, 1934; berg and Rodhe, 1942). In a review of the subject, Czecwga (959) presented some evidence from a lake in which, conversely, decomposition rates were greater in the epilimnion. However, renewal of epilimnetic oxygen and time of residence of sedimenting organic matter in the mctalimnion were not clearly delineated. It is clear that a balanced between  the transparency of the trophogenic zone and the depth of the metalimnion is important in relation to the depth at which photo- synthetic maxima occur and in relation to whether oxygen inputs are sufficient to offset decompositional consumption (Ruttner, 1933). It is easy to conceive of a situation in which a pronounced mctalimnetic maximum could shift to a minimum within a summer season (Fig. 9-6).In certain situations, concentrations of massive numbers of zooplanktonic microcrustacea in the metalimnion can contribute to a severe reduction of oxygen. Such respiratory consumption was most likely a major cause of the memlimnetic minima observed in Ziirichsee, Switzerland (Minder, 1923), and the same condition can prevail in Lake Washington (Fig. 9-5b), where non-migratting copepods often develop profusely in the summer (Shapiro, 1960). In the extreme case shown in Figure 9-6, less than 10 percent of the metalimnetic oxygen minimum was accounted for by zooplankton respiration (Mitchell and Bums, 1979). The metalimnetic anoxia was apparently caused mainly by intensive bacterial decomposition.The basin morphometry of a lake can also con-tribute to metalimnetic oxygen minima. In cases where the slope of the basin is gentle and where it coincides with the prevailing metalimnion, a greater area of the sediments with high bacterial utilization of oxygen will be in contact with metalimnetic water than is the case of sediments adjacent to the lower strata, Horizontal mixing and streaming of water from internal water movements is greatest in the metalimnion, where vertical density stability is greatest. As a result, reduction in oxygen content at the sediment-water interface is greater where the slope of the sediments is less. The oxygen reduction in the metalimnion may be sufficient to extend laterally over the entire lake, or, as in the case of Skiirshuhsjon, Sweden (Alstcrberg, 1927), it may occur more strongly in the meralimnion closer to the sediment-metalimnion interface. Biogenic oxidation of methane by methane·oxidizing bacteria can result in mctalimnetic oxygen minima in certain productive lakes (Kuznetsov, 1935, 1970; Ohle, 1958). Methane from anaerobic fermentation in sediments rises from the sediments and is carried to water strata such as the metalimnion, where, with warmer temperatures and greater oxygen, methane oxidation can occur rapidly. In such cases, severe oxygen reduction can occur in a few days if adequate dissolved inorganic nitrogen is available (Rudd e/ al., 1975,1976).*See top figure in PDF of a slide presentation by Michael Seewald for a diagram illustrating orthograde, positive heterograde and negative heterograde summer oxygen profiles.

The formation of a metalimnetic oxygen minimum exemplifies how ecosystem dynamics shape biogeochemical processes: A modelling study
Chenxi mi et al, 2020 in Water Research. “..we simulated the metalimnetic oxygen minimum (MOM) in the Rappbode Reservoir… The results indicated that around 60% of the total oxygen consumption rate in the MOM layer originated from benthic processes whereas the remainder originated from pelagic processes… Our research also confirmed the decisive role of water temperature in the formation of the MOM”

A metalimnetic oxygen minimum indirectly contributing to the low biomass of cladocerans in Lake Hiidenvesi – a diurnal study on the refuge effect
Jukka Horppila, et al., 2000 in Hydrobiologia. “In the study area, an oxygen minimum occurred in the metalimnion in the 10–15 m depth. No diurnal fluctuations in the position of the minimum were observed. Cladocerans inhabited the epilimnion throughout the study period and their vertical movements were restricted to above the thermocline and above theoxygen minimum. C. flavicans conducted a diurnal migration. During the day, the majority of the population inhabited the 12 – 15 m depth just in the oxygen minimum, while during darkness they were found in the uppermost8 m. Smelts started ascending towards the water surface before sunset and reached the uppermost 3 m around23:00. During daytime, the majority of smelts inhabited the depth of 7–9 m, where the water temperature was unfavourably high for them (18 ◦C). Smelts thus probably avoided the steep oxygen gradient in the metalimnion, whereas Chaoborus used the oxygen minimum as a refuge against predation. Those smelts that were found in the same water layers as Chaoborus used the larvae as their main prey. The metalimnetic oxygen minimum thus seemed to favour the coexistence of vertebrate and invertebrate predators, leading to a depression of cladoceran zooplankton.

The importance of physical transport and oxygen consumption for the development of a metalimnetic oxygen minimum in a lake
Julika Kreling et al. 2016 in Limnology & Oceanography. “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…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.”

Forty years record of the metalimnetic oxygen minimum in Germany’s largest drinking water reservoir
M Seewald et al., 2021. vEGU21, the 23rd EGU General Assembly, held online 19-30 April, 2021, id.EGU21-12937
View PDF of slide presentation “..The Rappbode Dam, Germany’s largest drinking water reservoir, forms a MOM every year and long-term observations indicate that the oxygen deficit may have increased in recent years…The results confirm increasing surfacewater temperatures and unchanged deepwater temperatures in summer (Mai to October) as well as an increasingly prolonged summer stratification in the course of global warming. In contrast to the previous working hypothesis, increasing stratification duration is not correlated with the significantly increasing (p 0.009; τ -0.26) annual maximum intensity of the MOM.

Metalimnetic oxygen minimum and the presence of Planktothrix rubescens in a low-nutrient drinking water reservoir
Valerie C. Wentzky et al., 2019 Water Research Volume 148, 1 January 2019, Pages 208-218.. From the Intro: Dissolved oxygen is a key variable for nearly all organisms in the aquatic environment. Especially in stratified lakes, vertical transport of dissolved substances is limited, which can result in sharp vertical gradients of oxygen concentration. Usually in direct contact with the atmosphere, the epilimnion shows a gas pressure that is close to equilibrium with the atmosphere, while the hypolimnion has trapped a limited amount of oxygen, which is subjected to depletion over the summer months until the thermal stratification breaks and deep recirculation recharges the hypolimnion with oxygen (e.g., Boehrer and Schultze, 2008). In many cases, however, a more complex picture is observed. Especially in the thermocline, where high density gradients restrict the vertical exchange, gradients of dissolved substances are formed and sustained due to small vertical transport. Both metalimnetic oxygen maxima and metalimnetic oxygen minima can be found. Though both features are commonly encountered, oxygen maxima have been dealt with in more detail in the literature (e.g., Wilkinson et al., 2015). Here we concentrate on the case of metalimnetic oxygen minima, which often get attributed to eutrophic lakes (e.g., Lake Arendsee: Boehrer and Schultze, 2008; in general see also Wetzel, 2001) or reservoirs (Zhang et al., 2015); however, also lakes of lower trophic state can show metalimnetic oxygen minima (e.g., Joehnk and Umlauf, 2001).

Response of Green Lake, Wisconsin, to Changes in Phosphorus Loading, With Special Emphasis on Near-Surface Total Phosphorus Concentrations and Metalimnetic Dissolved Oxygen Minima
Dale M. Robertson et al. 2022. USDI & USGS Scientific Investigations Report 2022–5003. “Green Lake is the deepest natural inland lake inWisconsin, with a maximum depth of about 72 meters. In the early 1900s, the lake was believed to have very good water quality (low nutrient concentrations and good water clarity) with low dissolved oxygen (DO) concentrations occurring in only the deepest part of the lake. Because of increased phosphorus (P) inputs from anthropogenic activities in its watershed, total phosphorus (TP) concentrations in the lake have increased; these changes have led to increased algal production and low DO concentrations not only in the deepest areas but also in the middle of the water column (metalimnion)…Data from previous studies showed that DO concentrations in the metalimnion decreased slightly as summer progressed in the early 1900s but, since the late 1970s, have typically dropped below 5 milligrams per liter (mg/L), which is the WDNR criterion for impairment.

Fondriest: Dissolved Oxygen
Below the epilimnion is the metalimnion, a transitional layer that fluctuates in thickness and temperature. The boundary between the epilimnion and metalimnion is called the thermocline – the point at which water temperature begins to steadily drop off ¹¹. Here, two different outcomes can occur. If light can penetrate beyond the thermocline and photosynthesis occurs in this strata, the metalimnion can achieve an oxygen maximum ¹¹. This means that the dissolved oxygen level will be higher in the metalimnion than in the epilimnion. But in eutrophic or nutrient-rich lakes, the respiration of organisms can deplete dissolved oxygen levels, creating a metalimnetic oxygen minimum ⁴².