Oxygen In Hypolimnion Research Articles

Isotope effects of O2 consumption in a deep lake as means for understanding partitioning of O2 demand among microorganisms, particles, and sediment

AbstractThe isotopic ratio 18O/16O of dissolved O2 in aquatic systems is affected by the preferential biological uptake of 16O (ε). Studies over the past six decades reveal that during incubation experiments, the isotopic effect of microorganism respiration (εorganism) varies in the range of −18‰ to −22‰. In contrast, natural variations in the deep‐ocean O2 concentration and δ18O levels show a considerably weaker effect (~ −10‰). The differences between these observations have been explained to result from either O2 uptake by sediments or organic particles that, due to diffusion‐limited respiration, are expected to weakly fractionate oxygen isotopes, by mixing processes or by weak fractionation at low temperatures. To gain better insight, we studied oxygen demand and δ18O in the deep, cold hypolimnion of Lake Stechlin between 2018 and 2021 as well as in various laboratory incubations. Our incubation results demonstrate an εorganism of about −24‰. Simple model calculations demonstrate a sediment O2 demand isotope effect (εSOD) of about −8.4‰, and a variated water‐column O2 demand isotope effect (εWOD) which is lower than εorganism, ranging from −13.9‰ in 2019 to −23‰ in 2021. Accompanying experiments indicate that the lower magnitude of εWOD may be related to respiration at organic particles lending to a weaker fractionation effect. Thus, variations in εWOD may reflect a changing partitioning of hypolimnion oxygen uptake between suspended particles and their containing microorganisms. Based on own incubation experiments with Daphnia carcasses, we discuss how possible changes of particle rigidity might influence εWOD.

Hypolimnetic oxygen depletion by sediment-based reduced substances in a reservoir formerly affected by acid mine drainage

Hypolimnion oxygen depletion (HOD) in lakes and reservoirs can degrade water quality and prevent recovery from eutrophication owing to the release of reduced metals and nutrients from sediments. However, oxygen consumption processes in the hypolimnia of reservoirs resulting from deep-water anoxia are not well understood. Here, we present the results from a mesotrophic reservoir, which had previously suffered from acid mine drainage, to demonstrate the process of hypolimnetic hypoxia. Sampling was conducted monthly from April 2021 to March 2022, and the sediment oxygen demand (SOD) and flux of reduced substances, including CH4, S2-, NH4+, and Mn2+ from the sediment core samples were analyzed intensively. Based on long-term monitoring data, we found that HOD in the reservoir began in early April and ended in early November. This led to a hypolimnetic hypoxic state in the lacustrine zone of the reservoir for eight months. The areal hypolimnetic oxidation demand (AHOD) and SOD were 0.21–1.29 gO2 m−2 d-1 and 0.06–0.84 gO2 m−2 d-1, respectively. This resulted in 30.45%–65.07% HOD due to oxygen depletion at the sediment–water interface. The highest fraction of SOD to AHOD occurred in the lacustrine zone samples of the reservoir. The flux of reduced substances in the sediment revealed that CH4 oxidation at the sediment–water interface dominated oxygen depletion in the lacustrine zone of the reservoir; additionally, S2- oxidation played an important secondary role in exacerbating hypolimnetic oxygen depletion in the mesotrophic reservoir. Sediments were a source of reduced substances. Organic matter content and SO42- and NO3- concentrations in the pore water were the most critical drivers of spatial variation in CH4 and S2- fluxes from the sediments. Temperature, dissolved oxygen concentration and saturation, and oxidation–reduction potential controlled temporal variations in CH4 and S2- concentrations in the overlying water. Compared with previous reports on the flux of reduced substances from sediments in lakes and reservoirs, HOD in the studies reservoir was not only affected by nutrient conditions but was also related to the sulfur cycle. Our findings indicate a spatial various of HOD mechanism in the reservoir and emphasized the importance of the sulfate reduction process in the reservoir formerly affected by acid mine drainage.

Accelerated sediment phosphorus release in Lake Erie's central basin during seasonal anoxia

AbstractEutrophication remains a serious threat to Lake Erie and has accelerated over past decades due to human activity in the watershed. Internal phosphorus (P) loading from lake sediment contributes to eutrophication, but our understanding of this process in Lake Erie is more uncertain than for its riverine P inputs. Past study has focused on incubating sediment cores in oxic or anoxic conditions, meaning we know little about sediment flux during state transitions. We used 56 controlled sediment core incubation experiments to quantify rates and onset of P release in Lake Erie's central basin as a function of depositional environment, season (spring, summer, and fall), temperature, and dissolved oxygen (DO) concentration. P flux under oxic or hypoxic (> 0 to ≤ 2 mg L−1 DO) conditions was slow (0.31–0.50 mg m−2 d−1) compared to anoxic P flux (5.19–30.7 mg m−2 d−1). The transition between slow and fast flux occurred within 24 h of anoxia (0 mg L−1 DO). Oxic or anoxic P flux was generally similar across seasons and incubation temperatures (8°C and 14°C). In 14°C incubated cores anoxic P flux onset was earliest in fall, when sediments had already been exposed to anoxic conditions in the lake. Re‐oxygenation of experimental cores that temporarily developed anoxia reversed the direction of P flux, but P release resumed at similar rates once the water returned to anoxia. Understanding the effects of hypolimnion oxygen conditions on internal P loading allows us to better constrain nutrients sources and implications for P budget management.

RESULTS OF MONITORING OF KALANOID COPEPODS EURYTEMORA LACUSTRIS (РОРРЕ, 1887) POPULATION STATE IN LAKE VECHELIE

Тhe spatial distribution of E. lacustris in the lake depends on the temperature and concentration of dissolved oxygen. Most of the individuals inhabit water layers 12–19 meters – the zone below thermocline. The water tempera-ture in these horizons remains constantly low (4,5–6,7 °С). Average content of dissolved oxygen in hypolimnion varies with 3,0–7,1 mg/l. When the concentration of dissolved oxygen in the hypolimnion layers increases, crustacean popula-tion concentrates more deeply.
 The population density of E. lacustris in lake Vechelye in different years of research consisted 584–2877 indi-viduals per m3. Although the temperature, transparency and concentration of oxygen in the water of lake Vechelye stay favorable for the species and characteristic of mesotrophic lakes, E. lacustris tends to reduce the number of its popula-tion.

Empirical modeling of hypolimnion and sediment oxygen demand in temperate Canadian lakes

ABSTRACT Sediment oxygen demand (SOD) and hypolimnetic oxygen demand (HOD) drive deep-water dissolved oxygen (DO) depletion in lakes, yet these parameters can be difficult to be measure routinely. To address this issue, we present an empirical DO depletion model from time-series measurements of hypolimnetic DO and water-column temperature profiles to estimate hypolimnion thickness (H). The model is based on a dataset that includes 3 temperate lakes (Lake Erie, Lake Simcoe, and Eagle Lake) in Ontario, Canada, of varying size and trophic state. We report SOD (mean [standard deviation]; 0.30 [0.07] g m−2 d−1) and HOD (0.08 [0.03] g m−3 d−1) values based on regression fits of (where t is time) and H from these lakes. The model shows that when vertical (through thermocline) and horizontal fluxes can be neglected in the DO budget during summer, SOD and HOD are the first-order parameters driving . The empirical model predicted hypolimnetic DO in the 3 lakes (root-mean-square error [RMSE] of DO < 3.58 g m−3) and was subsequently validated against observations from a fourth lake (Little Silver Lake, Ontario; RMSE of DO = 1.07 g m−3). The model provides insight into the importance of physical characteristics (i.e., 1/H) in the hypolimnetic DO budget and the relative impact of physical transport versus biogeochemical sources and sinks in the DO budget. When the spring turnover DO concentration is known, the model can be used to simulate depletion of DO in the hypolimnion, including the onset of hypoxia, using routinely collected temperature profile data. We suggest that the proposed values for SOD and HOD can serve as estimates for water quality model calibration when no information is available.

The influences of historic lake trophy and mixing regime changes on long-term phosphorus fraction retention in sediments of deep eutrophic lakes: a case study from Lake Burgäschi, Switzerland

Abstract. Hypolimnetic anoxia in eutrophic lakes can delay lake recovery to lower trophic states via the release of sediment phosphorus (P) to surface waters on short timescales in shallow lakes. However, the long-term effects of hypolimnetic redox conditions and trophic state on sedimentary P fraction retention in deep lakes are not clear yet. Hypolimnetic withdrawal of P-rich water is predicted to diminish sedimentary P and seasonal P recycling from the lake hypolimnion. Nevertheless, there is a lack of evidence from well-dated sediment cores, in particular from deep lakes, about the long-term impact of hypolimnetic withdrawal on sedimentary P retention. In this study, long-term sedimentary P fraction data since the early 1900s from Lake Burgäschi provide information on benthic P retention under the influence of increasing lake primary productivity (sedimentary green-pigment proxy), variable hypolimnion oxygenation regimes (Fe∕Mn ratio proxy), and hypolimnetic withdrawal since 1977. Results show that before hypolimnetic withdrawal (during the early 1900s to 1977), the redox-sensitive Fe∕Mn-P fraction comprised ∼50 % of total P (TP) in the sediment profile. Meanwhile, long-term retention of total P and labile P fractions in sediments was predominantly affected by past hypolimnetic redox conditions, and P retention increased in sedimentary Fe- and Mn-enriched layers when the sediment-overlaying water was seasonally oxic. However, from 1977 to 2017, due to eutrophication-induced persistent anoxic conditions in the hypolimnion and to hypolimnetic water withdrawal increasing the P export out of the lake, net burial rates of total and labile P fractions decreased considerably in surface sediments. By contrast, refractory Ca–P fraction retention was primarily related to lake primary production. Due to lake restoration since 1977, the Ca–P fraction became the primary P fraction in sediments (representing ∼39 % of total P), indicating a lower P bioavailability of surface sediments. Our study implies that in seasonally stratified eutrophic deep lakes (like Lake Burgäschi), hypolimnetic withdrawal can effectively reduce P retention in sediments and potential for sediment P release (internal P loads). However, after more than 40 years of hypolimnetic syphoning, the lake trophic state has not improved nor has lake productivity decreased. Furthermore, this restoration has not enhanced water column mixing and oxygenation in hypolimnetic waters. The findings of this study are relevant regarding the management of deep eutrophic lakes with mixing regimes typical for temperate zones.

Hypolimnetic oxygenation 2: oxygen dynamics in a large reservoir with submerged down-flow contact oxygenation (Speece cone)

Horne AJ, Jung R, Lai H, Faisst B, Beutel, M. 2019. Hypolimnetic oxygenation 2: oxygen dynamics in a large reservoir with submerged down-flow contact oxygenation (Speece cone). Lake Reserv Manage. 35:323–337. Low dissolved oxygen (DO) in the sediments of Camanche Reservoir, California (513 million m3, 31 km2), produced toxic hydrogen sulfide (H2S). Direct hypolimnetic oxygenation suppressed H2S without provoking early destratification and cold-water fish problems downstream. A cool, bubble-free plume directed horizontally over the sediments was chosen over a rising bubble plume. A submerged, down-flow contact oxygenator (Speece cone) pumped anoxic water from 5 m above the sediments to the top of a 7 m high cone to dissolve a counterflow of rising pure oxygen bubbles. The bubble-free, highly oxygenated discharge (80 mg/L) was diluted to fish-safe levels (8 mg/L) and directed up-reservoir via jets in a 45 m long manifold. Placing the cone on the bottom near the dam increased hydraulic pressure and doubled oxygen solubility. Poor-quality hypolimnion water (DO <2 mg/L, redox 18–100 mV, H2S odors) was converted to good-quality water (DO 3–7 mg/L, redox >300 mV, no H2S odors). Comparing preoxygenation hypolimnion DO decline (0.1 mg/L/d) with oxygenation temporarily switched off (0.23 mg/L/d) gave a full-scale estimate of induced hypolimnion oxygen demand. In 1994, the oxygenated plume moved 4.5 km upstream at 0.1 cm/s via natural water motion. No long pipes were needed. About 18% of the bottom hypolimnion was directly oxidized in the cone and 1.8 times the total volume was indirectly oxidized via entrainment in the plume. After 10 yr, oxygen additions were reduced by >50% with no deleterious effects.

Projecting the impact of climatic warming on coldwater fish habitat in Lake Jingpo

Climate warming would have a great impact on the water temperature and dissolved oxygen (DO) distribution of lakes in cold regions. It can have an effect on fish populations. To evaluate the effect of climatic change on coldwater fish habitat in cold regions of China, a fish habitat simulation model was developed. The fish habitat simulation model was developed based on the output of A vertical year-round water temperature and dissolved oxygen model. Lake Jingpo in Northeast China was chosen as the representive lake. The coldwater stenothermal fish called Hucho taimen was selected as the indicator species. Constant Values Method and oxythermal habitat parameter method were applied to identify the refuge habitat. The simulation results show that under the future climate scenarios, the uninhabitable space in winter due to low dissolved oxygen in hypolimnion will decrease, the uninhabitable space in summer due to high temperature in epilimnion will expand. Total habitat depth in summer is projected to decrease up to 60%-90%, which means high probability of summer kill. It is projected that multi-year average value of ATDO3 vb (highest average daily TDO3 over variable benchmark period) will have an increase of about 2.5℃, which means Hucho taimen juvenile survival rate will decrease about 35%. In the extreme warm year, the Hucho taimen juvenile survival rate will decrease to 9.0%. The oxythermal habitat parameter method is generic and can be applied to other lakes and reservoirs in Northeast China to identify and compare the refuge habitat of coldwater fishes.

High-resolution paleoecological records from Lake Malawi show no significant cooling associated with the Mount Toba supereruption at ca. 75 ka

The impact of the ca. 75 ka Mount Toba (Indonesia) supereruption on regional and global climate has been controversial. East Africa is pivotal for qualifying this impact, specifically in addressing a hypothesis that this eruption caused a volcanic winter leading to a genetic bottleneck in anatomically modern human populations. If the Toba eruption caused a volcanic winter in East Africa, effects should be recorded in sediments from climatologically sensitive ecosystems. In deep tropical lakes, cooler temperatures would have reduced the density contrast between the epilimnion and hypolimnion, causing increased mixing, hypolimnion oxygenation, and enhanced nutrient advection. Such environmental changes produce noticeable and predictable responses in lake fossil assemblages and sediment loading. Here we present the first paleoecological evidence from East Africa of environmental conditions surrounding the eruption in a high-resolution (∼8–9 yr) analysis of two cores collected by the Lake Malawi Drilling Project. These cores contain an undisturbed record of sedimentation before, during, and after deposition of the youngest Toba Tuff. Concentrations of climate-sensitive ecological indicators such as phantom midges, diatoms, and other algae through a >270 yr interval bracketing the Toba cryptotephra show no unusual or sustained deviations from background variability within Lake Malawi associated with the hypothesized post-Toba cooling. We find no evidence for significantly enhanced mixing or ecosystem disturbance that would be anticipated following a volcanic winter.

Modeling Lake Erie's hypoxia response to nutrient loads and physical variability

A 1-dimensional, linked hydrodynamic and eutrophication model was developed and calibrated with 19years of observations (1987–2005) for the summer stratification period in the central basin of Lake Erie, corroborated by comparison with observed process rates and areal hypoxic extents, and confirmed with observations from the 1960s and 1970s. The model effectively captures observations of both vertical and temporal trends in dissolved oxygen, as well as temporal trends in chlorophyll-a, phosphorus, zooplankton biomass, and several key processes. The model was used to develop a relationship between external phosphorus load and hypolimnion oxygen conditions, and then to establish load–response envelopes that account for inter-annual variability in physical conditions driven by variation in meteorological drivers. The curves provide a valuable tool for reassessing phosphorus loading targets with respect to reducing hypoxia in Lake Erie.

Lake microbial communities are resilient after a whole-ecosystem disturbance

Disturbances act as powerful structuring forces on ecosystems. To ask whether environmental microbial communities have capacity to recover after a large disturbance event, we conducted a whole-ecosystem manipulation, during which we imposed an intense disturbance on freshwater microbial communities by artificially mixing a temperate lake during peak summer thermal stratification. We employed environmental sensors and water chemistry analyses to evaluate the physical and chemical responses of the lake, and bar-coded 16S ribosomal RNA gene pyrosequencing and automated ribosomal intergenic spacer analysis (ARISA) to assess the bacterial community responses. The artificial mixing increased mean lake temperature from 14 to 20 °C for seven weeks after mixing ended, and exposed the microorganisms to very different environmental conditions, including increased hypolimnion oxygen and increased epilimnion carbon dioxide concentrations. Though overall ecosystem conditions remained altered (with hypolimnion temperatures elevated from 6 to 20 °C), bacterial communities returned to their pre-manipulation state as some environmental conditions, such as oxygen concentration, recovered. Recovery to pre-disturbance community composition and diversity was observed within 7 (epilimnion) and 11 (hypolimnion) days after mixing. Our results suggest that some microbial communities have capacity to recover after a major disturbance.

Effect of hypolimnetic oxygenation on oxygen depletion rates in two water-supply reservoirs

Oxygenation systems, such as bubble-plume diffusers, are used to improve water quality by replenishing dissolved oxygen (DO) in the hypolimnia of water-supply reservoirs. The diffusers induce circulation and mixing, which helps distribute DO throughout the hypolimnion. Mixing, however, has also been observed to increase hypolimnetic oxygen demand (HOD) during system operation, thus accelerating oxygen depletion. Two water-supply reservoirs (Spring Hollow Reservoir (SHR) and Carvins Cove Reservoir (CCR)) that employ linear bubble-plume diffusers were studied to quantify diffuser effects on HOD. A recently validated plume model was used to predict oxygen addition rates. The results were used together with observed oxygen accumulation rates to evaluate HOD over a wide range of applied gas flow rates. Plume-induced mixing correlated well with applied gas flow rate and was observed to increase HOD. Linear relationships between applied gas flow rate and HOD were found for both SHR and CCR. HOD was also observed to be independent of bulk hypolimnion oxygen concentration, indicating that HOD is controlled by induced mixing. Despite transient increases in HOD, oxygenation caused an overall decrease in background HOD, as well as a decrease in induced HOD during diffuser operation, over several years. This suggests that the residual or background oxygen demand decreases from one year to the next. Despite diffuser-induced increases in HOD, hypolimnetic oxygenation remains a viable method for replenishing DO in thermally-stratified water-supply reservoirs such as SHR and CCR.

Macroinvertebrate 13C variability analysis for the assessment of lake trophic functioning

The aim of this study was to better understand the relationships of within- and among-lake variability of macroinvertebrate carbon stable-isotope ratios with both lake trophic potential and lake trophic functioning. Two approaches based on the macroinvertebrate communities were undertaken on 11 French lakes: the Lake Biotic Index (LBI), a lake biological quality assessment method (Verneaux et al. 2004 a), and the carbon isotopic signatures of macroinvertebrates expressed by metrics of dispersion (standard deviation of the littoral macroinvertebrate mean δ 13 C values and differences between littoral and deep mean δ 13 C values (Δδ 13 C)). The littoral macroinvertebrate mean δ 13 C values obtained ranged from -11.7%o to -38.6‰. This great among-lake variability can be partially explained by different geological substrata on which the lakes are located and, potentially, by the lake organic matter recycling activity. Moreover, the depth-related differences in macroinvertebrate community δ 13 C (Δδ 13 C values) or macroinvertebrate taxa (taxon 13 C depletion: TΔδ 13 C) were highly correlated to the values of a LBI sub-index, which was itself correlated to hypolimnion oxygenation and deep sediment organic matter contents. This result suggests that the studied lakes showed various degrees of deep recycled carbon assimilation. The depth-related macroinvertebrate 13 C depletion may then be considered as describing the result of lake trophic functioning. However, because no correlation was observed between littoral macroinvertebrate δ 13 C variability and lake trophic potential, it seemed difficult to conclude as to the influence of different carbon sources on this potential.

Vertical distribution of pelagial zooplankton in a middle-sized dimictic valley reservoir

Our observations indicate the vertical distribution of zooplankton and its seasonal changes in Dubnik II reservoir (Slovakia) are determined mainly by the thermal regime of the reservoir, by transparency, and by fish and invertebrate predation. During periods of circulation, zooplankton vertical distribution in the whole water column was more homogeneous, whilst during summer temperature stratification zooplankton concentrated in the epilimnion — rotifers in higher layers than crustaceans. During summer stagnation a steep thermal gradient occurred at the boundary of the epi-and hypolimnion and low temperature and low dissolved oxygen in hypolimnion offered a refuge for Chaoborus flavicans larvae against fish, enabling coexistence of vertebrate and invertebrate predation. This evidence supports our previous findings concerning dominance of rotifers in zooplankton and representation of crustaceans by small-bodied species in the study reservoir. Steep thermal gradient and the presence of Chaoborus larvae caused very low zooplankton abundance in the lower part of the water column and a reduction of cladocerans refuges against fish to layers of thermocline or closely under thermocline where Daphnia cucullata and Daphnia parvula were found. Our previous assumptions about the high density of zooplanktivorous fish in Dubnik II reservoir are supported by the fact that these small cladocerans are represented by smaller individuals in the upper layers and bigger individuals in deeper layers.

Vertical distribution ofDaphniain Lake Berendonck (The Netherlands) during progressive hypolimnion oxygen depletion

Vertical distribution of<i>Daphnia</i>in Lake Berendonck (The Netherlands) during progressive hypolimnion oxygen depletion

Change of phytoplankton composition and biodiversity in Lake Sempach before and during restoration

Lake Sempach, located in the central part of Switzerland, has a surface area of 14 km2, a maximum depth of 87 m and a water residence time of 15 years. Restoration measures to correct historic eutrophication, including artificial mixing and oxygenation of the hypolimnion, were implemented in 1984. By means of the combination of external and internal load reductions, total phosphorus concentrations decreased in the period 1984–2000 from 160 to 42 mg P m−3. Starting from 1997, hypolimnion oxygenation with pure oxygen was replaced by aeration with fine air bubbles. The reaction of the plankton has been investigated as part of a long-term monitoring program. Taxa numbers, evenness and biodiversity of phytoplankton increased significantly during the last 15 years, concomitant with a marked decline of phosphorus concentration in the lake. Seasonal development of phytoplankton seems to be strongly influenced by the artificial mixing during winter and spring and by changes of the trophic state. Dominance of nitrogen fixing cyanobacteria (Aphanizomenon sp.), causing a severe fish kill in 1984, has been correlated with lower N/P-ratio in the epilimnion. Buoyant algae such as Planktothrix rubescens (syn. Oscillatoria rubescens) increased in abundance due to enlargement of the trophogenic layer and extended mixing depth during winter. The interactions between zoo- and phytoplankton seemed to be depressed as a result of restoration measures. Zooplankton composition changed to more carnivorous and less herbivorous species. Oxygenation of the hypolimnion induced bioturbation of sediments, mainly by oligochaetae worms, and stimulated germination of spores and cysts and hatching of resting eggs.

Hypolimnion oxygen concentrations and the abundance and size of bacteria

Hypolimnion oxygen concentrations and the abundance and size of bacteria

Wind-Induced Hypolimnion Exchange in Lake Ontario's Kingston Basin: Potential Effects on Oxygen

The hypolimnion of Lake Ontario's Kingston basin exhibited significant oxygen depletion during 1991. The degree of hypolimnion oxygen depletion was consistent with what would be expected on the basis of morphology. No such depletion was observed in 1992, a year during which hypolimnetic currents in the channel west of Galloo Island flowed predominantly from Lake Ontario into the Kingston basin. The prevailing southwest winds during the summer of 1991 were anomalously persistent. Wind and current data from 1992 suggest that winds from the south-southeast, departures from the prevailing southwest winds, enable hypolimnetic flow from the main basin of Lake Ontario to replace the hypolimnion. This apparently failed to occur during 1991.

数理モデルによるエアリフト循環の余呉湖水質改善効果の予知に関する研究

A mathematical model was developed to predict the effect of vertical water circulation caused by air-lift on water quality restoration in a lake. In this model, several ecosystem paths such as Algal growth, predation by zooplankton, degradation of organics and so on were combined with completely-mixing series tank model, in which a water body was divided vertically to two districts, epilimnion and hypolimnion. This model was applied to Lake Yogo, where two sets of the vertical water circulation unit were set, and verified by comparing the simulated results with observed data. The vertical circulation effects on lake restoration were discussed with this model from the viewpoints of concentrations of dissolved oxygen in hypolimnion, phytoplankton, total nitrogen and total phosphorus.

Dynamics of dissolved oxygen and vertical circulation in fish ponds

Dissolved oxygen dynamics and vertical water circulation in high density integrated fish culture ponds were monitored monthly at 2-h intervals for 26 h from March through September at the Pearl River Delta, China. Stable thermal stratification was found almost daily in May–September. Vertical circulation began daily between 8 and 10 p.m. The average depth for this turnover in summer can be as great as 0.7–1 m. Complete vertical circulation occurs only during major storms. A convection turbulence model was used to examine the vertical mixing process and showed that light wind (< 100 cm/s) and surface cooling (< 2°C) after sunset can substantially influence the depth of mixing. Photosynthesis by algae is the most important natural mechanism contributing oxygen to ponds; compared to photosynthesis, net oxygen gain and loss due to diffusion during daylight is small. The compensation depth corresponded to twice the Secchi disk depth, ranging from 50 to 80 cm. The dissolved oxygen (D.O.) concentration in the hypolimnion was usually less than 2 mg/l, but increased at night during the period of vertical circulation. Hypolimnion oxygen deficit (HOD) was explored with the in situ D.O. concentration. HOD was found to increase during the daylight period and decrease at night after destratification.