Does polymixis complicate prediction of high‐frequency dissolved oxygen in lakes and reservoirs?
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Abstract As lake and reservoir ecosystems transition across major environmental regimes (e.g., mixing regime) resulting from anthropogenic change, setting predictive expectations is imperative. We tested the hypothesis that (dissolved) oxygen is more predictable in monomictic reservoirs that thermally stratify throughout the summer (warm) season compared to polymictic reservoirs that stratify intermittently. Using two‐hourly vertical profiles of oxygen, we compared daily‐aggregated errors of oxygen predictions from random forests across and within two monomictic and two polymictic reservoirs in the south‐central (subtropical) USA. Although one monomictic reservoir was typically more predictable than the polymictic reservoirs, the hypereutrophic, small monomictic reservoir had less predictable oxygen patterns potentially related to rapid oxygen cycling and intrusions of oxygenated waters in the hypolimnion without mixing. Daily mixing did not relate strongly to model errors. Water temperature, depth, and wind were the most important predictors, but were not clearly related to season or mixing. Lastly, we compared multiple model types (regression, neural network, and process‐based) in one polymictic reservoir to test how our interpretations of oxygen predictability were sensitive to model type, finding that the models generally agreed; however, the process‐based model poorly predicted oxygen in the middle of the vertical profiles (5 m) where most models performed poorly due to a temporally unstable, vacillating metalimnion. Our results suggest predicting reservoir oxygen dynamics may be easier in stratified reservoirs, but eutrophication and complex hydrodynamics may cause forecasting surprises especially for those who use or manage water resources in mono‐ or dimictic reservoirs.Previous studies of the central basin of Lake Erie have indicated, on the basis lakewide budgets of heat and dissolved oxygen, that the thickness of the hypolimnion and the interaction of the hypolimnion with the overlying fluid are important factors governing the dissolved oxygen concentration in the near‐bottom water. Data collected during an intensive field program in 1979 contain an example of an event during which both the thickness and temperature of the hypolimnion increase due to an erosion of the thermocline from below. This thickening of the hypolimnion requires two conditions. First, the thermocline region, or metalimnion, must be thick enough so that the effects of the energetic surface mixing processes are confined to the upper portion of the thermocline. And second, the currents in the thin hypolimnion must be strong enough to entrain overlying metalimnion water down into the hypolimnion. An analysis of the current meter data suggests the source of turbulent energy driving the mixing is shearing stresses at the bottom. During the entrainment event, the contribution to bottom dissolved oxygen supply is about 10% of the daily demand.
Entrainment (biomusicology)
Epilimnion
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Vertical diffusion rates ( K ( K z ) were determined by measuring for several weeks the vertical spread of an injection of tritiated water into the thermoclines and hypolimnia of Lake 227 and Lake 224 in the Experimental Lakes Area (ELA) of northwestern Ontario. K z values of 5×10 −5 and 8×10 −4 cm 2 · s −1 were determined from the tracer experiments in the thermoclines of L227 and L224; in the hypolimnia, similar K z determinations of 1.7×10 −3 and 1.8×10 −2 cm 2 ·s −1 are 20−30 times greater than the thermocline rates. Vertical diffusion rates of heat were determined over the same time and depth intervals as the tracer experiments. In each lake, heat is diffusing vertically faster than mass in the thermocline and at more equal rates in the hypolimnion. The low K z values and the greater diffusion rate of heat than mass (tritium) indicate that molecular diffusion is important in determining the rate of vertical transport in the thermoclines of these highly stratified lakes. Vertical eddy diffusion rates ( K z ') determined by the tracer experiments show an inverse proportionality to the static stability of the water column (N 2 ), such that K z ′ ∝ (N 2 ) −0 · 8 . However, K z ′ values determined by measuring the hypolimnetic heating rates of eight ELA lakes (including L227 and L224) and three lakes outside ELA indicate that K z ′ ∝ (N 2 ) −0.44 . These observations suggest that in the absence of large shear where K z ′ < 10 −2 cm 2 · s −1 the vertical diffusion rates of mass and heat show different inverse correlations to the static stability of the water column.
TRACER
Eddy diffusion
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Epilimnion
Forcing (mathematics)
Stratification (seeds)
Mixed layer
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Marked differences in areal hypolimnetic oxygen deficits were found among three depressions of a single lake. These differences were similar to those measured 11 yr before and indicate that regional patterns of nutrient input, production, and eutrophication exist. The patterns are related to lakeshore land use and development. The rate of eutrophication has accelerated across the lake over the 11‐yr span. This increase in eutrophication rate would not have been detected had not the hypolimnetic oxygen deficit data also taken into account the differences in hypolimnetic temperatures between the two studies. In fact, without considering temperature, opposite conclusions would have been reached; i.e. that the rate of eutrophication was lessening.
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The complete climatic courses of the parameters of stable thermal stratification for the central part of Lake Ladoga, the largest European lake, are presented on the basis of empirical relationships, taking into account the physical processes governing water temperature variations. For the first time, the seasonal cycle of the surface water temperature, the temperature and the depth of the thermocline, and the hypolimnion temperature are calculated using the vertical profiles of the temperature obtained from the central area of Lake Ladoga. Temperature data are used for the period of in situ observations from 1897 to the present. The proposed functional forms of the temporal temperature cycle and the course of thermocline’s boundaries deepening are useful for examination and simulation of the heat vertical transport from air to water. Approximation curves for the parameters of heating and cooling periods were developed with high significant determination coefficients. Time dependencies of the climatic rates of change in water temperature and the depth of the thermocline boundaries were determined from the onset of stable stratification to its dissipation. The highest rate of water temperature change in the heating stage takes place in late June–early July, which at the water surface, is 0.32 °C/day, while in the thermocline layer, it is 0.18 °C/day. The peak velocity during the cooling stage at the surface occurs in late August–early September and is 0.14 °C/day, whereas in the thermocline, it is 0.08 °C/day and takes place between September and early October. During the period of heating, the deepening parameters of the thermocline layer do not fluctuate very much, only within the range of 0.1–0.3 m/day. During the cooling period, under the influence of free convection, rates increase drastically. The maximum rates of deepening during the period of full autumn mixing reach 1.8 m/day. When the autumn overturn occurs, the epilimnion thickness equals the bottom depth, and the bottom temperature reaches its maximum during the annual cycle. Climatic norms of the stratification parameters against which it is necessary to assess climate change are calculated.
Stratification (seeds)
Thermal Stratification
Lapse rate
Mixed layer
Annual cycle
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According to the vertical section monitoring data of Lake Lugu water temperature (WT), electrical conductivity (EC), dissolved oxygen (DO), pH and chlorophyll-a (Chl-a) parameters in January (winter), April (spring), July (summer), and October (autumn) in 2015, the vertical stratification structure of WT and the null seasonality of water chemistry were analyzed. The relationship between the seasonal variation of WT stratification and the spatial and temporal distribution of EC, pH, DO and Chl-a was explored. The relationship between EC and WT was found for the epilimnion, thermocline and hypolimnion. The results of the study showed that: (1) The Lake Lugu water body shows obvious thermal stratification in spring, summer and autumn. In winter, the WT is close to isothermal condition in the vertical direction; in summer, the thermocline is located at 10–25 m water depth; while in autumn, the thermocline moves down to 20–30 m. (2) The Hypolimnion WT was maintained at 9.5 °C~10 °C, which is consistent with the annual mean temperature of Lake Lugu, indicating that the hypolimnion water column is stable and relatively constant, and reflects the annual mean temperature of the lake. The thermally stratified structure has some influence on the changes of EC, DO, pH and Chl-a, resulting in the obvious stratification of EC, DO and pH in the water body. (3) Especially in summer, when the temperature increased, the thermal stratification phenomenon was significant, and DO and pH peaked in thermocline, with a decreasing trend from the peak upward and downward, and the hypolimnion was in an anoxic state and the pH value was small. Although chlorophyll a remained low below thermocline and was not high overall, there was a sudden increase in the surface layer, which should be highly warned to prevent a large algal bloom or even a localized outbreak in Lake Lugu. (4) There is a simple linear function between EC and WT in both vertical section and Epilimnion, thermocline and hypolimnion, which proves that Lake Lugu is still influenced by natural climate and maintains natural water state, and is a typical warm single mixed type of lake. (5) It is suggested to strengthen water quality monitoring, grasp its change pattern and influence factors, and take scientific measures to prevent huge pressure on the closed ecological environment of Lake Lugu, and provide scientific basis for the protection of high-quality freshwater lakes in the plateau.
Epilimnion
Stratification (seeds)
Seasonality
Thermal Stratification
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