Fall Turnover Research Articles

The influence of mixing on seasonal carbon dioxide and methane fluxes in ponds

Inland waters are important sources of the greenhouse gases carbon dioxide (CO2) and methane (CH4). Ponds have amongst the highest CO2 and CH4 fluxes of all aquatic ecosystems, yet seasonal variation in fluxes remain poorly characterized, creating challenges for accurately estimating annual emissions. Further, ponds can exhibit a range of mixing regimes, yet the impact of mixing regimes on gas emissions remains unclear. Here, we assessed annual dynamics of CO2 and CH4 in four temperate ponds (Minnesota, USA) that varied in mixing regimes. The ponds ranged from annual sinks to sources of CO2 (−1 to 15 mol m−2 yr−1) and were all significant sources of CH4 (4.3–8.2 mol m−2 yr−1), with annual fluxes in CO2 equivalents of 1.8–4.1 kg CO2-eq. m−2 yr−1. Mixing regimes impacted CO2 and CH4 dynamics, as stratified periods were associated with more anoxia, greater accumulation of gases in the bottom waters, higher emissions of CH4, and lower fluxes of CO2. Ponds with stronger summer stratification also had increased CO2 and CH4 fluxes associated with fall turnover. Overall, the two ponds with the strongest stratification had higher annual fluxes (2.6, 4.1 kg CO2-eq. m−2 yr−1) compared to the two ponds that more frequently mixed (1.8, 2.2 kg CO2-eq. m−2 yr−1).

Effect of a dense inflow on the stratification of a steep‐sided lake

AbstractWe detail the effect of a small stream of dense inflow that significantly altered the stratification and water quality in a constructed water body in northern British Columbia, Canada. As the dense inflow passed through the epilimnion of the steep‐sided lake, it entrained relatively large quantities of water. The resulting mixture of dense inflow and entrained epilimnetic water sank to the bottom of the lake. The removal of water from the epilimnion due to entrainment reduced the epilimnetic thickness. This opposes the normal process of epilimnetic deepening due to wind and convective cooling. The flux of fluid entrained into the dense inflow was calculated to be between 4 and 14 times the inflow, depending primarily on the thickness of the epilimnion. The entrainment had four major effects: (1) it reduced the residence time of the epilimnion from half a year to less than a month; (2) it removed the freshwater cap that resulted from spring ice melt; (3) it enabled fall turnover, which further enhanced deep oxygen content and helped to prevent meromixis from developing in the lake; and (4) it produced a rapid decline in contaminant (i.e., zinc) concentrations in the epilimnion, which received dissolved metals inputs from oxidized sulfide minerals exposed in the subaerial walls of the lake. Given the wide variety of inflows to inland water bodies, some of which are at least seasonally dense, an understanding of the mechanisms detailed here can inform lake management in general, and more specifically, management of water quality in mine‐impacted water bodies.

Investigating Multidecadal Trends in Ice Cover and Subsurface Temperatures in the Laurentian Great Lakes Using a Coupled Hydrodynamic–Ice Model

Abstract While changing lake surface conditions have received significant research scrutiny, changes in subsurface conditions, including stratification and heat content, remain largely unexplored. In this work, we highlight changes in thermal structure, stratification dynamics, and ice characteristics in the Laurentian Great Lakes (Lakes Superior, Huron, Michigan, Erie, and Ontario) as simulated between 1979 and 2021. Three-dimensional lake hydrodynamics and ice cover were modeled using the Finite Volume Community Ocean Model (FVCOM) coupled with the Los Alamos sea ice model (CICE). Analysis revealed significant increases in surface (0.4°–0.6°C decade−1) and subsurface (0.1°–0.4°C decade−1) temperatures as well as dramatic losses in ice cover (1%–8% decade−1) and ice volume (0–3 km3 decade−1) over the last 40 years. Estimated surface heating rates were strongest during the summer and fall, while subsurface warming was most rapid during the nearly isothermal winter and spring. Intensified (decreased) summer (winter) stratification led to shifts in lake turnover dynamics, with delayed fall turnover dates (2–6 days decade−1) and earlier spring overturn dates (2–9 days decade−1). Modeled surface temperatures (LST), bottom temperatures (LBT), and annual averaged ice cover (AAIC) were used to estimate low-frequency climate signals, which were highly correlated with the Atlantic multidecadal oscillation. Warming trends fit to residual climate signals (LST: 0.1°C decade−1; LBT: 0.03°C decade−1; AAIC: −1% decade−1), calculated by removing low-frequency variability from the raw climate signal, were lower than those fit to associated low-frequency components, suggesting that recent climate change in the Great Lakes may be strongly influenced by natural multidecadal climate variability.

Out of oxygen: Stratification and loading drove hypoxia during a warm, wet, and productive year in a Great Lakes estuary

Hypolimnetic hypoxia, or low oxygen in bottom waters, impairs ecosystem services of freshwater lakes and estuaries globally. Both hypoxia incidence and intensity are increasing around the world due to eutrophication and climate change. As the hypolimnion becomes hypoxic and ultimately anoxic, sediment-bound legacy phosphorus is released. Water column mixing due to large storm events or fall turnover entrains these nutrients to the surface, causing harmful algal blooms. To assess the dynamics of hypoxia throughout the growing season, we evaluated Muskegon Lake, where hypoxia recurs annually, utilizing high-frequency time-series data from the Muskegon Lake Observatory (MLO) buoy (https://www.gvsu.edu/wri/buoy/), biweekly nutrient sampling, and seasonal respiration experiments during 2021. While water-column stratification set the stage for hypolimnetic hypoxia, frequent wind-mixing events, and episodic intrusions of cold, oxygenated, upwelled Lake Michigan waters intermittently reduced the thickness or intensity of the hypoxic zone. Respiration experiments revealed that riverine and surface organic matter inputs contributed most to hypolimnetic hypoxia in the spring, whereas surface inputs did so during summer, and riverine inputs during fall, indicating seasonally variable sources drive hypoxia. Biweekly measurements indicated increased soluble reactive phosphorus in the hypolimnion during anoxia via internal phosphorus loading from the sediment with the potential for fueling surface blooms with net export of soluble reactive phosphorus and total phosphorus to nearshore Lake Michigan. Our findings on the role of seasonally changing temperature, loading, phytoplankton production, hypolimnetic respiration, and internal phosphorus loading in shaping hypoxia dynamics have relevance to similarly afflicted ecosystems in the Great Lakes Basin.

High-frequency sensor data capture short-term variability in Fe and Mn concentrations due to hypolimnetic oxygenation and seasonal dynamics in a drinking water reservoir

The biogeochemical cycles of iron (Fe) and manganese (Mn) in lakes and reservoirs have predictable seasonal trends, largely governed by stratification dynamics and redox conditions in the hypolimnion. However, short-term (i.e., sub-weekly) trends in Fe and Mn cycling are less well-understood, as most monitoring efforts focus on longer-term (i.e., monthly to yearly) time scales. The potential for elevated Fe and Mn to degrade water quality and impact ecosystem functioning, coupled with increasing evidence for high spatiotemporal variability in other biogeochemical cycles, necessitates a closer evaluation of the short-term Fe and Mn dynamics in lakes and reservoirs. We adapted a UV-visible spectrophotometer coupled with a multiplexor pumping system and partial least squares regression (PLSR) modeling to generate high spatiotemporal resolution predictions of Fe and Mn concentrations in a drinking water reservoir (Falling Creek Reservoir, Vinton, VA, USA) equipped with a hypolimnetic oxygenation (HOx) system. We quantified hourly Fe and Mn concentrations during two transitional periods: reservoir turnover (Fall 2020) and HOx initiation (Summer 2021). Our sensor system successfully predicted mean Fe and Mn concentrations and trends, ground-truthed by grab sampling and laboratory analysis. During fall turnover, hypolimnetic Fe and Mn concentrations began to decrease more than two weeks before complete mixing of the reservoir, with rapid equalization of epilimnetic and hypolimnetic Fe and Mn concentrations in less than 48 h after full water column mixing. During the initiation of HOx in Summer 2021, Fe and Mn displayed distinctly different responses to oxygenation, as indicated by the rapid oxidation of soluble Fe but not soluble Mn. This study demonstrates that Fe and Mn concentrations are sensitive to changes in redox conditions induced by stratification and oxygenation, although their responses to these changes differ. We also show that high spatio-temporal resolution predictions of Fe and Mn can improve drinking water monitoring programs and reservoir management practices.

Modeling changes in ice dynamics and subsurface thermal structure in Lake Michigan-Huron between 1979 and 2021

The world’s largest lakes, including the Laurentian Great Lakes, have experienced significant surface warming and loss of ice cover over the last several decades. Although changing surface conditions have received substantial research interest, changes below the surface remain largely unexplored, despite their importance for turbulent mixing, nutrient cycling, and primary production. In this study, we investigate changes in subsurface thermal structure and timing in Lake Michigan-Huron related to ongoing climate warming. This work utilizes atmospheric reanalysis data to drive the Great Lakes Finite Volume Community Ocean Model (GL-FVCOM), providing three-dimensional hydrodynamic and ice simulations between 1979 and 2021. Results are used to analyze trends in ice and temperature dynamics, revealing significant changes in annually averaged ice cover (− 2.1– − 5.2%/decade), ice thickness (− 0.68 – − 2.0 cm/decade), surface temperature (+ 0.47– + 0.51 ^circ{rm C}/decade), and bottom temperature (+ 0.26– + 0.29 ^circ{rm C}/decade) over the last 40 years, especially in ecologically important bays (e.g., Green Bay, Saginaw Bay). Significant warming was observed at all depth layers (0–270 m), with warming trends in the epilimnion and hypolimnion that agreed well with recent analysis of observational data in Lake Michigan. Shifting stratification dynamics led to dramatic changes in modelled overturning behavior, and earlier spring turnover dates (− 2.2– − 7.5 days/decade) and later fall turnover dates (+ 2.5– + 6.3 days/decade) led to a net lengthening of the stratified period. This study presents one of the most comprehensive analyses of changes in Great Lakes subsurface temperatures to date, providing important context for future climate modelling and coastal management efforts in the region.

Succession of bacteria and archaea involved in the nitrogen cycle of a seasonally stratified lake.

Human-driven changes affect nutrient inputs, oxygen solubility, and the hydrodynamics of lakes, which affect biogeochemical cycles mediated by microbial communities. However, information on the succession of microbes involved in nitrogen cycling in seasonally stratified lakes is still incomplete. Here, we investigated the succession of nitrogen-transforming microorganisms in Lake Vechten over a period of 19 months, combining 16S rRNA gene amplicon sequencing and quantification of functional genes. Ammonia-oxidizing archaea (AOA) and bacteria (AOB) and anammox bacteria were abundant in the sediment during winter, accompanied by nitrate in the water column. Nitrogen-fixing bacteria and denitrifying bacteria emerged in the water column in spring when nitrate was gradually depleted. Denitrifying bacteria containing nirS genes were exclusively present in the anoxic hypolimnion. During summer stratification, abundances of AOA, AOB, and anammox bacteria decreased sharply in the sediment, and ammonium accumulated in hypolimnion. After lake mixing during fall turnover, abundances of AOA, AOB, and anammox bacteria increased and ammonium was oxidized to nitrate. Hence, nitrogen-transforming microorganisms in Lake Vechten displayed a pronounced seasonal succession, which was strongly determined by the seasonal stratification pattern. These results imply that changes in stratification and vertical mixing induced by global warming are likely to alter the nitrogen cycle of seasonally stratified lakes.

Simulated impacts of climate change on Lake Simcoe water quality

ABSTRACT Lake Simcoe has undergone eutrophication and hypoxia since the 1960s. Climate change, leading to enhanced summer thermal stratification, has been identified as a key stressor. In this study, we modeled the impacts of climate change on hydrodynamics and biogeochemistry in Lake Simcoe by applying a 1-dimensional (vertical) model forced with A2 and B1 scenario outputs from a global climate model over 2000–2100. The model was calibrated in 2008 and validated in 2009, with maximum root mean square error (RMSE) of modelled temperature between 1.5 and 3.0 °C and dissolved oxygen RMSE between 0.5 and 2.5 mg L−1. Phytoplankton chlorophyll a was simulated with RMSE between 1.25 µg L−1 (large diatoms) and ∼0.5 µg L−1 (other groups). Interannual variability in spring water temperature and length of stratification were related to changes in the North Atlantic and Artic Oscillation indices, respectively. Under A2 and B1 forcing, the duration of stratification will increase by 45 and 38 days in summer between spring and fall turnover, respectively. The extended stratified period leads to a reduction in hypolimnetic dissolved oxygen from 3–7 to <3 mg L−1, thereby reducing the quality of cold-water fish habitat and increasing internal phosphorus loading from the benthos. These internal loads, combined with increased water temperatures, lead to increased cyanobacteria concentrations, beginning around 2070.

Seasonal Dynamics of Methanotrophic Bacteria in a Boreal Oil Sands End Pit Lake.

Base Mine Lake (BML) is the first full-scale demonstration end pit lake for the oil sands mining industry in Canada. We examined aerobic methanotrophic bacteria over all seasons for 5 years in this dimictic lake. Methanotrophs comprised up to 58% of all bacterial reads in 16S rRNA gene amplicon sequencing analyses (median 2.8%), and up to 2.7 × 104 cells mL-1 of water (median 0.5 × 103) based on qPCR of pmoA genes. Methanotrophic activity and populations in the lake water were highest during fall turnover and remained high through the winter ice-covered period into spring turnover. They declined during summer stratification, especially in the epilimnion. Three methanotroph genera (Methylobacter, Methylovulum, and Methyloparacoccus) cycled seasonally, based on both relative and absolute abundance measurements. Methylobacter and Methylovulum populations peaked in winter/spring, when methane oxidation activity was psychrophilic. Methyloparacoccus populations increased in the water column through summer and fall, when methane oxidation was mesophilic, and also predominated in the underlying tailings sediment. Other, less abundant genera grew primarily during summer, possibly due to distinct CH4/O2 microniches created during thermal stratification. These data are consistent with temporal and spatial niche differentiation based on temperature, CH4 and O2. This pit lake displays methane cycling and methanotroph population dynamics similar to natural boreal lakes. IMPORTANCE The study examined methanotrophic bacteria in an industrial end pit lake, combining molecular DNA methods (both quantitative and descriptive) with biogeochemical measurements. The lake was sampled over 5 years, in all four seasons, as often as weekly, and included sub-ice samples. The resulting multiseason and multiyear data set is unique in its size and intensity, and allowed us to document clear and consistent seasonal patterns of growth and decline of three methanotroph genera (Methylobacter, Methylovulum, and Methyloparacoccus). Laboratory experiments suggested that one major control of this succession was niche partitioning based on temperature. The study helps to understand microbial dynamics in engineered end pit lakes, but we propose that the dynamics are typical of boreal stratified lakes and widely applicable in microbial ecology and limnology. Methane-oxidizing bacteria are important model organisms in microbial ecology and have implications for global climate change.

Species-Level Spatio-Temporal Dynamics of Cyanobacteria in a Hard-Water Temperate Lake in the Southern Baltics.

Cyanobacteria are important primary producers in temperate freshwater ecosystems. However, studies on the seasonal and spatial distribution of cyanobacteria in deep lakes based on high-throughput DNA sequencing are still rare. In this study, we combined monthly water sampling and monitoring in 2019, amplicon sequence variants analysis (ASVs; a proxy for different species) and quantitative PCR targeting overall cyanobacteria abundance to describe the seasonal and spatial dynamics of cyanobacteria in the deep hard-water oligo-mesotrophic Lake Tiefer See, NE Germany. We observed significant seasonal variation in the cyanobacterial community composition (p < 0.05) in the epi- and metalimnion layers, but not in the hypolimnion. In winter—when the water column is mixed—picocyanobacteria (Synechococcus and Cyanobium) were dominant. With the onset of stratification in late spring, we observed potential niche specialization and coexistence among the cyanobacteria taxa driven mainly by light and nutrient dynamics. Specifically, ASVs assigned to picocyanobacteria and the genus Planktothrix were the main contributors to the formation of deep chlorophyll maxima along a light gradient. While Synechococcus and different Cyanobium ASVs were abundant in the epilimnion up to the base of the euphotic zone from spring to fall, Planktothrix mainly occurred in the metalimnetic layer below the euphotic zone where also overall cyanobacteria abundance was highest in summer. Our data revealed two potentially psychrotolerant (cold-adapted) Cyanobium species that appear to cope well under conditions of lower hypolimnetic water temperature and light as well as increasing sediment-released phosphate in the deeper waters in summer. The potential cold-adapted Cyanobium species were also dominant throughout the water column in fall and winter. Furthermore, Snowella and Microcystis-related ASVs were abundant in the water column during the onset of fall turnover. Altogether, these findings suggest previously unascertained and considerable spatiotemporal changes in the community of cyanobacteria on the species level especially within the genus Cyanobium in deep hard-water temperate lakes.

Turnover in a small Canadian shield lake

AbstractIn dimictic lakes, the stable density stratification during summer and winter inhibits vertical mixing of nutrients and oxygen. This favors the development of hypolimnetic hypoxia, which degrades cool‐water fish habitat and enhances nutrient mineralization from the sediments. Fall and spring turnover events, therefore, provide a crucial biannual link between surface and bottom waters. However, the physical processes occurring to mix lakes during turnover events remain comparatively uninvestigated. In this study, long‐term field observations were supplemented with output from a three‐dimensional numerical model, to better understand turnover events within a small temperate lake during 2011–2017. Mid‐basin penetrative convection, sidearm convection, and wind‐induced mixing were identified, with mid‐basin convection likely contributing the most to turnover events; a typical side‐arm convective plume had 2% (spring) to 4% (fall) of the mass flux compared to those at mid‐basin. During fall turnover, wind shear only mixed the upper 35% of the surface mixed layer, with convection acting to deepen below. During spring turnover, ice‐cover sheltered the lake from wind, causing convection to be the only process occurring and lengthening the turnover duration (~ 51 d) compared to fall (~ 13 d).

Comparing Spatial and Temporal Variation of Lake‐Atmosphere Carbon Dioxide Fluxes Using Multiple Methods

AbstractLakes emit globally significant amounts of carbon dioxide (CO2) to the atmosphere, but quantifying these rates for individual lakes is extremely challenging. The exchange of CO2 across the air‐water interface is driven by physical, chemical, and biological processes in both the lake and the atmosphere that vary at multiple spatial and temporal scales. None of the methods we use to estimate CO2 flux fully capture this heterogeneous gas exchange. Here, we compared concurrent CO2 flux estimates from a single lake based on commonly used methods. These include floating chambers (FCs), eddy covariance (EC), and two concentration gradient‐based methods labeled fixed (F‐pCO₂) and spatial (S‐pCO₂). At the end of summer, cumulative carbon fluxes were similar between EC, F‐pCO₂, and S‐pCO₂ methods (−4, −4, and −9.5 gC m−2), while methods diverged in directionality of fluxes during the fall turnover period (−50, 43, and 38 gC m−2). Collectively, these results highlight the discrepancies among methods and the need to acknowledge the uncertainty when using any of them to approximate this heterogeneous flux.

A Near‐Term Iterative Forecasting System Successfully Predicts Reservoir Hydrodynamics and Partitions Uncertainty in Real Time

AbstractFreshwater ecosystems are experiencing greater variability due to human activities, necessitating new tools to anticipate future water quality. In response, we developed and deployed a real‐time iterative water temperature forecasting system (FLARE—Forecasting Lake And Reservoir Ecosystems). FLARE is composed of water temperature and meteorology sensors that wirelessly stream data, a data assimilation algorithm that uses sensor observations to update predictions from a hydrodynamic model and calibrate model parameters, and an ensemble‐based forecasting algorithm to generate forecasts that include uncertainty. Importantly, FLARE quantifies the contribution of different sources of uncertainty (driver data, initial conditions, model process, and parameters) to each daily forecast of water temperature at multiple depths. We applied FLARE to Falling Creek Reservoir (Vinton, Virginia, USA), a drinking water supply, during a 475‐day period encompassing stratified and mixed thermal conditions. Aggregated across this period, root mean square error (RMSE) of daily forecasted water temperatures was 1.13°C at the reservoir's near‐surface (1.0 m) for 7‐day ahead forecasts and 1.62°C for 16‐day ahead forecasts. The RMSE of forecasted water temperatures at the near‐sediments (8.0 m) was 0.87°C for 7‐day forecasts and 1.20°C for 16‐day forecasts. FLARE successfully predicted the onset of fall turnover 4–14 days in advance in two sequential years. Uncertainty partitioning identified meteorology driver data as the dominant source of uncertainty in forecasts for most depths and thermal conditions, except for the near‐sediments in summer, when model process uncertainty dominated. Overall, FLARE provides an open‐source system for lake and reservoir water quality forecasting to improve real‐time management.

Phytoplankton production in relation to simulated hydro- and thermodynamics during a hydrological wet year – Goczałkowice reservoir (Poland) case study

Phytoplankton is one of the crucial components of water body ecosystems. Its presence and development depend on biological, physical and chemical factors and in consequence it is an important indicator of ecosystem condition. Monitoring of phytoplankton production, measured as chlorophyll a concentration, is a useful tool for assessing the status of dam reservoirs. Modeled chlorophyll a concentrations are used as water quality indicators in locations not included in monitoring systems, in situations when the temporal resolution of the monitoring is not enough, and in assessments of the impacts of future activities. Therefore, the aim of this study was to find correlations between hydro- and thermodynamics and the chlorophyll a concentration for possible application in reservoir monitoring and management, using an ELCOM-CAEDYM model. The analysis included summer and fall which are most prone to algal blooms, and four phytoplankton groups identified as dominant in the reservoir based on periodic observations.Comparisons of simulated water temperature and both observed and simulated chlorophyll a concentrations confirmed that these variables are significantly correlated (correlation of hourly chlorophyll a and water temperature was 0.70, ranging from 0.55 to 0.81 in the bottom and surface water layers, respectively, while for daily outputs it was 0.74, ranging from 0.60 to 0.83). This relation was stronger than that of chlorophyll a to nutrient (N, P and Si) concentrations. What is more, the method used allowed the assessment of a much more detailed spatial and temporal distribution of phytoplankton groups compared with conventional monitoring techniques.The study indicated that the phytoplankton community was dominated by Chlorophytes and Diatoms with a larger share of Chlorophytes in shallow parts of the reservoir. This domination was weaker after short water mixing events in summer and especially after the fall turnover. The increase in phytoplankton diversity was estimated to occur mainly near the surface and in shallow parts of the reservoir. Most of the observed concentrations of individual phytoplankton groups differed from simulation results by less than 25% and the model reflected accurately 74% of observed trends in concentrations. Calculated chlorophyll a concentration was well matched to hourly monitoring data (mean squared error = 5.6, Nash–Sutcliffe model efficiency coefficient = 0.51, Pearson correlation coefficient = 0.72 and p-value = 0.0007).High compatibility of the model to the values measured in the reservoir make it a promising tool for the prediction and planning of actions aimed at maintaining good functioning of the reservoir.

Seasonal stratification of a deep, high-altitude, dimictic lake: Nam Co, Tibetan Plateau

We investigate the seasonal evolution of stratification in a deep (100 m), high-altitude (4,730 m a.s.l.) dimictic lake on the Tibetan Plateau using three years of observation of Nam Co. The lake is situated at relatively low latitude (30 °N) where it receives high solar radiative forcing (observed maximum daily average 400 W m−2), yet the annual average air temperature is close to 0 °C at this high altitude. These features make Nam Co distinct from most well-documented dimictic lakes, which are usually located at higher latitude and lower altitude. We classify seasonal stratification into six phases based on strength of stratification, surface temperature relative to the temperature of maximum density, ice-cover, and heat and mixing dynamics, which we use to compare Nam Co with better-documented, higher-latitude dimictic lakes. While the three warm stratification phases (i.e. when water is warmer than the temperature of maximum density) in Nam Co are relatively cold, they are otherwise similar to that observed in high-latitude dimictic lakes. Conversely, two of Nam Co’s cold stratification phases are distinct from that reported in high-latitude lakes. These two phases are characterized by the interplay between the relatively strong radiative forcing and surface heat flux, and include the following: 1) during fall turnover, persistent winds aided by radiatively driven convection prolong vertical mixing (i.e. turnover) and surface heat loss such that the entire water column cools well below the temperature of maximum density (as low as 1 °C); and 2) in contrast, under ice-cover with relatively little snow, the entire water column of the lake warms continuously due to through-ice solar radiative flux. The intense cooling and heating during these two phases counteract each other such that hypolimnetic temperature at spring turnover is similar to that observed in high latitude lakes. Our observations highlight the relative importance of radiatively driven convection on the seasonal stratification dynamics of Nam Co, and underscore that these dynamics must be considered when attempting to predict climate change impacts on high-altitude, low-latitude lakes, including the >1100, largely unstudied, lakes on the Tibetan Plateau.

Impacts of Varying Dam Outflow Elevations on Water Temperature, Dissolved Oxygen, and Nutrient Distributions in a Large Prairie Reservoir

Dam operations are known to have significant impacts on reservoir hydrodynamics and solute transport processes. The Gardiner Dam, one of the structures that forms the Lake Diefenbaker reservoir located in the Canadian Prairies, is managed for hydropower generation and agricultural irrigation and is known to have widely altering temperature regimes and nutrient circulations. This study applies the hydrodynamic and nutrient CE-QUAL-W2 model to explore how various withdrawal depths (5, 15, 25, 35, 45, and 55 m) influence the concentrations and distribution of nutrients, temperature, and dissolved oxygen (DO) within the Lake Diefenbaker reservoir. As expected, the highest dissolved nutrient (phosphate, and nitrate, ) concentrations were associated with hypoxic depth horizons in both studied years. During summer high flow period spillway operations impact the distribution of nutrients, water temperatures, and DO as increased epilimnion flow velocities route the incoming water through the surface of the reservoir and reduce mixing and surface warming. This reduces reservoir concentrations but can lead to increased outflow nitrogen (N) and phosphorus (P) concentrations. Lower withdrawal elevations pull warmer surface water deeper within the reservoir and decrease reservoir DO during summer stratification. During fall turnover low outflow elevations increase water column mixing and draws warmer water deeper, leading to slightly higher temperatures and nutrient concentrations than shallow withdrawal elevations. The 15 m depth (540 m above sea level) outflow generally provided the best compromise for overall reservoir and outflow nutrient reduction.

Nitrate, ammonium, and phosphorus drive seasonal nutrient limitation of chlorophytes, cyanobacteria, and diatoms in a hyper‐eutrophic reservoir

AbstractNitrogen (N) and phosphorus (P) inputs influence algal community structure and function. The rates and ratios of N and P supply, and different N forms (e.g., NO3 and NH4), from external loading and internal cycling can be highly seasonal. However, the interaction between seasonality in nutrient supply and algal nutrient limitation remains poorly understood. We examined seasonal variation in nutrient limitation and response to N form in a hyper‐eutrophic reservoir that experiences elevated, but seasonal, nutrient inputs and ratios. External N and P loading is high in spring and declines in summer, when internal loading because more important, reducing loading N:P ratios. Watershed NO3 dominates spring N supply, but internal NH4 supply becomes important during summer. We quantified how phytoplankton groups (diatoms, chlorophytes, and cyanobacteria) are limited by N or P, and their N form preference (NH4 vs. NO3), with weekly experiments (May–October). Phytoplankton were P‐limited in spring, transitioned to N limitation or colimitation (primary N) in summer, and returned to P limitation following fall turnover. Under N limitation (or colimitation), chlorophytes and cyanobacteria were more strongly stimulated by NH4 whereas diatoms were often equally, or more strongly, stimulated by NO3 addition. Cyanobacteria heterocyte development followed the onset of N‐limiting conditions, with a several week lag time, but heterocyte production did not fully alleviate N‐limitation. We show that phytoplankton groups vary seasonally in limiting nutrient and N form preference, suggesting that dual nutrient management strategies incorporating both N and P, and N form are needed to manage eutrophication.

Decreased oxygenation demand following hypolimnetic oxygenation

Gantzer PA, Preece EP, Nine B, Morris J. 2019. Decreased oxygenation demand following hypolimnetic oxygenation. Lake Reserv Manage. 35:292–307. We analyzed long-term oxygenation dynamics in 3 waterbodies: Spring Hollow Reservoir (SHR) and Carvins Cove Reservoir (CCR), which are water supply reservoirs, and North Twin Lake (NTL), a natural lake. Bubble plume line diffuser hypolimnetic oxygenation system (HOS) operation was for 13, 10, and 7 yr for SHR, CCR, and NTL, respectively. Water column profiles were measured in all 3 waterbodies. Additionally, remotely deployed sensors were used to monitor dissolved oxygen (DO) in NTL year-round, specifically to capture winter conditions during ice cover. Hypolimnetic oxygen demand (HOD) was observed to decrease after several years of HOS operation in all 3 waterbodies. Annual HOD decreased more for HOS operated year-round compared to recovery-based operation where HOS was started after DO decreased below 5.0 mg/L. Additionally, HOD throughout the summer stratified period was lower following maintenance of higher DO during spring prior to onset of stratification. An analogous result was observed during winter ice cover relative to DO conditions following fall turnover. Prior work showed increased HOD during HOS operation compared to pre-HOS. The increased HOD during HOS operation was termed diffuser-induced oxygen demand and implied the HOS was inducing oxygen demand. However, the current work shows HOS operation is not actually inducing oxygen demand; rather, it is addressing the existing oxygen demand already in the sediments. Our data suggest that a HOS is an effective tool to maintain hypolimnion DO in stratified lakes and reservoirs. When operated year-round or before DO decreases, HOS can decrease HOD and reduce overall long-term operational costs.

Long-term impact of Central Basin hypoxia and internal phosphorus loading on north shore water quality in Lake Erie

ABSTRACTWe hypothesized that north shore water quality of Lake Erie’s Central Basin is impacted by Central Basin hypoxia and, by extension, related to redox-dependant internal phosphorus (P) loading from the profundal sediments. To evaluate this hypothesis, we first quantified Central Basin hypoxia as hypoxic factor (HF, days in Aug–Sep that a sediment area equal to the Central Basin surface area was hypoxic) from published annual hypoxic (<2.0 mg/L dissolved oxygen) areal extent and duration. From 1985 to 2012, mean HF values were 15.2 (SE 1.45) d/Aug–Sep and ranged between 0 (1996) and 34.3 (2012) d/Aug–Sep. Second, we estimated internal load from an areal P release rate of 8 mg/m2/d, multiplied by HF. The estimate of 122 (SE 11.6) mg/m2/Aug–Sep (1985–2012, n = 28) is supported by an independent estimate of 136 (SE 20.3) mg/m2/summer (1970–1986, n = 15) computed from total P concentration increases at fall turnover and by an estimate for 1970. Third, 2013 temperature and oxygen profiles demonstrated periodic upwelling on the north shore of the Central Basin, in agreement with other studies. Fourth, we analysed P, chlorophyll a (Chl-a) and phytoplankton species data at 2 north shore drinking water intake stations. HF and internal loading were significantly correlated with summer soluble reactive P, August–October Chl-a concentrations, and cyanobacteria abundance at the Central Basin site but not at the control site in the Western Basin. While these correlations are weak, suggesting other effects including benthic productivity, they may be partially caused by hypolimnetic water circulated toward the shoreline.

Assessment of methane and carbon dioxide emissions in two sub‐basins of a small acidic bog lake artificially divided 30 years ago

Abstract Although lakes are important sources of methane (CH4) and carbon dioxide (CO2) to the atmosphere contributing to global warming, their CH4 and CO2 emissions are rarely assessed. In particular, increasing inputs of terrestrial dissolved organic carbon (DOC) may affect gas dynamics and alter seasonal changes in gas production. Here, we analysed variations in CH4 and CO2 dynamics in sub‐basins of an acidic bog lake, which was artificially divided into four quarters three decades ago, leading to divergence in water chemistry and biology. In the divided lake, only the south‐west basin (SW) received DOC inputs from an adjacent peat bog, while the north‐east basin (NE) was hydrologically disconnected. A year‐long determination of CH4 and CO2 production and emission patterns in the two contrasting basins exposed the indirect mechanisms by which DOC supply exercised control on greenhouse gas dynamics in this shallow lake. In both basins, dissolved CH4 was negatively correlated with dissolved oxygen (O2) through the water column, suggesting that aerobic methanotrophy is an important regulator of CH4 emissions in this lake. In contrast, the amount of CO2 stored in oxic and anoxic layers was not significantly different between the basins, suggesting that O2 is not the most important driver of dissolved CO2. Estimated total CH4 and CO2 emissions were 2.1 and 1.7 times lower in the NE basin than in the SW basin, with major CH4 and CO2 emissions occurring during the fall turnover. The differences in CH4 and CO2 emissions suggest that the hydro‐physical properties, namely seasonal temperature, the duration of stratification and O2 availability, are the main drivers of CH4 and CO2 emissions to the atmosphere from small shallow lakes under the influence of DOC inputs under global warming pressure.