Reduction Of External Loading Research Articles

Decreasing denitrification rates poses a challenge to further decline of nitrogen concentration in Lake Taihu, China

Nitrogen (N) concentrations in many lakes have decreased substantially in recent years due to external load reduction to mitigate harmful algal blooms. However, little attention has been paid to the linkage between the lakes’ nitrogen removal efficiency and improved water quality in lakes, especially the variation of denitrification rate (DNR) under decreasing N concentrations. To understand the efficiency of N removal under improving water quality and its influence on the N control targets in Lake Taihu, a denitrification model based on in situ experimental results was developed and long-term (from 2007 to 2022) water quality and meteorological observations were used to estimate DNR and relate it to the amount of N removal (ANR) from the lake. The concentration of total nitrogen (TN) in Lake Taihu decreased from 3.28 mg L−1 to 1.41 mg L−1 from 2007 to 2022 but the reduction showed spatial heterogeneity. The annual mean DNR decreased from 45.6 μmol m−2 h−1 to 4.2 μmol m−2 h−1, and ANR decreased from 11.85×103 t yr−1 to 1.17×103 t yr−1 during the study years. N budget analysis suggested that the amount of N removed by denitrification accounted for 23.3 % of the external load in 2007, but decreased to only 4.0 % in 2022. Thus, the contribution of N removal by internal N cycling decreased significantly as water quality improved. Notably, the proportion of ANR in winter to total ANR increased from 14 % in 2007 to 23 % in 2022 due to warming. This could potentially lead to N deficiencies in spring and summer, thus limiting the availability of N to phytoplankton. A TN concentration of less than 1.0 mg L−1 in the lake and 1.5 mg L−1 in the inflowing lake zones in spring contribute to local N-limitation in Lake Taihu for cyanobacteria control. Our study revealed a general pattern that N removal efficiency decreases with improved water quality, which is instructive for eutrophic lakes in nitrogen management.

Recovery of lakes from eutrophication: changes in nitrogen retention capacity and the role of nitrogen legacy in 10 Danish lakes studied over 30 years

AbstractUsing data on 23 Danish lakes, we conducted mass balances to develop total nitrogen (TN) models for predicting annual mean TN in lakes based on external TN loading and found high predictability when including lake hydraulic retention time and mean depth in the model. We further used a unique 30-year mass balance data series from 10 Danish lakes with contrasting mean depths and hydraulic retention times to elucidate the effect of external TN loading reduction and N legacy on lake TN. We found that the TN retention percentage during the 30 years was generally not sensitive to an often major reduction in the external TN loading; it overall followed the pattern of the above model predictions, suggesting a low TN legacy effect. Moreover, the TN retention percentage was not affected by changes in TP. Our results, therefore, show a fast response to TN loading reduction, indicating that we can expect an immediate effect on lake water quality in shallow lakes suffering from internal phosphorus loading during re-oligotrophication provided that inorganic N is low enough to become a growth-limiting nutrient.

Community ecological response to polycyclic aromatic hydrocarbons in Baiyangdian Lake based on an ecological model

The dynamic response of a single population to chemicals can be represented by a Weibull function. However, it is unclear whether the overall response can still be represented in this manner when scaled up to the community level. In this study, we investigated the responses of biological communities to polycyclic aromatic hydrocarbons by using an ecological model of Baiyangdian Lake in northern China. The community dynamics process was divided into the following three stages. In the first stage, toxicity, played a dominant role and strong, medium, and weak species responses were observed according to the toxicity sensitivity. In the second stage, the dynamic process was dominated by the interaction strength with three alternative dynamic pathways comprising of direct response, no response, or inverse response. In the third stage, the toxicity was again dominant, and the biomasses of all species decreased to extinction. The toxicological dynamics were far more complex at the community level than those at the single species level and they were also influenced by the interaction strength as well as toxicity. The toxicological dynamic process in the community was constantly driven by the competing effects of these two forces. In addition to the total biomass, the interaction strength was identified as a suitable community-level signal because it exhibited good indicator properties regarding ecosystem steady-state transitions. However, we found that food web stability indicators were not suitable for use as community-level signals because they were not sensitive to changes in the ecosystem state. Some ecological management suggestions have been proposed, including medium to long-term monitoring, and reduction of external pollution loads and bioindicators. The results obtained in this study increase our understanding of how chemicals interfere with community dynamics, and the interaction strength and total biomass were identified as useful holistic indicators.

A modelling approach to evaluate the present and future effectiveness of hypolimnetic withdrawal for the restoration of eutrophic Lake Varese (Northern Italy)

Hypolimnetic withdrawal has been applied as a restoration measure in lakes subject to eutrophication together with external load reduction, to decrease internal load by removing limiting nutrient phosphorus (P) from anoxic deep waters and contributing to the unloading of bottom sediments from previously deposited nutrients and organic matter. The aim of this study is to evaluate the effect of hypolimnetic withdrawal on Lake Varese, a 24 m-deep and 14.8 km2-large subalpine lake in North-Western Italy. The lake suffered from extended eutrophication in the second half of the 20th century due to uncontrolled delivery of untreated urban sewage. Several restoration measures have been implemented during the years, including hypolimnetic withdrawal. In 2019, a cooperative programme for the protection and management of the lake and its surroundings was launched, establishing a systematic annual hypolimnetic withdrawal in the stratified season since 2020. In this research, we calibrated a one-dimensional (1D) coupled ecological-hydrodynamic model (General Lake Model/Aquatic EcoDynamics – GLM/AED2) of Lake Varese with data surveyed in the lake in 2019–2021. Model simulations of the period 2020–2021 with and without the performed withdrawal proved the effectiveness of this measure on hypolimnetic P concentration reduction. Then, future simulations of 2023–2085 were carried out to predict the future efficiency of hypolimnetic withdrawal and of reductions in external nutrient loads under climate change scenarios. Results show that the prescribed withdrawal increases hypolimnetic temperatures. This effect, coupled with thermocline deepening due to global warming, will possibly lead to decreasing water mass stability in autumn and shorter stratification in the moderately deep Lake Varese, with an eventual decrease of P concentrations in the water column. The future effectiveness of hypolimnetic withdrawal is further discussed considering the possible role of dry periods.

Maintenance of internal load despite a stepwise reduction in external load during moderate intensity heart rate clamped cycling with acute graded normobaric hypoxia in males

ObjectivesTo investigate the acute effects of graded hypoxia on external and internal loads during 60 min of endurance cycling at a clamped heart rate. DesignRepeated measures. MethodsOn separate visits, 16 trained males cycled for 60 min at a clamped heart rate corresponding to 80 % of their first ventilatory threshold at sea-level and 2500 m, 3000 m, 3500 m and 4000 m simulated altitudes (inspired oxygen fractions of 20.9 %, 15.4 %, 14.5 %, 13.6 % and 12.7 %, respectively). Markers of external (power output) and internal (blood lactate concentration, tissue saturation index, cardio-respiratory and perceptual responses) loads were measured every 15 min during cycling. Neuromuscular function of knee extensors was characterised pre- and post-exercise. ResultsCompared to sea-level (101 ± 22 W), there was a stepwise reduction in power output with increasing hypoxia severity (−17.9 ± 8.9 %, −27.1 ± 10.7 %, −34.2 ± 12.0 % and − 44.6 ± 15.1 % at 2500 m, 3000 m, 3500 m, and 4000 m, respectively, all p < 0.05). Blood lactate and tissue saturation index were not different across hypoxia severities, and perceptual responses were exacerbated at 4000 m only, with increased breathing difficulty. Knee extensor torque decreased post-exercise (−14.5 ± 9.0 %, p < 0.05), independent of condition. ConclusionsIncreasing hypoxia severity reduces cycling power output and arterial oxygen saturation in a stepwise fashion without affecting exercise responses between sea-level and simulated altitudes up to 3500 m despite breathing difficulty being elevated at 4000 m.

Quantitative Assessment of Water Quality Improvement by Reducing External Loadings at Lake Erhai, Southwest China.

To quantitatively evaluate the effects on water quality improvement caused by reducing external loadings entering Lake Erhai through inflow rivers, a one-dimensional hydrodynamic and ecological model (DYRESM-CAEDYM) was set up to simulate the water quality and water level variations. The calibrated and validated model was used to conduct six scenarios for evaluating the water quality responses to different amounts of external loading reduction at Lake Erhai. The results show (1) the total nitrogen (TN) concentration of Lake Erhai will be higher than 0.5 mg/L without any watershed pollution control during April-November 2025, which cannot meet Grade II standard of the China Surface Water Environmental Quality Standards (GB3838-2002). (2) External loading reductions can significantly reduce the concentrations of nutrients and Chla at Lake Erhai. The effects of water quality improvement will be proportional to the reduction rate of external loading reductions. (3) Internal release might be an important source of pollution It needs to be seriously considered as well as external loading for mitigating the eutrophication at Lake Erhai in the future.

Modelagem da qualidade da água em um reservatório tropical usando CE-QUAL-W2: lidando com a escassez de dados, poluição urbana e sazonalidade hidroclimática

ABSTRACT This study applies a 2-D hydrodynamic model (CE-QUAL-W2) for simulating water quality dynamics in a tropical reservoir located in Fortaleza, Ceará, Brazil. While rainfall concentrates basically in the first semester, this reservoir receives untreated sewage from an urban catchment throughout the year. To deal with data scarcity, model simplifications are justified and several adjustments are carried out, so that only the parameters temperature (T), dissolved oxygen (DO), chlorophyll a (Chla) and phosphate (PO4) are kept in the modeling process. Additionally, different assumptions are performed regarding the time-evolution of reservoir inlet concentrations: constant values, step and linear variations. The results indicate that the simplified model can predict well the seasonal variations of T, DO, Chla and PO4. The best fitting between model results and measurements are obtained with the assumption of linear variation in inlet concentrations, followed by the assumptions of constant values and step variation. Moreover, the results reveal that while PO4 presents a complete mixing behavior with a clear increase in concentration from the wet to the dry season, T, DO and Chla show an alternating stratification-destratification patter during the day-night but without relevant variations throughout the year. Model simulations of different scenarios also indicate a significant reduction in Chla concentration in the second semester, but external load reduction has a stronger impact on model outputs than hydroclimatic variability. The modeling approach developed in the present study is proposed as a simple way to cope with data scarcity, urban pollution and hydroclimatic seasonality in tropical reservoirs.

Reducing nutrient increases diatom biomass in a subtropical eutrophic lake, China–Do the ammonium concentration and nitrate to ammonium ratio play a role?

Response of aquatic organisms to eutrophication have been well reported, while less studies are available for the recovery of eutrophic lakes following a reduction in the external loading, especially for systems where nitrogen is reduced but the phosphorus concentration is maintained high due to internal loading. Diatoms are nitrate (NO3-N) opportunists but can also use ammonium (NH4-N). They may, therefore, be more sensitive to nitrogen reduction than other algae that typically prefer NH4-N. We document the variations of nutrients and diatoms in subtropical, eutrophic Lake Taihu over 28 yr during which a reduction of the external loading resulted from lake management. According to the results of change point analysis, data on environmental variables were divided into two periods (P1: 1992–2006; P2: 2007–2019) with two different seasons (WS: Winter-Spring; SA: Summer-Autumn), respectively. Compared with P1-WS, the concentration of NH4-N decreased significantly whereas NO3-N showed no significant change in P2-WS. In contrast, NH4-N concentrations were low and showed no significant changes in P1-SA and P2-SA and NO3-N decreased significantly in the latter period. Accordingly, NO3-N: NH4-N mass ratios in P1-SA and P2-WS were all significantly higher than those in P2-SA and P1-WS, respectively. The biomass of WS diatom increased significantly and the timing of the peak biomass shifted from P1-SA to P2-WS since 2007. The SEM analysis showed that NO3-N was retained as a statistically significant predictor for diatom biomass in P1-SA and significant effects of windspeed, zooplankton and NH4-N on diatom biomass in P2-WS. Windspeed and zooplankton have further changed the biomass of diatoms in the case of declining inorganic nitrogen. We conclude that the magnitude of vernal suppression or stimulation of diatom assemblages has increased, concomitant with the variations of NH4-N and NO3-N: NH4-N mass ratios. Diatoms response to NH4-N or NO3-N is apparently changing in response to water temperature in this eutrophic shallow lake. Thus, parallel reductions in external nitrogen loading, along with variations in dominant inorganic nitrogen, will stimulate the growth of diatom and therefore increase the total biomass of phytoplankton in still high internal phosphorus loading, which is should be regarded as a good sign of restoration measures.

A critical review on operation and performance of source water control strategies for cyanobacterial blooms: Part I-chemical control methods

Cyanobacterial blooms produce nuisance metabolites (e.g., cyanotoxins and T&O compounds) thereby posing water quality management issues for aquatic sources used for potable water production, aquaculture, and recreation. A variety of in-lake/reservoir control measures are implemented to reduce the abundance of nuisance cyanobacteria biomass or decrease the amount of available phosphorous (P). This paper critically reviews the chemical control strategies implemented for in-lake/reservoir management of cyanobacterial blooms, i.e., algaecides and nutrient sequestering coagulants/flocculants, by highlighting (i) their mode of action, (ii) cases of successful and unsuccessful treatment, (iii) and factors influencing performance (e.g., water quality, process control techniques, source water characteristics, etc.). Algaecides generally result in immediate improvements in water quality and offer selective cyanobacterial control when peroxide-based alagecides are used. However, they have a range of limitations: causing cell lysis and release of cyanotoxins, posing negative impacts on aquatic plants and animals, leaving behind environmentally relevant treatment residuals (e.g., Cu in water and sediments), and offering only short-term bloom control characterized by cyanobacterial rebound. Coagulants/flocculants (alum, iron, calcium, and lanthanum bentonite) offer long-term internal nutrient control when external nutrient loading is controlled. Treatment performance is often influenced by background water quality conditions, and source water characteristics (e.g., surface area, depth, mixing regimes, and residence time). The reviewed case studies highlight that external nutrient load reduction is the most fundamental aspect of cyanobacterial control. None of the reviewed control strategies provide a comprehensive solution to cyanobacterial blooms.

Evaluation of reduction of external load of total phosphorus and total suspended solids for rehabilitation of urban lakes

Urban lakes provide important services to cities, but they suffer from en-vironmental problems, mainly resulting from degradation of the watershed. Rehabilitation of degraded lakes is based on the recovery of water quality and reduction of external pollution, the latter usually taking priority. The Lagoa da Pampulha (LP) is an artificial lake located in Belo Horizonte, Minas Gerais - Brazil, that has suffered eutrophication and silting for decades and has been undergoing rehabilitation for some years. The objective of this study was to evaluate the importance of sanitary, environmental and urban improvements implemented in the LP watershed. For this purpose, total phosphorus (TP) and total suspended solid (TSS) loads transported to LP, between 2016 and 2017, obtained from hydrological and water quality monitoring data, were estimated. Additionally, simulations were carried out using the Storm Water Management Model (SWMM) to evaluate TP and TSS loads in hypothetical scenarios. The conservative scenario proposed a reduction of 50 % in sanitary sewage discharge to water courses and an increase of 30 % and 10 % in the urban area and non-vegetated soil, respectively, in the watershed. In an op-timistic scenario, a reduction of 95 % in sewage discharge and a reduction of 50 % in non-vegetated soils was proposed. According to the results obtained, between October 2016 and September 2017, about 38 tons of TP and 3000 tons of TSS were transported into LP. The results of the SWMM simulations indicated high loads entering the lake, ranging from about 23,000 to 13,000 ton/year for TP and from 3200 to 2400 ton/year for TSS, in actual and optimistic scenario respectively. Despite presenting significant reductions, of up to 60 % for TP and 25 % for TSS, the loads transported to LP remained high even in the most optimistic scenario, indicating that implementation of the evaluated improvements only not sufficient to guarantee rehabilitation of the lake, and must be associated with other measures.

Sensitivity Analysis of Boundary Load Reduction in a Large Shallow Lake Water Quality Model

To explore the spatial and temporal response of water quality to external load reduction in Lake Taihu, Jiangsu Province, China, and clarify the exogenous load reduction under different water inflow and pollution conditions, a water quality model was constructed and the inflow boundaries were divided into seven groups based on the EFDC model. Taking COD and ammonia nitrogen as output targets, the sensitivities of Taihu Lake water quality boundaries were analyzed using a local sensitivity analysis. The results showed that COD and ammonia nitrogen concentrations of each lake area were more sensitive to the boundary load of the lake area than the rest of the lake area, and the sensitivity index was the highest in the Northwest Lake area. Furthermore, the improvement rates of mean COD concentrations in the whole lake decreased by 28.40%-34.71% in the dry season relative to the wet season, and the ranked sensitivity order of the boundaries was as follows:Northwest Lake boundary > Zhushan Lake boundary > Gonghu Lake boundary > Meiliang Bay boundary > Southwest Lake area boundary > Eastern Lake area boundary > East Lake Taihu boundary. The average improvement rates of ammonia nitrogen concentrations in the whole lake were 41.59%-42.34% higher in the dry season relative to the wet season, and the ranked boundary sensitivity order was as follows:Northwest Lake boundary > Meiliang Bay boundary > Zhushan Lake boundary > Gonghu Lake boundary > Southwest Lake boundary > East Lake Taihu boundary > Eastern Lake area boundary. This difference was affected by algal growth and metabolism, and artificial water diversion and drainage. Therefore, it is necessary to consider the reduction period and inflow location according to different water-quality indicators when planning external prevention and control measures in large lakes.

Internal positive feedback promotes water quality improvement for a recovering hyper-eutrophic lake: A three-dimensional nutrient flux tracking model

Nutrient dynamics in lakes are determined by the combined effects of external and internal nutrient fluxes. However, the feedback loop of nutrient fluxes and water quality changes is still an open question. An integrated three-dimensional flux tracking approach based on the Environmental Fluid Dynamics Code model was established to quantify the long-term dynamic changes in external and internal processes in Lake Dianchi (one of the three most hyper-eutrophic lakes in China), and to explore the potential causes of water quality improvement during 2012–2018. The long-term trends and relative influences of nutrient fluxes on water quality were identified. The results showed that the inflow flux was the largest input source and declined by 50% between 2003 and 2012, which was followed by a stable trend from 2012 to 2018. The second largest input source was benthic release, which exhibited a significant interannual reduction. Algae sedimentation was the largest removal process, and declined by 45% between 2012 and 2018. An integrated analysis demonstrated that, following an external loading reduction prior to 2012, the positive feedback of internal fluxes promoted water quality improvement during 2012–2018. Considering the long timescale of the nutrient-flux feedback mechanism, reducing external loading is still the top priority for a long-term virtuous cycle of water quality in the process of eutrophic lake restoration.

Eutrophication management in a Great Lakes wetland: examination of the existence of alternative ecological states

AbstractThe degradation and loss of ecologically important wetlands has been a topical issue in the Great Lakes region, where 60–80% of the coastal wetlands have been lost since the 1800s. The present modeling study aims to guide the restoration efforts in Cootes Paradise marsh, one of the most degraded shallow wetlands in Southern Ontario. We use a process‐based eutrophication model designed to reproduce the biotic competition among multiple phytoplankton and macrophyte functional groups. Our primary focus is to offer guidelines for wetland restoration by characterizing the ecophysiological processes of the autotrophic assemblage, such as the nutrient uptake from the water column and/or the sediment pore waters, the relative ability to harvest light and fuel photosynthesis, and temperature control of the algal/macrophyte growth and basal metabolism. We predict that the additional reduction of external phosphorus loading in Cootes Paradise could induce an abrupt, non‐linear shift from the current turbid phytoplankton‐dominated state to a desirable clear macrophyte‐dominated state. The emergence of this critical (or tipping) point, where the shift to another ecological state may occur, can be accelerated by the presence of a thriving macrophyte community with an enhanced ability to sequester phosphorus. However, it may also be delayed by the presence of a suite of biogeochemical mechanisms (often referred to as “feedback loops”), such as the remobilization of legacy P due to sediment diagenetic processes, wind resuspension, bioturbation, hydraulic loading from local tributaries, water‐level fluctuations, and the leachable P pool of dead plant material that can be returned into the water column through senescence. Our study identifies the restoration actions required to minimize the likelihood of prolonged hysteresis and to facilitate a shift to a desirable ecological state in the foreseeable future. The areal expansion of aquatic vegetation will not only lead to the establishment of a thriving meadow and emergent vegetation community, but may also pave the way for submerged macrophytes through a suite of synergistic mechanisms. Additional point‐source loading reductions will facilitate the transition to an alternative stable clear macrophyte‐dominated state, but could also consolidate the future resilience of the marsh.

Review of characterization, factors, impacts, and solutions of Lake eutrophication: lesson for lake Tana, Ethiopia

Lake eutrophication and water quality deterioration have become a major environmental problem in urban areas and fertilized basins in developing countries across the world. This paper reviews the characterization, driving factors, and impacts of lake eutrophication as well as the mechanism of preventing and recovering lake eutrophication with case studies of eutrophic lakes across the world including Lake Tana, Ethiopia. In most waterbodies including lakes and reservoirs, total phosphorus concentration, chlorophyll a concentration, and Secchi disk visibility in association with species composition are the common criteria to classify lakes and reservoir as oligotrophic, mesotrophic, and eutrophic. Nutrient-rich runoff from cultivated land and industrialized and urbanized cities concentrated in phosphorus are the critical factors that drove eutrophication in water bodies. Among others, controlling external loading of nutrient, ecological, and mechanical methods were found to be common mechanisms to prevent and recover lake eutrophication. Avoiding the factors that are under human control, i.e., a reduction of external loading of nutrients especially targeted on phosphorus reduction into the water basins, relocates sewage, industrial and domestic waste discharges to be lined out of the catchment of the lake. Furthermore, motivating the community to use less phosphorus-containing fertilizers and promoting phosphorus-free detergents are suggested solutions to sustainably prevent and reduce eutrophication in the long run. These could be some possible measures to safeguard endangered Lake Tana of Ethiopia.

Optimizing reservoir operations for tradeoffs between economic objectives and legacy phosphorus management

Dam impoundment causes significant phosphorus (P) retention in reservoirs, leading to the emergence of legacy P release as a water quality threat under external pollution load reduction measures. Reservoir operations impact sediment P release through their complex influence on hydrodynamic and biogeochemical processes, but applicatory reservoir operation strategies for legacy P management have rarely been proposed. This study introduces a reservoir operation optimization approach for balancing economic objectives (i.e., water supply and hydropower generation) and P release mitigation. A coupled hydrodynamic-eutrophication-sediment model based on the Environmental Fluid Dynamics Code platform is used to simulate sediment P release under multiple reservoir operation scenarios. The resultant simulation data are used to train a dynamic artificial neural network (ANN) to model the response of P release to reservoir operations. By coupling the ANN with a reservoir operation model, we develop a multi-objective optimization model to refine reservoir operations for a three-tier tradeoff (i.e., water supply, hydropower generation and P release mitigation). China's Danjiangkou Reservoir is chosen as a case study to generate optimal operation rules. The optimal operation scheme decreases multi-year averaged annual P release by 19% (from 6688 tons to 5436 tons) while inversely increasing annual economic loss caused by water supply shortages by 14% and decreasing hydropower generation by 5%. Pareto-optimal solutions are subsequently generated to analyze tradeoffs between water supply/hydropower generation and P release mitigation. This study demonstrates the efficiency of using reservoir operations for legacy P management and offers an optimization tool for reservoir operation decision-making processes.

Restoring shallow lakes impaired by eutrophication: Approaches, outcomes, and challenges

Shallow lake ecosystems are widely impaired by eutrophication. Lake restoration projects frequently fail or are only partially successful, reflecting that environmental managers face difficult challenges in addressing this complex problem. Key uncertainties relate to selecting appropriate methods and predicting the resulting response trajectory, that is, “what should be done, how much improvement should be expected, and how long will it take?” This review critically evaluates approaches applied to address eutrophication in shallow lakes. A set of shallow lake restoration case studies from across the globe, involving moderate to high effort monitoring, was evaluated to determine common features and to identify advantages/disadvantages and drivers of successful restoration projects. Approaches are broadly grouped into four categories: 1) external nutrient load reductions, 2) internal nutrient load reductions, 3) biomanipulation, and 4) hydrological manipulations. Case studies highlight that external load reduction is an essential component of successful restoration, although supplementary measures can support restoration goals. To inform planning and help set realistic expectations, the following key challenges to the restoration of shallow lakes were identified: 1) insufficient reductions in external nutrient loads, 2) biogeochemical lags, 3) resistance of a stable, algal-dominated state due to hysteresis in the turbidity–nutrient load relationship, 4) delayed macrophyte reestablishment, 5) climate change, 6) need for lake-specific approaches, and 7) social and political shortcomings. Managing these challenges is crucial to the success of shallow lake restoration projects.

How the integrated engineering solutions can support the lakes restoration?

ABSTRACT A method of the simultaneous reduction of external and internal load of phosphorus as a new solution for lake restoration is presented. A flow-through, stratified lake, with diverse morphometry was selected for the study. In all three parts of the lake, an oxygen deficit was noted at the lake bottom at peak summer stagnation, with a simultaneous increase in mineral forms of phosphorus (the concentration of orthophosphates at sites I, II and III reached 0.301, 0.318 and 0.395 mg P l−1, respectively). Additionally, a large load of nutrients originating from the main inflow (annual load of 14395.9 kg P y−1 and 16 4829.8 kg N y−1), indicates that the conditions present in the lake are conducive to more intensive eutrophication. In this situation, reduction of the main loads of nutrients can be achieved by the simultaneous employment of pipelines delivering oxygenated water to the lake bottom during summer stagnation and a pipeline withdrawing deoxygenated and nutrient-rich water from the bottom in the vicinity of the outflow from the lake. It has been calculated that during the period of summer stagnation (VI-IX), the operation of delivering pipelines can reduce the phosphorus load from the main inflow by 5800.5 kg (40.3%). Using the pipeline withdrawing (site III), the amount of phosphorus removed from the lake would increase by 30.4% (from 2259.6 kg to 3247.8 kg). An additional solution, can be dosing the precipitating substances into the pipes delivering water to the lake, which will enable precise inactivation of phosphorus.

Decreasing nitrogen loading and climate change promotes the occurrence of nitrogen-fixing cyanobacteria in a restored city lake

External loading reduction has been implemented to improve water quality in China since the 2000s. To evaluate the effects of nutrient reduction on phytoplankton, long-term monitoring data for Lake Wuli from 1998 to 2017 were analyzed. After the restoration since 2003, nitrogen decreased significantly from 8 to 2 mg/l until 2008 and then remained stable until 2017, while total phosphorus declined progressively from 0.2 mg/l to approximately 0.08 mg/l during 2003 and 2017. Biological data indicated that phytoplankton biovolume decreased after restoration until 2010 and then increased slightly from 2010 to 2017, mainly due to increasing of cyanobacterial biovolume. The dominant taxa shifted from cryptophyta to cyanobacteria since 2009. The results of both nonmetric multidimensional scaling and random forest regression showed that changes in nutrients, especially decreases in nitrogen, were the main drivers for phytoplankton community variation and increasing of cyanobacterial biovolume to the total phytoplankton biovolume ratio in summer and autumn. In addition to nutrients reduction, climatic variables such as wind speed decrease also played vital roles in increasing of cyanobacterial dominance in Lake Wuli. Our study confirmed that phytoplankton community respond strongly to nitrogen reduction in the subtropics and that climate change mediates the response.

Internal phosphorus loading due to sediment anoxia in shallow areas: implications for lake aeration treatments

Shallow lake sediments may be anoxic despite overlying aerated water. In the current study, we aimed to ascertain the contribution of shallow areas to internal phosphorus (P) loading due to sediment anoxia in stratifying lakes. Moreover, we analyzed relationships of the key water quality variables with internal P loading due to sediment anoxia originating solely from stratifying areas (IPobs) and that accounting also for the shallow areas (IPpred) for a set of Finnish lakes, including intentionally aerated and non-aerated lakes. Finally, using a broader set of lakes worldwide, we established a specific combination of lake characteristics that predict sediment P release due to sediment anoxia and linked it to the practices of aeration. Our results showed that shallow lake areas (a difference between IPpred and IPobs) contributed about half of the total P flux due to sediment anoxia. While all of the studied water quality variables related significantly to IPpred, only the concentration of total phosphorus (TP) in the near-bottom water layer related significantly to IPobs. This indicates the key importance of P release of shallow areas for water quality. The concentrations of TP in the surface water layer and chlorophyll a were significantly dependent on IPpred irrespectively of the treatment (aerated lakes or not). P supply from shallow areas may affect aeration effectiveness in stratifying lakes. IPpred was found to be dependent on the specific combination of lake characteristics (including mean and maximum depth, lake and catchment area, external P loading) PC3, driven mainly by external P loading. Hence, external load reduction should be considered as the first priority in lake water quality management. By linking the dependence of IPpred on PC3 to aeration practices, we determined the conditions that promise increased effectiveness of aeration treatments.

Time scales of external loading and spatial heterogeneity in nutrients-chlorophyll a response: Implication on eutrophication control in a large shallow lake

Nutrient over-enrichment of phosphorus (P) and nitrogen (N) makes phytoplankton proliferate, and is closely linked to algal blooms. Identifying effects of nutrients on algal biomass is necessary for eutrophication control. Long term (1994–2018) monitoring data of chlorophyll a (Chl-a), in-lake nutrient concentrations, and external phosphorus loading were leveraged in four ecological zones of Lake Okeechobee. Generalized Additive Models (GAMs) were developed to detect the response of Chl-a to nutrients. Term of cumulative loading with different time scales was added for the first time into GAMs to study the legacy effect of P accumulation. We find that the added term of cumulative total phosphorus (TP) loading integrated over four years to one decade produced the GAM model with the best explanatory power. This result suggests that the long-term TP loading, rather than short-term external loading, has a more significant role on algal biomass in Lake Okeechobee. Our study also demonstrated the inhibition effect of ambient TP concentration and cumulative TP loading on Chl-a at their high values, while a positive impact of total nitrogen concentration in most circumstances. In addition, a threshold value of nitrate and nitrite in the response curve indicated the nitrogen fixing ability of some algae species. These findings imply that the delayed effects of restoration strategies can be up to one or two decades after external phosphorus loading reduction, and that inhibition phenomenon during high TP concentrations and cumulative loadings should be paid attention to when dealing with eutrophication problems in large and shallow lakes.