Excess nutrient loads to Lake Taihu: Opportunities for nutrient reduction

Unravelling of the interaction mechanism of PFOA with submerged macrophytes and epiphytic biofilms at gene and molecular level

Perfluorooctanoic acid (PFOA) is a global concern for the remediation of aquatic environments due to its toxicity, persistence and bioaccumulation. To gain a comprehensive understanding of the effects of PFOA on submerged macrophytes and epiphytic biofilms in eutrophic waters, as well as the mechanisms of PFOA and nutrient removal by submerged macrophyte-biofilm system, plant-attached biofilms were cultured under PFOA stress. Results indicated that PFOA exposure leads to an increase in ROS in plants, seriously damaging cellular systems, activating enzyme antioxidants to protect plant cells from oxidative damage. Additionally, PFOA affects microbial diversity, structure and function in biofilms, with Cyanobacteria and Proteobacteria being the dominant groups. While impacting both plants and biofilms, PFOA will be absorbed by them. PFOA binds to channel proteins (ULX0, VIH9) on plant cells and enters the plant, thus removing it from the environment. Plant co-cultivation has the highest pollutant removal rate of PFOA, reaching to 33.00 %. Furthermore, metagenomics analyses revealed that alterations in genes associated with N (nxrB, nxrA) and P (phoR, pstA) cycling of in the biofilm microorganisms facilitated the removal of N and P, with removal rates of 98.00 % and 99.42 %, respectively. These findings contribute to the ecological assessment of PFOA and provide valuable sights into the mechanisms of PFOA, N and P removal in eutrophic lakes by plants and epiphytic biofilms.

Lacustrine groundwater discharge as an important hidden source of nutrients to a large eutrophic lake: Implications for eutrophication management

Lake eutrophication driven by excessive nutrient inputs has become a global issue, but the potential impact of lacustrine groundwater discharge (LGD) as a nutrient source on lake eutrophication remains largely unknown. This study assessed the contribution of LGD-derived nutrient loads and revealed their potential impact on lake eutrophication in Taihu Lake, a typical large shallow and eutrophic lake in China, based on the segmented radon mass balance model and nutrient data. The total LGD flux was estimated to be 6.59 × 109 m3 a-1, representing 57.8 % of the annual flux from inflowing rivers. LGD was a significant hidden nutrient source, contributing total nitrogen (TN) and total phosphorus (TP) loads to the entire lake comparable to those of the inflowing rivers. Dissolved inorganic forms dominated these LGD-derived nutrient loads. Spatially, the majority of TN (59.9 %) and TP (62.4 %) loads derived from LGD originated from sub-area III (southwest), which differed from the dominant area of riverine inputs, sub-area II (northwest). In addition, the significant enrichment of nitrogen observed in LGD suggests its potential to mitigate nitrogen limitation in the lake. The increasing nitrogen limitation in Taihu Lake and the prevalence of nitrogen limitation in eutrophic lakes worldwide indicate that nitrogen is a key nutrient in managing LGD-derived nutrient loads. This study highlights the importance of integrating LGD-derived nutrient loads into nutrient reduction strategies to reverse eutrophication in large eutrophic lakes, especially those with nitrogen limitation.

Effects of artemisinin sustained-release algaecides on in-situ cyanobacterial inhibition and microbes-floating plants dominated ecosystem functions in artificial landscape lake

The artemisinin sustained-release algaecides (ASAs) have been proven to be a safe and effective mean of inhibiting cyanobacteria in laboratory experiments. However, their effectiveness and impacts on ecosystem functions (EF) in natural waters are still unclear. In this study, the in-situ inhibitory effect of ASAs on cyanobacteria in natural waters was investigated over a period of 110 days to assess EF impacts dominated by microbes and floating plants. The results indicated that ASAs had a long-term inhibitory effect on cyanobacteria. ASAs did not affect sediment but increased TOC and TP in the water column in 2-10 days. Microbial diversity and network analyses indicated that ASAs enhanced bacterial diversity, network complexity, and hub-bacteria in networks. Metabolic pathway predictions and CCA analysis showed that ASAs maintained the stability of EF by enhancing the metabolic capacities of bacteria, and the relationships between metabolic microorganisms and environmental factors. PLS-PM revealed that ASAs primarily drove bacterial resistance to cyanobacteria, which was the key mechanism for its long-term inhibition of cyanobacteria. However, the early outbreak of floating plants was not conducive to the long-term inhibition of cyanobacteria by ASAs. This study provides new insights into the mechanisms and ecological impacts of cyanobacterial inhibition by ASAs in complex aquatic environments.

Long-term responses of internal environment dynamics in a freshwater lake to variations in external nutrient inputs: A model simulation approach

Lake restoration usually focuses on reducing external nutrient sources. However, when sediments contain nutrients accumulated over multiple years, internal nutrient release can delay restoration progress. In lake restoration and management, it is important to understand the dynamic relationship between nutrient concentrations in a lake and internal and external nutrient sources. In this study, we quantified external nutrient inputs through measurements and compared them with internal sediment release from simulation using the PCLake+ model. Additionally, we evaluated alterations in the internal nutrient release, lake nutrient concentrations, and algae biomass (chlorophyll-a) within the lake following varying degrees of reduction in external nutrient loads. The results demonstrate that the PCLake+ effectively simulated the lake's nutrient concentration and algae biomass. Based on the PCLake+ estimates, internal nutrient loads accounted for 51 % of the total nitrogen (N) and 80 % of the total phosphorus (P) loadings in Lake Erhai in 2019. In 2020, the total contributions were 43 % for TN and 72 % for TP. We simulated four scenarios where external nutrient inputs were reduced to 25 %, 50 %, 75 %, and 99.99 % of their original levels. The 40-year simulation showed that the lake's ecological system initially exhibited a fast internal response but reached equilibrium after eight years. P concentrations took longer to reach equilibrium compared to N concentrations, probably due to the stronger binding characteristics of P. To meet the water quality target in the future, it is necessary to reduce external N and P inputs into Lake Erhai by at least 23 % and 15 %, respectively, under current conditions. Although reducing external nutrient loads can indirectly lower internal nutrient loads, water management should address both external and internal loads simultaneously, as internal release cannot be effectively reduced by external reductions alone. Additionally, the lake's internal release may continue for several years, even with reductions in external inputs.

Sewage-and fertilizer-derived nutrients alter the intensity, diversity, and toxicity of harmful cyanobacterial blooms in eutrophic lakes

Cyanobacterial harmful algal blooms (CHABs) are promoted by excessive nutrient loading and, while fertilizers and sewage are the most prevalent external nutrient sources in most watersheds, the differential effects of these nutrient sources on CHABs are unknown. Here, we tracked CHABs and performed experiments in five distinct lakes across the Northern US including Lake Erie. Fertilizers with ammonium and orthophosphate, membrane (0.2 μm)-filtered sewage (dominated by reduced forms of nitrogen) sand-and membrane-filtered sewage (dominated by nitrate), and an inorganic nutrient solution of ammonium and orthophosphate were used as experimental nutrient sources for CHABs at N-equivalent, environmentally realistic concentrations. Phytoplankton communities were evaluated fluorometrically, microscopically, and via high throughput sequencing of the 16S rRNA gene, and levels of microcystin and the δ15N content of particulate organic nitrogen (δPO15N) were quantified. Fertilizer and both sources of wastewater increased the abundance of cyanobacteria in all experiments across all five lakes (p < 0.05 for all) whereas effects on eukaryotic phytoplankton were limited. Sand-filtered sewage contained less P, organic matter, and ammonium but more nitrate and had a 25% less potent stimulatory effect on cyanobacteria than membrane-filtered sewage, suggesting nitrification may play a role in reducing CHABs. Fertilizer increased microcystin levels and decreased the δPO15N whereas wastewater increased δPO15N (p < 0.05 for all). Microcystis was the genus most consistently promoted by nutrient sources (p < 0.05 in all experiments), followed by Cyanobium (p < 0.05 in 50% of experiments), with increases in Microcystis biomass consistently elicited by membrane-filtered wastewater. Collectively, results demonstrate that differing types of sewage discharge and fertilizers can promote CHAB intensity and toxicity, while concurrently altering CHAB diversity and δPO15N. While membrane-filtered sewage consistently favored Microcystis, the discharge of sewage through sands muted bloom intensity suggesting sand-beds may represent a tool to remove key nutrients and partially mitigate CHABs.

Ecological restoration for eutrophication mitigation in urban interconnected water bodies: Evaluation, variability and strategy

Many urban water bodies grapple with low flow flux and weak hydrodynamics. To address these issues, projects have been implemented to form integrated urban water bodies via interconnecting artificial lake or ponds with rivers, but causing pollution accumulation downstream and eutrophication. Despite it is crucial to assess eutrophication, research on this topic in urban interconnected water bodies is limited, particularly regarding variability and feasible strategies for remediation. This study focused on the Loucun river in Shenzhen, comprising an pond, river and artificial lake, evaluating water quality changes pre-(post-)ecological remediation and establishing a new method for evaluating the water quality index (WQI). The underwater forest project, involving basement improvement, vegetation restoration, and aquatic augmentation, in the artificial lake significantly reduced total nitrogen (by 43.58%), total phosphorus (by 79.17%) and algae density (by 36.90%) compared to pre-remediation, effectively controlling algal bloom. Rainfall, acting as a variable factor, exacerbated downstream nutrient accumulation, increasing total phosphorus by 4.56 times and ammonia nitrogen by 1.30 times compared to the dry season, and leading to algal blooms in the non-restoration pond. The improved WQI method effectively assesses water quality status. The interconnected water body exhibits obvious nutrient accumulation in downstream regions. A combined strategy that reducing nutrient and augmenting flux was verified to alleviate accumulation of nutrients downstream. This study provides valuable insights into pollution management strategies for interconnected pond-river-lake water bodies, offering significant reference for nutrient mitigation in such urban water bodies.

Sources of nitrogen in reservoirs of the Haihe basin (China) 2012-2017

Nitrogen (N) is essential for agricultural production. However, too much N can pollute waters. The Chinese government published several policies to reduce N losses from agricultural production to waters since 2015, which may influence river export of N to reservoirs and lakes and their pollution sources. This study aimed to quantify the trends of river export of N to five reservoirs in the Haihe basin and analyze the main sources of this N pollution from 2012 to 2017. This was done by upscaling the MARINA-Lakes (Model to Assess River Inputs of Nutrients to lAkes) model to the Haihe basin, including 22 sub-basins. From 2012 to 2017, river export of total dissolved nitrogen (TDN) to the Haihe reservoirs decreased by 11-51%, associated with a decreased contribution of point sources and an increased contribution of diffuse sources for the whole study area Sub-basins draining into Reservoir Pan-Da contributed over one-third to the total TDN export by rivers in 2012 and 2017. The share of diffuse sources in river export of TDN to the Guanting reservoir reached 63% in 2017. Among the TDN diffuse sources, the contribution of animal manure (a diffuse source) to river export of diffuse TDN increased to 28%, 25%, and 23% for the sub-basins of Reservoir Miyun, Pan-da, and Guanting from 2012 to 2017, respectively. Among the TDN point sources, direct manure discharges were the main contributors to the river export of point TDN to the Haihe reservoirs in 2012. By 2017, direct discharges of untreated human waste became another important point source, especially for the Lake Baiyangdian and Reservoir Gang-Huang. This study concludes the need for specific agricultural N management options for different reservoirs of the Haihe basin.

Analysis of future nitrogen and phosphorus loading in watershed and the risk of lake blooms under the influence of complex factors: Implications for management

For the management of eutrophic lakes, watershed nitrogen and phosphorus control is oriented to future water quality. Assessing future nutrient dynamics and the risk of lake eutrophication is necessary. However, current assessments often lack integrated consideration of socioeconomic and climatic factors, which reduces the reference value of the results. In this study, a typical large shallow lake Chaohu, which is highly influenced by human activities, was selected as the study area, and the current and future total nitrogen (TN) and total phosphorus (TP) loading in the basin were analysed using the improved MARINA model, and the risk of water bloom were assessed. The results showed that socioeconomic factors alone varied future TN and TP loading by -24% to 32% and -40% to 34%, respectively, under different development patterns. After considering the effect of increased precipitation, the changes of TN and TP loading became -10% to 163% and -29% to 108%, respectively. The effect on loading reduction under the sustainable development pattern was weakened (58% and 28% for TN and TP loading, respectively) and the increase in loading under the brutal development pattern was significantly amplified (409% and 215% for TN and TP loading, respectively). The adoption of active environmental policies remained an effective way of loading control. However, the risk of water bloom in local lake areas might persist due to factors such as urbanization. Timely and comprehensive assessments can provide managers with more information to identify key factors that contribute to the risk of water blooms and to develop diverse water quality improvement measures. The insights from our study are applicable to other watersheds around the world with similar socio-economic background and climatic conditions.

Relative impact of environmental variables on the lake trophic state highlights the complexity of eutrophication controls

For the effective management of lakes apart from defining and monitoring their current state it is crucial to identify environmental variables that are mostly responsible for the nutrient input. We used interpretative machine learning to investigate the environmental parameters that influence the lake's trophic state and recognize their patterns. We analysed the influence of the 25 environmental variables on the commonly used trophic state indicators values: total phosphorus (TP), Chlorophyll-a (Chl-a) and Secchi depth (SD) of 60 lakes located in the Central European Lowlands. We attempted to delineate the lakes into groups due to the influence of common prevailing environment variable/variables on the water trophic state reflected by each indicator. The results indicated that the relative impact of environmental variables on the lake trophic state has an individual hierarchy unique for each indicator. The most important are variables related to catchment impact on the lake, Ohle ratio (L. catchment area/L. area) for TP and Schindler ratio (L. area + L. catchment area)/L. volume for Chl-a and SD. There are also few variables strongly influential only for small sub-groups of lakes that stand out: lake maximum depth, catchment slope steepness expressed by the height standard deviation. The methods used in the study enabled the assessment of the character of the influence of the environmental variables on the indicator value and revealed that most essential variables (Ohle ratio for TP and Schindler ratio for Chl-a and SD) have bimodal distribution with a clear threshold value. These findings contribute to a better understanding of the drivers shaping the lake trophic status and have implication for planning effective management strategies.

Temporal characteristics and trends of nitrogen loadings in lake Taihu, China and its influencing mechanism at multiple timescales

Climate warming impact on excessive nitrogen (N) load in sediment favours cyanobacterial blooms in eutrophic waters. The nitrate (NO3--N) and ammonium (NH4+-N) are two forms of N loads that contribute to algae blooms. However, little attention is paid to the impact of environmental factors on N loads variations at different time scales. This paper used a well-calibrated and validated EFDC model to investigate the temporal patterns and trends of ammonium and nitrate from June 2016 to June 2017. This paper presented the relationship and effects between these variations and environmental factors using data from satellite and reanalysis-based observations obtained for six meteorological parameters. The relationship and effects between these variations and environmental factors were also examined at different timescales (i.e., daily, monthly and seasonal scales). Model calibration results indicated that measured values reasonably matched simulated values. The validation results revealed that relative error (RE) values were within an acceptable range. The REs of ammonium at East Taihu (S12) and Xu Lake (S23) sampling sites were 55.83% and 57.61%, while that of nitrate was 24.37% (S12) and 41.08%, respectively. The daily analysis of NH4+-N and NO3--N variations was 7.318 ± 3.876 (g/m2/day) and 0.0275 ± 0.222 (g/m2/day), respectively. The monthly analysis showed NH4+-N and NO3-N range from 2.04 to 12.04 (g/m2/day) and 0.0008 to 0.064 (g/m2/day), respectively. The magnitude NH4+-N and NO3--N varied and showed distinct inter-monthly variations. , The relationship between sediment fluxes and meteorological parameters showed the magnitude of correlation coefficient (r) and strength of correlation varied significantly. At daily scales, the relationship of NH4+-N and NO3--N had a significant positive correlation with all meteorological parameters. At monthly, the correlation coefficient (r) of NH4+-N and NO3-N were heterogenous. At daily and monthly scales, air temperature and wind speed are the main drivers affecting sediment N loads' dynamics; however, the influence of relative humidity, precipitation, and evaporation on N loads are smaller. The study demonstrates the contribution of meteorological conditions to the magnitude and timing of N loadings variability in water bodies. The findings provide more insight into lake ecosystem protection and environmental remediation.

A raster-based estimation of watershed phosphorus load and its impacts on surrounding rivers based on process-based modelling

Quantifying phosphorus (P) load from watersheds at a fine scale is crucial for studying P sources in lake or river ecosystems; however, it is particularly challenging for mountain-lowland mixed watersheds. To address this challenge, we proposed a framework to estimate the P load at the grid scale and assessed its risk to surrounding rivers in a typical mountain-lowland mixed watershed (Huxi Region in Lake Taihu Basin, China). The framework coupled three models: the Phosphorus Dynamic model for lowland Polder systems (PDP), the Soil and Water Assessment Tool (SWAT), and the Export Coefficient Model (ECM). The coupled model performed satisfactory for both hydrological and water quality variables (Nash-Sutcliffe efficiency >0.5). Our modelling practice revealed that polder, non-polder, and mountainous areas had P load of 211.4, 437.2, and 149.9 t yr^-1, respectively. P load intensity in lowlands and mountains was 1.75 and 0.60 kg ha^-1 yr^-1, respectively. A higher P load intensity (>3 kg ha^-1 yr^-1) was mainly observed in the non-polder area. In lowland areas, irrigated cropland, aquaculture ponds and impervious surfaces contributed 36.7%, 24.8%, and 25.8% of the P load, respectively. In mountainous areas, irrigated croplands, aquaculture ponds, and impervious surfaces contributed 28.6%, 27.0%, and 16.4% of the P load, respectively. Rivers with relatively high P load risks were mainly observed around big cities during rice season, owing to a large contribution of P load from the non-point source pollution of urban and agricultural activities. This study demonstrated a raster-based estimation of watershed P load and their impacts on surrounding rivers using coupled process-based models. It would be useful to identify the hotspots and hot moments of P load at the grid scale.

The evolution of a typical plateau lake from macrophyte to algae leads to the imbalance of nutrient retention

Long-term anthropogenic nitrogen (N) and phosphorus (P) inputs have led to lake eutrophication and decreased environmental quality. However, the imbalance in nutrient cycling caused by ecosystem transformation during lake eutrophication is still unclear. The N, P, organic matter (OM) and their extractable forms in the sediment core of Dianchi Lake were investigated. Combining ecological data and geochronological techniques, a coupling relationship between the evolution of lake ecosystems and nutrient retention was established. The results show that the evolution of lake ecosystems promotes the accumulation and mobilization of N and P in sediments, leading to an imbalance in nutrient cycling in the lake system. From the "macrophyte-dominated" period to the "algae-dominated" period, the accumulation rates of potential mobile N and P (PMN, PMP) in sediments have significantly increased, and the retention efficiency of total N and P (TN, TP) has decreased. The increased TN/TP ratio (5.38 ± 1.52 ‒ 10.19 ± 2.94) and PMN/PMP ratio (4.34 ± 0.41 ‒ 8.85 ± 4.16), as well as the reduced humic-like/protein-like ratio (H/P, 11.18 ± 4.43 ‒ 5.97 ± 3.67), indicated an imbalance in nutrient retention during sedimentary diagenesis. Our results show that eutrophication has resulted in the potential mobilization of N in sediments exceeding P, providing new insights for further understanding the nutrient cycle in the lake system and strengthening lake management.

Legume cover with optimal nitrogen management and nitrification inhibitor enhanced net ecosystem economic benefits of peach orchard

Peach (Prunus persica L.), as a traditional kind of fruits in China, was extremely dependent on large application of nitrogen (N) fertilizer to maintain high fruit yield and commercial income, resulting in raising environmental damage risk. Therefore, a three-year field trail was conducted to clarify the environmental N loss under conventional management, investigate the positive effects of optimal N management, legume cover and 3,4-dimethylpyrazole phosphate (DMPP) on N input/output and the net ecosystem economic benefits (NEEB). There are four treatments in this study: conventional fertilizer management with 521.1 kg N ha-1 yr-1 input (CU); optimal N management including 406.4 kg N ha-1 yr-1 input and deep fertilization (OP); DMPP was added to OP at rate of 1 % (w/w) (OPD); legume (white clover) was covered to OPD (OPDG). Results showed 102.9 kg N ha-1 was removed by annual fruit and residues (including pruned branches, pruned and fallen leaves), while 70.2 kg N ha-1 was lost to the environment by ammonia (NH3), nitrous oxide (N2O) and N runoff loss under the conventional fertilizer management. While, the optimal N management mitigated NH3 volatilization about 49.3 %, further added DMPP abated N2O emission by 61.4 %, besides covered white clover lowered N runoff loss by 64.5 %. The NEEB results revealed that optimal N management combined with added DMPP and covered white clover could minimize the production cost, reduce environmental damage cost by 35.9 %, increase fruit yield by 10.3 % and achieved the maximum NEEB with improvement of 27.1 %, in comparison of the conventional fertilizer management. Generally, conventional peach cultivation constituted overwhelming N loss to raise potential environmental risk. While, extending mode of optimized N management combined with DMPP and legume cover could not only realize high fruit revenue, but also abate environmental N losses, thereby should be considered as effective strategy for sustainable fruit cropping systems.

Trends in nutrients in the Changjiang River

Better documentation and understanding of long-term temporal dynamics of nutrients in watersheds are necessary to support effective water quality management. We examined the hypothesis that the recent management of fertilizer use and pollution control in the Changjiang River Basin could govern the fluxes of nutrients from the river to the sea. Results based on historical data since 1962 and surveys in recent years show that concentrations of dissolved inorganic nitrogen (DIN) and phosphorus (DIP) in the mid- and downstream reaches were higher than those in the upper reaches due to intensive anthropogenic activities, while dissolved silicate (DSi) was distributed evenly from the up- to downstream reaches. Fluxes of DIN and DIP increased rapidly, and DSi declined during the 1962-1980 and 1980-2000. After the 2000s, concentrations and fluxes of DIN and DSi remained almost unchanged; those of DIP remained stable until the 2010s and slightly decreased afterward. The decline in fertilizer use explains 45 % of the variance in the decline of DIP flux, followed by pollution control, groundwater and water discharge. As a result, the molar ratio of DIN:DIP, DSi:DIP and ammonia:nitrate varied largely during 1962-2020, and the excess DIN relative to DIP and DSi lead to increased limitations of silicon and phosphorus. A turning point probably occurred for nutrient fluxes in the Changjiang River in the 2010s, with the pattern of DIN from continuous increase to stability and DIP from increase to decrease. This decline in phosphorus in the Changjiang River has many similarities with the rivers worldwide. The continued basin nutrient management is likely to have a major influence on river nutrient delivery and therefore may control coastal nutrient budget and ecosystem stability.

Long-term remote observations of particulate organic phosphorus concentration in eutrophic Lake Taihu based on a novel algorithm

Monitoring the long-term spatiotemporal variations in particulate organic phosphorus concentration (C_POP) is imperative for clarifying the phosphorus cycle and its biogeochemical behavior in waters. However, little attention has been devoted to this owing to a lack of suitable bio-optical algorithms that allow the application of remote sensing data. In this study, based on Moderate Resolution Imaging Spectroradiometer (MODIS) data, a novel absorption-based algorithm of C_POP was developed for eutrophic Lake Taihu, China. The algorithm yielded a promising performance with a mean absolute percentage error of 27.75% and root mean square error of 21.09 μg/L. The long-term MODIS-derived C_POP demonstrated an overall increasing pattern over the past 19 years (2003-2021) and a significant temporal heterogeneity in Lake Taihu, with higher value in summer (82.06 ± 3.81 μg/L) and autumn (78.74 ± 3.8 μg/L), and lower C_POP in spring (79.52 ± 3.81 μg/L) and winter (81.97 ± 3.8 μg/L). Spatially, relatively higher C_POP was observed in the Zhushan Bay (85.87 ± 7.5 μg/L), whereas the lower value was observed in the Xukou Bay (78.95 ± 3.48 μg/L). In addition, significant correlations (r > 0.6, P < 0.05) were observed between C_POP and air temperature, chlorophyll-a concentration and cyanobacterial blooms areas, demonstrating that C_POP was greatly influenced by air temperature and algal metabolism. This study provides the first record of the spatial-temporal characteristics of C_POP in Lake Taihu over the past 19 years, and the C_POP results and regulatory factors analyses could provide valuable insights for aquatic ecosystem conservation.

Comparing critical source areas for the sediment and nutrients of calibrated and uncalibrated models in a plateau watershed in southwest China

Controlling non-point source pollution is often difficult and costly. Therefore, focusing on areas that contribute the most, so-called critical source areas (CSAs), can have economic and ecological benefits. CSAs are often determined using a modelling approach, yet it has proved difficult to calibrate the models in regions with limited data availability. Since identifying CSAs is based on the relative contributions of sub-basins to the total load, it has been suggested that uncalibrated models could be used to identify CSAs to overcome data scarcity issues. Here, we use the SWAT model to study the extent to which an uncalibrated model can be applied to determine CSAs. We classify and rank sub-basins to identify CSAs for sediment, total nitrogen (TN), and total phosphorus (TP) in the Fengyu River Watershed (China) with and without model calibration. The results show high similarity (81%-93%) between the identified sediment and TP CSA number and locations before and after calibration both on the yearly and seasonal scale. For TN alone, the results show moderate similarity on the yearly scale (73%). This may be because, in our study area, TN is determined more by groundwater flow after calibration than by surface water flow. We conclude that CSA identification with the uncalibrated model for TP is always good because its CSA number and locations changed least, and for sediment, it is generally satisfactory. The use of the uncalibrated model for TN is acceptable, as its CSA locations did not change after calibration; however, the TN CSA number changed by over 60% compared to the figures before calibration on both yearly and seasonal scales. Therefore, we advise using an uncalibrated model to identify CSAs for TN only if water yield composition changes are expected to be limited. This study shows that CSAs can be identified based on relative loading estimates with uncalibrated models in data-deficient regions.

Stream water quality optimized prediction based on human activity intensity and landscape metrics with regional heterogeneity in Taihu Basin, China

The driving effects of landscape metrics on water quality have been acknowledged widely, however, the guiding significance of human activity intensity and landscape metrics based on reference conditions for water environment management remains to be explored. Thus, we used the self-organized map, long- and short-term memory (LSTM) algorithm, and geographic detectors to simulate the response of human activity intensity and landscape metrics to water quality in Taihu Lake Basin, China. Fitting results of LSTM displayed that the accuracy was acceptable, and scenario 2 (regional heterogeneity) was more efficient than scenario 1 (regional consistent) in the improvement of water quality. In the driving analysis for the reference conditions, clusters I and II (urban agglomeration areas) were mainly affected by the amount of production wastewater per unit of developed land and the amount of livelihood wastewater per unit of developed land, respectively. Their optimal values were 0.09 × 10³ t/km² (reduction of 35.71%) and 0.2 × 10³ t/km² (reduction of 4.76%). Cluster III (agricultural production areas) was mainly affected by interference intensity, and the optimal value was 2.17 (increased 38.22%), and cluster IV (ecological forest areas) was mainly affected by the fragmentation of cropland, and the optimal value was 1.14 (reduction of 1.72%). The research provides a reference for the prediction of water quality response and presents an ecological and economic sustainability way for watershed governance.

The Spatiotemporal Characteristics of Water Quality and Main Controlling Factors of Algal Blooms in Tai Lake, China

Taking Tai Lake in China as the research area, a 3D water environment mathematical model was built. Combined with the LHS and Morris uncertainty and sensitivity analysis methods, the uncertainty and sensitivity analysis of total phosphorus (TP), total nitrogen (TN), dissolved oxygen (DO), and chlorophyll a (Chl-a) were carried out. The main conclusions are: (1) The performance assessment of the 3D water environment mathematical model is good (R2 and NSE > 0.8) and is suitable for water quality research in large shallow lakes. (2) The time uncertainty study proves that the variation range of Chl-a is much larger than that of the other three water quality parameters and is more severe in summer and autumn. (3) The spatial uncertainty study proves that Chl-a is mainly present in the northwest lake area (heavily polluted area) and the other three water quality indicators are mainly present in the center. (4) The sensitivity results show that the main controlling factors of DO are ters (0.15) and kmsc (0.12); those of TN and TP are tetn (0.58) and tetp (0.24); and those of Chl-a are its own growth rate (0.14), optimal growth temperature (0.12), death rate (0.12), optimal growth light (0.11), and TP uptake rate (0.11). Thus, TP control is still the key treatment method for algal blooms that can be implemented by the Chinese government.

Spatial characteristics of nutrient budget on town scale in the Three Gorges Reservoir area, China

Accurately quantifying nutrient budget is an essential step toward sustainable nutrient management in large watersheds increasingly disturbed by human activity. A town-scale nutrient budget framework based on the Soil and Water Assessment Tool was developed for 2010-2012 in the Three Gorges Reservoir area in China (TGRA). Moran's I spatial correlation test and Geodetector spatial heterogeneity test were employed to systematically analyze the spatial characteristics of the resulting nutrient budget. The Moran's I value of total nitrogen (TN) and total phosphorus (TP) gradually increased from input to output in the range of 0.091-0.232 and 0.102-0.484, respectively. Towns with higher TN and TP inputs were largely concentrated in the main urban area of Chongqing because of its high population density. By contrast, towns with higher TN and TP outputs were concentrated in the head of the TGRA. The Moran's I values of the TN and TP retention coefficients (R) were 0.433 and 0.524, respectively, demonstrating clear spatial consistency. Towns with a "High-high" spatial consistency pattern and positive R value were concentrated in the tail and hinterland, while those with a "Low-low" spatial consistency pattern and negative coefficient value were located mainly in the head of the TGRA. This phenomenon was mostly caused by differences in regional elevation, the normalized difference vegetation index, and soil erosion factor. The interaction effect between any two of these three factors on nutrient retention (Geodetector q-value) was greater than 60%. Therefore, future nutrient management should be based on a full understanding of regional biophysical conditions, especially in large areas. These findings provide a new perspective on fine nutrient management.

Measuring the zonal responses of nitrogen output to landscape pattern in a flatland with river network: a case study in Taihu Lake Basin, China

Landscape pattern changes induced by rapid urbanization and intensified agricultural activities have exerted great pressure on regional water purification services. Relationship between landscape metrics and nitrogen-related ecosystem services has been a major concern of many scholars and has been widely used for guidance for land use and cover (LULC) management. However, clear zonal differences may exist, especially in highly developed reticular river network area, thus limiting our understanding of nitrogen output (NOP) to landscape pattern in the details. The spatial distribution of regional NOP was obtained based on the InVEST model. The zonal responses of NOP to landscape patter were examined under hydraulic subregions and subbasin scale. The results show that the unit value of average NOP in the Taihu Lake Basin (TLB) was 146.14 (kg/km²), and the total output reached 23677.92 t in 2020. The simulation NOP showed reasonable agreement with verified water quality observations in the lake inlet stations, with an R² of 0.76. In terms of space composition, merely cropland have significant effects on NOP in the whole basin scale, while the explanatory variables include cropland and developed land in Pudong (PD), Puxi (PX), Wuchengxiyu (WC), and Hangjiahu (HJ) regions. In Huxi (HX) and Yangchengdianmao  (YC) regions, cropland and forest are the significant impact types, while in (Zhexi) ZX region, cropland, developed land, and forest are significant impact types. In the space configuration, the percentage of landscape (PLAND) or largest patch index (LPI) of cropland showed positive effects about NOP, whether in the whole basin or the hydraulic subregions. Edge density (ED) (-3.48), number of patches (NP) (-3.91), and percentage of like adjacencies (PLAND) (-2.80) of the forest exhibit negative correlations with NOP, in the HX, ZX, and YC region, respectively. It displays diversiform in the response of NOP to the landscape metric of developed land, which speculate that the heterogeneity of developed land can also have a constraint on NOP, in the highly urbanized areas with less forest area. In addition, the total nitrogen output of the TLB needs to be controlled, especially in HJ region which was identified as the sensitive area of pollution sources with the largest NOP and should be paid more attention to. Compared with the administrative management unit, it is more reasonable to control and manage the pollution sources by referring to the hydraulic subregions and subbasin units. Senior managers are required to strengthen communication and cooperation with hydraulic subregions across administrative regions. However, when managing NOP through the landscape modifications, measures should be taken to reduce the aggregation of nitrogen sources and increase the fragmentation of nitrogen sinks. As for high aggregation developed and agricultural land regions, the types of land used should be enriched to help the sustainable development.

Occurrence and driving forces of different nitrogen forms in the sediments of the grass and algae-type zones of Taihu Lake

Excessive nitrogen (N) load in sediments is at risk of release resulting in the degradation of grass-type lake ecosystems. At present, the occurrence characteristics of N forms and the driving forces of organic N (ON) hydrolysis in the sediments of Taihu Lake were still unclear. Here, 52 sampling sites in 7 lake areas in Taihu Lake were investigated to compare the spatial occurrence characteristics of the sedimentary free N (FN), exchangeable N (EN), acid hydrolyzable N (HN), and residual N (RN) and their associated driving forces. The results showed that the total N contents in the dry sediment ranged from 1811.56 to 5594.06 mg kg^-1, and the contribution was in the order of RN > HN > EN > FN. Spatially, RN and total organic carbon were significantly consistently influenced by dam construction and deposition algal residue. The HN concentration was high in the estuaries affected by N inputs from the rivers. The coupling relationship of spatial distribution between ON and N forms was revealed. The factors, i.e., algal residue deposition and terrigenous N inputs, were considered as the main driving forces stimulating the ON hydrolysis in the algae-type lake zones. It can be deduced that controlling terrigenous N inputs and sediment suspension may be the key to inhibiting the transformation from grass-type to algae-type lake ecosystem.

Remote monitoring of total dissolved phosphorus in eutrophic Lake Taihu based on a novel algorithm: Implications for contributing factors and lake management

Understanding the spatiotemporal dynamics of total dissolved phosphorus concentration (C_TDP) and its regulatory factors is essential to improving our understanding of its impact on inland water eutrophication, but few studies have assessed this in eutrophic inland lakes due to a lack of suitable bio-optical algorithms allowing the use of remote sensing data. We developed a novel semi-analytical algorithm for this purpose and tested it in the eutrophic Lake Taihu, China. Our algorithm produced robust results with a mean absolute square percentage error of 29.65% and root mean square error of 9.54 μg/L. Meanwhile, the new algorithm demonstrates good portability to other waters with different optical properties and could be applied to various image data, including Moderate Resolution Imaging Spectroradiometer (MODIS), Medium Resolution Imaging Spectrometer (MERIS), and Ocean and Land Color Instrument (OLCI). Further analysis based on Geostationary Ocean Color Imager observations from 2011 to 2020 revealed a significant spatiotemporal heterogeneity of C_TDP in Lake Taihu. Correlation analysis of the long-term trend between C_TDP and driving factors demonstrated that air temperature is the dominant regulating factor in variations of C_TDP. This study provides a novel algorithm allowing remote-sensing monitoring of C_TDP in eutrophic lakes and can lead to new insights into the role of dissolved phosphorus in water eutrophication.

What is the pollution limit? Comparing nutrient loads with thresholds to improve water quality in Lake Baiyangdian

Ecological thresholds are useful indicators for water quality managers to define limits to nutrient pollution. A common approach to estimating ecological thresholds is using critical nutrient loads. Critical nutrient loads are typically defined as the loads at which the phytoplankton chlorophyll-a exceeds a certain concentration. However, national policies, such as in China, use chemical indicators (nitrogen and phosphorus concentrations) rather than ecological indicators (phytoplankton chlorophyll-a) to assess water quality. In this study, we uniquely define the critical nutrient loads based on maximum allowable nutrient concentrations for lake Baiyangdian. We assess whether current and future nutrient loads in this lake comply with the Chinese Water Quality standards. To this end, we link two models (MARINA-Lakes and PCLake+). The PCLake+ model was applied to estimate the critical nutrient loads related to ecological thresholds for total nitrogen, total phosphorus and chlorophyll-a. The current (i.e., 2012) and future (i.e., 2050) nutrient loads were derived from the water quality MARINA-Lakes model. Nitrogen loads exceeded the nitrogen threshold in 2012. Phosphorus loads were below all ecological thresholds in 2012. Ecological thresholds are exceeded in 2050 with limited environmental policies, and urbanization may increase nutrient loads above the ecological thresholds in 2050. Recycling and reallocating animal manure is needed to avoid future water pollution in Lake Baiyangdian. Our study highlights the need for effective policies for clean water based on policy-relevant indicators.

Characterizing 19 thousand Chinese lakes, ponds and reservoirs by morphometric, climate and sediment characteristics

Chinese lakes, including ponds and reservoirs, are increasingly threatened by algal blooms. Yet, each lake is unique, leading to large inter-lake variation in lake vulnerability to algal blooms. Here, we aim to assess the effects of unique lake characteristics on lake vulnerability to algal blooms. To this end, we built a novel and comprehensive database of lake morphometric, climate and sediment characteristics of 19,536 Chinese lakes, including ponds and reservoirs (>0.1 km²). We assessed lake characteristics for nine stratification classes and show that lakes, including ponds and reservoirs, in eastern China typically have a warm stratification class (T_avg>4 °C) and are slightly deeper than those in western China. Model results for representative lakes suggest that the most vulnerable lakes to algal blooms are in eastern China where pollution levels are also highest. Our characterization provides an important baseline to inform policymakers in what regions lakes are potentially most vulnerable to algal blooms.

Uncertainty and Sensitivity Analysis of Input Conditions in a Large Shallow Lake Based on the Latin Hypercube Sampling and Morris Methods

We selected Tai Lake in China as the research area, and based on the Eco-lab model, we parameterized seven main external input conditions: discharge, carbon, nitrogen, phosphorus, wind speed, elevation, and temperature. We combined the LHS uncertainty analysis method and the Morris sensitivity analysis method to study the relationship between water quality and input conditions. The results showed that (1) the external input conditions had an uncertain impact on water quality. Among them, the uncertainties in total nitrogen concentration (TN) and total phosphorus concentration (TP) were mainly reflected in the lake entrance area, and the uncertainties of chlorophyll-a (Chl-a) and dissolved oxygen (DO) were mainly reflected in the lake center area. (2) The external input conditions had different sensitivities to different water layers. The bottom layer was most clearly and stably affected by input conditions. The TN and TP of the three different water layers were closely related to the flux into the lake, with average sensitivities of 83% and 78%, respectively. DO was mainly related to temperature and water elevation, with the bottom layer affected by temperatures as high as 98%. Chl-a was affected by all input factors except nitrogen and was most affected by wind speed, with an average of about 34%. Therefore, the accuracy of external input conditions can be effectively improved according to specific goals, reducing the uncertainty impact of the external input conditions of the model, and the model can provide a scientific reference for the determination of the mid- to long-term governance plan for Tai Lake in the future.

Quantifying and optimizing agroecosystem services in China's Taihu Lake Basin

This study examines rice-wheat agroecosystems in the Taihu Lake Basin: one of China's largest commercial grain-farming areas and a region that has faced severe deterioration in water quality. Spatiotemporal changes over the period 1986-2015 in four key ecosystem services (ESs) - grain yield, nitrogen loss, N₂O emission, and soil organic carbon (SOC) accumulation - were examined by applying the Agricultural Production Systems Simulator (APSIM) across the basin at county level. Two straw return modes (namely, full straw return versus no return) and three fertilizer-use reduction modes (-5%, -10%, and -20%) were set up to generate six combined scenarios, to propose pathways that reduce the variability of grain production and improve water quality by reducing loss of nitrogen (N loss) - in consideration of the Basin's vital role in agricultural production and the need to protect water quality. Results show that annual grain yield and net five-year difference in SOC accumulation exhibited an overall downward trend from 1986 to 2015, while N₂O emission and N loss increased. Two pairs of ESs showed desirable synergies (increasing grain yield and increasing SOC accumulation; decreasing N₂O emission and decreasing N loss), encompassing 45.8% and 2.4% of total cultivated land area respectively. Another two pairs exhibited desirable trade-offs (increasing SOC accumulation and decreasing N loss; increasing SOC accumulation and decreasing N₂O emission), accounting for 19.0%, and 2.4% of total cultivated land area respectively. There was considerable overlap within counties, which showed high values of grain yield, N₂O emission, nitrogen loss, and SOC accumulation in the Basin; but values were relatively high in the east and relatively low in the west. Fertilizer use has significant positive correlations with grain yield and SOC accumulation, and it reduces N loss and N₂O emission. Straw return was predicted to raise grain yields and net five-year difference in SOC accumulation and to reduce N loss, but also to increase N₂O emissions. Recommended strategies to reduce N loss and stabilize grain supply in the study area are 1) reducing fertilizer use by 20% in areas where N application was above 490 kg N/ha, and 2) implementing straw return and reducing fertilizer use by 5% for areas where N application ranged between 380 and 490 kg N/ha.

Water and Land as Shared Resources for Agriculture and Aquaculture: Insights from Asia

Although agriculture and aquaculture depend on access to increasingly scarce, shared water resources to produce food for human consumption, they are most often considered in isolation. We argue that they should be treated as integrated components of a single complex system that is prone to direct or indirect tradeoffs that should be avoided while also being amenable to synergies that should be sought. Direct tradeoffs such as competition for space or the pollution of shared water resources usually occur when the footprints of agriculture and aquaculture overlap or when the two practices coexist in close proximity to one another. Interactions can be modulated by factors such as hydropower infrastructure and short-term economic incentives, both of which are known to disrupt the balance between aquaculture and agriculture. Indirect tradeoffs, on the other hand, play out across distances, i.e., when agricultural food sources are diverted to feed animals in aquaculture. Synergies are associated with the culture of aquatic organisms in rice paddies and irrigation waters, seasonal rotations of crop cultivation with aquaculture, and various forms of integrated agriculture–aquaculture (IAA), including jitang, a highly developed variant of pond-dike IAA. Policy decisions, socioeconomic considerations, and technology warrant increased scrutiny as determinants of tradeoffs and synergies. Priority issues for the future include guiding the expansion of aquaculture from its traditional base in Asia, taking advantage of the heterogeneity that exists within both agricultural and aquaculture systems, the development of additional metrics of tradeoffs and synergies, and adapting to the effects of climate change.

Assessing seasonal nitrogen export to large tropical lakes

Rivers are exporting increasing amounts of nitrogen (N) to lakes, which is leading to eutrophication. However, the seasonality apparent in nutrient loading, especially in tropical areas, is thus far only partially understood. This study aims to better understand the seasonality and the sources of dissolved inorganic nitrogen (DIN) inputs from sub-basins to tropical lakes. We integrated existing approaches into a seasonal model that accounts for seasonality in human activities, meteorology and hydrology, and we applied the model to the sub-basins of a representative tropical lake: Lake Tana, Ethiopia. The model quantifies the river export of DIN by season, source and sub-basin and also accounts for open defecation to land as a diffuse source of N in rivers. Seasonality parameters were calibrated, and model outputs were validated against measured nitrogen loads in the main river outlets. The calibrated model showed good agreement with the measured nitrogen loads at the outflow of the main rivers. The model distinguishes four seasons: rainy (July-September), post-rainy (October-December), dry (January-March) and pre-rainy (April-June). The river export of DIN to Lake Tana was about 9 kton in 2017 and showed spatial and temporal variability: It was highest in the rainy and lowest in the dry seasons. Diffuse sources from agriculture were important contributors of DIN to rivers in 2017, and animal manure was the dominant source in all seasons. Our seasonal sub-basins and rivers model provides opportunities to identify the main nutrient sources to the lake and to formulate effective water quality management options. An example is nutrient application level that correspond to the crop needs in the sub-basins. Furthermore, our model can be used to analyse future trends and serves as an example for other large tropical lakes experiencing eutrophication.

Wind Effects on the Water Age in a Large Shallow Lake

As the third largest fresh water lake in China, Taihu Lake is suffering from serious eutrophication, where nutrient loading from tributary and surrounding river networks is one of the main contributors. In this study, water age is used to investigate the impacts of tributary discharge and wind influence on nutrient status in Taihu Lake, quantitatively. On the base of sub-basins of upstream catchments and boundary conditions of the lake, multiple inflow tributaries are categorized into three groups. For each group, the water age has been computed accordingly. A well-calibrated and validated three-dimensional Delft3D model is used to investigate both spatial and temporal heterogeneity of water age. Changes in wind direction lead to changes in both the average value and spatial pattern of water age, while the impact of wind speed differs in each tributary group. Water age decreases with higher inflow discharge from tributaries; however, discharge effects are less significant than that of wind. Wind speed decline, such as that induced by climate change, has negative effects on both internal and external nutrient source release, and results in water quality deterioration. Water age is proved to be an effective indicator of water exchange efficiency, which may help decision-makers to carry out integrated water management at a complex basin scale.

Production of the neurotoxin beta-N-methylamino-l-alanine may be triggered by agricultural nutrients: An emerging public health issue

Diverse taxa of cyanobacteria, dinoflagellates and diatoms produce β-N-methylamino-l-alanine (BMAA), a non-lipophilic, non-protein amino acid. BMAA is a neurotoxin in mammals. Its ingestion may be linked to human neurodegenerative diseases, namely the Amyotrophic lateral sclerosis/Parkinsonism dementia complex, based on epidemiological evidence from regions where cyanobacterial harmful algal blooms occur frequently. In controlled environments, cyanobacteria produce BMAA in response to ecophysiological cues such as nutrient availability, which may explain the elevated BMAA concentrations in freshwater environments that receive nutrient-rich agricultural runoff. This critical review paper summarizes what is known about how BMAA supports ecophysiological functions like nitrogen metabolism, photosyntheis and provides a competitive advantage to cyanobacteria in controlled and natural environments. We explain how BMAA production affected competitive interactions among the N₂-fixing and non-N₂-fixing populations in a freshwater cyanobacterial bloom that was stimulated by nutrient loading from the surrounding agricultural landscape. Better control of nutrients in agricultural fields is an excellent strategy to avoid the negative environmental consequences and public health concerns related to BMAA production.

Towards the comprehensive water quality control in Lake Taihu: Correlating chlorphyll a and water quality parameters with generalized additive model

In this study, the generalized additive model (GAM) was used to analyze seasonal monitoring data from Lake Taihu, collected from 2010 to 2014, with the aim to explore the correlation between chlorophyll a (Chla) and other water quality parameters. The selected optimal multivariable GAM could effectively explain the concentration variation of Chla occurring during each season, and the interpretation degree followed the order: summer > autumn > spring > winter. The fitting results indicated that the concentration variation of Chla could reflect that of biochemical oxygen demand and chemical oxygen demand in all seasons. In addition, the total phosphorus showed strong ability to explain the concentration change of Chla in spring and summer, as the growth of algae would be affected when the concentration of phosphorus shifted high or low. Nitrogen showed strong ability to explain the variations in Chla concentration in autumn. The conclusions of the optimal multivariable GAM could provide decision basis for the eutrophication control. In other words, the prevention of eutrophication outbreaks could be carried out via the targeted control of key water pollutants. According to these results, the concentration of Chla was higher in northern and western lake during summer and autumn, the management should focus on nutrient input of adjacent rivers.

Spatiotemporal Variations in Seston C:N:P Stoichiometry in a Large Eutrophic Floodplain Lake (Lake Taihu): Do the Sources of Seston Explain Stoichiometric Flexibility?

Although sources of seston are much more complicated in lakes compared to oceans, the influences of different sources on the spatiotemporal variations in seston stoichiometry are still underexplored, especially in large eutrophic floodplain lakes. Here, we investigated seston stoichiometric ratios across a typical large eutrophic floodplain lake (Lake Taihu, East China) over one year. In addition, we used the n-alkane proxies to examine the influence of the seston source on seston stoichiometry variation. Throughout the study, the average value of the C:N:P ratio of 143:19:1 across Lake Taihu was close to the canonical lake’s ratios (166:20:1). Similar to other eutrophic lakes, seston C:N ratios varied the least across all environments, but C:P and N:P ratios varied widely and showed a strong decreasing trend in ratios of N:P and C:P from growing season to senescence season. This seasonal change was mainly associated with the decreasing contribution from algal-derived materials in seston pools because the non-algal dominated seston exhibited significantly lower ratios than algal-dominated seston. Furthermore, the spatial heterogeneity of stoichiometric ratios was also related to the seston source. During the senescence season, the terrestrial-dominated seston from agricultural watershed exhibited the lowest ratios in estuary sites compared with other areas. Statistically, the predictive power of environmental variables on stoichiometric ratios was strongly improved by adding n-alkanes proxies. Apart from source indicators, particulate phosphorus (PP) contents also partly explained the spatiotemporal variations in stoichiometric ratios. This study, thus, highlights the utility of multiple-combined n-alkane proxies in addition to simple C:N ratios to get more robust source information, which is essential for interpreting the spatiotemporal variations in seston stoichiometric ratios among eutrophic floodplain lakes and other freshwater ecosystems.

Success of lake restoration depends on spatial aspects of nutrient loading and hydrology

Many aquatic ecosystems have deteriorated due to human activities and their restoration is often troublesome. It is proposed here that the restoration success of deteriorated lakes critically depends on hitherto largely neglected spatial heterogeneity in nutrient loading and hydrology. A modelling approach is used to study this hypothesis by considering four lake types with contrasting nutrient loading (point versus diffuse) and hydrology (seepage versus drainage). By comparing the longterm effect of common restoration measures (nutrient load reduction, lake flushing or biomanipulation) in these four lake types, we found that restoration through reduction of nutrient loading is effective in all cases. In contrast, biomanipulation only works in seepage lakes with diffuse nutrient inputs, while lake flushing will even be counterproductive in lakes with nutrient point sources. The main conclusion of the presented analysis is that a priori assessment of spatial heterogeneity caused by nutrient loading and hydrology is essential for successful restoration of lake ecosystems.