Lake Eutrophication Management Research Articles

Combining effects of submerged macrophytes and lanthanum-modified bentonite on sediment enzyme activity: Evidence from mesocosm study

Lanthanum-modified bentonite (LMB) combined with submerged macrophytes (SM) has been a conventional means of eutrophication management in lakes in recent years, and is one of the most important methods for P removal. However, trends in nutrients and sediment enzymes at the water-sediment interface during this process have not been systematically assessed, and there are still some gaps in how abiotic properties drive changes in enzyme activity. Here, we show changes in aquatic environmental conditions under the action of different ratios of modified bentonite (0, 10%, 20%, and 30%) in combination with SM (Vallisneria natans, Potamogeton lucens, and Hydrilla verticillate) and quantify their effects on sediment enzyme activities. The results showed that the nutrient cycling at the water-sediment interface was facilitated by the combined effect of SM and LMB, which effectively reduced the overlying water nutrient concentration, increased the sediment enzyme activity and enhanced the N cycling process. Partial least squares structural equation model (PLS-SEM) showed that sediment parameters strongly influenced changes in enzyme activity, with NO3–N as the main controlling factors. Our study fills in the process of changing environmental conditions in lake water under geoengineered materials combined with macrophyte measures, especially the changes in biological properties enzyme activities, which contributes to a clearer understanding of nutrient fluxes during the management of eutrophication in lakes.

Trophic status of a shallow lake in Inner Mongolia: long-term, seasonal, and spatial variation

The study of lake-trophic status drivers and their variable effects over space and time can assist in the management of lake eutrophication. Winter dynamics in lake trophic status are rarely evaluated, yet they could vary drastically from the more commonly sampled summer patterns. This represents a key blindspot in our knowledge because ice sheets affect nutrient distribution (e.g., nitrogen and phosphorus), light infiltration (translucency), and phytoplankton propagation (Chl-a concentration). We examined long-term (2011–2020) changes in the trophic status (trophic level index; TLI) of Lake Wuliangsuhai, a shallow lake situated in a cold and arid region, using Generalized Additive Models (GAMs). The results suggested a significant decrease in eutrophication of Wuliangsuhai Lake over the study period, with TLI decreasing from 61.7 (moderately eutrophic) in 2011 to 49.9 (mesotrophic) in 2020. TLI was significantly higher over winter (p < 0.05), with the rank of seasons in terms of TLI of: winter (58.7) > summer (54.7) > autumn (54.2) > spring (54.0). TLI of the ice-covered period (58.7) significantly exceeded that of the ice-free period (54.3) (p < 0.05). There were also significant spatial differences in TLI (p < 0.05). Factors driving the spatial and temporal heterogeneity in lake trophic levels included the water supplement, water depth, water temperature, external inputs of nutrients, hydrodynamic conditions (flow velocity), distribution of phragmites, and the location of the main diversion channel.

Trophic status and lake depth play important roles in determining the nutrient-chlorophyll a relationship: Evidence from thousands of lakes globally

A fundamental problem in lake eutrophication management is that the nutrient-chlorophyll a (Chl a) relationship shows high variability due to diverse influences of for example lake depth, lake trophic status, and latitude. To accommodate the variability induced by spatial heterogeneity, a reliable and general insight into the nutrient-Chl a relationship may be achieved by applying probabilistic methods to analyze data compiled across a broad spatial scale. Here, the roles of two critical factors determining the nutrient-Chl a relationship, lake depth and trophic status, were explored by applying Bayesian networks (BNs) and a Bayesian hierarchical linear regression model (BHM) to a compiled global dataset from 2849 lakes and 25083 observations. We categorized the lakes into three groups (shallow, transitional, and deep) according to mean and maximum depth relative to mixing depth. We found that despite a stronger effect of total phosphorus (TP) and total nitrogen (TN) on Chl a when combined, TP played a dominant role in determining Chl a, regardless of lake depth. However, when the lake was hypereutrophic and/or TP was >40 μg/L, TN had a greater impact on Chl a, especially in shallow lakes. The response curve of Chl a to TP and TN varied with lake depth, with deep lakes having the lowest yield Chl a per unit of nutrient, followed by transitional lakes, while shallow lakes had the highest ratio. Moreover, we found a decrease of TN/TP with increasing Chl a concentrations and lake depth (represented as mixing depth/mean depth). Our established BHM may help estimating lake type and/or lake-specific acceptable TN and TP concentrations that comply with target Chl a concentrations with higher certainty than can be obtained when bulking all lake types.

팔당호의 호소 부영양화 관리를 위한 수질인자 분석

It is necessary to comprehensively understand about lake water quality, environmental factors to successfully manage environmental problems such as eutrophication and mass propagation of phytoplankton in lakes. When it comes to a method of eutrophication evaluation, Carlson’s trophic state index (TSI) has been used so far but was not suitable for artificial lakes in Korea. Accordingly, a new Korean trophic state index (TSIKO), was developed and utilized by National Institute of Environmental Research. The purpose of this research is to identify current situation of water quality in the six spots of artificial lakes, to measure eutrophication through Korean TSIKO, to finally provide information on lake eutrophication management. As a result, Chl-a (Chlorophyll a) of KAC9 showed higher than others. According to Korean TSIKO, nutritional status by lake In 2016 showed that Paldang Lake was mesotrophic·eutrophic In the long-term from 2008 to 2016, Korean TSIKO of Paldang Lake ranged from 44 to 50 and it was assessed mesotrophic·eutrophic as well. To conclude, in the Han River That is why going downstream and increasing inflow link to increasing retention time in the lake as well as an increase of algae, nutrients and so on, it could be reasons of eutrophication and needs management continuously with nonpoint pollution. This paper could be applied a basic resource for comprehensive management of lake eutrophication by types of lake.

Alleviating eutrophication by reducing the abundance of Cyanophyta due to dissolved inorganic carbon fertilization: Insights from Erhai Lake, China

The eutrophication of lakes is a global environmental problem. Regulating nitrogen (N) and phosphorus (P) on phytoplankton is considered to be the most important basis of lake eutrophication management. Therefore, the effects of dissolved inorganic carbon (DIC) on phytoplankton and its role in mitigating lake eutrophication have often been overlooked. In this study, the relationships between phytoplankton and DIC concentrations, carbon isotopic composition, nutrients (N and P), and hydrochemistry in the Erhai Lake (a karst lake) were investigated. The results showed that when the dissolved carbon dioxide (CO2(aq)) concentrations in the water were higher than 15 µmol/L, the productivity of phytoplankton was controlled by the concentrations of TP and TN, especially by that of TP. When the N and P were sufficient and the CO2(aq) concentrations were lower than 15 µmol/L, the phytoplankton productivity was controlled by the concentrations of TP and DIC, especially by that of DIC. Additionally, DIC significantly affected the composition of the phytoplankton community in the lake (p<0.05). When the CO2(aq) concentrations were higher than 15 µmol/L, the relative abundance of Bacillariophyta and Chlorophyta was much higher than those of harmful Cyanophyta. Thus, high concentrations of CO2(aq) can inhibit harmful Cyanophyta blooms. During lake eutrophication, when controlling N and P, an appropriate increase in CO2(aq) concentrations by land-use changes or pumping of industrial CO2 into water may reduce the proportion of harmful Cyanophyta and promote the growth of Chlorophyta and Bacillariophyta, which may provide effectively assist in mitigating water quality deterioration in surface waters.

Evaluation of the effectiveness and effects of long-term ecological restoration on watershed water quality dynamics in two eutrophic river catchments in Lake Chaohu Basin, China

The ecological restoration projects are important in alleviating the ecological degradation conditions. The evaluation of the long-term effects and effectiveness of ecological restoration projects could provide an empirical foundation for future projects and assessment of watershed water environment restoration. However, empirical research, based upon long-term monitoring data on this topic is relatively lacking. In this study, the effectiveness of restoration projects on water quality improvement of two eutrophic rivers in Lake Chaohu Basin (LCB) was investigated. The spatiotemporal dynamics of water quality from 2000 to 2021 were studied and a delayed water quality response to restoration projects was revealed. In the past two decades, especially since 2014, the water quality in the LCB improved significantly whereas restoration projects have been implemented since late 2012. Additionally, a sharp regression discontinuity (RD) design was constructed to confirm the effectiveness of restoration projects on water quality improvement successfully. Moreover, the positive effects of restoration projects on water quality dynamics were quantified and distinguished from the effects of negative human activities and climate change using the residual trend method. During 2014–2020, ecological restoration projects contributed more than half (64.3 % SWL1 and 58.2 % HB2) of water quality variation in two eutrophic river catchments. Finally, a framework from the perspective of lake eutrophication management with effective measures is proposed for maintaining the sustainable development of ecological restoration projects in watersheds. The empirical study here provides scientific support for achieving better effectiveness of ecological restoration projects in watersheds with water pollution problems worldwide.

Assessing the role of internal phosphorus recycling on eutrophication in four lakes in China and Malaysia

Internal phosphorus recycling (IPR) is an important nutrient source driving algal growth and eutrophication in lakes. The complexity of eutrophication behaviours caused by high IPR complicates lake management and undermines restoration efforts. Hence, knowledge about the possible types of bifurcation behaviours caused by high IPR is essential for effective and sustainable lake eutrophication management. For this purpose, numerical bifurcation analysis is performed on an algae‑phosphorus model to investigate how IPR drives complex and rich eutrophication behaviours in two tropical and two subtropical lakes. The two tropical lakes are Tasik Harapan and Sunway Lagoon in Malaysia, while the two subtropical lakes are Lake Fuxian and Lake Taihu in China. For each specified level of IPR, co-dimension one bifurcation analysis is performed by means of XPPAUT. Co-dimension two bifurcation analysis is then carried out by means of MatCont. At low IPR, Lake Fuxian exhibits reversible behaviour, accompanied by higher external phosphorus loading (EPL) thresholds. Lake Fuxian is also more conducive to stable equilibrium and its lake dynamics are easily predictable. At moderate IPR, Sunway Lagoon is likely to exhibit stable equilibrium, accompanied by possible shifting between two stable steady states (hysteresis behaviour) and oscillations. With higher IPR, Lake Taihu and Tasik Harapan are prone to irreversibility, accompanied by lower EPL thresholds. Because of increased complexity in lake dynamics in Lake Taihu and Tasik Harapan, small changes in EPL or in algal mortality rates could trigger various transitions in lake dynamics. Overall, high IPR can trigger unexpected sharp increases in algal concentration and can reduce the resilience of an oligotrophic lake. For shallow lakes, high IPR would cause unexpected sharp increases in algal concentrations, undermine resilience of lakes, complicate lake management, and delay lake recovery process.

Dissimilatory nitrate reduction to ammonium (DNRA) potentially facilitates the accumulation of phosphorus in lake water from sediment.

Nitrogen (N) and phosphorus (P) are crucial nutrients for eutrophication in the lacustrine ecosystem and attract the attention worldwide. However, the interaction between them need further clarification. This study aimed to assess the influence of dissimilatory nitrate reduction to ammonia (DNRA) on the cycle of P in lacustrine sediment. Different fractions of N and P in the pore water were measured using high-resolution in-situ measurement techniques, HR-Peeper and DGT, coupling with sequential extraction for solid sediment from a shallow freshwater lake. The results showed that elevated nitrate (NO3-) reduction via DNRA rather than denitrification was verified at deeper sediment layer, suggesting the generation of inorganic ammonia (NH4+) as electron donor under anaerobic episodes. High abundance of DNRA bacteria (nrfA gene) obtained using high-throughput sequencing analysis were detected at upper layer and responsible for the accumulation of NH4+ in the sediment coupling with chemolithoautotrophic metabolism. Additionally, significant desorption of ionic ferrous iron (Fe2+) and dissolved reactive phosphate (DRP) from solid phase and the enrichment in the solution was simultaneously detected. Higher concentration of solid Fe bound P (Fe-P) at deeper layer indicated the potential re-oxidation of Fe2+ as electron donor during DNRA process and sorption of DRP toward the Fe-containing minerals. However, obvious evidence of desorption proved by DGT indicated that higher NH4+ concentrations favored the reduction of Fe(III) oxy(hydr)oxides and the desorption of DRP into the pore water and diffusion toward the overlying water. Finally, noteworthy S2- release from solid sediment was speculated to stimulate the DNRA and facilitated the accumulation of NH4+ in the solution, which further induced the enrichment of DRP in water from the solid phase. Overall, DNRA potentially facilitates the accumulation of P in lake water, and the synchronous control of N and P is important for the eutrophication management and restoration of lake eutrophication.

Elucidating phytoplankton limiting factors in lakes and reservoirs of the Chinese Eastern Plains ecoregion.

Knowledge of phytoplankton limiting factors is essential for cost-efficient lake eutrophication management. Herein, we propose a statistical framework to explore site-specific phytoplankton limiting factors and their dependence on water depth (WD) in 54 lakes in the Chinese Eastern Plains ecoregion. First, the maximal chlorophyll a (Chla) response to total N (TN) or P (TP), representing a region-specific "standard" model where phytoplankton were primarily N- or P-limited, was quantified using a 95% quantile regression. Second, site-specific limiting factors were identified using analogical residual analysis. N- or P-limitation was inferred if FractionTN (i.e. fraction of Chla observed and predicted by the "standard" model for a given TN)>0.95 or FractionTP >0.95; if both FractionTN and FractionTP <0.95 in a specific environmental condition (e.g. high non-algal turbidity), light limitation was suggested. As a result, 5%, 7%, 4%, 36%, 16%, 2%, and 30% of the sampling sites were limited by N, P, N+P, light availability, rapid flushing, abundant macrophytes, and unmeasured factors, respectively. Bloom control suggestions in the short run are proposed considering these actual limiting factors. Furthermore, the maximal FractionTN or FractionTP response to WD was explored, reflecting the effect of WD on FractionTN (or FractionTP) without significant confounders. The results indicated that phytoplankton in the studied freshwaters would be potentially light-limited, N-limited, N+P-co-limited, or P-limited depending on WD (<1.8, 1.8-2.1, 2.1-5.2, or >5.2m, respectively), because N will gradually become surplus with increasing WD, while at very shallow depths, strong sediment re-suspension induces light limitation. This finding implies that long-term nutrient management strategies in the studied freshwaters that have WDs of 0-2.1, 2.1-5.2, and >5.2m can entail control of N, N+P, and P, respectively. This study provides essential information for formulating context-dependent bloom control for lakes in our study area and serves as a valuable reference for developing a cost-efficient eutrophication management framework for other regions.

Spatiotemporal variation of phosphorus use efficiency across 70 lakes in China: Implications for lake eutrophication management

Reducing phosphorus input is currently the main lake eutrophication management strategy, but algal blooms persist in many lakes despite effective phosphorus abatement measures. This study analyzed monitoring data across 70 Chinese lakes using statistical methods to reveal the spatiotemporal characteristics of the algae production efficiency of total phosphorus (ETP) and the driving factors of ETP changes. The results showed that ETP ranged from 0.01 to 1.87 across 70 Chinese lakes from 2016 to 2018. Spatially, lakes with higher ETP values were concentrated in the Yungui Plateau region due to its natural conditions, which were suitable for phytoplankton growth. Temporally, the ETP values from July to October were higher than in other months. In addition, over 30% of lakes exhibited rising ETP values, and these were mainly located in the Eastern Plain region. These spatiotemporal differences reveal the reason for the failure of phosphorus control based on a universal regression equation in eutrophication management for some individual lakes. Nitrogen-limited or co-limited lakes were also found mainly located in the Northeast Plain region, and it was confirmed that alkalinity was the main factor affecting algae growth in the Qingzang Plateau region. Our results indicate that TP reduction alone, based on the empirical relationship between chlorophyll-a and TP, is insufficient to accurately control eutrophication at the lake-specific scale, and individualized lake management strategies considering the variations in ETP are recommended.

Chlorophyll-nutrient relationship changes with lake type, season and small-bodied zooplankton in a set of subtropical shallow lakes

Phytoplankton biomass is generally measured as chlorophyll-a concentration (Chla), and is well known to be sensitive to nutrients and large zooplankton. However, whether nutrient was still be a better predictor of Chla when potential affecting variables were taken together, and whether the critical TN:TP (total nitrogen: total phosphorus, by mass) threshold for classifying nutrient limitation varied with lake type remain uncertain. Furthermore, it remains largely unknown to what extent small zooplankton (SZ) can affect the nutrients-Chla relationships. Here, an investigation on how Chla responded to abiotic factors and SZ dynamics was performed in 21 subtropical shallow lakes along the lower reach of Chinese Yangtze River. These lakes were subdivided into two lake types including urban lakes featured by a low water exchange rate, and suburban lakes with hydrological connectivity. Chla and TP:Chla ratio were generally higher and lower in urban lakes than those in suburban lakes, respectively. Various analyses revealed that TP and water temperature were the key factors regulating Chla in urban and suburban lakes, respectively. Across urban lakes, TP accounted for >50% of the Chla variances in spring, autumn, and winter, but only explained 28% of summer Chla. However, if the sampling sites where SZ density was >17.5 times higher than that in spring were removed, the determination coefficient (R2) in the linear regression analysis between TP and summer Chla increased (from 0.28 to 0.57), and the Akaike Information Criterion decreased. Furthermore, TP:Chla was positively correlated with SZ in urban lakes in spring when the highest density of SZ (HSZ): the lowest density of SZ (LSZ) ratio was 456 (>17.5), while that interrelationship was not found in summer with the HSZ:LSZ ratio of 17.5. Therefore, the weak TP-summer Chla relationship in urban lakes was primarily caused by SZ when their density exceeded a 17.5-fold increase, i.e. small zooplankton could adopt “quantity-driven” strategy for phytoplankton control. For further strengthening the eutrophication management and remediation in subtropical urban lakes, it is critical to recover aquatic vegetation that can not only absorb P directly but provide refuge for zooplankton instead of dredging the sediments completely. In addition, we recommended that TN:TP ratio exceeding 20.0 could be the threshold identifying P-limitation in urban lakes rather than suburban lakes, and transparency can be used as rough indicators for measuring TP and Chla concentrations (<25 μm) in urban lakes.

Temporal dependence of chlorophyll a–nutrient relationships in Lake Taihu: Drivers and management implications

Eutrophication and its associated algal blooms are principal environmental challenges confronting lakes worldwide. The empirical relationships between nutrient (total nitrogen, TN; total phosphorus, TP) and chlorophyll a (Chla) level are widely used as a theoretical basis for lake eutrophication management. Here, seasonal environmental variables and Chla from 2005 to 2020 in Chinese shallow eutrophic Lake Taihu were examined and Chla-nutrient equations in the entire period and annually from 2005 to 2020 were explored using 95% quantile regression model. The results showed robust linear relationships of logChla-logTN and logChla-logTP in the vast majority of cases. Based on Chla-nutrient equations in the entire study period, 0.69 mg/L TN and 52 μg/L TP are recommended as nutrient threshold in Lake Taihu. Furthermore, the results revealed increasing Chla sensitivity to nutrient for each study month (i.e. February, May, August, and November) from 2005 to 2020, whose drivers included increase in water temperature and water level, decrease in wind speed, mass ratio of nitrogen to phosphorus, and grazing effect. It is noteworthy that atmospheric stilling is likely to be the key climatic factor promoting annual peak Chla in Lake Taihu. For one, the deviations of the sub-index of Trophic State Index indicated that light is a critical limiting factor of summer Chla in Lake Taihu. For another, calmer water mainly due to atmospheric stilling decreased near 40% non-algal turbidity and a substantially increased buoyant cyanobacteria during the study period, improving phytoplankton “light niche”. Thus, increasing algal sensitivity to nutrient occurred until the additional algal-turbidity induce further light limitations or the exhaustion of TN (or TP) cause nutrient limitation. Given atmospheric stilling is a global phenomenon, this study would affect future algal bloom mitigation efforts in shallow lakes as temperature is always the focus in the recent literatures on global climate change.

Internal phosphorus cycling in macrophyte-dominated eutrophic lakes and its implications

Macrophyte-dominated eutrophication (MDE) generally exhibits different characteristics from phytoplankton-dominated eutrophication (PDE). However, the significance of P cycling on eco-environmental management of MDE lakes is still not fully recognized. In this study, P-cycling mechanism was studied in a typical MDE lake (Wuliangsu Lake, China) based on a Before-After-Control-Impact design, taking advantage of the contrasting states between its 13 sub-lakes (with/without macrophytes and with/without external water flow). Our study demonstrated that P cycling predominantly occurs through “macrophyte ↔ sediment” in the MDE lakes, rather than “water ↔ sediment” as in PDE lakes; the biodynamics of the macrophytes acts as a primary driving force for this self-enforced P cycling. Our findings challenge the present lake eutrophication management strategies, which primarily limited to the water nutrient content, and demonstrate that successful MDE lake restoration should focus on stressors caused by the sustainable “macrophyte-sediment” P cycling. Macrophyte harvesting immediately before withering is recommended as the most sustainable technique in environment management for periodically frozen shallow MDE lakes. By this technique, sediment P can be gradually pumped up by the overgrown macrophytes each year until the advent of an alternative stable state (low sediment P, small biomass, and higher diversity), thereby forming a positive feedback loop “macrophyte harvesting → less sediment P → less macrophyte → higher diversity.” Also, the catastrophic shift from MDE to PDE is no longer a concern. Furthermore, the macrophyte debris will not pose a problem as long as the macrophytes are removed during the harvest.

Review on the control and mitigation strategies of lake cyanobacterial blooms

由于人类活动和全球气候变化的叠加影响，湖泊富营养化和蓝藻水华仍是未来相当长一段时间内的水生态环境问题.蓝藻水华暴发会引发湖泊生态系统的灾害和饮用水安全风险，因此湖内蓝藻水华防控必不可少.现有蓝藻水华防控长效方法主要基于营养盐控制理论、浅水湖泊稳态转换理论和生物操纵理论，技术措施包括内源营养盐控制、生态修复、生物操纵.应急处置方法可以削减局部水域蓝藻水华强度，主要包括物理、化学方法.本文基于国内外研究治理案例，梳理了单项蓝藻防控技术运用的边界条件、蓝藻削减效果及防控成本.湖泊富营养化控制和蓝藻水华防控是一个长期而艰巨的系统工程，必须采取流域污染削减和湖内防控相结合的治理策略.湖泊水质与水生态的持续跟踪监测以及水质和水生态预测模型的构建，是动态调整治理方案、保障治理效果长效稳定的基础.;Lake eutrophication and cyanobacterial blooms are water ecological and environmental problems for a long time to come, due to the synergistic effects of human activities and global climate change. The outbreak of lake cyanobacterial blooms would entail water ecological risk and drinking water crisis, thus curative methods to suppress the proliferation of cyanobacteria bloom are imperative. Nutrient control, shallow lake regime shifts and biological manipulation are theoretical basis for the long-term strategies for cyanobacteria blooms prevention and control. Accordingly, external and internal nutrient loading reduction, ecological restoration and biomanipulation are sustainable control and management approaches. In addition, methods used for emergency situations can mitigate the intensity of cyanobacterial bloom, mainly including physical and chemical approaches. Based on the review of the domestic and international cases on cyanobacterial bloom control, we sort out the applicable conditions, cyanobacterial mitigation efficiency and performance cost of each approach. In fact, lake eutrophication management and cyanobacterial bloom mitigation are a long-term and systematic process. These in-lake strategies should invariably be accompanied by nutrient input reductions. The continuous monitoring of lake water quality and water ecology as well as the building of prediction model support the further adjustment of strategies and measures to ensure long-term success and sustainability.

A statistical framework to track temporal dependence of chlorophyll–nutrient relationships with implications for lake eutrophication management

A reliable chlorophyll–nutrient relationship (CNR) is essential for lake eutrophication management. Although the spatial variability of CNRs has been extensively explored, temporal variations of CNRs at the individual lake scale has rarely been discussed. The paucity of information about temporal dependence in CNRs may in part be due to the lack of a suitable statistical framework that helps guide such investigations. In order to reveal temporal dependence of CNR, this study develop a novel statistical framework. In the framework, we employ quantile regression to generate overall (the entire dataset), annual (subsets for each year), and accumulative (subsets collected before a certain year) CNRs. We aim to 1) show biases of annual relationships by comparing the overall and annual relationships and 2) determine whether or not data accumulation is enough to develop a reliable CNR. We use Lake Champlain and Lake Kasumigaura as case studies to illustrate the necessary steps needed to utilize this novel framework. Results show that large interannual variations exist for CNRs. Accumulative relationships tend to converge to the overall relationship, indicating that overall relationships are reliable for informing lake-specific eutrophication management in the two case study lakes. The novel statistical framework that we propose for a procedure to estimate reliable CNRs is important for informing lake-specific eutrophication control decision-making processes.

Internal phosphorus recycling promotes rich and complex dynamics in an algae-phosphorus model: Implications for eutrophication management

Internal phosphorus recycling in lakes is an important nutrient source that promotes algal growth. Its persistence impedes the effort to improve water quality and thus poses a challenge to the management of eutrophication in lakes, especially in shallow lakes where the occurrence of internal phosphorus recycling is reportedly more common. This paper aims to provide crucial insights on the effects of internal phosphorus recycling on eutrophication dynamics for effective management of lake eutrophication. For this purpose, a mathematical model for lake eutrophication, comprising two compartments of algae and phosphorus, is first formulated for application to a eutrophic tropical lake named Tasik Harapan in Universiti Sains Malaysia. Numerical bifurcation analysis of the model is then performed to assess the combined influences of internal phosphorus recycling, algal mortality and external phosphorus loading on Tasik Harapan eutrophication dynamics. Specifically, co-dimension one bifurcation analysis of algal mortality rate is carried out by means of XPPAUT for various external phosphorus loading rates. The emergence of limit cycle for a certain range of algal mortality rate could be related to the hydra effect (i.e., algal concentration increases in response to greater algal mortality) and the paradox of enrichment (i.e., destabilization of algae in nutrient rich environment). To trace the locus of co-dimension one bifurcation, co-dimension two bifurcation analysis is performed by means of MatCont. The analysis demonstrated that the inclusion of the internal phosphorus recycling term induces rich and complex dynamics of the model. These dynamics include saddle-node bifurcation, cusp, Bogdanov-Takens bifurcation, Generalized Hopf bifurcation and limit point bifurcation of cycles. The results suggest that high internal phosphorus recycling rate promotes bistability and catastrophic shift in a shallow and tropical lake like Tasik Harapan. Hence, the key to effective management of eutrophication in shallow and tropical lakes is the control of internal phosphorus recycling.

New insights into eutrophication management: Importance of temperature and water residence time

Eutrophication and harmful cyanobacterial blooms threaten water resources all over the world. There is a great controversy about controlling only phosphorus or controlling both nitrogen and phosphorus in the management of lake eutrophication. The primary argument against the dual nutrients control of eutrophication is that nitrogen fixation can compensate the nitrogen deficits. Thus, it is of great necessary to study the factors that can significantly affect the nitrogen fixation. Due to the difference of climate and human influence, the water quality of different lakes (such as water temperature, N:P ratio and water residence time) is also quite different. Numerous studies have reported that the low N:P ratio can intensify the nitrogen fixation capacities. However, the effects of temperature and water residence time on the nitrogen fixation remain unclear. Thus, 30 shallows freshwater lakes in the eastern plain of China were selected to measure dissolved N2 and Ar concentrations through N2: Ar method using a membrane inlet mass spectrometer to quantify the nitrogen fixation capacities and investigate whether the temperature and water residence time have a great impact on nitrogen fixation. The results have shown that the short lake water residence time can severely inhibit the nitrogen fixation capacities through inhibiting the growth of nitrogen-fixing cyanobacteria, changing the N:P ratio and resuspending the solids from sediments. Similarly, lakes with low water temperature also have a low nitrogen fixation capacity, suggesting that controlling nitrogen in such lakes is feasible if the growth of cyanobacteria is limited by nitrogen.

A method for reconstructing past lake water phosphorus concentrations using\xa0sediment geochemical records

An existing steady state model of lake phosphorus (P) budgets has been adapted to allow reconstruction of long-term average historic lake water total phosphorus (TP) concentrations using lake sediment records of P burial. This model can be applied without site-specific parameterisation, thus potentially having universal application. In principle, it is applicable at any site where there is both a sediment P burial record and knowledge of the current water budget, although we advise caution applying it to problematic sediment records. Tested at six published case study sites, modelled lake water TP concentrations agree well with water-quality monitoring data, and limited testing finds good agreement with wholly independent diatom inferred lake water TP. Our findings, together with a review of the literature, suggest that well preserved lake sediments can usefully record a long-term average P burial rate from which the long-term mean lake water TP can be reliably estimated. These lake water TP reconstructions can provide meaningful site-specific reference values to support decision making in lake eutrophication management, including establishing targets for lake restoration.

A framework to develop joint nutrient criteria for lake eutrophication management in eutrophic lakes

Nutrient criteria provide the numeric basis for lake eutrophication management. However, there are two obstacles that can hinder the effective application of nutrient criteria, including 1) although total phosphorus (TP) and total nitrogen (TN) might co-limit phytoplankton biomass in eutrophic lakes, their criteria are often developed independently; and 2) the linkage between nutrient criteria and the percentile-based compliance assessment method of chlorophyll a (CHL; as a measure of phytoplankton biomass) has not been well established. To resolve these obstacles, we propose a novel analytical framework of nutrient criteria development, by which joint nutrient criteria are developed using quantile regression (QR). We demonstrated the steps necessary to utilize this novel approach using TP, TN, and CHL data from Lake Dianchi, a hypereutrophic lake located in southwestern China. First, we built candidate QR models to quantify the nutrient-CHL relationship at six regression quantiles. Next, we conducted the sequential Wald test to select the “best” model for each regression quantile. Finally, we visualized the joint nutrient criteria surface using a contour map. The contour map effectively illustrated the joint nutrient criteria by showing the linkage of TP and TN criterion. In addition, based on the QR, it was easy to deduce nutrient criteria which met the requirement of percentile-based compliance assessment. We further found that joint nutrient criteria could help the selection of an efficient load reduction strategy in the watershed. The proposed method can be generalized to other systems and may facilitate site-specific lake eutrophication management.

Regulation and management of lake eutrophication in urban regions based on an improved model for aquatic ecosystem simulations

The construction of a model based on an analytic system and dynamic structure is an important technique in lake manage-ment. The Yan-Model conducts system simulations based on the dynamics of the main aquatic species but does not include filter-feeding fish and N elements, which have important effects on eutrophication. Based on the Yan-Model, modules for the filter-feeding fish A. nobilis and the N cycle were constructed and combined to form a new model, the Yan-Model II. After model calibration and validation, a scenario analysis was performed to simulate eutrophication regulation and external pollution impacts. The results show the following: (1) the simulated and measured values of the lake ecosystem were highly consistent (R > 0.9, RSR < 0.7); (2) an eutrophic lake could be effectively restored by planting Vallisneria natans (Lour.) Hara with low biomass density; (3) assuming that urban sewage drained into the lake, equal or more than 600 m3 of urban sewage would lead to an increase in the Chl-a concentration to a level that exceeded the eutrophication threshold; and (4) an Aristichthys nobilis density of 50 g/m3 had a strong ability to control phytoplankton growth. Using the improved model, this study successfully guided the ecological restoration of Dongshan Lake. This model can be used for lake management and sustainable development in urban ecosystems.