Lake Phosphorus Research Articles

Variations of phosphorus in sediments and suspended particulate matter of a typical mesotrophic plateau lake and their contribution to eutrophication

Internal phosphorus loading (IPL), as an important part of lake phosphorus cycle and the key to solve the eutrophication problem, is still an important cause of regional and seasonal algal blooms for some mesotrophic lakes located in plateau areas. We investigated the composition, distribution of P fractions in sediments and suspended particulate matter (SPM) of Erhai Lake, southwest China, and explored the relationships between environmental variables and spatial-temporal variations of P fractions. The total P (TP) in surface sediments ranged from 817 to 1216 mg/kg, with inert Ca-P (32%) and Res-P (24%) predominating, at a moderate level. The comparison of short-term release fluxes (0.08 mg/(m2·d)) and long-term release fluxes (0.09 mg/(m2·d)) reflected that the northern region was recovering slowly from the previous P pollution. Mobile-P (the sum of loosely adsorbed P, iron bound P, and organic P) accounted for 52.3% of the TP in SPM and showed high spatial-temporal variations, which were closely related to the growth of algae throughout the investigation. The results suggested that sediments could make a sustained contribution to IPL, and that the P in SPM was highly active and significantly contributed to eutrophication in Erhai Lake especially at the time of seasonal alternations. Our data provided important theoretical bases for the relationship between internal phosphorus loading and eutrophication in plateau lakes.

Stratification controls the magnitude of in-lake phosphorus cycling: insights from a morphologically complex eutrophic lake

AbstractUsing a combination of sediment trap experiments, sedimentary biogeochemical analyses and mass balance calculations, we conducted a comprehensive quantitative evaluation of the in-lake phosphorus (P) cycles including in both the water and sediment phases for Lake Hiidenvesi, a dimictic eutrophic lake in southern Finland. We explicitly demonstrated the heterogeneity of the in-lake P cycles between basins with distinct morphological features. Enhanced interactions between waters and sediments occur in shallow and non-stratified areas, as evidenced by the magnitudes of gross sedimentation and total internal P loading. In such shallow areas, sediment resuspension contributes over 60% of the total internal P loading throughout the entire open water season. In contrast, sedimentary P cycling is less intensive in deep and stratified areas, where diffusive fluxes account for an average of 70% of total internal P loading. We show that sedimentary P burial plays a key role in controlling the in-lake P cycle. Permanent burial of P showing higher rates and efficiencies tends to occur in deeper areas. Overall, sediments in Lake Hiidenvesi act as a net P sink under modern biogeochemical settings; the lake is in the process of long-term recovery from eutrophication due to the larger annual P output than external loading.

The Critical Role of Hydrological Distance in Shaping Nutrient Dynamics Along the Watershed‐Lake Continuum

AbstractTerrestrial hydrological and nutrient cycles are subjected to major disturbances by agricultural operations and urbanization that profoundly influence freshwater resources. Non‐point source pollution is one of the primary causes for water quality deterioration, and thus an emerging imperative in limnology is establishing empirical models that connect watershed attributes and hydrological drivers with lake nutrient dynamics. Here, we compiled three nation‐wide nutrient, meteorological, and watershed‐landscape data sets, to develop Generalized Linear Models that predict lake phosphorus and nitrogen concentrations as a function of the surrounding watershed characteristics within various hydrological distances across 104 Chinese lakes and reservoirs. Our national‐scale investigation revealed that lake nutrient concentrations can be satisfactorily predicted by proxies of natural drivers and anthropogenic activities, reflecting the properties of the surrounding watershed. Counter to previous studies, we found that China's lake nutrient concentrations strongly depend on watershed characteristics within a hydrological distance of less than 45 km rather than the entire watershed. Furthermore, extensive human activities in watersheds not only compromise our predictive capacity, but also increase the hydrological distance that is relevant to predict lake nutrients. This national‐scale characterization can inform one of the most contentious issues in the context of China's lake management, that is, the determination of the extent of the nearshore area, where nutrient control should be prioritized. As far as we know, our study represents the first attempt to apply the concept of hydrological distance and establish statistical models that can delineate the critical spatial domain primarily responsible for the nutrient conditions along the watershed‐lake continuum.

Investigating nearshore spatial and temporal trends in nutrient concentrations along an urban northern shoreline, Lake Ontario

The nearshore zone of the Laurentian Great Lakes is of significant ecological importance, providing critical biogeochemical processes supporting spatially diverse habitats for a variety of species. Extreme plant and algal growth are common due to excessive anthropogenic eutrophication, with exacerbations from dreissenid establishment and lake mixing (e.g., coastal upwelling events and nearshore-offshore water exchanges), resulting in nuisance algal blooms across the nearshore area adjacent to Western Durham, Ontario, Canada. Thus, our main goal was to characterize the trends in essential nutrients (i.e., phosphorus and nitrogen) within the nearshore by applying general additive models on irregular time-series from 2011 to 2022 and to identify plausible contributing factors using exploratory principal component analysis. Increasing trends for total phosphorus were observed in surface (0.5 m below the surface) and benthic (0.5 m above the bottom) waters despite decreases in point source loading to the area. Most notably, the local water pollution control plant (WPCP) outfall did not seem to drive lake phosphorus concentrations across the study area, depicted by an orthogonal relationship within the principal component analysis. While the WPCP is a point source to the nearshore, it does not appear to be the primary driver of temporal lake phosphorus trends. Our results suggest that weather, inertial forcings and lake hydrodynamics in combination with traditional point-sources across Western Durham are contributing to the increasing total phosphorus trends observed as the peak period of wave spectra, wave height, alongshore wind speeds, watershed loadings, and total phosphorus concentrations within Lake Ontario were positively associated.

Historical Phosphorus Mass and Concentrations in Utah Lake: A Case Study with Implications for Nutrient Load Management in a Sorption-Dominated Shallow Lake

Utah Lake is unusual due to its large surface area, shallow depth, phosphorus-rich sediments, and well-mixed, unstratified waters. This creates conditions where water column phosphorous concentrations tend toward equilibrium, with lake sediments containing high concentrations of geologic phosphorus. To help understand the potential impact of phosphorous load reductions, we computed a time history of phosphorus mass in the lake using state and federal records of lake volume, dissolved phosphorus concentrations, and outflow. We show that historically, Utah Lake phosphorus concentrations have remained stable over time, in the range of 0.02 to 0.04 mg/L, despite large changes in lake volume and internal phosphorus mass. We performed sorption calculations using data from the literature, demonstrating that it would take unrealistically large load changes to alter water column phosphorus concentrations under sorption processes. The sorption model produces results consistent with historical data that show relatively constant phosphorous concentrations despite large lake volume changes. We show, through several lines of evidence, that water column phosphorus concentrations are insensitive to external loads. Phosphorous load reduction is unlikely to have a significant effect on phosphorus concentrations in Utah Lake and, by extension, in other sorption-dominated shallow lakes with phosphorus-rich sediment.

Spatiotemporal variation reconstruction of total phosphorus in the Great Lakes since 2002 using remote sensing and deep neural network

Total phosphorus (TP) is non-optically active, thus TP concentration (CTP) estimation using remote sensing still exists grand challenge. This study developed a deep neural network model (DNN) for CTP estimation with synchronous in-situ measurements and MODIS-derived remote sensing reflectance (Rrs) (N = 3916). Using DNN, the annual and intra-annual CTP spatial distributions of the Great Lakes since 2002 were reconstructed. Then, the reconstructions were correlated to nine potential factors, e.g., Chlorophyll-a, snowmelt, and cropland, to explain seasonal and long-term CTP variations. The results showed that DNN reliably estimated CTP from MODIS Rrs, with R2, mean absolute error (MAE), root mean squared error (RMSE), mean absolute percentage error (MAPE), and root mean squared logarithmic error (RMSLE) of 0.83, 1.05 μg/L, 2.95 μg/L, 9.92%, and 0.13 on the test set. The near-surface CTP in the Great Lakes decreased significantly (p < 0.05) during 2002 − 2022, primarily attributed to cropland reduction, coupled with improvements in basin natural ecosystems. The sensitivity analysis verified the model robustness when confronted with input feature changes < 35%. This result along with the marginal difference between CTP derived from two sensors (R2 = 0.76, MAE = 2.12 μg/L, RMSE = 2.51 μg/L, MAPE = 11.52%, RMSLE = 0.24) suggested the model transferability from MODIS to VIIRS. This transformation facilitated optimal usage of MODIS-related archive and enhanced the continuity of CTP estimation at moderate resolution. This study presents a practical method for spatiotemporal reconstruction of CTP using remote sensing, and contributes to better understandings of driving factors behind CTP variations in the Great Lakes.

Interpretable CEEMDAN-FE-LSTM-transformer hybrid model for predicting total phosphorus concentrations in surface water

The complexity of the biogeochemical cycle of phosphorus in lakes makes it challenging to produce efficient and accurate predictions of total phosphorus (TP) concentrations. In this study, a hybrid model is developed for TP predictions. This model combines the Complete ensemble empirical mode decomposition with adaptive noise, Fuzzy entropy, Long short-term memory, and Transformer (CF-LT). The introduction of data split-frequency reconstruction effectively solves the problems of over- and underfitting suffered by previous machine learning models in the face of high-dimensional data, while an attention mechanism overcomes the inability of these models to establish long-term dependencies between data when making long-term predictions. The CF-LT model is applied to predict TP concentrations from January 1, 2015, to December 31, 2020, at Yaoxiangqiao, Zhihugang, and Guanduqiao, three national water quality monitoring stations at the inlet of Taihu Lake, China. Moreover, the Shapley additive explanations are used to interpret the CF-LT model and identify the essential input features. The prediction results demonstrate that the CF-LT model achieves a coefficient of determination (R2) of 0.37–0.87 on the test dataset, representing an improvement of 0.05–0.17 (6%-85%) over the control models. In addition, the CF-LT model provides the best peak value predictions. The model interpretation results indicate that the turbidity and total nitrogen are the essential factors influencing TP predictions. This demonstrates that the TP concentrations at the inlet of Taihu Lake are closely related to the non-point pollution discharge and the status of aquatic plants. It's worth noting that these two indicators exert a more significant influence on the prediction of TP during wet season. This work provides a viable modeling strategy for predicting TP concentrations and guidance for early warning and treatment of surface water eutrophication in the Taihu Lake basin.

Flocculating and dewatering of lake sediment: An in-situ pilot study comparing synthetic polymers and biopolymers for restoring lake water quality and reusing phosphorus

Dredging of lake sediment is a method to remove accumulated phosphorus and nitrogen in lakes and thereby reducing the risk of eutrophication. After dredging, the sediment is dewatered to reduce the volume. It is important to get a high dry matter content and ensure that the filtrate does not contain harmful compounds so it can be returned to the lake. A pilot-scale belt filter and flexible intermediate bulk containers (FIBC) were used for dewatering lake sediment with the sediment treated with a synthetic polymer or three different biopolymers. The goal of the study was to retain the phosphorus in the filter cake while returning the filtrate to the lake with a minimal phosphorus content. Results showed dry matter content of up to 16 % in the dewatered sediment and the sediment retained 96–99 % of the phosphorus. Furthermore, nitrogen was reduced by 27–71 % in the filtrate water. Toxicity tests found low ecotoxicity for most biopolymer filtrates, whereas synthetic polymer showed the highest potential ecotoxicity. Consequently, biopolymers provided satisfactory results, proving more environmentally friendly despite requiring longer filtration time.

Recent increase in sediment dry matter, carbon, and phosphorus accumulation in small boreal lakes with clayey catchments

This study estimated the mass accumulation rates of sediment (MAR), carbon (CMAR), and phosphorus (PMAR) in small Finnish lakes with agricultural clayey catchments over a 25-year period (1986–2011) and compared these with the conditions before major agricultural land use. Twenty-two lakes were cored for recent and reference (pre-disturbance) sediments. The recent sediment section was selected based on the 1986 137Cs fallout peak (TOP), whereas the pre-disturbance section (REF) was selected immediately below the first signs of human-induced erosion. The 50-cm reference section was dated with 14C at both ends. The mass accumulation rates were estimated based on dating, weighing, and chemical analysis for both sediment sections. Furthermore, sediment-penetrating echo soundings were used to estimate the amount of sediment in the whole lake basins. These data were used to examine area-specific loading from clay-rich catchments. The average whole-basin pre-disturbance MAR, PMAR, and CMAR were 62 g m−2 a−1, 0.06 g m−2 a−1, and 4.7 g m−2 a−1, respectively. The corresponding recent rates were 11, 13, and 8-fold (693 g m−2 a−1, 0.79 g m−2 a−1, and 37 g m−2 a−1). In the recent conditions, sediments were generally more minerogenic and MAR, PMAR, and CMAR were higher in lakes with more arable fields in their catchments. Average area-specific suspended sediment load from the catchment for the region (∼39% clayey soils) was approximately 69–137 kg ha−1 a−1 in the undisturbed state and 767–1534 kg ha−1 a−1 in recent conditions based on 100–50% retention. The results demonstrate that the increases in sediment, nutrient, and organic matter accumulation due to agriculture can be several fold over undisturbed state.

Revealing the hidden burden for lake management: the sediment phosphorus storage pools in Eastern Plain Lake Zone, China.

As one of the essential components in ecosystems, lakes play a major role in the global phosphorus (P) cycle. It is helpful for further understanding of the inside lake P geochemical cycle to research P pollution and storage in lakes, which is of positive significance for lake eutrophication restoration. In this study, we investigated the total phosphorus concentrations (TPC) of water and sediments from 37 lakes in the Eastern Plain Lake Zone (EPL) of China, evaluated the P pollution degree of lakes, and estimated P storage in lake sediments with quantitative data of lake area and number. The results indicate that the total phosphorus concentrations of water (TPCW) and total phosphorus concentrations of the surface sediments (0-1cm, TPCSS) in EPL were high, the mean values were 0.11mg·L-1 and 869.85mg·kg-1 respectively, with obvious differences between urban and rural areas, as well as between different river basins. Over half (56.76% and 70.27% respectively) of the lakes reached severe pollution levels in water and surface sediments. There were 16224 lakes (> 0.01 km2) with a total area of 21662.37 km2 in the EPL, and the P storage in the lake sediments (0-30cm) was about 4.87 ± 2.08 Tg (1 Tg = 1 × 1012g), accounting for about 2.74% of the basin soil. TPCW and TPCSS of lakes in the EPL were significantly positively correlated, may suggest a close nutrient cycling relationship between the lake water and the sediment. During periods of high winds and waves, the stored P in the top sediments in the EPL may continue to participate in the internal P geochemical cycle and migrate to the overlying water, posing a potential pollution hazard. Therefore, it is crucial to take into account the sediment P pools when formulating effective lake phosphorus management strategies.

Contrasting Impacts of Agricultural Intensification and Urbanization on Lake Phosphorus Cycling and Implications for Managing Eutrophication

AbstractSediment core and water quality data, together with historical information on land use/land cover (LULC), were used to reconstruct changes in phosphorus (P) loading and cycling in Lake Wilcox, Ontario, Canada, since the early 1920s. After first being cleared for farming, the originally forested watershed subsequently underwent urbanization. The large increase in P loading accompanying agricultural intensification after World War II caused the eutrophication of the lake. However, improved soil conservation since the 1980s and urban stormwater management since the 1990s have brought watershed P loading and sediment accumulation down to levels comparable to the early 1900s. Yet, the lake continues to exhibit eutrophication‐like symptoms, especially the intensification of hypoxia in the hypolimnion. Post‐2000 water quality data indicate that the latter is not driven by external P loading from the watershed, but rather by rapid salinization that strengthens the lake's summer stratification and enhances internal P loading. Salinization is caused by the increasing application of deicing agents in the expanding urban area. Curbing salt inputs will therefore be essential to restore the lake. Overall, our results provide new insights into the shifts in lake biogeochemistry associated with LULC changes and the implementation of best management practices. The approaches and findings of our case study have broad applicability for the large number of freshwater ecosystems worldwide that are experiencing salinization.

Nutrient explorer: An analytical framework to visualize and investigate drivers of surface water quality

Excess nutrients (nitrogen and phosphorus) in lakes can lead to eutrophication, hypoxia, and algal blooms that may harm aquatic life and people. Some U.S. states have established numeric water quality criteria for nutrients to protect surface waters. However, monitoring to determine if criteria are being met is limited by resources and time. Using R code and publicly available lake data, we introduce a downloadable interactive user interface for modeling relationships between watershed land use, climate, and other variables and surface water nutrient concentrations. Random Forest modeling identified watershed agricultural and forest land coverage, fertilizer inputs, and lake depth as the most important predictors of total phosphorus. The analytical framework implemented in this application can be applied to different locations and other surface water types to be leveraged by decision makers to identify the most influential drivers of excess nutrient concentrations and to prioritize watersheds for restoration.

A mass-balance approach for predicting lake phosphorus concentrations as a function of external phosphorus loading: Application to the Lake St. Clair – Lake Erie System (Canada – USA)

Abstract A mass balance model is presented that links the total phosphorus concentration in lakes to the water residence time, Rw (lake volume divided by the annual water inflow) and the total phosphorus residence time, Rp (average standing stock of lake total phosphorous divided by the external annual total phosphorus input). Following a change in the external load, the lake total phosphorus concentration asymptotically approaches a new value that depends on the Rp:Rw ratio, with the rate of approach controlled by Rp. We applied this approach to a recent reanalysis of the water and total phosphorus budgets of the Lake Erie system of the Laurentian Great Lakes for the 2003-2016 period. We generated load–response relationships and response matrices that relate the steady state total phosphorus concentrations to external total phosphorus loads, for the whole Lake Erie system and for the individual basins (Lake St. Clair, western basin, central basin, eastern basin) and connecting channels (St. Clair River, Detroit River). These relationships and matrices provide a simple but robust framework to gauge the potential response of total phosphorus concentrations to total phosphorus load reductions, such as the 40% reduction proposed for Lake Erie under the Canada-United States Great Lakes Water Quality Agreement. The mass balance analysis further highlights the importance of inter-basin total phosphorus transfers. For example, around 70% of the total phosphorus concentration in the eastern basin is contributed by inflow from the central basin. Consequently, total phosphorus load abatements in watersheds upstream of the eastern basin, rather than in the direct watershed itself, will have the greatest impact on the eastern basin's concentration. Overall, our results illustrate how simple mass balance calculations can provide useful guidance to efforts to manage phosphorus enrichment of lakes.

Reconstruction and Trends of Total Phosphorus in Shallow Lakes in Eastern China in The Past Century

Lake eutrophication due to excessive nutrient enrichment by human activity is one of the most studied ecosystem regime shifts. The suddenness and irreparability of such eutrophication in shallow lakes cause substantial socio-economic losses, especially in fast-developing areas in eastern China. Although eutrophication has been well documented in many lakes, a regional assessment of the eutrophication process is still missing. Here, we provided a regional assessment of water phosphorus changes since 1900 in eutrophic lakes in eastern China using paleolimnological records and diatom-/chironomid-TP transfer functions. We collated the reconstructed water total phosphorus (TP) of ten lakes and reconstructed the other five records based on identified diatom compositions in sediment cores from previous papers. We found three trend types of decrease, increase and fluctuate in the fifteen TP reconstructions according to cluster analysis of the data correlation results. Increase is the dominated trend, in which TP changes are highly correlated. Among eight lakes with an increasing nutrient, the time-series TP data of six lakes fit step functions better than linear regression models, indicating the main non-linear change in lake nutrient levels over time. Our results show how integrating spatial information on a large scale from paleolimnological records highlights the eutrophication process and further benefits current lake management.

Vertical distribution analysis and total mass estimation of nitrogen and phosphorus in large shallow lakes

Analysing the vertical distribution of nutrient salts and estimating the total mass of lake nutrients is helpful for the management of lake nutrient status and the formulation of drainage standards in basins. However, studies on nitrogen (N) and phosphorus (P) in lakes have focused on obtaining measures of N and P concentrations, but no understanding exists on the vertical distribution of N and P in the entire water column. The present study proposes algorithms for estimating the total masses of N/P per unit water column (ALGO-TNmass/ALGO-TPmass) for shallow eutrophic lakes. Using Lake Taihu as an example, the total masses of nutrients in Lake Taihu in the historical period were obtained, and the algorithm performance was discussed. The results showed that the vertical distribution of nutrients decreased with increasing depth and exhibited a quadratic distribution. Surface nutrients and chlorophyll-a concentrations play important roles in the vertical distribution of nutrients. Based on conventional surface water quality indicators, algorithms for the vertical nutrient concentration in Lake Taihu were proposed. Both algorithms had good accuracy (ALGO-TNmass R2 > 0.75, RMSE <0.57; ALGO-TPmass R2 > 0.80, RMSE ≤0.50), the ALGO-TPmass had better applicability than the ALGO-TNmass, and had good accuracy in other shallow lakes. Therefore, deducing the TPmass using conventional water quality indicators in surface water, which not only simplifies the sampling process but also provides an opportunity for remote sensing technology to monitor the total masses of nutrients, is feasible. The long-term average total mass of N was 11,727 t, showing a gradual downward trend before 2010, after which it stabilised. The maximum and minimum intra-annual total N masses were observed in May and November, respectively. The long-term average total mass of P was 512 t, showing a gradual downward trend before 2010, and a slow upward trend thereafter. The maximum and minimum intra-annual total masses of P occurred in August and February or May, respectively. The correlation between the total mass of N and meteorological conditions was not obvious, whereas some influence on the total mass of P was evident, particularly water level and wind speed.

Bloom-induced internal release controlling phosphorus dynamics in large shallow eutrophic Lake Taihu, China

High phosphorus (P) concentrations are commonly observed in lakes during algal blooms despite massive efforts on external nutrient reduction. However, the knowledge about the relative contribution of internal P loading linked with algal blooms on lake phosphorus (P) dynamics remains limited. To quantify the effect of internal loading on P dynamics, we conducted extensive spatial and multi-frequency nutrient monitoring from 2016 to 2021 in Lake Taihu, a large shallow eutrophic lake in China, and its tributaries (2017–2021). The in-lake P stores (ILSP) and external loading were estimated and then internal P loading was quantified from the mass balance equation. The results showed that the in-lake total P stores (ILSTP) ranged from 398.5 to 1530.2 tons (t), and exhibited a dramatic intra- and inter-annual variability. The annual internal TP loading released from sediment ranged from 1054.3 to 1508.4 t, which was equivalent to 115.6% (TP loading) of the external inputs on average, and responsible for the fluctuations of ILSTP on a weekly scale. High-frequency observations exemplified that ILSTP increased by 136.4% during algal blooms in 2017, while by only 47.2% as a result of external loading after heavy precipitation in 2020. Our study demonstrated that both bloom-induced internal loading and storm-induced external loading are likely to run counter significantly to watershed nutrient reduction efforts in large shallow lakes. More importantly, bloom-induced internal loading is higher than storm-induced external loading over the short term. Given the positive feedback loop between internal P loadings and algal bloom in eutrophic lakes, which explains the significant fluctuation of P concentration while nitrogen concentration decreased. It is emphasized that internal loading and ecosystem restoration are unignorable in shallow lakes, particularly in the algal-dominated region.

Long-term phosphorus mass-balance of Lake Erie (Canada-USA) reveals a major contribution of in-lake phosphorus loading

While early efforts to reduce external phosphorus (P) loads to Lake Erie eliminated the worst eutrophication symptoms in the 1970s and 1980s, the observed intensification of these symptoms in recent decades is difficult to explain by changes in the external P loading alone. We hypothesized that, in addition to external inputs, in-lake processes that release P to the water column may be contributing to the re-eutrophication of Lake Erie. Directly measuring these internal inputs is challenging and their scaling up to the entire lake system remains fraught with uncertainty. To constrain the magnitude of the net P input associated with in-lake processes, we generated a long-term annual total P (TP) budget for the Lake St. Clair–Lake Erie system, averaged over a 14-year period, from 2003 to 2016. The budget shows that the TP output from the system substantially exceeds the sum of the external TP inputs. To balance the budget, we estimate that in-lake processes add a net internal TP load of 3783 metric tons per year (MTA) to the water column. In comparison, the mean yearly external TP load to the Lake St. Clair–Lake Erie system is 11,703 MTA. Thus, overall, in-lake processes account for about a quarter of the total (external plus internal) TP input. The internal input is not evenly distributed among the system's basins, however, with Lake St. Clair and Lake Erie's western, central, and eastern basins receiving 6, 13, 21 and 60% of the total internal P input, respectively. Our results highlight the importance of accounting for the internal P loading in nutrient cycling modeling and load reduction strategies for large lake systems.

Sudden eutrophication of an aluminum sulphate treated lake due to abrupt increase of internal phosphorus loading after three decades of mesotrophy

Aluminum salts are widely used to immobilize phosphorus (P) in lakes suffering from internal loading. However, longevity of treatments varies among lakes; some lakes eutrophy faster than others. We conducted biogeochemical investigations of sediments of a closed artificial Lake Barleber, Germany that was successfully remediated with aluminum sulfate in 1986. The lake became mesotrophic for almost 30 years; a rather rapid re-eutrophication took place in 2016 leading to massive cyanobacterial blooms. We quantified internal loading from sediment and analyzed two environmental factors that might have contributed to the sudden shift in trophic state. Increase in lake P concentration started in 2016, reaching 0.3 mg L−1, and remained elevated into the spring of 2018. Reducible P fraction in the sediment was 37 - 58% of total P, indicating a high potential for mobilization of benthic P during anoxia. Estimated P release from sediments for 2017 was approximately 600 kg for the whole lake. This is consistent with sediment incubation results; higher temperature (20°C) and anoxia contributed to release of P (27.9 ± 7.1 mg m−2 d−1, 0.94 ± 0.23 mmol m−2 d−1) to the lake, triggering re-eutrophication. Loss of aluminum P adsorption capacity together with anoxia and high water temperatures (organic matter mineralization) are major drivers of re-eutrophication. Accordingly, treated lakes at some time require a repeated aluminum treatment for sustaining acceptable water quality and we recommend regular sediment monitoring in treated lakes. This is crucial given the effects of climate warming on duration of stratification in lakes which may result in the need for treatment of many lakes.

Evaluating the effectiveness of sediment retention by comparing the spatiotemporal burial of sediment carbon, nitrogen and phosphorus in a plateau lake and its affiliated reservoirs

Evaluating the effectiveness of sediment retention by comparing the spatiotemporal burial of sediment C, N, and P in a plateau lake and its affiliated reservoirs is critical in dealing with global eutrophication in freshwater ecosystems. The objective of the study was to estimate the spatial–temporal of C, N and P accumulation rates (CAR, NAR and PAR) in sediment cores of a plateau Lake Dianchi (DC) and its four affiliated reservoirs (Dashiba-DSB, Xibaisha-XBS, Baoxianghe-BXH, and Shuanlong-SHL). The results indicated that the average sediment accumulation rates (SARs) ranged from 0.040 to 1.187 g cm−2 a−1 in DC and 0.201 to 3.965 g cm−2 a−1 in its affiliated reservoirs. Spatially, all of the values of CAR, NAR and PAR exhibited similar distribution characteristics, along with two high-value regions concentrated in the southern and northern parts of the lake DC. Temporally, these three values of Lake DC and its affiliated reservoirs showed upward trends from bottom to top over the past ∼ 150 years, especially occurring in the 1950s. The change in the C/N ratio indicated that intensive anthropogenic factors promoted the CAR, NAR and PAR in sediments. Based on a developed proportional model, the ratios of sediment retention by each reservoir on TOC, TN and TP were calculated to be 2.15 %, 4.38 % and 9.01 % for DSB, 0.14 %, 0.18 % and 0.14 % for XBS, 1.61 %, 5.77 % and 1.23 % for BXH, and 0.97 %, 2.07 % and 0.46 % for SHL, respectively. Reservoirs located in the upper and middle reaches of the lake catchment play a critical role in sediment retention and prevent a potential risk from water deterioration of the downstream Lake DC. Linkages between reservoir-sources and lake-sinks not only provide valuable information for managing the environmental changes within the catchment, but also contribute to new insight into the regulation of water eutrophication in global lake ecosystem.

Potential risk of eutrophication in the deepest lake of Southwest China: Insights from phosphorus enrichment in bottom water

Large deep lakes in plateau regions provide crucial ecosystem services but are susceptible to eutrophication due to their long water residence time. To date, the water quality of deep lakes has not received as much attention as that of shallow lakes owing to logistical challenges. This study investigated the seasonal variation and vertical distribution of phosphorus and related environmental variables in a large deep lake in the Yunnan Plateau, China (Fuxian Lake). Generally, the concentrations of total phosphorus (TP, R2 = 0.862), total dissolved phosphorus (TDP, R2 = 0.922), and dissolved inorganic phosphorus (DIP, R2 = 0.889) exhibited a linear increase with the greater water depth, whereas the pH and dissolved oxygen (DO) showed decreasing trends. The TP, TDP, and DIP values were 0.012, 0.006, and 0.004 mg/L, respectively, in surface waters (0.5 m depth), and increased to 0.074, 0.065, and 0.062 mg/L, respectively, at 140.0 m depth. The averaged over ordering method demonstrated that DO and air temperature accounted for a higher proportion of the explained variance of TP, TDP, and DIP in the shallow water layer (0.5–20.0 m). In contrast, DO and pH accounted for a higher proportion of the explained variance of TP, TDP, and DIP in deeper water layers (40.0–150.0 m). As a warm monomictic lake, the higher observed phosphorus concentrations in deeper water and sediment potentially pose a risk of future eutrophication in the Fuxian Lake. Our findings demonstrate that more efficient technical and management measures should be taken to reduce the external phosphorus load to Fuxian Lake, so that the load to and from the sediment will decrease eventually.