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Freshwater chemistry across the circumpolar region was characterised using a pan‐Arctic data set from 1,032 lake and 482 river stations. Temporal trends were estimated for Early (1970–1985), Middle (1986–2000), and Late (2001–2015) periods. Spatial patterns were assessed using data collected since 2001.Alkalinity, pH, conductivity, sulfate, chloride, sodium, calcium, and magnesium (major ions) were generally higher in the northern‐most Arctic regions than in the Near Arctic (southern‐most) region. In particular, spatial patterns in pH, alkalinity, calcium, and magnesium appeared to reflect underlying geology, with more alkaline waters in the High Arctic and Sub Arctic, where sedimentary bedrock dominated.Carbon and nutrients displayed latitudinal trends, with lower levels of dissolved organic carbon (DOC), total nitrogen, and (to a lesser extent) total phosphorus (TP) in the High and Low Arctic than at lower latitudes. Significantly higher nutrient levels were observed in systems impacted by permafrost thaw slumps.Bulk temporal trends indicated that TP was higher during the Late period in the High Arctic, whereas it was lower in the Near Arctic. In contrast, DOC and total nitrogen were both lower during the Late period in the High Arctic sites. Major ion concentrations were higher in the Near, Sub, and Low Arctic during the Late period, but the opposite bulk trend was found in the High Arctic.Significant pan‐Arctic temporal trends were detected for all variables, with the most prevalent being negative TP trends in the Near and Sub Arctic, and positive trends in the High and Low Arctic (mean trends ranged from +0.57%/year in the High/Low Arctic to −2.2%/year in the Near Arctic), indicating widespread nutrient enrichment at higher latitudes and oligotrophication at lower latitudes.The divergent P trends across regions may be explained by changes in deposition and climate, causing decreased catchment transport of P in the south (e.g. increased soil binding and trapping in terrestrial vegetation) and increased P availability in the north (deepening of the active layer of the permafrost and soil/sediment sloughing). Other changes in concentrations of major ions and DOC were consistent with projected effects of ongoing climate change. Given the ongoing warming across the Arctic, these region‐specific changes are likely to have even greater effects on Arctic water quality, biota, ecosystem function and services, and human well‐being in the future. [ABSTRACT FROM AUTHOR]

• Drinking water treatment residual (DWTR) inactivated phosphorus (P) in lake sediment. • An empirical model was developed to describe P inactivation by DWTR. • Model inputs were initial mobile P in sediment and oxalate-extractible aluminum in DWTR. • The model was able to successfully predict sediment mobile P following DWTR dose. • The model can be used to determine DWTR dose for lake restoration. Drinking water treatment residual (DWTR) derived from flocculation and sedimentation of raw water using aluminum coagulants is a valuable environmental remediation byproduct capable of inactivating phosphorus (P). However, no generalizable model exists in the literature to describe reduction of releasable (mobile) P in lake sediment as a result of DWTR addition. The reduction of mobile P (sum of labile P and reductant soluble P) was investigated in over 100 sub-samples using five sediment samples from two lakes and three DWTRs from different water treatment plants. A consistent relationship was determined across a range of mobile P contents (0.23 g/m2/cm to 0.92 g/m2/cm, or 15.8 to 186.1 µg/g DW) and DWTRs. The relationship was best described as a function of the mobile P content of the sediment and the oxalate-extractable aluminum content of the DWTR. An empirical model was developed to predict the immediate reduction in mobile P following the addition of DWTR containing aluminum. This model was validated using two additional lake sediments and one additional DWTR (R² = 0.995). Thus, the immediate inactivation of P in lake sediment following DWTR addition can be predicted with this model, which can be used with internal P loading or other water quality goals to determine an appropriate DWTR dose. Further recommendations were made about dosing DWTRs for lake restoration, allowing practitioners to use DWTR to inactivate P in lake sediment without conducting individual sorption experiments. [Display omitted] [ABSTRACT FROM AUTHOR]

We present an analysis of long-term (1988–2013; 26 years) total phosphorus (TP) concentration trends in 81 Swedish boreal lakes subject to minimal anthropogenic disturbance. Near universal increases in dissolved organic carbon (DOC) concentrations and a widespread but hitherto unexplained decline in TP were observed. Over 50% of the lakes (n = 42) had significant declining TP trends over the past quarter century (Sen's slope = 2.5% y − 1 ). These declines were linked to catchment processes related to changes in climate, recovery from acidification, and catchment soil properties, but were unrelated to trends in P deposition. Increasing DOC concentrations appear to be masking in-lake TP declines. When the effect of increasing DOC was removed, the small number of positive TP trends (N = 5) turned negative and the average decline in TP increased to 3.9% y − 1 . The greatest relative TP declines occurred in already nutrient poor, oligotrophic systems and TP concentrations have reached the analytical detection limit (1 μg L − 1 ) in some lakes. In addition, ongoing oligotrophication may be exacerbated by increased reliance on renewable energy from forest biomass and hydropower. It is a cause of significant concern that potential impairments to lake ecosystem functioning associated with oligotrophication are not well handled by a management paradigm focused exclusively on the negative consequences of increasing phosphorus concentrations. [ABSTRACT FROM AUTHOR]

114 lakes treated with aluminum (Al) salts to reduce internal phosphorus (P) loading were analyzed to identify factors driving longevity of post-treatment water quality improvements. Lakes varied greatly in morphology, applied Al dose, and other factors that may have affected overall treatment effectiveness. Treatment longevity based on declines in epilimnetic total P (TP) concentration averaged 11 years for all lakes (range of 0–45 years). When longevity estimates were used for lakes with improved conditions through the end of measurements, average longevity increased to 15 years. Significant differences in treatment longevity between deeper, stratified lakes (mean 21 years) and shallow, polymictic lakes (mean 5.7 years) were detected, indicating factors related to lake morphology are important for treatment success. A decision tree developed using a partition model suggested Al dose, Osgood index (OI, a morphological index), and watershed to lake area ratio (related to hydraulic residence time, WA:LA) were the most important variables determining treatment longevity. Multiple linear regression showed that Al dose, WA:LA, and OI explained 47, 32 and 3% respectively of the variation in treatment longevity. Other variables (too data limited to include in the analysis) also appeared to be of importance, including sediment P content to Al dose ratios and the presence of benthic feeding fish in shallow, polymictic lakes. [ABSTRACT FROM AUTHOR]

Both in-lake and catchment measures designed to reduce phosphorus (P) loading were implemented as part of a 12.3 million USD restoration project for the Minneapolis Chain of lakes in Minnesota (USA). Treatment wetlands, ‘in-pipe’ measures, and in-lake aluminum sulfate (alum) treatment were applied to restore water quality in the four urban lakes. Different alum dosing methods led to between 4 and 20+ (modeled) years of water quality improvements in these lakes after treatment and only one of the four lakes continues to meet water quality goals approximately 25 years after the project started. Due to limited space and poor performance, reduction of total external loads was low (1–13%) for three lakes. Changes to internal P sediment release rates after application of alum correlated well with epilimnetic total P (TP) concentrations in these lakes, indicating that improvements in water quality were mainly driven by reduced internal loading via in-lake measures. Substantial reductions to external P loading were only achieved at Cedar Lake (49%) via conversion of an existing natural area to a treatment wetland, but even Cedar Lake is no longer meeting management goals. When expressed in terms of dollars spent per unit P removed, in lake alum treatment was on average 50 times more effective than in-catchment measures. The results of this study indicate that substantial external nutrient reductions may not be adequate to sustainably maintain water quality in urban lakes and that continued in-lake management of P accumulated in lake sediment will not only be necessary, but will also be more cost efficient relative to in-catchment measures. [ABSTRACT FROM AUTHOR]

The common carp ( Cyprinus carpio) is a globally distributed, benthivorous fish that is capable of substantially altering lake ecology. Although dozens of studies have demonstrated that it has the potential to negatively affect submersed vegetation and water quality, little attention has been paid to effects of carp on sediment mixing and sediment phosphorus (P) in lakes. This study examined carp effects on sediment mixing and mobile (mainly iron bound and pore water) sediment P in a shallow, Midwestern lake (Kohlman Lake, MN, USA) undergoing restoration with aluminum (Al) to reduce internal P loading. Using Al as a tracer, we determined that the sediment mixing depth was at least 2.5 times greater in areas with carp (13.0 ± 3.7 cm) than in areas from which carp had been excluded (5.0 ± 1.2 cm) using exclosures. Vertical sediment profiles of P mass suggested that the increase in sediment mixing depth caused by carp increased the amount of mobile P potentially available for release by 55-92%, depending on location within the lake. The increase in sediment mixing depth is likely to have a negative effect on the efficacy of management methods designed to reduce mobile sediment P availability in lakes. [ABSTRACT FROM AUTHOR]

Restoration of acidified lakes by liming does not, in many cases, improve productivity to a pre-acidified state. We hypothesize that the poor recovery detected in many of these lakes is due to constrained in-lake phosphorous (P) cycling caused by enhanced precipitation of metals in higher pH, limed waters. Long-term (1990-2012) data for 65 limed, circum-neutral (pH 6-8), and acidified lakes in Sweden were analyzed to determine trends for P and potential drivers of these trends. Limed lakes not only had lower mean values and stronger decreasing trends for total P than non-limed lakes, but they also had the highest percentage of decreasing trends (85 %). A P release factor (Hypolimnetic P/Epilimnetic P) was developed to elucidate differences in internal P cycling between lake groups. Consistently, lower P release factors in limed lakes show limitation of internal P cycling during summer months that may be a factor limiting P bioavailability and thus productivity of these systems. [ABSTRACT FROM AUTHOR]

Abstract: The conversion of mobile phosphorus (P) to aluminum bound P (Al–P) after addition of Al to over 300 sub-samples from 35 sediment cores collected from 20 lakes in the upper Midwest, United States was investigated in this study. Consistent relationships between mobile P reduction and Al–P formation were detected across a broad range of mobile sediment P contents (0.04–2.8 g P m−2 cm−1 or 0.083–2.8 mg P g−1DW) and lake types. The conversion of mobile P to Al–P was dependent on the initial mobile sediment P content and the amount of Al added to the sediment. An empirical model was then developed to predict the formation of Al–P based on the amount of Al added relative to the initial mass of mobile P in the sediment. The results were compared to sediment collected from an Al treated lake and good agreement was found between the model and in-situ changes to sediment P fractions caused by Al treatment. The model developed in this study, unlike previous models with extreme, singular endpoints, allows for a continuum of estimates for mobile P conversion to Al–P, along with efficiency of P binding by Al, as Al dose varies. Model results can be used in conjunction with mobile sediment P based predictions for internal P loading to calculate an Al dose required to meet internal phosphorus loading goals for lake management and restoration without the need for expensive, time consuming Al additions to sediment. [Copyright &y& Elsevier]

• A drinking water treatment residual (DWTR) was washed and heated without oxygen. • The DWTR sank Microcystis colonies in lake water more effectively than a local soil. • Ballast dose, alkalinity, pH, and chlorophyll-a affected sinking efficacy. • Leaching of metals and nutrients from the pre-treated DWTR was low. • The DWTR had a high P sorption capacity, sufficient for inactivating P in lake sediment. The combination of a low dose of coagulant with a ballast that can inactive phosphorus (P) in lake sediment—a technique known as "flock and lock"—is one method for restoration of eutrophic lakes. The effectiveness of a drinking water treatment residual (DWTR) as a ballast in flock and lock was assessed using assays of eutrophic lake water from Thailand dominated by Microcystis aeruginosa cyanobacteria colonies by measuring changes in chlorophyll-a, pH, and zeta potential. P sorption isotherms were developed from long-term batch equilibrium experiments; desorption of nutrients and metals was assessed via leaching experiments; and morphological changes to cellular structure were assessed using scanning electron microscopy. Results showed that combining DWTR with a low dose of aluminum sulfate (0.6-4.0 mg Al/L) effectively sank 74-96% of Microcystis , with DWTR dose (50-400 mg/L), initial chlorophyll-a concentration (92-976 µg/L), pH (7.4-9.3), and alkalinity (99-108 ppm CaCO 3) identified as factors significantly associated with sinking efficacy. P sorption capacity of the DWTR (7.12 mg/g) was significantly higher than a local soil (0.33 mg/g), enabling the DWTR to inactivate P in lake sediment. Desorption of Al, Fe, Ca and N from the DWTR was estimated to contribute to a marginal increase in concentrations of those compounds in the water column of a small shallow lake (1.2, 0.66, 53.4, and 0.07 µg/L, respectively) following a simulated application. Therefore, pre-treated DWTRs may be a viable alternative ballast in the flock and lock approach to lake restoration, supplementing or replacing modified local soils or lanthanum modified clays. [Display omitted] [ABSTRACT FROM AUTHOR]

The relationship between total phosphorus (TP) and chemical, climatic, morphological, and geographic variables was examined for over a thousand reference lakes across Sweden. A significant relationship was found between TP and both absorbance of irradiance (at 420 nm, filtered) and altitude for all lakes. These two variables alone, however, were not able to adequately predict TP concentrations in naturally turbid lakes. A natural particulate matter factor (PMn) was developed as part of this study to incorporate the effect of natural suspended matter on lake phosphorus concentration. Variability in TP concentration was well explained with the addition of PMn to the model (R2 = 0.71) even though conditions external and internal to the lakes varied greatly. The ability of the three parameter model to identify culturally eutrophic systems was then successfully tested using a data set of lakes with known anthropogenic phosphorus loads. Thus, the model allows for estimation of reference TP concentration and by extension can be used to identify when a reference concentration has been exceeded due to anthropogenic phosphorus loading. The model output also provides a realistic end point to which phosphorus concentrations should be reduced to achieve a natural trophic state in both clear water and naturally turbid lakes. [ABSTRACT FROM AUTHOR]

Abstract: Sediment cores from six aluminum sulfate treated lakes in Minneapolis, MN were analyzed to determine the effectiveness of phosphorus (P) binding by aluminum (Al). Two of the study lakes are polymictic and the remaining four are dimictic. Above background concentrations of Al and Al-bound-P (PAl) were detected in all six lakes at varying sediment depths. In contrast to previous studies, however, the binding relationship between Al and P was not consistent between lakes and substantial variation was also detected within each sediment profile. Average lake sediment Al:PAl ratios ranged from 5.6 to 15 (molar) with higher ratios, or less efficient P binding, generally being detected in deep, dimictic lakes with high sediment Al content due to treatment. Multiple linear regression was used to explain the variability among average Al:PAl ratios detected in each core and a lake morphometry index (Al Depth Index, core collection depth divided by the square root of lake area) along with Al dose described most of the variation (92%). Even though P bound to the added Al appears to be permanently removed from the internal P cycle in each lake (as evidenced by burial with new sediment), the differences in binding efficiency may indicate lower P inactivation, on a per unit Al basis, when elevated amounts of Al are added to the sediment, especially in deeper areas of lakes where sediment focusing may cause elevated Al accumulation to occur. [Copyright &y& Elsevier]

A method is described to evaluate two methods of phosphorus (P) management in lakes using aluminum sulfate (alum)—in-lake and tributary (inflow) treatment—and compare the resulting in-lake P levels. For in-lake treatment, a technique is described to calculate the optimum alum dose based on measurement of “mobile P” in lake sediments. Mobile P is defined as loosely sorbed and Fe–P, the fraction of sediment P subject to release under anoxic conditions. A linear relationship was found between P-release rate and the mobile-P content in sediment cores. Addition of alum to aliquots of sediment showed predictable relationships between (i) alum dose and aluminum-bound P (Al–P) formed and (ii) mobile-P loss and Al–P formation. The decrease in sediment P release that would result from in-lake alum treatment was estimated from the residual mobile P after treatment. A method also is presented to estimate the amount of alum needed to bind potentially mineralizable sediment organic P. For inflow treatment, jar tests with urban runoff in metropolitan St. Paul and Minneapolis, Minnesota (USA) were used to study effects of alum dose on P removal from water. With sufficient mixing, a dose of reduced total P (TP) and soluble reactive P to low levels regardless of pH, TSS, and TOC, but doses did not significantly reduce TP compared with settling alone. A modeling approach is described to compare the effects of inflow and in-lake treatment, and a case study is developed to illustrate the approach for typical eutrophic lakes. [Copyright &y& Elsevier]

Acidification of lakes exposed to acid deposition is generally accompanied by a severe decrease in production (oligotrophication). In this study, we examined sediment from Lakes Gårdsjön and Härsvatten, Sweden, to determine whether sediment phosphorus (P) retention increased during the years corresponding to lake acidification. Sediment from both lakes had increases in aluminum (Al) in the upper 10 cm, and dating of Lake Gårdsjön sediment revealed that the Al increase occurred from 1950 to 2001 in this lake. The increase in Al input caused an increase in Al-bound P (Al–P) formation and overall sediment total phosphorus retention during the same period. Lake Gårdsjön received an additional 12.9 g·m–2 of Al, above preacidification background levels, that bound 1.1 g·m–2 of P and removed it from the in-lake P cycle from 1950 to 2001. A substantial portion (up to 76%) of the total external P load eventually was converted to Al–P and buried in the sediment over this period. The increase in sediment P burial due to increased formation of Al–P in systems similar to Lake Gårdsjön may have detrimental effects on nutrient cycling, and as a result, on productivity within the lake, leading to acido-oligotrophication. [ABSTRACT FROM AUTHOR]

Sustainable management of lakes requires us to overcome ecological, economic, and social challenges. These challenges can be addressed by focusing on achieving ecological improvement within a multifaceted, co-beneficial context. In-lake restoration measures may promote more rapid ecosystem responses than is feasible with catchment measures alone, even if multiple interventions are needed. In particular, we identify restoration methods that support the overarching societal target of a circular economy through the use of nutrients, sediments, or biomass that are removed from a lake, in agriculture, as food, or for biogas production. In this emerging field of sustainable restoration techniques, we show examples, discuss benefits and pitfalls, and flag areas for further research and development. Each lake should be assessed individually to ensure that restoration approaches will effectively address lake-specific problems, do not harm the target lake or downstream ecosystems, are cost-effective, promote delivery of valuable ecosystem services, minimize conflicts in public interests, and eliminate the necessity for repeated interventions. Achieving optimal, sustainable results from lake restoration relies on multidisciplinary research and close interactions between environmental, social, political, and economic sectors. This article is categorized under: Science of Water > Water Quality Water and Life > Stresses and Pressures on Ecosystems Water and Life > Conservation, Management, and Awareness [ABSTRACT FROM AUTHOR]

Eutrophication of lake ecosystems is a pervasive global environmental problem, exacerbated by urbanization, industrialization, and intensification of agriculture. Excess loading of the macronutrients nitrogen and phosphorus from a myriad of human activities in catchment areas has forced many lake ecosystems into turbid, eutrophic states from which natural recovery is slow. A wide array of tools is available to lake managers to accelerate the process of lake restoration. These range from external measures to reduce nutrient loading, to internal measures designed to either retain nutrients in sediments or remove them from the lake ecosystem. In this preface to the Special Issue Restoration of eutrophic lakes: current practices and future challenges, we briefly review the currently available approaches to lake restoration and assess their global prevalence through a literature survey. The results show that traditional restoration methods such as aeration and chemical inactivation of phosphorus by aluminum continue to be widely used and reported in the scientific literature. The popularity of biomanipulation appears to be in decline, while studies into newly developed, in-lake nutrient inactivation methods are expanding. Hypolimnetic withdrawal, on the other hand, remains a comparatively rare technique. The 16 original research articles in the Special Issue showcase the current state-of-the-art in knowledge of these approaches and their efficacy. We conclude by discussing the key relevant future challenges in the field of lake restoration science. Of these, the need to unite diverse fields of knowledge to develop quantitative models of lake ecosystem responses to restoration measures under a changing climate is paramount. We also emphasize the ongoing need for interaction between scientists, managers, and stakeholders in lake restoration. [ABSTRACT FROM AUTHOR]

Eutrophication of lake ecosystems is a pervasive global environmental problem, exacerbated by urbanization, industrialization, and intensification of agriculture. Excess loading of the macronutrients nitrogen and phosphorus from a myriad of human activities in catchment areas has forced many lake ecosystems into turbid, eutrophic states from which natural recovery is slow. A wide array of tools is available to lake managers to accelerate the process of lake restoration. These range from external measures to reduce nutrient loading, to internal measures designed to either retain nutrients in sediments or remove them from the lake ecosystem. In this preface to the Special Issue Restoration of eutrophic lakes: current practices and future challenges, we briefly review the currently available approaches to lake restoration and assess their global prevalence through a literature survey. The results show that traditional restoration methods such as aeration and chemical inactivation of phosphorus by aluminum continue to be widely used and reported in the scientific literature. The popularity of biomanipulation appears to be in decline, while studies into newly developed, in-lake nutrient inactivation methods are expanding. Hypolimnetic withdrawal, on the other hand, remains a comparatively rare technique. The 16 original research articles in the Special Issue showcase the current state-of-the-art in knowledge of these approaches and their efficacy. We conclude by discussing the key relevant future challenges in the field of lake restoration science. Of these, the need to unite diverse fields of knowledge to develop quantitative models of lake ecosystem responses to restoration measures under a changing climate is paramount. We also emphasize the ongoing need for interaction between scientists, managers, and stakeholders in lake restoration. [ABSTRACT FROM AUTHOR]

Aluminum (Al) treatment is one of the most commonly used approaches to reduce internal phosphorus (P) loading in lakes. However, the adequate amount of Al that should be added to permanently inactivate mobile (releasable) sediment P can be determined using many different methods. These methods differ substantially in their specified design sediment depth, targeted P pool(s), and expected binding ratio. In this study, Al doses for inactivating sediment P in Beung Gii Lake of Thailand were determined using the most commonly used methods reported in literature and then compared. Experimental procedures included sediment P fractionation, Al assay experiments, and a geochemical model. Mobile P was detected in the lake's sediment at 2.52, 5.42, and 7.65 g/m2 in the upper 4, 10, and 15 cm, respectively, with additional P contained in labile organic form. Comparing the resulting Al doses for the lake, it was found they varied by nearly an order of magnitude (45–306 g Al/m2). This result highlights the importance of choosing a dosing method, because such a range of Al doses would likely result in highly variable levels of effectiveness and longevity, including both under- and overdosing. Based on the results of this study and a review of literature, a conservative, fixed ratio between Al and mobile plus labile organic sediment P (11:1) is recommended. All potentially releasable P (both mobile organic and inorganic forms) within the active sediment layer should be used to determine the total Al dose. Finally, the calculated Al dose in most cases will need to be split into sub-doses, based on lake morphology and total Al dose, to ensure maximum binding efficiency. Al dosing strategy should seek to minimize the risk for overdosing, maximize binding efficiency, and ensure all potentially releasable P forms are targeted during treatment. Image 1 • The first comparison of Al dosing methods for lake treatment is reported. • Al doses varied by nearly an order of magnitude depending on dosing method. • Al dose based on in-vitro Al addition was highest and likely overestimated Al dose. • An empirical model was validated as substitute for Al addition experiments. • A fixed ratio of Al to P, based on in-situ binding, is recommended for Al dose. [ABSTRACT FROM AUTHOR]

The article discusses the importance of having a confident communication to address the eutrophication faced by lakes. Topics discussed include the comprehensive understanding of the site being required in the manipulation of the biogeochemical processes successfully, the interaction of the amendment to lakes with the physicochemical environment, and the flexibility in the management offered by geo-engineering.