Global Lake Health in the Anthropocene: Societal Implications and Treatment Strategies

The world's 1.4 million lakes (≥10 ha) provide many ecosystem services that are essential for human well‐being; however, only if their health status is good. Here, we reviewed common lake health issues and classified them using a simple human health‐based approach to outline that lakes are living systems that are in need of oxygen, clean water and a balanced energy and nutrient supply. The main reason for adopting some of the human health terminology for the lake health classification is to increase the awareness and understanding of global lake health issues. We show that lakes are exposed to various anthropogenic stressors which can result in many lake health issues, ranging from thermal, circulatory, respiratory, nutritional and metabolic issues to infections and poisoning. Of particular concern for human well‐being is the widespread lake drying, which is a severe circulatory issue with many cascading effects on lake health. We estimated that ∼115,000 lakes evaporate twice as much water as they gain from direct precipitation, making them vulnerable to potential drying if inflowing waters follow the drying trend, putting more than 153 million people at risk who live in close vicinity to those lakes. Where lake health issues remain untreated, essential ecosystem services will decline or even vanish, posing a threat to the well‐being of millions of people. We recommend coordinated multisectoral and multidisciplinary prevention and treatment strategies, which need to include a follow‐up of the progress and an assessment of the resilience of lakes to intensifying threats. Priority should be given to implementing sewage water treatment, mitigating climate change, counteracting introductions of non‐native species to lakes and decreasing uncontrolled anthropogenic releases of chemicals into the hydro‐, bio‐, and atmosphere.


Introduction
The human population has now reached 8 billion people (United Nations, 2022) all of whom need clean water.Of the approximately 1.4 million unevenly distributed lakes around the world (Figure 1), many play a fundamental role in providing humans with water and food, thereby contributing to the achievement of the 17 global sustainable development goals (SDGs), in particular SDGs 1: no poverty, 2: zero hunger, 3: good health and wellbeing, 6: clean water and sanitation, and 14: life below water (Ho & Goethals, 2019).If lakes and the organisms living in and around them stay healthy, humans in their vicinity have increased prospects for a healthy life, a concept known as One Health (Adisasmito et al., 2022).There is, however, increasing evidence that many lakes on Earth, including the large ones that provide people with extensive ecosystem services, are no longer healthy, owing to local, regional and global stressors (Jenny et al., 2020).
Defining lake health is challenging.In 1998, the U.S. Environmental Protection Agency described a healthy lake as a lake with clean water, balanced algal growth, adequate oxygen levels and abundance and diversity of fish, bottom-dwelling invertebrates and native plants (Environmental Protection Agency, 1998).This general definition is based on undefined terms like "clean," "balanced," "adequate" and "diversity," where unhealthy conditions might be recognized as algal blooms, fish kills, foams, smells, oil films, litter etc.Many health problems are, however, invisible, such as contamination by disease-causing microorganisms, mercury, persistent organic pollutants or microplastics, among many others.These invisible health problems are usually only detectable by diagnostic tests.There is a wide range of diagnostic tests available, for example, more than 100 variables were measured to assess the health status of hundreds of lakes across Canada (Huot et al., 2019).Performing all of these measurements is costly and consequently not feasible to implement for the majority of the world's lakes.To overcome the lack of data from comprehensive lake sampling programs, there are increasing efforts to retrieve lake and watershed data by the interpretation of satellite images (Alsdorf et al., 2007;Dornhofer & Oppelt, 2016;Huot et al., 2019;Yin et al., 2005).
Over the past decades, many countries have made substantial progress in assessing the health status of their freshwaters and there are even efforts, such as the European Water Framework Directive (https://environment. ec.europa.eu/topics/water/water-framework-directive_en), to harmonize assessments across countries.A variety of authorities regularly update the assessments, for example, the U.S. National Lake Assessment (https:// www.epa.gov/national-aquatic-resource-surveys/nla), the Australian Healthy Land and Water program (https:// reportcard.hlw.org.au/), the European Environment Agency (https://www.eea.europa.eu/data-and-maps/),and the China National Environmental Monitoring Center (https://szzdjc.cnemc.cn:8070/GJZ/Business/Publish/Main.html).A key concept of lake health assessments is a comparison of the present status to reference conditions, often defined as conditions that prevail in the absence or near absence of human disturbance, and thus corresponding to pre-industrial conditions (Bouleau & Pont, 2015).Although the determination of reference conditions is still highly uncertain and requires more research (Bouleau & Pont, 2015;Noges et al., 2009), the approach to assess deviations from reference conditions has similarities to practices used in the human healthcare.In medical sciences, health can be conceptualized as the capability to react to all kinds of environmental stressors with desired emotional, cognitive and behavioral responses, and to avoid undesirable ones (Leonardi, 2018), where the delimitation between desired and undesired responses is given by reference values.
Despite major improvements in the assessment of lake health, especially in high-income countries, there is not yet a global classification system for lake health available and links to human well-being have been postulated but often remain unclear.Here, we introduce a simple classification system which has similarities to the international classification system for human health, developed by the World Health Organization (https://www.who.int/standards/classifications/classification-of-diseases).In our approach, lake health issues are classified into thermal, circulatory, respiratory, nutritional and metabolic issues, infections, poisoning and other harmful disturbances (Figure 2).The choice of using anthropomorphic analogies is based on studies showing that such an approach might enhance connectedness to and protectiveness toward nature (Chan, 2021;Tam et al., 2013).

Common Lake Health Issues and Their Societal Implications
In this review, we focus on lake health issues that are widespread, human-induced (including climate change) and that can have major implications for human well-being (Figure 3).Lake health issues can vary substantially in their severity.In human health, the severity of a disease is commonly assessed by monitoring vital signs.For lakes, vital signs might correspond to measurements of oxygen saturation, nutrient (phosphorus and nitrogen) concentrations, water temperature, pH, and water clarity (e.g., Secchi depth).Using this approach and adopting the concept of ecological status of the European Water Framework Directive, the range of lake health conditions can vary from being excellent to critical (Table 1).Presently, it is not known how many of the 1.4 million lakes ≥10 ha (0.1 km 2 ) suffer from one or more health issues, mainly because of undefined reference conditions and missing data on lake physical, chemical and biological conditions.Here, we use rough estimates on the likelihood of lakes to suffer from one or more health issues by analyzing some of the lake and watershed data compiled in LakeATLAS (Lehner et al., 2022), available at https://www.hydrosheds.org/hydroatlas.In LakeATLAS, water bodies with a surface area ≥10 ha (0.1 km 2 ) are considered, encompassing both lakes and reservoirs, of which less than 200,000 are larger than 1 km 2 (Messager et al., 2016).Their combined surface area covers 2.9 × 10 6 km 2 which is about 2.0% of the global land area.This  .Approximately 5% of the lakes are reservoirs (only the largest ∼7,000 are shown on this map).Data for the map are from LakeATLAS (Lehner et al., 2022).Of the 1,427,688 lakes, 994,072 are located in North America, 54,048 in South America, 281,956 in Europe,15,964 in Africa,68,169 in Asia and 13,479 in Oceania.
Earth's Future 10.1029/2023EF004387 WEYHENMEYER ET AL. selection has been made for reasons of data availability.The total number of lakes on Earth is not exactly known but has been estimated to be around 3.4 million when including those ≥3 ha, based on satellite imagery and deep learning methods (Pi et al., 2022), and around 21 million when including those ≥1 ha, based on extrapolation techniques (Messager et al., 2016).Considering even smaller lakes and ponds, estimates reach between 117 million (Verpoorter et al., 2014) and 304 million lakes (Downing et al., 2006).For the work presented here, we used version 1.0 of LakeATLAS that contains data from a total of 1,427,688 lakes.According to the LakeATLAS database, almost 965 million people live within 3 km of a lake, which is more than 12% of the world's population.A detailed description of available variables, their sources and abbreviations are available in Lehner et al. (2022).
In the following sections we address and classify lake health issues which have been observed in many lakes around the world.By outlining their consequences for human well-being, we demonstrate the need for improved prevention and treatment strategies.Lakes are living systems that can suffer from a large variety of health issues, similar to humans.A lake has a health issue if conditions deviate from reference conditions, that is, conditions that prevail in the absence or near absence of human disturbance, often referred to as pre-industrial conditions.Many, but not all, of the health issues are visible, demonstrating the importance of applying adequate diagnostic test.Lakes often experience multiple concurrent or serial health issues.The health issue of heat accumulation includes prolonged and intensified thermal stratification as a response to climate change.

Thermal and Circulatory Issues
Climate change has resulted in pronounced changes in thermal stratification and water column mixing in lakes around the world (Adrian et al., 2009).Water column mixing is a key regulator of lake health, as it determines the replenishment of life-sustaining oxygen concentrations (Boehrer & Schultze, 2008).Full water column mixing can occur several times a year in polymictic lakes, during winter in monomictic lakes, during spring and autumn

Critical
One or more vital signs such as oxygen saturation, nutrient concentrations, temperature, pH and water clarity of a lake are far outside reference conditions.According to the guidelines given by the European Water Framework Directive, this condition corresponds to a bad ecological and chemical status of a lake.

Serious
One or more vital signs such as oxygen saturation, nutrient concentrations, temperature, pH and water clarity of a lake are clearly outside reference conditions.According to the guidelines given by the European Water Framework Directive, this condition corresponds to a poor ecological and chemical status of a lake.

Fair
One or more vital signs such as oxygen saturation, nutrient concentrations, temperature, pH and water clarity of a lake are outside reference conditions.According to the guidelines given by the European Water Framework Directive, this condition corresponds to a moderate ecological and chemical status of a lake.

Good
All vital signs such as oxygen saturation, nutrient concentrations, temperature, pH and water clarity are close to reference conditions.According to the guidelines given by the European Water Framework Directive, this condition corresponds to a good ecological and chemical status of a lake.

Excellent
All vital signs such as oxygen saturation, nutrient concentrations, temperature, pH and water clarity are fully within reference conditions.According to the guidelines given by the European Water Framework Directive, this condition corresponds to a high ecological and chemical status of a lake.

Note.
To judge the severeness of a lake health issue, reference conditions need to be known.The colors correspond to the colors used in the European Water Framework Directive.
Earth's Future  (Bégin et al., 2021), dimictic lakes can become monomictic lakes (Shatwell et al., 2016), and circulation in some large lakes that had mixed once in an annual cycle may no longer occur at all, as strengthening vertical density gradients can resist the seasonal cooling and wind turbulence that would normally result in full circulation (Mesman et al., 2021;Sahoo et al., 2016).From a human health perspective, the latter case could be likened to the loss of circulation from a limb; not necessarily life threatening, but severely limiting functional ability.Even some shallow polymictic lakes may change to become monomictic with climate change and therefore lose connectivity of deep waters to atmospheric reaeration when they are stratified.Human activities associated with salts from de-icing roads, connection of coastal canals to lakes and mining activities can also increase the density of water inflows to lakes (Ladwig et al., 2023).This water can accumulate at the lake bottom and make these lakes permanently stratified, with major effects on dissolved oxygen, circulation and nutrition (Boehrer & Schultze, 2008).
Below we focus on four critical widespread thermal and circulatory issues: heat accumulation that includes prolonged and intensified thermal stratification, loss of ice cover, drying-out and flooding.

Heat Accumulation Including Prolonged and Intensified Thermal Stratification
Many lakes accumulate heat as a response to global warming, with an increasing occurrence of heat waves, defined as a period in which lake surface temperatures exceed a local and seasonally varying 90th percentile threshold for at least 5 days relative to a baseline climatological mean (Woolway, Jennings, et al., 2021;Woolway, Sharma, et al., 2021).Direct effects of heat waves can include a loss of habitat (Kraemer et al., 2021), deoxygenation (Jane et al., 2021) and an accelerated growth of potentially harmful lake organisms, for example, disease-causing microorganisms, invasive species and toxin-producing cyanobacteria (Wilk-Wozniak, 2020).
Heat waves are commonly associated with prolonged and intensified thermal stratification (Woolway, Jennings, et al., 2021;Woolway, Sharma, et al., 2021).The cascading effects of longer and stronger stratification on chemical and biological processes in lakes with an increasing occurrence of harmful algal blooms and fish kills are well known (Shimoda et al., 2011).As long as global warming proceeds, the duration and intensity of thermal stratification will most likely further increase in lakes around the world (Woolway, Jennings, et al., 2021;Woolway, Sharma, et al., 2021).Such a trend might not only cause new health issues but will most probably also intensify the severity of existing issues.

Ice Cover Loss
The majority of lakes on Earth are still periodically covered by ice.Climate change has, however, caused rapid ice cover loss, which is projected to continue, with another approximately 35,000 lakes losing their seasonal ice cover in a 2°C warmer world (Sharma et al., 2019).Additionally, the quality of ice is changing, resulting in a higher frequency of unstable ice conditions (Weyhenmeyer et al., 2022).Unstable ice conditions can cause substantial economic losses due to delays in winter ice road construction (Hori et al., 2017), as well as cancellations of ice fishing and ice-skating tournaments (Knoll et al., 2019).Under projected warmer climatic conditions, ice is forecasted to be too thin to be safe for winter transportation without engineering adaptation solutions (Woolway et al., 2022).Changes in ice cover and quality also entail direct threats to human health by increasing the occurrence of drownings, especially of small children (Sharma et al., 2020).Furthermore, the rapid decline in lake ice cover can threaten human health by affecting various cultural, recreational and spiritual ecosystem services (Knoll et al., 2019).Additionally, there will be far-reaching ecological consequences because winter conditions strongly influence how lake conditions, dynamics and functionality will unfold over the following seasons (Hampton et al., 2017).Conversely, some of the ecological changes from lake ice cover loss may increase lake ecosystem services, for example, from an increase in lake productivity (Weyhenmeyer et al., 2013).

Drying-Out of Lakes
Severe water exploitation and climate change have caused many lakes around the world to rapidly lose water, resulting in a substantial decline in key lake ecosystem services such as the availability of water for drinking, irrigation and fisheries (Rodell et al., 2018;Vörösmarty et al., 2000).Famous examples of lakes that have Earth's Future 10.1029/2023EF004387 WEYHENMEYER ET AL.
rapidly lost water over a relatively short time period with fatal consequences for human well-being are the Aral Sea in Central Asia (Micklin, 1988), Lake Urmia in Northern Iran (Rahimi & Breuste, 2021), Lake Chapala in Mexico (von Bertrab, 2003), Lake Chilwa in Malawi (Njaya et al., 2011), Lake Chad in West Central Africa (Lemoalle et al., 2012), and the Great Salt Lake in the USA (Wurtsbaugh & Sima, 2022).The process of drying-out is analogous to dehydration and potential death for humans.Sustained loss of water security has become a major constraint to socio-economic development and a threat to livelihood in many parts of the world (Liu et al., 2017).Water scarcity can also increase salinity, often making lakes unusable as a water and food resource with direct effects on human well-being (Kafumbata et al., 2014;Wurtsbaugh & Sima, 2022; see also section below on salinization).Despite the well-known ripple effects of drying-out of lakes, many lakes on Earth continue to rapidly lose water (Yao et al., 2023;Zhao et al., 2022).Using global lake and watershed data from LakeATLAS (Lehner et al., 2022), we estimated that about 8% of the worlds' lakes ≥10 ha (i.e., 115,179 out of 1,427,688 lakes) evaporate at least twice as much water as they gain from direct precipitation, which corresponds to an aridity index of ≤0.5 (Zomer et al., 2022; Figure 4a).Lakes that evaporate substantially more water than they directly receive are highly dependent on inflows from upstream or groundwater sources, making them particularly vulnerable to alterations due to climate or human water use change.Such lakes are present across all continents, but they dominate in densely populated low-income countries (Figure 4a).Based on data from LakeATLAS, more than 153 million people live in close vicinity (3 km radius) to a lake with an aridity index ≤0.5.As long as global warming and human water consumption continues to increase, many lakes will continue to lose water and some lakes might even dry-out completely.Globally, the loss of lakes in warm and dry geographical regions might, to some extent, be compensated by the formation of new lakes when permafrost or glaciers melt (Shugar et al., 2020) but these lakes are usually far away from densely populated regions where freshwater is most needed.

Flooding
Lakes around the world suffer not only from drying-out but also from flooding (Tellman et al., 2021), in particular in the tropics where heavy tropical storms increasingly occur under climate change with fatal consequences for the survival of flora and fauna (Reyer et al., 2017).Floods are frequently monitored with data being available and regularly updated in repositories such as the global flood database (https://global-flooddatabase.cloudtostreet.ai/#),yet predicting their effect on individual lakes remains challenging.Floods often classify as a severe circulatory issue of a lake but flooding can sometimes also be beneficial to replenish aquifers, to reconnect lakes with their natural floodplains and to provide suitable habitat for fish to spawn (Talbot et al., 2018).Here, we fous on extreme flooding events that in the worst case can result in loss of homes, property, livestock and lives.Extreme flooding is not only climate-driven but can also occur when dams break or when they are built, famous examples being the dam building projects in James Bay, Canada (Roebuck & Virginia, 2010) and Three Gorges in China (New & Xie, 2008; see section on hydromorphological modifications below).Extreme flooding events are often a direct threat to lake and human health by substantially increasing contaminant loads, macropollutants and disease-causing microorganisms, in particular when flood water encroaches into agricultural land or when there are overflows from sewers or manufacturing facilities into lakes and reservoirs (Talbot et al., 2018;Zaher & Aly, 2021).The litter washed into lakes and onto shores by a flooding event can substantially impair recreational values of a lake due to unsightly debris and malodor.These tangible threats to lake health may be perceived by lake users as more harmful than invisible threats such as heat accumulation, disease-causing microorganisms, oxygen depletion, acidification, etc.The frequency of lake flooding may increase in the future due to an increase in climate change driven extreme precipitation events or rapid glacial melt (Paprotny et al., 2018).Extreme flooding events might also increase due to an increase in landslides that can result in dam breaks (Fischer et al., 2021).Extreme lake flooding events may, however, also decrease because of improved hydromorphological modifications that can regulate the water flow through the landscape in a more efficient way (see also section on hydromorphological modifications below).

Nutritional Issues
The nutritional balance of a lake is disturbed when its nutrient concentrations are either too high or too low compared to reference conditions.Such disturbances commonly result in a decline of lake ecosystem services, for example, the productivity of a lake might rapidly decrease due to nutrient deficiency or it might rapidly increase due to nutrient excess, a process which has been known for many decades as eutrophication (Le Moal et al., 2019).
Earth's Future  Eutrophication is often associated with phytoplankton blooms which occur when microscopic or filamentous algae or cyanobacteria aggregate and float to the surface and become visible to the naked eye.Algal blooms have been observed for centuries; for example, in 1188 Geraldus Cambrensis reported from a lake in Wales: "The lake has many miraculous properties-it sometimes turns bright green, and in our days it has been known to become scarlet, not overall, but as if blood were flowing along certain currents and eddies."Blooms can also involve benthic biofilms of algae and filamentous forms of cyanobacteria in littoral habitats (Vadeboncoeur et al., 2021).Algal blooms might directly threaten human health as certain cyanobacterial strains produce toxins, causing respiratory, gastrointestinal, and dermatological issues (Funari & Testai, 2008).A wide range of symptoms can be triggered by cyanotoxins, with hepatotoxicity (Hernandez & Bessone, 2022) and neurotoxicity (Hinojosa et al., 2019) effects leading to cellular and genomic damage, protein synthesis inhibition and potential carcinogenesis in humans and wildlife (Funari & Testai, 2008).Cyanobacterial blooms have been related to the death of a large variety of organisms (Benayache et al., 2022;Carmichael & Boyer, 2016;Chen et al., 2009;Codd et al., 2015;Lugomela et al., 2006;Trevino-Garrison et al., 2015).Another effect of algal blooms is their contribution to methane production through various pathways, including organic matter degradation and photosynthesis-related processes (Bartosiewicz et al., 2021;Yao et al., 2016) with potential to influence global warming (Bartosiewicz et al., 2021).
The key driver for the global expansion and intensification of algal blooms and toxin production is the cultural eutrophication from domestic, industrial and agricultural waste (Carpenter et al., 1998) which is likely exacerbated by climate change.Eutrophication is very common in lakes that have substantial agricultural land in their watersheds (Arbuckle & Downing, 2001;Keatley et al., 2011).Taking more than 75% cropland in a lake watershed as a proxy for a high risk of eutrophication and the development of harmful algal blooms, we found globally that about 0.9% of the 1.4 million lakes ≥10 ha might be eutrophic as a consequence of extensive agricultural acitivities in the watershed, most of them located in India (3,043 lakes).This number increases to 8% when a threshold of more than 20% cropland in the watershed is used, with most lakes located in USA (18,581 lakes; Figure 4b).According to the LakeATLAS database, more than 590 million people live around lakes (3 km radius) which have >20% cropland in the watershed.

Respiratory Issues
Clear signs of lake respiratory issues are dissolved oxygen concentrations far below reference conditions.Oxygen desaturation in a lake is primarily related to algal blooms, warmer water temperatures, and insufficient water circulation due to stronger and longer stratification (Jane et al., 2023).Among the most severe consequences of oxygen desaturation in a lake is the death of oxygen demanding organisms, with massive fish kills being a sign of critical lake health conditions.Fish kills have been reported from lakes all around the world (Fukushima et al., 2017;Hoyer et al., 2009;Kangur et al., 2016;Ochumba, 1990;Rao et al., 2014;Roelke et al., 2011;Sayer et al., 2016;Smith et al., 2016).Fish kills usually result in the loss of many ecosystem services, the most apparent being a loss in the provision of food from lakes and a loss of recreational value.Fish kills are commonly related to the decay of massive algal blooms in highly eutrophic waters with low flushing rates (Zhou et al., 2015) and subsequent oxygen depletion (Rao et al., 2014).They have also been linked to cyanobacterial toxins (Carmichael & Boyer, 2016), infections (Scott & Bollinger, 2014), acidification episodes (Rosseland, 1986), exceptionally high organic carbon inputs related to browning (Brothers et al., 2014), high rainfall events (Kragh et al., 2020), pollutants, loss of habitat connectivity (Mendoza et al., 2022) or a combination of factors often related to heat waves (Kangur et al., 2013).Fish kills usually generate considerable attention from the public, putting pressure on managers to identify causal factors and implement treatment strategies.Fish kills are expected to increase in a warmer future, especially during heat waves when thermal stratification is expected to intensify and lengthen (Woolway, Jennings, et al., 2021;Woolway, Sharma, Figure 4. Lake and watershed characteristics of about 1.4 million lakes.Lakes that are shown have a surface area ≥10 ha (0.1 km 2 ) and include reservoirs.The data are taken from LakeATLAS (Lehner et al., 2022).Panel (a) Global aridity index for lake surface area, defined as the ratio between annual precipitation that falls directly onto the lake surface and potential evapotranspiration from the lake.An aridity index >1 (blue color) represents lakes that receive more water from direct precipitation than they lose through evaporation, whereas an aridity index ≤0.5 (orange and red color) represents lakes that evaporate at least twice as much water as they receive from direct precipitation.Most lakes in Africa and Australia, and many lakes in Central Asia evaporate much more water than they receive.Panel (b) Extent of cropland in lake watersheds.Lakes that have >20% cropland in the watershed (yellow, orange and red color) are regarded as being at high risk of eutrophication and algal blooms.Panel (c) Human development index within 3 km vicinity of a lake.A human development index ≤0.5 (dark purple color) is regarded as being critical to lake health due to the high probability of uncontrolled inputs of untreated sewage and unmanaged waste.Most lakes in Africa and many in Asia are surrounded by people with a low human development index.
Earth's Future  et al., 2021), resulting in elevated risk for oxygen depletion (Jane et al., 2021).Globally, oxygen depletion in lakes is spreading quickly, even faster than in the oceans and with no signs of recovery (Jane et al., 2021).Hypoxia is even returning to lakes with good water quality, like observed in Lake Geneva due to milder winters that result in incomplete lake overturn, causing long-term isolation of the deepest part of these lakes from the atmosphere, threathening benthic fauna (Mesman et al., 2021).The loss of dissolved oxygen from deeper waters has important ramifications for lake nutrition, as it is associated with the release of dissolved phosphorus from bottom sediments (Sondergaard et al., 2003), linked to reduction and dissolution of binding metal ions (e.g., iron, manganese) and shutdown of nitrification (Small et al., 2014).Thus, the loss of dissolved oxygen in a lake should be viewed similarly to any impairment in human respiratory function, with potential for farreaching consequences on health and nutrition.

Metabolic Issues
Acidification, salinization and browning with elevated energy and ion inputs into lakes can all disturb the metabolic balance of a lake.Because those processes also occur naturally, we only consider them as health issues when pH/acidity, salinity and color/dissolved organic matter are outside of reference conditions.High densities of naturally saline and acid-saline lakes are usually found in regions with extreme aridity and specific geological conditions.Those lakes commonly show high variability of salinity and acidity with seasonal rains, often with high levels of endemism and acting as important bird migration flyovers (Pedler et al., 2014).

Acidification
Mining activities, industrial pollution, atmospheric deposition of sulfur and nitrogen compounds and afforestation can all result in a pH drop to critical levels below 6 or, in the presence of strong nitric and sulfuric acids, even lower (Schindler, 1988).Acidification can be chronic or episodic, causing a large variety of chemical changes in lake waters, including the release of metals, with pronounced effects on the growth and reproduction of pH sensitive microorganisms, plants and animals (Muniz, 1990;Vrba et al., 2016).In severe cases, fish kills can occur.In general, acidification shifts the balance between acid-sensitive and acid-tolerant species (Muniz, 1990), thereby altering the structure of aquatic food webs.Acidification affects human well-being not only by reducing food resources but often results in substantial economic losses due to reduced or extirpated fish stocks of commercial or recreational value (Caputo et al., 2017;Tammi et al., 2003).Despite efforts to raise the pH in lakes by controlling emissions of sulfate and nitrogen oxides at national (e.g., U.S.A.'s Clean Air Act) and international levels (e.g., United Nations regulations) and by treating lakes and surrounding watersheds with calcium carbonate (e.g., Nordic countries) acidification remains a current issue because biological recovery has often not shown the desired response to a pH increase (Evans et al., 2001).In addition, acidification is expected to increase again in the future due to projected population and consumption increases that lead to an increase in mining, smelting, fossilfuel combustion, food production, use of nitrogen fertilizers, deforestation and other processes (Rice & Herman, 2012).Another factor behind increasing acidification with effects on biota is an increase in carbon dioxide concentrations in lakes (Hasler et al., 2016), although trends of carbon dioxide concentrations in freshwaters are highly variable, sometimes even decreasing over time (Nydahl et al., 2017).

Salinization
Salinization can be severe when lakes dry out.Salinization of lakes is of particular concern due to its strong effects on the structure of biological communities and ecosystem health (Williams, 1998).The process of salinization by rapid evaporative water loss can be amplified by salts that enter lakes due to agricultural activities in the watershed (Wakeel, 2013), by pressures associated with dryland and wetland salinity (De Sousa et al., 2023), and/ or by hydrological changes that cause incursion of coastal waters (Tibby et al., 2020).Apart from the rapid increase in salinity in lakes of the semi-arid and arid regions of the world, with no signs of improvement, salinization has also become a concern for lakes located in cold geographical regions, mainly due to an increased use of road salts (Dugan et al., 2017).In those regions, current water quality guidelines are inadequate to protect lake biota from harm (Hintz et al., 2022).For the salinization process in cold geographical regions mitigation measures such as using less road salt in winter or the application of salt alternatives, such as the utilization of sand and heating of roads, are presently under consideration by policy makers.

Browning of Lakes
Many lakes across boreal, temperate and arctic regions have become browner due to increased human-induced (i.e., climate change, acidification, forestry) inputs of dissolved organic matter and iron (Kritzberg et al., 2020;Weyhenmeyer et al., 2014).When lake waters become browner, increased light limitation can reduce primary and fish production (Karlsson et al., 2009;van Dorst et al., 2019).In contrast, the growth and reproduction rates of microorganisms are usually increased in browner lakes, with an increased risk of oxygen depletion (Brothers et al., 2014) and increased internal nutrient loading (Kazanjian et al., 2021).These changes can favor the production of methane in lakes with effects on global warming (Dean et al., 2018).The increased growth and reproduction of (pathogenic) microorganisms is also a major challenge for the generation of safe drinking water (Edge et al., 2013).There are treatment options to prevent bacterial presence in drinking water such as chlorine additions but the disinfection by-products are often carcinogenic (Eikebrokk et al., 2004).
Exposure to UV-light or membrane filtration are other treatment options, but are costly (Eikebrokk et al., 2004).
Although browning is often regarded as a lake health issue, mainly due to its effect on drinking water quality, reference conditions remain undefined, and the extent of its effect on lake health is therefore uncertain.Browning is common for lakes that are surrounded by soils that contain a high amount of organic carbon in the top layer (Weyhenmeyer et al., 2012).According to LakeATLAS, ∼837,500 lakes are surrounded by a watershed in which soils contain more than 100 tonnes of organic carbon per hectare in the top 5 cm soil layer.These lakes are mainly located in regions with a low population density.Browning might continue into the future, driven by climate change and forestry, but there are also indications that the process of browning has slowed down in geographical regions that have recovered from acidification (Riise et al., 2018;Worrall et al., 2018).

Infections
Infections can occur when there is a massive input or a rapid growth of disease-causing microorganisms in a lake.The infections might spread among lake organisms, but also among humans when they consume or are in contact with non-purified lake water.Disease-causing microorganisms are primarily found in untreated wastewater which enters lakes (Ford, 2016), but they can sometimes enter lakes also through discharge of ballast water (Ruiz et al., 2000) and defecation by animals such as water fowl, cattle and dogs (Graczyk et al., 2009).Globally, only about 56% of the world's household domestic wastewater is presently adequately treated to be at safe levels for consumption (UN Habitat and WHO, 2021).This kind of lake health issue poses a major challenge for countries that still do not have an adequate infrastructure for the treatment of wastewater.Improvement of water sanitation and hygiene has been shown to be a very efficient method to reduce waterborne infections (Ford, 2016).Each dollar invested in water sanitation and hygiene interventions gives approximately 4.3 dollars in return from preventative healthcare costs (World Health & Water, 2015).People who live in countries that have a low human development index, a summary measure of the average achievement in key dimensions of human development, including standard of living, access to education and having a long and healthy life (Kummu et al., 2018) are unlikely to have adequate infrastructure for the treatment of sewage water with the occurrence of many waterborne diseases (Ford, 2016).Using a very low human development index as a rough estimate for a high likelihood of untreated sewage water to enter lakes with infections as a consequence, we found approximately 0.5% of the 1.4 million lakes ≥10 ha where the human development index was ≤0.5 (Figure 4c), potentially affecting more than 44 million people in their close vicinity (3 km radius).Of these 44 million people, 5% live in Asia and 46% in Africa.Raising the human development index with adequate treatment of wastewater should be of highest priority, given that the growth and reproduction of microorganisms are expected to further increase in a warmer world, as metabolic rates of microorganisms are highly sensitive to temperature increases (Brown et al., 2004;Yvon-Durocher et al., 2012).Together with eutrophication and an increased cycling of organic matter, often linked to diffuse pollution from intensive agricultural practices, the growth and reproduction rates of microorganisms are likely to further accelerate, with an increased risk for a higher abundance of disease-causing microorganisms.

Poisoning and Other Harmful Disturbances
Many lake health issues are related to inputs of human-made chemicals and materials, overexploitation, introductions of non-native species and/or hydromorpholocial modifications.We classify these issues as poisoning and other harmful disturbances.This classification has similarities to the category "Injury, poisoning or certain other consequences of external causes" in the human health classification system (https://icd.who.int/en).

Accumulation of Hazardous Substances
The list of hazardous substances that can poison lakes is long and rapidly increasing.Hazadous substances commonly spread via air and water and are consequently found in lakes all around the world (Wang et al., 2019), and even in drinking water (Bao et al., 2012;Fick et al., 2009;Yadav et al., 2015).A well-known hazardous substance where the spread, fate and consequences have been intensively studied, is mercury (Ma et al., 2021;Meili et al., 2003).Mercury is of primary concern as it occurs in a highly toxic form (i.e., methylmercury) and can easily be taken up by organisms, thus finding its way into the food chain.When humans consume fish with high mercury levels, it may occasionally cause irreversible and fatal neurological diseases, in particular in vulnerable subpopulations, such as pregnant women, infants and young children.An example of a catastrophic outcome of mercury pollution is the poisoning event in the Japanese city Minamata, which was caused by methylmercury in the industrial wastewater released from a chemical factory from 1932 to 1968 (Harada, 1995).In general, mercury exposure to freshwater organisms has a strong potential for deleterious effects and ecological risks to sensitive fauna, and it alters biochemical, physiological, hematological and behavioral conditions (Chan et al., 2003).
Although mercury emissions have successfully been reduced in some countries following international legislation such as the Minamata Convention, the presence of mercury in everyday products and processing operations constitutes an ongoing challenge (Selin & Selin, 2022).This mercury primarily comes from mining, as well as fossil fuel combustion, forestry including deforestation and open burning of waste and industrial processes such as chlor-alkali manufacturing (Obrist et al., 2018).
Numerous other hazardous substances besides mercury are found in lake water, ranging from pharmaceutical residues, endocrine disrupters, personal care products to industrial chemicals and pesticides.Of particular concern are persistent organic pollutants (POPs) and per-and polyfluoroalkyl substances (PFAS) because of their persistence and potential to cause toxic reactions (Daughton & Ternes, 1999;Ibor et al., 2019;Vandenberg et al., 2012).Apart from direct toxic reactions, hazardous substances can also cause additional problems; there is, for example, a high risk that antibiotic resistance increases when antibiotics are released into the environment (Ben et al., 2019).Many regional and global regulations have been implemented to decrease the spread but despite legislation, more and more hazardous substances become detectable in lakes all around the world and in drinking water (Morin-Crini et al., 2022), in particular in low-income countries (Wee & Aris, 2023).

Accumulation of Microplastic and Nanomaterials
Micromaterials such as microplastics occur in lakes all around the world (Nava et al., 2023) and have become a major concern because of their persistence with complex effects on aquatic ecosystems (Sarijan et al., 2021).Plastic production reached 359 million tonnes worldwide in 2018 and is projected to increase (Plastics Europe, 2019).Microplastics are defined as synthetic polymers with an upper size limit of 5 mm and without a specified lower limit (Eerkes-Medrano et al., 2015), although the smallest fractions might be classified as nanoplastics that pose additional health risks due to their very small size (Lai et al., 2021), see below.Well established sources for microplastics are commercial and sport fishing, boats, textile industries, personal care products, air-blasting processes, improperly disposed plastics, car tires and leachates from landfills (Yang et al., 2021).Microplastics can act as substrate for microorganisms and pollutants, including antibiotic resistant bacteria (Di Cesare et al., 2021) and are suggested to negatively impact the health of fish communities as well as human health through the consumption of fish and water from lakes containing microplastics (Angnunavuri et al., 2023;Azizi et al., 2021).Although many microplastics, together with their associated bacteria and pollutants, will end up in marine systems, they can also accumulate in lakes, in particular in those with an endorheic watershed (Cai et al., 2022).Since the lake health issue of microplastics has only recently been recognized, there are not yet commonly used treatment strategies in place.
In addition to microplastics, an increasing global release of engineered nanomaterials into aquatic environments poses another growing concern (Reidy et al., 2013).Broadly defined, the term "nano" refers to any material with at least one dimension that measures 100 nm or less (American Society for Testing and Materials, 2006), implying that they show large variations in size, toxicity and coating materials.The main sources of nanomaterials found in lakes are products used by humans that enter waterways via wastewater treatment plants, industrial effluent, atmospheric deposition, and surface water runoff (Malakar et al., 2021).The risks of nanomaterials to lake health are numerous with observed effects on the growth and reproduction of, for example, fish (Martin et al., 2018).Human health implications include those from nanoparticle ingestion through drinking water.The effects of Earth's Future 10.1029/2023EF004387 WEYHENMEYER ET AL.
nanomaterials found in purified drinking water, however, are still not well understood, ranging from being judged as posing a low risk to human health (Westerhoff et al., 2018) to a relatively high risk (Sousa & Teixeira, 2020;Zhang et al., 2021).As with microplastics, the lake health issue of nanomaterials has only recently gained broader attention, a reason why common treatment strategies are still at the very beginning.

Macropollutants
Macropollutants are visible pollutants in particulate form, such as litter and macroplastics.Most macropollutants found in lakes come from waste dumped at the shore or dumped from ships.Macropollutants can also enter lakes when wastewater is not adequately treated (Aragaw, 2021;Merga et al., 2021).Macropollutants cause a substantial loss in recreational, spiritual, esthetic and cultural values of lakes (Wood et al., 2021) and they can reduce the quality of drinking water resources (Cera et al., 2023).Macropollutants can also affect lake organisms when they digest the smaller sized fractions (Cera et al., 2023).To treat the lake health issue of macropollutants, which is particularly common in low-income countries, it may be most efficient to educate people on the consequences of spreading macropollutants into nature (Irfan et al., 2020).In addition, adequate infrastructure to manage waste sewage water can substantially decrease the lake health issue of macropollutants (Baron et al., 2002).

Overexploitation
Overexploitation of lakes in the form of overfishing and excessive water removal for domestic, industrial and agricultural use commonly results in obvious lake health issues which have been intensively reviewed in other studies (Beeton, 2002;Coble et al., 1990;Ogutu-Ohwayo et al., 1997;Winfield, 2016).Overexploitation is frequently occurring in densely populated low-income countries where food shortage is common and legislations might not be followed if they at all exist (Odada et al., 2020).

Hydromorphological Modifications
Hydromorphological modifications such as dams, weirs, sluices, locks, channelization, decoupling of floodplains from active river channels, shoreline destruction and many more human alterations are widespread globally, occurring in both low-and high-income countries (Zarfl et al., 2015).Such modifications, in particular hydropower plants for the provision of electricity, have tradeoffs that can be beneficial for human health (Avtar et al., 2019), but at the same time, they have been linked to a severe reduction in habitat diversity, connectivity and complexity within the global river system, as well as the loss of specific habitats such as boulders and rocks, coarse woody debris, submerged tree roots, and macrophyte stands, with pronounced effects on invertebrate and fish communities (Cebalho et al., 2017;Poikane et al., 2020;Ziv et al., 2012).Apart from habitat degradation, several other ecosystem services are negatively affected by hydromorphological modifications, that is, the loss of recreational, spiritual, esthetic and cultural values (Lin & Qi, 2017), reduction in navigation and transport capacity (von Sperling, 2012) and enhanced greenhouse gas emissions (Borges et al., 2015;Deemer et al., 2016).Hydromorphological modifications can lead to conflicts over water at regional and national scales, demonstrating the need to undertake modifications without impacting social values or compromising environmental resilience, defined here as the capacity to retain a functional ecosystem while under stress (Scheffer & Carpenter, 2003).Hydromorphological modifications are manifold, with the establishment of large dams turning lakes and rivers into reservoirs.Human-made reservoirs can be found all around the world (Figure 1).The majority of large dams has been built since 1950 (Lehner et al., 2011), and there are no signs that the establishment of new dams is slowing down, particularly not in low-income countries, due to the worldwide rapid expansion of human activities, land use change, urbanization and, in particular, the demand for hydropower (Zarfl et al., 2015).

Invasive Species
Intensified trade, tourism, and recreational activities, as well as climate change have been linked to an increasing invasion of non-native species that can harm native species and entire ecosystems (Rahel & Olden, 2008).The direct effects of invasive species on human health range from physical effects (e.g., allergies, poisoning, and bites) to psychological effects (e.g., phobias, discomfort, loss of recreation; Mazza et al., 2014).One well studied example known to directly affect human health is the invasion of the red swamp crayfish, which now can be found Earth's Future on all continents except Oceania and Antarctica (Oficialdegui et al., 2020).This crayfish forms poisonous spines that can cause respiratory issues, arterial hypotension and an irregular heartbeat in anglers when they come into contact with the spines (Lodge et al., 2012).Another globally widespread invader is the Dreissena mussels, both zebra and quagga.Dreissenids spread quickly and, as a filter feeder, can change the functioning of lakes, with resources being funneled from pelagic to benthic communities (Karatayev & Burlakova, 2022).It has been suggested that Dreissena can facilitate harmful cyanobacteria blooms by removing competitors (Vanderploeg et al., 2001) and by concentrating bioavailable nutrients previously bound in phytoplankton during excretion (Raikow et al., 2004), but no support for this was found by Pires et al. (2005).Dreissena settlement often leads to enormous costs to human infrastructure, like blocking cooling systems of power plants and industry (Karatayev & Burlakova, 2022).In some lakes, these mussels may, however, also help to enhance water clarity, control algal blooms, promote macrophyte growth and in some shallow lakes they have become a staple food for large numbers of diving ducks (Ibelings et al., 2005).The lake health issue of invasive species is rapidly increasing along with the general trend of globalization (Sentis et al., 2021).

Co-Existence and Interactions of Lake Health Issues
Lake health issues may occur in isolation but most health issues co-exist and interact due to close connections between physical, chemical and biological processes in lakes (Shimoda et al., 2011).In medical sciences, the presence of two or more long-term health conditions is defined as multimorbidity which has gained increasing attention across nations over the past years (Chua et al., 2021).It has been estimated that almost a third of the world's population (2-3 billion individuals) suffers from multimorbidity with the co-occurrence of more than five ailments (Vos et al., 2015).Such estimates are not available for lakes but it is highly likely that multimorbidity is the norm.A classic example of multimorbidity in lakes is the occurrence of thermal and circulatory issues that coexist with, for example, oxygen depletion and nutritional imbalances (see sections above).Although many coexisting and interacting lake health issues are known the health risks of mixtures of thousands of interacting chemical compounds in lakes is poorly understood.Some of the harmful substances might be flushed out or deposited on sediments at the bottom of lakes.Lake sediments are, however, not a safe final repository because harmful substances can be remobilized when redox conditions change, for example, when lakes suffer from oxygen depletion or acidification or when the sediments are resuspended (Weyhenmeyer, 1998).In addition, nutrients and warming can enhance negative effects of chemical mixtures (Vijayaraj et al., 2022).The increasing accumulation of interacting harmful substances constitutes a global lake health issue that has gained far too little attention, probably because it is a major challenge to implement prevention and treatment strategies for a threat that is not yet fully understood and invisible.

Treatment Strategies
To treat lake health issues is complex, particularly when several health issues co-exist and interact and where the causes for the health issues can range from global, for example, climate change (Adrian et al., 2009;Woolway et al., 2020) and/or atmospheric deposition (Baron et al., 2011;Elser et al., 2009;Meili, 1992;Weyhenmeyer, 2008) to regional and local causes, for example, intensive land-use, in particular agriculture (Carpenter et al., 1998), forestry (Kritzberg et al., 2020) and urbanization (Hall et al., 1999), water regulations (Zarfl et al., 2015), wildfires (Scordo et al., 2021), wars (Shumilova et al., 2023), contamination by disease-causing microorganisms (Ford, 2016) and local exploitation such as water removal for domestic, industrial and agricultural use, chemical pollution and overfishing (Micklin, 1988).Additional challenging aspects of treating lake health issues are potential conflicts with human demands such as maximizing food productivity, energy generation, economic prosperities etc.To find sustainable solutions, lake health issues need to be diagnosed and related to tradeoffs among energy, food and water.We suggest applying strategies that are similar to those used in human healthcare: (a) intervention and preventative actions before health problems occur by, for example, nature conservation efforts, (b) regular screening and early identification of lake health issues and (c) remediation and mitigation efforts at an appropriate scale, spanning from local to global (Figure 5).Ultimately, we need to act locally to treat most lake health issues where local community involvement and participation in both prevention and treatment of lake health issues is critical to ensuring appropriate and sustainable solutions (Cianci-Gaskill et al., 2024).

Intervention and Preventative Actions
Many efforts have been made to protect lakes around the world which is an efficient primary prevention measure, provided that regulations are in place and followed.Data from LakeATLAS (Lehner et al., 2022), indicate that approximately 14% of the 1.4 million lakes ≥10 ha are to some extent protected, with almost 12% being fully protected, that is, where the protection is 100% of the lake surface area.Other efficient prevention strategies comprise the management and treatment of waste and sewage water as well as education of society, which goes along with a rise of the general socioeconomic status of a country (United Nations, 2018).Another example of a preventative action is the Water Safety Plan (WSP) which has been advocated by the World Health Organisation (Gunnarsdottir et al., 2012).The overall goal of WSPs is to protect consumers against pollutants in their drinking water, be it persistent organic pollutants, mercury, cyanotoxins or other contaminants.The WSPs provide a comprehensive framework for risk assessment and management, where barriers to exposure are implemented at multiple levels encompassing all steps in the drinking water supply chain, captured in the popular catchphrase "from catchment to consumer."The protection measures taken at catchment scales immunizes lakes from a variety of health issues.The World Health Organization and the International Water Association have developed a manual to guide authorities in the implementation of WSPs.

Regular Screening and Early Identification of Lake Health Issues
Regular screening and early identification of lake health issues using consistent indicators, metrics and classification systems, is poorly developed globally.Many more harmonized diagnostic measurements and reliable reference conditions for lakes around the world are needed to provide evidence of lake health problems.One effective way to increase the reliability of reference conditions is to assess them by applying different methods, for example, through paleolimnology and coupled catchment-lake modeling (Abell et al., 2019).Recently, some improvements have been made in harmonizing diagnostics on a global scale by analyzing satellite images.Many lake health issues are, however, invisible and thus cannot be diagnosed by the interpretation of satellite images.Those invisible lake health issues require additional screening methods, mainly by taking water samples that can be screened for contaminants of emerging concern, such as pharmaceutical residues, endocrine disruptors, POPs, PFAS etc. Another option to screen for critical health conditions is the use of inexpensive sensors that can provide reliable measurements of multiple vital signs of a lake, such as temperature, oxygen saturation, water clarity and pH.In the future, lake health might be assessed digitally in similar ways as the digital assessment of human health.Treatment strategies of lake health issues.In the best case, lake health issues can be prevented by, for example, nature conservation efforts.Independent of such efforts, lakes need to be screened for health issues on a regular basis, which might be done by interpreting satellite images, using high-frequency sensor technology, taking water and/or sediment samples etc. From those screening data, lake health issues need to be identified at an early stage so that cost-efficient treatments can be started.Whenever treatments are started, there is an urgent need to follow-up the progress of the treatments and to adjust the treatments if necessary.

Remediation and Mitigation Efforts
Worldwide, many lake health issues have already been diagnosed, but only some are adequately treated, and even then only in a few lakes.When treatments are chosen, it is of utmost importance to identify the causes of the health issues, which is challenging because they are usually manifold, ranging from global to regional and local causes.
The easiest to treat are local causes, in particular when a lake health issue can be related to a local point source.
Many countries have come far in identifying and successfully treating local point sources, the implementation of waste water treatment plants that cured many lakes from infections and eutrophication being an example (Krantzberg, 2012;Yu et al., 2023).Sometimes the causes of a lake health issue might not be known, and for this reason only the symptoms can be treated.Symptoms might also be treated if the causes of health issues are too expensive and complex to treat, for example, cyanobacteria are removed from lakes by using hydrogen peroxide (H 2 O 2 ) instead of addressing the causes of the nutrient excess in the lake (Matthijs et al., 2012).Likewise, aerators or fountains are installed in lakes to successfully cure lakes from oxygen desaturation (Mackay et al., 2014), but the actual causes of the oxgen depletion are not treated.
Despite well-known cost-effective local treatment options such as the investment in adequate domestic and industrial sewage and stormwater infrastructure, including runoff from agricultural landscapes, there are still many vulnerable or degraded lakes that are untreated, in particular in densely populated low-income countries where resources are low and many other problems supersede lake health (Jamu et al., 2011).A concept which has been used to overcome some of the socioeconomic hindrances is active participation of society in managing their water resources (United Nations, 2018).Engagement to improve the health of a lake can be maximized by compensating lakeside communities for any efforts they make to improve the health of lakes that sustain them.The mobilization of multiple sectors, disciplines and communities in society to work together corresponds to the One Health approach, which has the overall goal to balance and optimize the health of people, animals and the environment (Adisasmito et al., 2022).
Apart from the large variety of local and regional efforts, many lake health issues, in particular those that spread via the air, may be helped by national and international legislation.The list of national and international environmental laws and agreements is very long, with new laws and agreements constantly being added.The success rate of these laws and agreements to improve lake health is mixed.A good example for successful legislation has been the international regulation of atmospheric sulfur dioxide emissions, which cured many lakes in the Northern Hemisphere from acidification (Baldigo et al., 2021).Other agreements are less successful, for example, the legally binding international treaty on climate change.Without additional multilateral and international efforts to combat climate change, lake water volume in many parts of the world will further decline, causing multiple cascading effects such as salinization, oxygen depletion with fish kills and accumulation of harmful substances.
The global problem of water scarcity has been outlined many times before (Masson-Delmotte et al., 2021;Rodell et al., 2018) but to date there are no signs of improvement.To better treat the rapid spread of water scarcity, there is also an urgent need to reduce the human water consumption, in particular by persons who have a high living standard, as this group consumes disproportionally large amounts of water (Savelli et al., 2023).The reduction can thus be done either directly or, more effectively, indirectly by reducing the demand for products that require large amounts of water for their production (i.e., decreasing the water footprint of food).
Although legislation might be an efficient treatment strategy for many lake health issues, additional actions are required.Presently, there is, for example, too much focus on the decline in lake health caused by individual harmful substances, often not taking account of legacy effects.Lake health is usually not a matter of single substances posing a threat and it is no longer solely a local or national issue.Instead, it is a cocktail of chemical substances from various sources worldwide that determines the health status of a lake.According to Rockström et al. (2009) and Wang-Erlandsson et al. (2022) chemical pollution and green water have been identified as planetary boundaries that should not be crossed in order to safeguard humanity.Far too little is known on the complex transformations that chemical substances undergo in the atmosphere and in water.Since climate and the spread of harmful substances via the atmosphere are not independent, a coupling of mitigation strategies for anthropogenic influences on physical, chemical and biological processes in the atmosphere and freshwaters is needed to achieve substantial improvement in global lake health.
An additional issue with legislation is the large variability of environmental laws and enforcement across countries where low-income countries often lack appropriate laws.Such inequalities are problematic because countries with strict environmental laws increasingly rely on the production of-and trade in-harmful substances, Earth's Future 10.1029/2023EF004387 WEYHENMEYER ET AL.
with countries that do not yet have regulations, a practice that is globally not sustainable (Goulson, 2020).Since there are no signs that global production and human consumption are decreasing (Wang et al., 2022), we have to find strategies to change human behavior toward a more sustainable pattern.No degree of legislation or technical clean up measures can replace the long-term effectiveness of changing our ways of living.
The list of treatment options for lake health issues is long, ranging from nature-based solutions, physical, chemical and biological treatments to legislation.It is often a combination of treatments which is needed to cure lakes from health problems.Whenever treatments are chosen, there is an urgent need to move to watershedoriented treatment strategies as practiced by, for example, the European Union.Such strategies can imply big challenges, in particular when watersheds cross jurisdictional or national boundaries.It is important to tackle these challenges, because globally only relatively few nations presently have laws that acknowledge the role of watershed hydrology and riparian buffers in the movement of pollutants from anthropogenic hotspots across watersheds into the adjacent water bodies (Owokotomo et al., 2020).Once treatments have been chosen and started, the progress of the treatments needs to be followed-up, a step which is commonly not yet done.Here, lake managers can learn from the human healthcare system-a medical doctor usually closely follows the outcome and effectiveness of a treatment and makes adjustments whenever needed.Thus, in the human healthcare system, strong efforts are made to find the optimal dose and duration of a treatment, efforts which are also urgently needed for treating lake health issues.

Conclusions
Lakes need to be recognized as living systems that can suffer from a large variety of health issues with similarities to human health issues.Despite increasing preventation and treatment efforts in many countries, we were not able to find evidence for a substantial improvement in the overall global lake health status.Thus, there is a high risk that more and more lake health issues will become chronic.Chronic health issues caused by climate change, human consumption, intensive agriculture, deforestation, mining, dams, industrial emissions, urbanization, wars etc. are complex and interconnected and consequently very challenging to treat.A balance between trade-offs from those activities and water ecosystem services must be presented to governments to find a viable way to sustain lake health and the consequential ecosystem services.There is an urgent need for international treaties, a coalition of economy and society and an engagement of local citizens and non-governmental organizations.For example, the World Water Quality Alliance of the University National Environment Program (https://www.unep.org/explore-topics/water/what-we-do/improving-and-assessing-world-water-quality-partnership-effort) seeks to improve access to safe, clean water using a coalition of local communities, non-governmental organizations, national administrations, water authorities, farmers and fishers.Such a model could be applied to lakes, particularly those shared across borders, and lead to improved coordination of national legislation and a common understanding of lake health and management strategies.
In this study, we took a human-centric approach, where we outlined that hundreds of millions of people are living around lakes that most likely suffer from one or more health issues.This human-centric approach may give a biased view (Schroter et al., 2014), where it remains unknown how many other living organisms are affected by the various lake health issues.Instead, our focus on humans acknowledges us as both the agents of harm and the potential agents of remediation, in a classification framework that will hopefully resonate to stimulate prompt action.While we acknowledge that many additional human threat databases exist they are not always compatible, which makes the task of mapping the spread of health issues challenging and highlights the need for improved global data alignment and co-ordination.
At present, many lake health issues are well-known but not yet treated.Thus, there is an urgent need to start treatments, particularly in densely populated low-income countries.When lakes and their watersheds are treated the outcome and effectiveness of the treatments need to be well-documented, with adjustments being made whenever needed, so that treatment strategies can be refined and promoted.This work is the result of an international team science effort facilitated by the Global Lake Ecological Observatory Network (GLEON).

Figure 1 .
Figure 1.World's distribution of lakes ≥10 ha (0.1 km 2).Approximately 5% of the lakes are reservoirs (only the largest ∼7,000 are shown on this map).Data for the map are from LakeATLAS(Lehner et al., 2022).Of the 1,427,688 lakes, 994,072 are located in North America, 54,048 in South America, 281,956 inEurope, 15,964 in Africa, 68,169 in Asia and  13,479 in Oceania.

Figure 2 .
Figure2.Lakes are living systems that can suffer from a large variety of health issues, similar to humans.A lake has a health issue if conditions deviate from reference conditions, that is, conditions that prevail in the absence or near absence of human disturbance, often referred to as pre-industrial conditions.Many, but not all, of the health issues are visible, demonstrating the importance of applying adequate diagnostic test.Lakes often experience multiple concurrent or serial health issues.The health issue of heat accumulation includes prolonged and intensified thermal stratification as a response to climate change.

Figure 3 .
Figure 3. Possible threats to human well-being as a result of lake health issues.The threats differ widely across geographical regions, both in occurrence and severity.Threats that pose the highest likelihood of directly endangering human survival such as water and food shortage, contaminated drinking water, waterborne diseases, water-related wars and floods are highlighted in red in the color gradient.

Figure 5 .
Figure 5. Treatment strategies of lake health issues.In the best case, lake health issues can be prevented by, for example, nature conservation efforts.Independent of such efforts, lakes need to be screened for health issues on a regular basis, which might be done by interpreting satellite images, using high-frequency sensor technology, taking water and/or sediment samples etc. From those screening data, lake health issues need to be identified at an early stage so that cost-efficient treatments can be started.Whenever treatments are started, there is an urgent need to follow-up the progress of the treatments and to adjust the treatments if necessary.
GAW acknowledges financial support for this study from the Swedish Research Council (Grant 2020-03222) and Swedish Research Council for Environment, Agricultural Sciences and Spatial Planning (FORMAS; Grant 2020-01091), BL and HPG from the German Science Foundation (DFG; project Pycnocline GR1540/37-1), MAX from the Canada's Research Chairs program, KK from the Estonian Research Council (Grant PRG 1266), RM from Alter-C (PID2020-114024GB-C32/AEI/10.13039/ 501100011033), RLN from the IGB Senior Research Fellows Program, FS from IAI-CONICET, PICT-2020-Serie A-00548, and PGI Piccolo UNS, SAW from the New Zealand Ministry of Business, Innovation and Employment research programme-Our lakes, Our future (CAWX2305), JH and MCPM Ma.Cristina Paule-Mercado from TAČR/Norway Grant (No. TO01000202) and from Czech Science Foundation (No. 22-33245S), DPH from the Australian Research Council (DP210102575), and YZ from the National Natural Science Foundation of China(42322104).We are thankful to the reviewers who provided very constructive comments.
dimictic lakes, or it can be absent in meromictic lakes.With climate change, some lakes have started to shift to a new category of lake circulatory, for example, permanently ice-covered meromictic lakes in the Arctic can nowadays undergo circulation as ice cover is lost WEYHENMEYER ET AL. in