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Review

An Invisible Threat to Natural Heritage: Examples of Large Protected Areas with Hg-Enriched Freshwater Environments

by
Anna V. Mikhailenko
1 and
Dmitry A. Ruban
2,*
1
Department of Physical Geography, Ecology, and Nature Protection, Institute of Earth Sciences, Southern Federal University, Zorge Street 40, Rostov-on-Don 344090, Russia
2
Department of Management Technology in Tourism Industry, Institute of Tourism, Service and Creative Industries, Southern Federal University, 23-ja Linija Street 43, Rostov-on-Don 344019, Russia
*
Author to whom correspondence should be addressed.
Heritage 2025, 8(9), 384; https://doi.org/10.3390/heritage8090384
Submission received: 7 July 2025 / Revised: 9 August 2025 / Accepted: 14 September 2025 / Published: 16 September 2025
(This article belongs to the Section Biological and Natural Heritage)
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Abstract

Freshwater environments of large protected areas such as national parks and biosphere reserves concentrate a significant amount of natural heritage. An active release of mercury (Hg) to the global environment may challenge the state of this heritage. The present work synthesizes tentatively the information on Hg-enrichment in freshwater environments of large protected areas. A major bibliographical database was used to find the related literature (articles in international journals), which then was filtered to leave only the most relevant sources. Their content was analyzed to extract the necessary information. This bibliographical survey permitted us to find a few dozen examples of protected areas with freshwater environments enriched in mercury and methylmercury. These areas are present in the different parts of the world, and most commonly the Americas. The researchers paid more attention to mercury in biota than in water and sediments. The reported factors of Hg-enrichment differ, with the prevalence of those anthropogenic. The role of volcanism and long-distance dispersal of mercury by air and water is also significant. Interpreting the examples faces various uncertainties, but it is generally clear that Hg-enrichment can be regarded as a potential threat to natural heritage of protected areas on the global scale. It is proposed that Hg-hotspots (e.g., in Nova Scotia in Canada and Patagonia in Argentina) are rare phenomena constituting a new category of heritage. This interpretation extends the vision of the overall natural heritage of national parks and biosphere reserves. Several recommendations to natural heritage management in large protected areas with Hg-enriched freshwater environments are specified.

1. Introduction

Natural heritage includes both abiotic and biotic heritage elements. The value of this heritage to the contemporary civilization and, particularly, its utility to the imperatives of sustainability make urgent its efficient conservation. The related ideas were developed by Groves et al. [1], Reyes-Fornet et al. [2], J. Zhang et al. [3], Z. Zhang et al. [4], and Zhao et al. [5]. Apparently, the broad public attention to natural heritage enhances conservation programs, rational usage of natural resources, and territorial environmental management and planning; moreover, this attention stimulates more correct perception of human–environment relationships and interactions, which is important for pro-environmental thinking and behavior. One should add the relevance of natural heritage to the achievement of several United Nations Sustainable Development Goals [6,7,8]. Indeed, the knowledge of natural heritage requires a regular reconsideration due to the massive accumulation of new lines of evidence.
Large protected areas such as national parks or biosphere reserves provide effective organizational, socio-economic, and legal approaches to natural heritage conservation and management. The Mornington Peninsula and Westernport Biosphere Reserve (Australia) gives a bold example of the monitoring program development, which is essential to maintain the state of the local natural heritage [9]. The previous research in the Tatra National Park (Slovakia) indicated the very high economic value of its natural heritage [10]. Nonetheless, the efficacy of protected areas, the exact nomenclatures and regimes of which differ between the countries, is limited to their boundaries and (in ideal cases) vicinities (e.g., buffer zones). Various anthropogenic influences on natural heritage located outside protected areas are much stronger. Moreover, these influences may have serious threats to natural heritage within protected areas due to the large-scale continuity of environmental processes. For instance, air and water pollution outside a protected area would result to a delivery of dangerous substances to the latter, and it would be difficult (if possible) to prevent these secondary effects. Moreover, such external negative influences may be almost invisible (but not less stressful) compared to those, leading to sharp changes in animal populations or forested areas. A typical example can be found in the Burabay State National Natural Park (Kazakhstan), which possesses rich natural heritage attracting crowds of tourists: the moss biomonitoring studies revealed pollution by heavy metals to levels risky to human health, which is associated with regional mining activity [11].
The knowledge of heavy metal enrichment and, particularly, anthropogenic pollution in large protected areas was presented in numerous publications, and, thus, it requires systematization and regular update. It is unlikely that a single review paper or even book can achieve these highly ambitious tasks, and, thus, synthetic studies should be more specific. On the one hand, these studies can focus on particular substances, and, on the other hand, they can deal with particular environments of protected areas (e.g., freshwater, marine, coastal environments). The present work deals with mercury (Hg), which is dangerous to many organisms and human health even in small amounts [12,13,14,15,16]; it is actively cycled in the environment and accumulated through a food chain [17,18,19]. Presently, this pollutant requires special attention due to the economically driven increase in its use in gold mining (artisanal and small-scale extractive activities contribute substantially, and large-scale production can also release significant amounts of mercury to the environment) [20,21,22,23,24] and the global-scale air dispersal [25,26,27]. Regarding these reasons, one may suppose that even anthropogenically undisturbed natural heritage in remote and pristine regions can be prone to Hg-enrichment.
The other focus of the present work is freshwater environments. These are not only common in large protected areas but often determine the very presence of natural heritage (e.g., unique bird species or plant communities). Moreover, freshwater objects such as lakes and streams are themselves valuable abiotic heritage elements. The related theoretical developments can be found, particularly, in the seminal work by Abell et al. [28]. Two bold examples of the relevance of freshwater environments to the goals of nature protection and biological conservation are linked to Yangtze finless porpoise [29] and maintaining freshwater species diversity in Europe [30]. Indeed, some pitfalls and failures have become unavoidable [31,32,33], and they stress the need for some improvements in the creation and the management of protected areas with freshwater environments.
The objective of the present work is to characterize the contemporary Hg-enrichment of freshwater environments in large protected areas using examples from the scientific literature. Methylmercury is also considered where reported in the literature. The bibliographical survey carried out permitted finding many examples, which can be employed for making some general judgments of the reviewed phenomenon. A summary of these examples can also be used as a reference, i.e., for finding the initially dispersed information in one place. It should be stressed that the work does not aim at reviewing the entire problem of Hg-enrichment and anthropogenic pollution in freshwater environments of large protected areas, which would require a lot of information, including that unpublished and/or contained in various reports and the other “grey” literature. The actual purpose of the paper is to summarize some previous research findings presented to the international research audience. The reviewed examples can stimulate conceptualization and facilitate the understanding of natural heritage in large protected areas.

2. Literature Selection Principles

The present review presents a tentative synthesis of the information on Hg-enriched freshwater environments in large protected areas, and it focuses on finding examples of this phenomenon from across the globe in the scientific literature. The latter was selected in three steps (Figure 1). The PRISMA guidelines [34] were taken into account. One should note that various terminological and conceptual limitations considered below allowed creation of a representative, not comprehensive, dataset, which, nonetheless, seems to be enough for the purpose of the pioneering synthesis.
The first step was a search in the major bibliographical database “Scopus” known for the extensive coverage of the international environmental research outlets and, first of all, the leading scientific journals. Some other specialists also preferred this database in their reviews of environmental themes [35,36]. The search formula was designed to consider, on the one hand, both mercury and methylmercury and, on the other hand, some principal and most common large protected areas such as national parks and biosphere reserves (Figure 1). The search was carried out at the end of June 2025, and, thus, the completeness of the bibliographical information in the database is limited to this time. The only literature sources published in the past ten years were considered to exclude the outdated information about mercury pollution.
The next two steps served to specify the most relevant sources, and they both were performed “manually”. The second step was an initial filtering. Titles and abstracts of the literature sources found with “Scopus” were checked to select only those that presented original results of empirical research, in particular, of protected areas, and were published as journal articles (Figure 1). Importantly, it was decided to select the sources dealing with freshwater environments at this step (not earlier) so as not to occasionally miss some sources due to the very different naming of such environments in the scientific literature (especially when articles were written by the authors whose native language was not English). The initial tests of the “manual” check indicated that this was more efficient in sorting out the sources dealing with the other types of environments.
The most challenging was the third step, when the full texts of the literature sources were checked with attention (Figure 1). The main purposes were (a) the exclusion of the remaining thematically irrelevant sources and (b) the selection of only those sources where Hg-enrichment was considered. The latter was difficult to conduct due to several limitations. First, some (if not many) authors did not make definite conclusions in their articles about the presence/absence of Hg-enrichment. Second, the criteria for registering the latter differed between the studies depending on local geochemical setting formed under the influence of natural and anthropogenic processes (present and historical), officially established norms (law concentrations), and particular research tasks. Interestingly, some specialists from one country chose law concentrations of the other country or proposed the own criteria. Third, Hg-enrichment is a whole phenomenon, but it can be registered sometimes for only some components of freshwater environments (biota, sediments, water) due to the research purposes or the absence of enrichment in other components. In order to overcome the noted limitations, the final selection of the literature was performed as follows (Figure 1). An in-depth examination of the full texts of all sources selected at the second step permits finding the authors’ notions (if even brief) that could be understood as indications on anthropogenic or natural Hg-enrichment. Special attention was paid to how the reported mercury concentrations were described in the text. The works, where they were interpreted in the terms of enrichment, pollution, danger/threat, or significant risks, were left for the present analysis (the above-mentioned terms differ essentially, but their use by the authors in relation to the reported mercury concentrations enhances the selection of the relevant literature). In other words, the third step dealt with the narratives of Hg-enrichments in the scientific literature. Indeed, this step was subjective to a certain degree, but this seemed to be unavoidable regarding the representation of the reviewed phenomenon in the scientific literature. Moreover, the work aims at finding only examples of Hg-enriched freshwater environments in large protected areas, not a comprehensive synthesis of the related information (presently, the latter seems to be an unrealistic task).
A provisional term “notable concentration” was employed in this paper. This is not a common term in environmental geochemistry, but it is suitable to the present work to signify those concentrations that deserve attention when the problem of Hg-enrichment in protected areas is considered. In the literature, mercury/methylmercury concentrations in water, sediments, and biota of freshwater environments were reported, and, thus, these three major components of freshwater environments were addressed. Importantly, the present work did not establish any strict criteria for what a notable concentration of mercury or methylmercury is. The notable nature of these concentrations results chiefly from the context in which they were treated in the original texts of the selected sources. If one selected literature source dealt with two or all three components, only that with notable concentrations was considered. Indeed, such a procedure (if even very careful) can be only provisional, but permits us to find the examples for a subsequent discussion.
The selection of the most relevant literature sources, i.e., dealing with Hg-enriched freshwater environments in large protected areas, permitted us to extract and to systematize various information from these sources. First of all, this information was summarized using protected areas and components of freshwater environments (water, sediments, and biota). Mercury concentrations were converted to the same measurement units for better comprehension. The geographical distribution of the considered protected areas in the world’s space was registered. Then, attention was paid to the factors of mercury enrichment and delivery to the considered protected areas. The above-mentioned procedures formed a new foundation for a subsequent discussion of the addressed environmental phenomenon in the terms of natural heritage.

3. Results: Found Examples and Tentative Synthesis of the Extracted Information

The present work deals with 32 sources containing the information about Hg-enriched freshwater environments of large protected areas. Such a limited number of the sample (Figure 1) is not surprising because the considered environmental phenomenon is rather specific, whereas the related investigations are difficult to carry out and, thus, rare.

3.1. Geographical Pattern

The collected literature sources give examples of Hg-enrichment in freshwater environments in about three dozens of protected areas (Table 1). These are chiefly national parks and biosphere reserves. They occur in different countries and represent different geographical domains, including contrasting territories such as the Alps, Central Africa, and Russian Siberia (Table 1).
Importantly, the large protected areas with Hg-enriched freshwater environments are found in the different parts of the world (Figure 2). They are most abundant in the Americas, where they were examined by the different research teams (Table 1). These areas are distributed rather evenly and, thus, situated in a broad spectrum of the physical geographical settings. The relative abundance of such areas in Africa is explained by the wide geographical scope of one research project [38]. A bit surprising is the scarcity of the large protected areas with freshwater environments polluted by mercury in Eurasia and their absence in Australia. On the one hand, these parts of the world host numerous protected areas, and, on the other hand, they possess strong research centers. Apparently, the registered pattern can be explained by either a weaker Hg-enrichment or the specifics of interests of research communities (see also the discussion below). The latter explanation is hypothetically more realistic taking into account the high degree of the overall mercury pollution, at least, in Asia [68]. Nonetheless, the considered examples (Figure 2) seem to be enough to realize that the Hg-enrichment in freshwater environments of protected areas can happen in many places on Earth. The causes differ (e.g., wide distribution of gold mining, massive coal combustion, generally intensified mercury cycling), but it is evident that large protected areas are chiefly prone to Hg-enrichment passively, i.e., together with the global environment.

3.2. Water

Water enrichment in mercury can be deduced from the considered literature for only seven large protected areas (Table 2). Concentrations of not only total mercury, but also methylmercury were addressed. The former differ within a very wide range, i.e., from <1 ng/L to >10,000 ng/L. The studied water objects also differ, and include streams, lakes, and even meltwater streams as in the case of the Grand Teton National Park (USA) [45].
The geographical distribution of the large protected areas Hg-enriched water is limited to chiefly North America, with a few examples from South America and Southeast Asia (Figure 3). Two bold examples were found in the Sierra Gorda Biosphere Reserve (Mexico) and the Singalila National Park (Mexico). In the both, the concentrations measured by thousands of ng/L were registered (Table 2). The former represents several types of forests with rich biodiversity, and pollution by different substances of its territory was reported [60,69]. The latter represents unique Eastern Himalayan biomes and rich flora and fauna, and it is polluted strongly by not only mercury, but also pesticides [61,70]. Principally, this means that Hg-enrichment in water of these protected areas is one aspect of a complex anthropogenic stress.

3.3. Sediments

Sediments of water bodies in large protected areas are prone to Hg-enrichment, and several related examples were found (Table 3). The concentrations of total mercury vary and exceed several hundreds and even first thousands of ng/g. Commonly, they refer to the upper layers of bottom sediments, but elevated concentrations of mercury were also established in beach sand in the Baikal National Park (Russia) [37].
The geographical distribution of the large protected areas with Hg-enriched sediments is rather broad, and such areas are known chiefly from western South America and Eurasia, although the lines of evidence from eastern North America and South Africa also exist (Figure 4). The example with the most remarkable concentrations is the Farallones de Cali National Park (Colombia), where the maximum concentrations exceed 2000 ng/g (Table 2). This protected area represents unique vegetation and rich ecosystems of the Colombian Andes, but illegal gold mining in the nearest past affected freshwater environment and, particularly, caused mercury pollution [43].

3.4. Biota

The majority of the collected literature sources deal with Hg-enrichment of freshwater environments in large protected areas as evidenced by biota (Table 4). Notable concentrations of mercury and methylmercury in different organisms (from microorganisms to crocodiles) and different organs (e.g., brain, blood, intestine) were established. Fishes and crocodiles were among the most intensively studied organisms, although insects and birds should also be noted. Importantly, animals were studied much more frequently than plants. The examples of enrichment from the different trophic levels were collected (Table 4). The concentrations vary, and they can be compared directly only for the same organisms and same organs. Nonetheless, one should note that some concentrations are measured by thousands of ng/g (Table 4).
Geographically, the large protected areas with Hg-enriched biota concentrate in the Americas and Africa, with a single example from Europe (Figure 5). Three regions seem to be of special interest. The first of them corresponds to a part of Central Africa, where several national parks (many are located in Gabon) host crocodiles with elevated concentrations of mercury in their blood [38]. The second interesting object is the Kejimkujik National Park and Historic Site (Canada), which represents unique landscapes and biodiversity of Nova Scotia. This seems to be the protected area, where Hg-enrichment is the best studied; this national park is often treated as a mercury hotspot [47,48,49,50,51,52]. The other hotspot of this kind is located in Patagonia [55,56,58,59], where two Argentinian national parks (Los Alerces National Park and Nahuel Huapi National Park) boast notable concentrations of mercury in biotic (plants) and abiotic (water and sediments) components of the local environments (Table 2, Table 3 and Table 4).

3.5. Factors of Pollution

The literature considered for the purpose of the present work often indicates or hypothesizes the factors of Hg-enrichment of freshwater environments in large protected areas (Table 5). In some cases, these factors are mentioned too generally, i.e., as natural or anthropogenic. However, many articles provide more detailed explanations. Among them, one can note extractive activities (including artisanal and small-scale gold mining, sometimes illegal), transport (both land and water), agriculture, combustion of fuel, and volcanism; long-distance mercury transportation by air or rivers and subsequent deposition are often proposed (Table 5). Of special interest are such phenomena as cold trapping [61], when mercury is redistributed from ice and snow of elevated domains. In some cases, several factors were proposed (Table 5).
A two-folded synthesis of the information of the possible factors of Hg-enrichment can be offered. First, one should distinguish natural (e.g., from volcanism), anthropogenic (e.g., from gold mining), and complex (e.g., atmospheric deposition of mercury initially delivered from different sources) factors. It is not surprising that the anthropogenic factors of Hg-enrichment of freshwater environments in the large protected areas are the most common (Table 5). However, one should note the relative importance of natural and complex factors. They matter in strongly Hg-enriched protected areas as the Nahuel Huapi National Park (Argentina) [59] and the Singalila National Park (India) [61].
Second, it is possible to distinguish local, regional, and large-scale factors, such as fuel combustion within protected areas, delivery from mining enterprises in the vicinity of protected areas, and global (at least, quasi-global or continental-scale) air transport, respectively. The collected information (Table 5) implies that all three categories matter, but regional and large-scale factors prevail. Principally, this means that any strict control of anthropogenic disturbance within the limits of large protected areas does not prohibit their freshwater environments from Hg-enrichment. This problem was noted, particularly, by du Preez et al. [54], who studied mercury in crocodile eggs in the Kruger National Park (South Africa).

4. Discussion: Applications to Natural Heritage

4.1. Threat to Natural Heritage: Principal Questions and Research Biases

The synthesized information on Hg-enriched freshwater environments of several large protected areas raises questions of a threat to natural heritage. If these areas are essential to natural heritage conservation, Hg-enrichment is a direct indication of risks to this heritage. Such risks seem to be especially high due to the anthropogenic pollution of water objects or freshwater biota, which are among the common elements of this heritage. Two principal questions are about the magnitude of this Hg-enrichment and its spatial extent. Although the present review deals with only the selected examples from the scientific literature (Figure 1), the amount of the collected information seems to be enough to give tentative answers.
The concentrations of total mercury and methylmercury vary within broad ranges in the considered entity of the protected areas (Table 2, Table 3 and Table 4). They reach the relatively high levels in biota in a bigger number of areas than in sediments and water, although the examples of strong enrichment are found for all three components. Apparently, Hg-enrichment becomes a serious threat in only some areas, and it affects commonly organisms, and, thus, becomes a factor of stress to biodiversity with its high heritage value. These interpretations imply that the general magnitude of the threat is apparently moderate, but the associated risks cannot be ignored.
The examples of the large protected areas with Hg-enriched freshwater environments demonstrate a worldwide distribution (Figure 1). At the first glance, this implies the global extent of the related threat. However, such an interpretation should be performed with three cautions. First, the number of terrestrial protected areas exceeds 175 × 103 [71], and even national parks and biosphere reserves are measured by thousands. This means that the considered examples of Hg-enrichment (Table 1) refer to a tiny little portion of them. Second, there are large protected areas without mercury pollution. For instance, the study of fishes from the Tablas de Daimiel National Park (Spain) by Fernández-Trujillo et al. [72] indicated the absence of threats related to mercury. Moreover, national parks were considered by some researchers as pristine areas, and mercury concentrations from there were treated as background—a typical example is the study by Tomiyasu et al. [73]. Third, one should note that Hg-enrichment in some protected areas is natural or complex (Table 5). This means that it can be related to the long-existing peculiarities of the local environments, and, thus, the elevated concentrations are not anomalous on the small scale. If so, local geochemical processes may remain to be balanced, and biota has adapted to such an enrichment. In other words, Hg-enrichment does not necessary mean a threat as in the case of the strong anthropogenic pollution from mining.
The above-mentioned cautions mean that the reported Hg-enrichment of freshwater environments in the large protected areas may reflect a discrete series of local-scale phenomena, i.e., the true spatial extent of the threat may be limited. More definite judgments are impossible until measurements of mercury concentrations in the bigger number of large protected areas with freshwater environments are made. The outcomes of these measurements will have to be compared to the actual knowledge of heterogeneous distribution and deposition of mercury in the world’s space [74]. Evidently, when Hg-enrichment is absent or minimal, there is no need for special research projects focused on mercury in such protected areas. Thus, the bibliographical survey cannot answer the question about the ratio between the freshwater environments protected with Hg-enrichment and those without this enrichment. Nonetheless, the registered geographical patterns (Figure 2) can be understood so that the threat of Hg-enrichment to natural heritage represented in large protected areas has become real on the global scale (if even discrete, not ubiquitous), and the related risks are significant.
The other important question is about the sources (factors) of pollution. Although the collected information specifies some of them (Table 5), the relative importance of these sources and their relevance to the other large protected areas remain unclear. A separate investigation would be required to clarify these aspects, and it cannot be sure that such an investigation would be realistic regarding the present state of the knowledge. Nonetheless, it is clear that many considered protected areas experience external pollution, i.e., the latter is either sourced outside of these areas or related to large-scale (territorial, regional, even global) processes. This proves the idea (see above) that protected areas can become affected due to a general Hg-enrichment of the global environment.
Indeed, the present synthesis of the knowledge reveals significant biases and gaps in the knowledge of Hg-enriched freshwater environments of large protected areas. Three main issues are the geographical bias (research focused on particular domains that missed information from some parts of the world—Table 1 and Figure 2), the overemphasis on biota (if even organisms are really more affected than water and sediments, the prevalence of the works focusing on only biota complicates objective judgments), and the different understanding of what Hg-enrichment is and how it corresponds to the anthropogenic pollution. The latter is especially important because the lines of evidence extracted from the collected literature (Table 2, Table 3 and Table 4) are highly heterogeneous, and the only tentative and qualitative inferences are possible. The high value of natural heritage from large protected areas, the richness of freshwater biodiversity, the complexity (and, thus, fragility) of freshwater ecosystems, and the active cycling of mercury in freshwater ecosystems would require development and application of stricter guidelines and norms for the registration of Hg-enrichment relative to non-protected areas. Undoubtedly, the situations with water in the Singalila National Park [61] and crocodiles in the Gabonese national parks [38] are alarming, but the true magnitude of the threat in many other large protected areas is difficult to realize.

4.2. Geographical Biases?

The distribution of the considered large protected areas with Hg-enrichment (Figure 2) reveals some geographical irregularities. For instance, mercury has actively been mined in the circum-Mediterranean region and, particularly, in such countries as Spain, Italy, and Slovenia. There, environmental pollution by this metal or, at least, its risks were already discussed [75,76,77,78,79,80,81,82]. Some extractive activities has been stopped in the nearest past, but abandoned mining sites have remained dangerous to certain degree. However, the considered literature sources do not provide evidence of the related Hg-enrichment of the large protected areas (Table 1). This pattern is intriguing and, thus, needs special discussion.
Three explanations of the above-mentioned pattern can be proposed. First, the present review focuses on only some examples established by the bibliographical survey, which means that the entity of the protected areas is not complete by definition (in other words, making this entity fully complete is beyond the scope of the analysis and, possibly, even beyond the present potential of bibliographical surveys). Second, it cannot be excluded that Hg-enrichment of freshwater environments in the large protected areas of the circum-Mediterranean region has been reflected in the scientific literary with certain biases, i.e., it has been understudied. Third, one should note that the protected areas can initially be planned so to avoid polluted plots or the regime of their functioning mitigates influences of the local anthropogenic pollution. This means that the irregularities in the distribution of the considered large protected areas with Hg-enriched freshwater environments (Figure 2) can either reflect unintentional geographical biases or not.
Although these three proposals are mere hypotheses, two lines of evidence can be taken into account. First, it has been known that Hg-mining and the related incidents affect environment of some protected areas as evidenced from the Doñana National Park (Spain) [83]. However, a tentative search for the relevant literature (not necessary on freshwater environments) with the bibliographical database “Scopus” did not permit to find a large number of sources for the circum-Mediterranean region or beyond. Second, the example of the Tablas de Daimiel National Park (Spain) shows that even the relatively close location of the famous Almadén Hg-mining district does not elevate mercury concentrations in fish above the maximum residue levels [72]. The other example from the Somiedo Natural Reserve (Spain) demonstrates that an abandoned mercury mine can remain a source of Hg-pollution of soils, whereas water and sediments do not exhibit high concentrations [84]. These lines of evidence make urgent serious attention to the second and third proposals presented above. Indeed, further investigations (also based on the “grey” literature) are necessary to fill the knowledge gaps or to postulate the true irregularities of Hg-enrichment in large protected areas on the global scale.

4.3. Heritage-Related Inferences

Shifting the vision of Hg-enriched freshwater environments of protected areas from a pure ecological to heritage perspectives advances conceptualization of the related threat. First of all, natural heritage means that natural objects have their own values, and they are of utmost importance to the society. If so, the considered threat diminishes such values (down to zero) and stresses, even disrupts nature–society connections. The latter are highly complex and essential to protected areas [85,86,87,88,89]. Freshwater environments form pillars of territorial environmental and socio-economical sustainability due to their direct relevance to biodiversity [90,91], as well as tourism and recreation [92,93]. Generally, the threat has a socio-economical expression, which makes it not only more appealing to the society, but also easier to present to policy-makers. Protected areas are essential for the world’s biodiversity conservation [94,95], and, hypothetically, representing the need in this conservation in the terms of natural heritage may improve its acceptance by society and policy-makers. It should be added that Hg-enrichment (especially when it occurs thanks to anthropogenic pollution) is an invisible threat, and attracting attention to it requires special efforts. A solution of this task can be facilitated by the explanations of this threat in the terms of heritage.
The collected literature sources imply the existence of two territories that have attracted a significant research attention (Table 1). The first of them corresponds to the Kejimkujik National Park and Historic Site in Nova Scotia (Canada), and the second hosts the Los Alerces and Nahuel Huapi national parks in Patagonia (Argentina). The both were considered as mercury hotspots [48,59]. Their origin is linked to the natural and complex factors, and the anthropogenic factors also contributed (Table 5). Although (methyl)mercury hotspots can be linked to various aquatic environments, including mangroves [96] and aquatic wetlands [97], an additional search in the bibliographical database “Scopus” showed that the areas of Nova Scotia and Patagonia are among a few of the number of localities that can be treated as mercury hotspots [48,59,98,99,100,101]. If so, they represent highly peculiar, almost unique phenomena. Therefore, it is reasonable to understand them as heritage with the overall natural heritage of the large protected areas. This proposal echoes the earlier ideas of sites/areas serving as natural laboratories/test sites for studying (bio)geochemical phenomena [102,103,104,105,106]. Hg-enrichment can simultaneous be a threat to natural heritage and part of this heritage.
Conceptually, the initial attention to abiotic (e.g., peculiar lakes or landforms) and biotic (e.g., rare birds or plants) heritage elements leads to the creation of protected areas. Their detailed examination may result in finding unusual (bio)geochemical phenomena extending the overall heritage value of national parks or biosphere reserves (Figure 6). Indeed, this scheme functions with a single condition of the diverse and intense research activities in large protection areas. Particularly, scientists need to broaden their interests in potential environmental threats to natural heritage. A further growth of the knowledge of Hg-enriched freshwater environments of large protected areas can trigger a shift from the biodiversity- and landscape-centered vision of natural heritage to its more complex vision.

4.4. Managerial Perspectives and Recommendations

Mitigation of Hg-enrichment of freshwater environments of large protected areas is necessary when it is caused by local or distant anthropogenic pollution. As explained above, mercury easily crosses boundaries of protected areas and enters their freshwater environments. Realizing the scale of the related threat and taking into account the global cycling of mercury [74], managers of protected areas should act on three levels (Figure 7). First of all, they have to focus locally via an implementation of monitoring strategies and prohibition of the factors of pollution within national parks and biosphere reserves (i.e., pollution from mining, transport, tourism, etc.). A higher, regional level of action requires a deeper integration of the protected area management into the large-scale governance that should establish mechanisms of anthropogenic pollution mitigation with territorial continuity. The urgency of freshwater (and not only) environmental conservation has to be considered seriously in entire regions (provinces, states), not only in protected areas. General greening of the regional governance and its imperatives would help.
The highest level of actions is international. If anthropogenic mercury pollution affects freshwater environment of any given protected area, the latter can become a locality (test site) for international research. The present tentative synthesis of the knowledge shows that this research is not abundant in the world (Table 1). The explanations are methodological complexities and dependence on specific knowledge. If so, international collaborations would help. Investigations should focus not only Hg-enrichment itself, but also development and testing of techniques and equipment for anthropogenic pollution mitigation and environmental remediation. Special attention should be paid to the development and the international unification of guidelines and norms for the correct understanding of mercury and methylmercury concentrations in the terms of threats, risks, and dangers. Indeed, stricter requirements should be established for protected areas with their unique natural heritage. A creation and expansion of an international, permanently updating database of mercury concentrations in freshwater environments of the large protected areas will help significantly. The creation of the above-mentioned test sites will permit to develop educational programs at the local, regional, and international scales, which are essential to increase the awareness of threats to natural heritage.
The fact that Hg-enrichment of freshwater environments in some large protected areas can be unique (see above) means that heritagizing the related phenomena should also be among managerial tasks. It is reasonable to recommend a support of investigations linked to socio-economic estimates of the negative effects of Hg-enrichment (especially due to anthropogenic pollution) on heritage elements of protected areas. This support can be provided by governments, professional associations, and various research funders.

5. Conclusions

Large protected areas possess precious elements of natural heritage. However, their status and regime does not permit them to avoid Hg-enrichment (often in the form of anthropogenic pollution). On the one hand, mercury is often delivered from the outside with air or rivers. On the other hand, anthropogenic mercury pollution is an invisible threat that cannot be observed in the same manner as more common threats such as agriculture-driven deforestation or illegal hunting. The examples considered in the present review permit making five general conclusions, as follows.
First, notable concentrations of mercury and methylmercury in biota and less commonly water and sediments were reported from several dozens of large protected areas situated in the different parts of the world.
Second, the collected literature knowledge demonstrates significant uncertainty and biases in the understanding of Hg-enrichment of freshwater environments as a threat to natural heritage in national parks and biosphere reserves. Nonetheless, this threat and its global distribution should be considered seriously.
Third, the considered examples imply that the anthropogenic factors of Hg-enrichment prevail, but the natural and complex factors are also rather common. The long-distance transportation of mercury by air and rivers is responsible for Hg-enrichment in many cases.
Fourth, a few large protected areas host mercury hotspots, which are rare phenomena that deserve heritagizing.
Fifth, mitigating anthropogenic mercury pollution of freshwater environments in large protected areas requires specific, multi-level actions and international cooperation. Putting this environmental task into the heritage perspective seems to be helpful.
The present paper shows that the consideration of even selected examples of Hg-enrichment of freshwater environments can be important to realize its relevance to natural heritage. Indeed, this work has limitations. First of all, it deals with only selected examples reported in the scientific literature. There may be other lines of evidence, which were (a) not published, (b) appeared in journals or books not covered by the major bibliographical databases (or communicated in a form, which cannot be registered by the employed search procedures), or (c) presented in the so-called “grey literature”. Their omission is unimportant for the present, tentative review, but attention should be paid to them in future investigations. The other limitation is an uncertainty in the understanding of Hg-enrichment by in the international research community. Apparently, this limitation cannot be addressed fully in the contemporary science. One should also note a need for investigations aimed at the understanding of natural and anthropogenic Hg-enrichment and heritage value of freshwater environments by various stakeholders linked to protected area management (managers, policy-makers, local communities). Generally, there is a large space for further theoretical, methodological, and empirical research projects.

Author Contributions

Conceptualization, A.V.M. and D.A.R.; investigation, A.V.M. and D.A.R.; writing—original draft preparation, investigation, A.V.M. and D.A.R.; visualization, D.A.R.; project administration, A.V.M. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Data Availability Statement

All data are provided directly in the main paper.

Acknowledgments

The authors thank deeply several researchers who kindly helped with the literature or explained some places in their works.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Procedures of the literature selection for the purpose of the present work. See the text for the explanation of the tentatively used term “notable concentration”. It should be stressed that the deep filtering step focused more on how Hg-enrichment was treated in the texts of the selected sources than on the concentrations reported there.
Figure 1. Procedures of the literature selection for the purpose of the present work. See the text for the explanation of the tentatively used term “notable concentration”. It should be stressed that the deep filtering step focused more on how Hg-enrichment was treated in the texts of the selected sources than on the concentrations reported there.
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Figure 2. Global occurrence of large protected areas with notable mercury concentrations in freshwater environments. See Table 1 for IDs.
Figure 2. Global occurrence of large protected areas with notable mercury concentrations in freshwater environments. See Table 1 for IDs.
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Figure 3. Global occurrence of large protected areas with notable mercury concentrations in the water component of freshwater environments. See Table 1 for IDs. Bold font marks the examples of strong Hg-enrichment considered in the text.
Figure 3. Global occurrence of large protected areas with notable mercury concentrations in the water component of freshwater environments. See Table 1 for IDs. Bold font marks the examples of strong Hg-enrichment considered in the text.
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Figure 4. Global occurrence of large protected areas with notable mercury concentrations in sediments of freshwater environments. See Table 1 for IDs. Bold font marks the examples of strong Hg-enrichment considered in the text.
Figure 4. Global occurrence of large protected areas with notable mercury concentrations in sediments of freshwater environments. See Table 1 for IDs. Bold font marks the examples of strong Hg-enrichment considered in the text.
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Figure 5. Global occurrence of large protected areas with notable mercury concentrations in biota of freshwater environments. See Table 1 for IDs. Dashed lines delineate the examples of strong Hg-enrichment considered in the text.
Figure 5. Global occurrence of large protected areas with notable mercury concentrations in biota of freshwater environments. See Table 1 for IDs. Dashed lines delineate the examples of strong Hg-enrichment considered in the text.
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Figure 6. The proposed idea of heritagized mercury hotspots.
Figure 6. The proposed idea of heritagized mercury hotspots.
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Figure 7. The proposed multi-level managerial actions aimed at mitigation of the considered threat to natural heritage.
Figure 7. The proposed multi-level managerial actions aimed at mitigation of the considered threat to natural heritage.
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Table 1. Large protected areas considered in the present work.
Table 1. Large protected areas considered in the present work.
IDProtected AreaCountryArea, km2Geographical DomainConsidered Literature on Mercury in Freshwater Environments
B1Baikal (Pribaikalskiy) National ParkRussia4173Russian Siberia[37]
B2Bui National ParkGhana1821West African savannah[38]
C1Cajas National ParkEcuador285Andes[39]
C2Chimanimani national parks *Zimbabwe-Southern Africa[40,41]
E1Everglades National ParkUSA6105Florida[42]
F1Farallones de Cali National ParkColombia1500Andes[43]
G1Giant Panda National Park **China-Qinling Mountains[44]
G2Grand Teton National ParkUSA1255Rocky Mountains[45]
I1Isle Royale National ParkUSA2314Great Lakes[46]
I2Ivindo National ParkGabon3000Central Africa[38]
K1Kejimkujik National Park and Historic SiteCanada404Nova Scotia[47,48,49,50,51,52]
K2Kidepo Valley National ParkUganda1442Central Africa[38]
K3Krka National ParkCroatia109Western Balkans[53]
K4Kruger National ParkSouth Africa19,485Southern Africa[54]
L1Loango National ParkGabon1550Central Africa[38]
L2Lope National ParkGabon4912Central Africa[38]
L3Los Alerces National ParkArgentina2596Patagonia[55,56]
M1Manu National ParkPeru17,163Eastern Andes–Western Amazonia[20]
M2Montes Azules Biosphere ReserveMexico3312Chiapas[57]
M3Moukalaba-Doudou National ParkGabon4500Central Africa[38]
N1Nahuel Huapi National ParkArgentina7050Patagonia[56,58,59]
P1Plateau Bateke National ParkGabon2034Central Africa[38]
S1Sierra Gorda Biosphere ReserveMexico3836Central Mexico[60]
S2Singalila National ParkIndia204Darjeeling (West Bengal)[61]
S3Southwest Alaska national parks *USA-Alaska[62]
T1Tagus Estuary Natural ReservePortugal142Iberian Peninsula[63]
T2Triglav National ParkSlovenia880Alps[64]
V1Volga-Akhtuba Natural ParkRussia1539Caspian Region[65]
V2Voyageurs National ParkUSA883West of Great Lakes (Minnesota)[46]
W1Wood Buffalo National ParkCanada44,741Boreal Plains[66]
Y1Yabotí Biosphere ReserveArgentina2216Misiones’s forests[67]
Notes: * these objects include several protected areas, for which the information from the considered sources cannot be separated; ** this is a whole chain of protected areas, and the information from the considered source addresses the only part of this chain.
Table 2. Examples of large protected areas with notable Hg concentrations in water. See Table 1 for IDs. The concentrations are given for a general reference, and they were not checked with any strict criteria (e.g., law concentrations prescribed nationally).
Table 2. Examples of large protected areas with notable Hg concentrations in water. See Table 1 for IDs. The concentrations are given for a general reference, and they were not checked with any strict criteria (e.g., law concentrations prescribed nationally).
IDConcentrations *, **Explanations ***Literature
E1Hg = 0.59–1.08 ng/L
MeHg = 0.05–0.28 ng/L		Different water objects; range reflects differences in mean values for 11 years between geographic regions[42]
G1Hg = 30–250 ng/LStreams; range reflects differences in mean values between areas[44]
G2Hg ~ 0.2–0.6 ng/LMeltwater and streams; range reflects differences in values between different types of water objects, different localities, and different times of measurement[45]
K1MeHg = 0.05–0.46 ng/gLakes and stream; range reflects differences in values between samples and sites[47]
Hg = 1–2.28 ng/L
MeHg = 0.03–0.09 ng/L		Lakes; range reflects differences in average values between lakes[50]
N1Hg = 40.7–363 ng/L
MeHg = 0.01–0.30 ng/L		Streams; range reflects differences in values between samples[58]
Hg = 16.8–268 ng/L
MeHg = 0.01–0.16 ng/L		Lakes; range reflects differences in values between samples
Hg = 1.73–61.5 ng/LLakes; range reflects differences in values between samples[59]
S1Hg = 7000 ng/LTap water; maximum value[60]
S2Hg = 10,000–16,000 ng/LLakes, man-made water body; range reflects differences in values between sampled water objects[61]
Notes: * measurement units are converted to ng/L for consistency; ** Hg—total mercury, MeHg—methylmercury; *** see sources for more information on what are these concentrations and how they should be understood.
Table 3. Examples of large protected areas with notable Hg concentrations in sediments of water objects. See Table 1 for IDs. The concentrations are given for a general reference, and they were not checked with any strict criteria (e.g., law concentrations prescribed nationally).
Table 3. Examples of large protected areas with notable Hg concentrations in sediments of water objects. See Table 1 for IDs. The concentrations are given for a general reference, and they were not checked with any strict criteria (e.g., law concentrations prescribed nationally).
IDConcentrations *, **Explanations ***Literature
B1Hg = 10–25 ng/gIsland beach sand; range reflects differences in values between samples[37]
C1Hg ~ 150–200 ng/gUpper layers of lake sediments; range reflects differences in values between lakes[39]
C2Hg = 40 ng/gStream sediments; mean value[41]
F1Hg = 5.3–2200 ng/gStream sediments; range reflects differences in values between samples[43]
K1MeHg = 0.02–28.94 ng/gLake and stream sediments; range reflects differences in values between samples and sites[47]
K3Hg = 36 ng/gLake sediments; mean value[53]
L3Hg = 81 ng/gUpper layers of lake sediments; maximum value[55]
T1Hg < 1000 ng/g
MeHg < 4.4 ng/g		Estuary sediments; common values[63]
T2Hg = 30 ng/gLake sediments; mean value[64]
V1Hg = 180–750 ng/gShallow channel sediments; range reflects differences in values between channels[65]
Notes: * measurement units are converted to ng/g for consistency; ** Hg—total mercury; MeHg—methylmercury; *** see sources for more information on what are these concentrations and how they should be understood.
Table 4. Examples of large protected areas with notable Hg concentrations in biota of water objects. See Table 1 for IDs. The concentrations are given for a general reference, and they were not checked with any strict criteria (e.g., law concentrations prescribed nationally).
Table 4. Examples of large protected areas with notable Hg concentrations in biota of water objects. See Table 1 for IDs. The concentrations are given for a general reference, and they were not checked with any strict criteria (e.g., law concentrations prescribed nationally).
IDConcentrations *, **Explanations ***Literature
B2Hg = 258–495 ng/gCrocodiles, blood; range reflects differences in values between samples[38]
E1MeHg = 59.8–163.5 ng/gFishes; range reflects differences in mean values for 2018 between geographic regions and species [38]
F1Hg = 0.4–42.6 ng/gFrogs, muscle tissues; range reflects differences in values between species and sites[43]
I1Hg = 1034 ng/g
MeHg = 311 ng/g		Fishes, liver; mean value[46]
I2Hg = 2025–10,162 ng/gCrocodiles, blood; range reflects differences in values between samples[38]
K1MeHg = 14.28–276.96 ng/gInsects; range reflects differences in values between samples, sites, and groups of insects[47]
MeHg = 380–2000 ng/gFishes, brain; range reflects differences in values between samples[48]
Hg = 180–2130 ng/gFishes, muscles; range reflects differences in values between samples
Hg = 310–980 ng/gFishes, liver; range reflects differences in mean values between sites
Hg = 270–340 ng/gFishes, muscles; range reflects differences in mean values between species[49]
MeHg = 40–320 ng/gInvertebrates; range reflects differences in mean values between groups of invertebrates[51]
MeHg = 40–2180 ng/gEpilithic biofilm, macroinvertebrates, zooplankton; range reflects differences in mean values between organisms[52]
K2Hg = 253–1002 ng/gCrocodiles, blood; range reflects differences in values between samples[38]
K3Hg ~ 50 ng/gFishes, intestine; approximate maximum value[53]
Hg ~ 120 ng/gParasites (acanthocephalans); approximate maximum value
K4Hg = 30–1800 ng/gCrocodiles, eggs; range reflects differences in values between samples[54]
L1Hg = 390–11228 ng/gCrocodiles, blood; range reflects differences in values between samples of different species[38]
L2Hg = 450–2328 ng/gCrocodiles, blood; range reflects differences in values between samples[38]
L3 + N1Hg = 2000–7600 ng/gAquatic plants; range reflects differences in values between samples[56]
M1Hg = 320–3370 ng/gFishes, muscle tissues; range reflects differences in mean values between sites[20]
M2Hg = 827.7 ng/gCrocodiles; mean value[57]
M3Hg = 739–4993 ng/gCrocodiles, blood; range reflects differences in values between samples[38]
P1Hg = 928–8608 ng/gCrocodiles, blood; range reflects differences in values between samples[38]
S3Hg = 101–3046 ng/gFishes, muscles; range reflects differences in values between samples[62]
V2Hg = 2922 ng/g
MeHg = 177 ng/g		Fishes, liver; mean value[46]
W1Hg = 160–1440 ng/gBirds, eggs; range reflects differences in values for 2017 between samples[66]
Y1Hg = 1340 ng/gFishes, muscles; maximum value[67]
Notes: * measurement units are converted to ng/g for consistency; ** Hg—total mercury; MeHg—methylmercury; *** see sources for more information on what are these concentrations and how they should be understood.
Table 5. Proposed sources/mechanisms of Hg delivery to freshwater environment of large protected areas. See Table 1 for IDs.
Table 5. Proposed sources/mechanisms of Hg delivery to freshwater environment of large protected areas. See Table 1 for IDs.
IDProposed Sources *Principal Factors of Pollution **Literature
NaturalAnthropogenicComplex
B1Anthropogenic + [37]
B2Natural and anthropogenic++ [38]
C1Atmospheric deposition, mining, road transport + [39]
C2Illegal gold mining + [41]
E1Atmospheric deposition +[42]
F1Illegal gold mining + [43]
G1Combustion of coal, waste, fuel + [44]
G2Atmospheric deposition +[45]
I1Atmospheric deposition +[46]
I2Natural and anthropogenic, also oil extraction, agriculture++ [38]
K1Atmospheric deposition, local gold mining ++[50,52]
K2Natural and anthropogenic++ [38]
K3Anthropogenic + [53]
K4Long-distance river transportation +[54]
L1Natural and anthropogenic, also oil extraction, agriculture++ [38]
L2Natural and anthropogenic++ [38]
L3Anthropogenic fires, volcanism++ [55]
M1Artisanal and small-scale gold mining + [20]
M2Boat transport + [57]
M3Natural and anthropogenic++ [38]
N1Volcanism+ [59]
P1Natural and anthropogenic++ [38]
S1Mercury mining + [60]
S2Atmospheric deposition, cold trapping +[61]
S3Volcanism, other natural+ [62]
T1Local industrial activity + [63]
T2Geological, agriculture and sewage leaching++ [64]
V1Anthropogenic + [65]
V2Atmospheric deposition +[46]
W1Various anthropogenic, long-distance river transportation ++[66]
Y1Anthropogenic + [67]
Notes: * sources as indicated in the considered literature (sources are not specified or specified too generally in some publications; ** interpretations for the purpose of the present work.
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Mikhailenko, A.V.; Ruban, D.A. An Invisible Threat to Natural Heritage: Examples of Large Protected Areas with Hg-Enriched Freshwater Environments. Heritage 2025, 8, 384. https://doi.org/10.3390/heritage8090384

AMA Style

Mikhailenko AV, Ruban DA. An Invisible Threat to Natural Heritage: Examples of Large Protected Areas with Hg-Enriched Freshwater Environments. Heritage. 2025; 8(9):384. https://doi.org/10.3390/heritage8090384

Chicago/Turabian Style

Mikhailenko, Anna V., and Dmitry A. Ruban. 2025. "An Invisible Threat to Natural Heritage: Examples of Large Protected Areas with Hg-Enriched Freshwater Environments" Heritage 8, no. 9: 384. https://doi.org/10.3390/heritage8090384

APA Style

Mikhailenko, A. V., & Ruban, D. A. (2025). An Invisible Threat to Natural Heritage: Examples of Large Protected Areas with Hg-Enriched Freshwater Environments. Heritage, 8(9), 384. https://doi.org/10.3390/heritage8090384

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