Mercury ( Hg ) researches in Brazilian biomes : A scientometric analysis between the years 1991 and 2018

Mercury is a metal present in natural sources and its concentration have been allocated by human activities, posing impacts in environmental matrices and organisms. In this study, we highlight the panorama of Hg researches in Brazilian biomes in the scientific literature published between 1991 and 2018. Although the appreciated attention to the mercury researches in Amazon context, there has not been an assessment of Hg research within other Brazilian biomes. With this in mind, we searched for articles in the periodic database the Thomson’s ISI Web of Science database and observed an oscillated trend of the number of publications throughout this period. The low percentage of papers performed in Pantanal (8%), Caatinga (4%), Atlantic Rainforest (4%), Pampa (3%) and Cerrado (0.36%) was contrasted to majority academic works performed in Amazon regions (83%), indicating the asymmetrical geographical distribution in Hg researches. The five top keywords of research topics were mercury, Amazon, Brazil, river, Brazilian and fish. The top authors formed 19 different clusters, with different research directions and strengths. Our data shows that Hg studies have been geographically biased. The lack of Hg studies on other Brazilian biomes may mask the extent and intensity of the impacts related to the Hg mobilization in these areas. Concerning Hg concentration is context-dependent, we emphasize the need for investment in Hg research in different tropical biomes in order to overcome spatial bias of Hg knowledge, particularly with organisms from different environmental matrices.


Introduction
Mercury (Hg) is a metal present in natural sources; however, a considerable proportion of Hg have been allocated by burning of fossil fuels, mining, and other human actions [1,2].
Mercury is found in three valence states (0, +1 and +2), and in several inorganic and organic complexes [3]. The physical-chemical properties and interactions of these species with environmental matrices determine the mechanisms of transport, transformation and removal of mercury [4]. The proportion of Hg Page: 2 www.raftpubs.com species varies in different environmental compartments [5] and may affect its bioavailability and toxicity [6]. The [Hg] in the atmosphere is appreciable due to its volatility be directly proportional to the temperature increase [7] and more than 95% of it is found in Hg 0 steam [8,9]. The largest [Hg] present in the abiotic part is in the inorganic form of Hg (II), while methylmercury (MeHg) predominates in biota [10]. Mercury is biomagnified in most food chains [11] and its methylation increases the potential for biomagnification through the trophic food chain [12,13]. Environmental Hg loadings have had significant impacts on wildlife populations and increased overall human risk population-level health [14]. Mercury studies over the last decade are considerably due to the advances basically on three major fields: new data, new models and new techniques and analytical and dynamic tools [10,[15][16][17]. Considering this, scientometric studies have been crucial to identify trends, gaps and biases in the scientific development, as well as guide future topic researches [18][19][20]. Although the welcome attention to the mercury researches in Amazon context [20] there has not been an assessment of Hg research within other Brazilian biomes. Brazil is the largest country in Latin America and houses a great variety of ecosystems ranging from grasslands and savannas to wetlands and dense tropical rainforests [21][22][23]. Considering this gap in knowledge, the great interest of Hg impacts on different environmental matrices across the country and the importance of evaluating the scientific production, we aimed to provide a panorama Hg researches in Brazilian biomes throughout the last three decades (1991 to 2018). We also aimed to answer the following questions: (i) what is the status of Hg researches in Brazilian biomes? (ii) The topic studies vary between biomes? and (iii) Are the major Hg fields related to land use intensity, economic activity or public health? Then, our study aims to contribute to a better understanding of Hg studies in Brazilian Biomes.

Materials and Methods
In order to evaluate the Hg research in Brazilian biomes, we employed a combination of bibliometric citation and content analysis techniques. We used the Thomson ISI Web of Science database (ISI WoS) to search for articles published from 1991 to 2018. The year 1991 was chosen as the initial by the fact that the abstracts were only available for articles published from 1991. First, we performed a Boolean search for articles in September of 2019 using a combination of the following terms: TI (Title) = (*mercur* OR Hg) and TS (Topic) = (Amazon*) for Amazon, (Brazil* semi arid* and Brazil* northeast*) for Caatinga, (Savanna*) for Cerrado, (Brazil* Atlantic forest*) for Atlantic Rain Forest, (Pantanal*) for Pantanal and (Paraná and Rio Grande do Sul) for Pampa in the ISI Web of Science database. The six terrestrial biomes identified in this study (Amazon, Caatinga, Cerrado, Atlantic Rainforest, Pampa and Pantanal) followed the 2004 habitat classification from the Brazilian Institute of Geography and Statistics (IBGE-acronym in Portuguese). The selected articles were included in the compilation only when it was explicitly indicated that the study was fully or partially performed within Brazilian biomes. The results were then filtered by language (English) and document type (articles) in text files organized by biomes, which were then inserted individually into the free HistCite™ software (version 9.8.24 (Philadelphia, PA, USA) for bibliometric article analysis [24]. which provided the name of the first author, the journal names, the keywords/words of the title and the abstract of each article. We also used the VOS Viewer™ software, version 1.6.8 [25], which builds network connections of scientific publications, academic journals, researchers, countries, keywords (or terms based on coauthorship), citation, and bibliographic coupling. With this software, we could find published mercury-related research in the Brazilian biomes, the most significant authors in each area and how co-authorship networks have been established; and how publications Page: 3 www.raftpubs.com and authors were grouped according to different research topics.

Authorship and co-authorship analysis in all Biomes
After a refined analysis of abstracts, 281 academic works fitted the objectives of our study in international peer reviewed journals (Web of Science-WoS). The evaluation of selected articles showed an oscillating trend with a remarkable growth in the 2018 year ( Figure 1a). About 83% of the academic works produced were related to Hg and Amazon Biome throughout this period. The 17% of the works were corresponded respectively to Pantanal (8%), Caatinga (4%), Atlantic Rainforest (4%), Pampa, (3%) and Cercado (0,36%) (Figure 1b). The number of citations of the publications reveals the leading authors and the key issues of interest in Hg research conducted in Brazil's territory, as shown in table 1. It is important to note that the 6,825 references were analyzed from the 281 articles in the database and resulted in figure 2. Page: 4 www.raftpubs.com  Page: 5 www.raftpubs.com

Keywords Analysis
There were 952 words listed in the title and abstract from analyzed articles. Following the criteria that the word should appear in at least ten publications and removing synonymous words and terms unrelated to the research topic [20,26], we had 39 words with 708 links grouped in three main topic clusters from all academic works studied ( Figure 5). The ten most used keywords ranked by frequency for each biome are listed in table 1. Considered all selected academic works, the five top keywords were respectively: mercury (254), Amazon (141), Brazil (86), river (57), Brazilian (56) and fish (55). The six most used keywords from academic works conducted in Amazon biome were very similar to the overall biomes. In contrast, the most frequent keywords in other Brazilian biome showed differences ( Table 1). The main keywords in the studies related to the Hg and Brazilian Biomes ( Figure 5) revealed three clusters: the blue group (n=8) with common terms Hg concentration, species, muscle and Madeira basin. The red group (n=18) with population, fish consumption, exposure, child and blood terms commonly used and the green cluster (n=17) which most pronounced words were sediment, water, river, soil, environment and data. The terms that linked the different groups were risk and environment. The main keywords presented in the academic works related to the research of Hg in the Amazon showed three clusters: the blue one (n=9) with species, blood, liver, muscle, turtle, and Amazon basin, as common terms. The red group (n=18) presented child, mother, community, fish consumption, and protein, and the green cluster (n=10) included river, site, water, soil and sediment ( Figure 6). The main keywords in the studies related to the search for Hg in the Caatinga (Figure 7) revealed four clusters, with different foci: the yellow cluster (n=12) with gold mining, dissolved organic matter, and other heavy metals and the most commonly used term was carbonate. The green one (n=15) showed volcanic activity and sediment and the blue cluster (n=7) aggregated the river and land use and occupation terms. The red cluster (n=10) included mercury concentration in oysters. The terms that linked the academic works in this cluster were Amazon, Brazil, contamination, pollution, atmospheric mercury and accumulation. The main key words in the academic works related to the research of Hg in the Cerrado (Figure 8) showed only one cluster with commonly used terms were those related to soil and mineral sorption. Concerning Hg in the Atlantic Forest, the main keywords showed five clusters ( Figure 9): The red cluster (n=15) encompassed fluxes, biodiversity and bioindicators; the green one (n=14) with litterfall, forest ecosystem and isotopic composition, as commonly used terms. The blue one (n=11) included atmospheric deposition, canopy, methylmercury and accumulation and the yellow one (n=7) aggregated photosynthesis and microclimate parameters while the purple one (n= 7) included land use change and ecosystem. With Pampa, the main keywords in the studies related to Hg research ( Figure 10) showed two clusters: the green cluster (n=23) included water composition and maximum depth and the red one (n=30) included basically fish species. With Pantanal, the main keywords in the studies related to the Hg research ( Figure 11) showed four clusters: the blue one (n=7) had methylmercury, methylation, soil, lake and root. The green one (n=9) aggregated fish, fish consumption, risk and mercury concentration. The red one (n=11) covered mining and mercury concentration terms and the yellow one (n=4) included mercury geochemistry, watershed pollution and reservoir contamination, as well as their association with atmospheric concentration. The term that linked the academic works in this cluster was sediment. Page: 6 www.raftpubs.com  Page: 7 www.raftpubs.com  Page: 8 www.raftpubs.com  Page: 9 www.raftpubs.com  Page: 10 www.raftpubs.com

Conclusions
Hg researches in tropical environments are crucial for a better understanding of its ecological processes in local, regional and ecosystem scale. In this sense, this comprehensive scient metric evaluation of mercury research in Brazilian biomes from 1991 to 2018 resulted in the following findings.

Findings
-Annual publications concerning mercury in the Brazilian biomes presented an oscillated trend over the years and the low number of academic works performed in Pantanal, Caatinga, Atlantic Rainforest, Pampa and Cerrado indicate the need for strategic studies in these biomes through national policies of research promotion. -Authors formed 19 different clusters, with different research directions and strengths. We also identified the top influential scholars in the context of this study, such as Bastos, W.R. and Dórea, J.G. -The five top keywords of research topics during the period considered were respectively mercury, Amazon, Brazil, river, Brazilian and fish. Integrating scientific interest in Hg publications in the Brazilian biomes, our study identified strengths and weaknesses of Hg research areas, as well as a tool for future Hg research in Brazil´s territory. Page: 11 www.raftpubs.com