Research hotspots and current challenges of lakes and reservoirs: a bibliometric analysis
Scientometrics https://doi.org/10.1007/s11192-020-03453-1 Research hotspots and current challenges of lakes and reservoirs: a bibliometric analysis Long Ho1 · Peter Goethals1 Received: December 2019 © Akadémiai Kiadó, Budapest, Hungary 2020 Abstract Despite covering a small portion of the earth’s surface, lakes and reservoirs offer enormous benefits to human society, environmental well-being, and economic welfare Previous studies have provided insights into specific subjects, yet integrated perspectives on the development of the two waterbodies are missing To this end, we conducted a bibliometric analysis as a systematic data gathering to perform a large-scale overview and assess global trends of their scientific publications Moreover, a second goal is to differentiate their research hotspots and current challenges given the different nature of their origin and functionality 147,811 publications from 1955 to 2019 were retrieved from the database of the Science Citation Index Expanded, and then, divided into four research lines, (1) design and operation; (2) environment and ecology; (3) sanitation and human health; (4) socioeconomics Bibliometric indicators showed that the number of publications sustained a rapid growth, from 100 during the 1950s to around 7800 publications per year during the past few years The United States and EU 28 have long been world leaders in lake and reservoir research yet China has tremendously boosted its publications within the past 20 years, advancing this nation to the new world leader in both categories in 2019 Taking a closer look at research hotspots, design and operation have been the main topics for reservoir research while environment and ecology topics are the hotspots in lakes-related studies This reflected the intensive human interventions in reservoirs, whose major purposes are to supply hydropower energy, irrigation, water storage, and aquaculture Conversely, the impacts of eutrophication, heavy metals, and climate change have become more severe with the increase of species extinction and biodiversity loss, leading to urgent needs for lake restoration Both freshwater bodies show comparable attention on their roles in socioeconomics while much higher concerns about sanitation and human health have been paid in reservoirs compared to its counterpart Clear obtained distinctions in the hotspots and challenges of lake and reservoir research can contribute to better decision support systems of the two waterbodies Electronic supplementary material The online version of this article (https://doi.org/10.1007/s1119 2-020-03453-1) contains supplementary material, which is available to authorized users * Long Ho Long.TuanHo@Ugent.be Department of Animal Sciences and Aquatic Ecology, Ghent University, Ghent, Belgium 13 Vol.:(0123456789) Scientometrics Keywords Lakes · Reservoirs · Research hotspots · Scientometrics · Text mining Introduction Occupying a small fraction of around five million km2, natural lakes and artificial reservoirs are considered a minor element of the hydrosphere, yet they play a crucial role in the biogeochemical cycles of the Earth (Verpoorter et al 2014) New findings indicated the considerable contribution of the inland waterbodies in carbon and nitrogen storage, oxidation, and emission pathways (Cole and Caraco 2001; Sobek et al 2003; Bastviken et al 2004; Raymond et al 2013) Thanks to the fast growth of remote sensing data extraction from satellites, the inventory of the inland waterbodies has become more accurate (Verpoorter et al 2012) According to Verpoorter et al (2014), the Earth has around 117 million lakes (> 0.002 km2) covering around 3.7% of the non-glaciated land surface of the Earth This high-resolution mapping was proved able to reduce significantly the uncertainty in global estimates of the total lakes-occupied areas, their abundance and size distribution As such, the number of small waterbodies could be estimated in a more precise way compared to previous statistical extrapolation methods, e.g Pareto distribution, which were error-prone (Lehner et al 2011b) Moreover, lake volume and mean depth at the global scale have recently been calculated with the new method including statistical topographic theory within and across diverse regions in the study of Cael et al (2017) From that, total lake volume was estimated at about 199,000 km3 and the mean depth was about 41.8 m These new findings were significant contributions to advance the current knowledge in the limnology field Equally important, the functions of lakes and reservoirs are very vital for human society as a main source of freshwater Besides, thanks to the complexity of biochemical and hydrodynamic processes, these waterbodies can offer a great ability of self-clarification In fact, they reduce significantly the load and concentration of waste (water) discharged from human activities, which is very essential in many places where the application of wastewater treatment plants (WWTPs) remains low because of the bottleneck of affordability (Ho et al 2019) Furthermore, lakes and reservoirs also serve as the water resource of irrigation, industrial applications, fishery-nourishment, and hydroelectricity generation In fact, a number of hydropower dams have accelerated to 50,000 dams (higher than 15 m) capable of accumulating from 7000 to 8300 km3 of freshwater (Lehner et al 2011a, b) Apart from these services, the inland waterbodies also deliver substantial benefits with respect to cultural services including ceremonial practices, recreational and educational activities However, the calculation of their total values has not yet defined due to the diversity of their services and the complexity of assessment approaches (Reynaud and Lanzanova 2017) Numerous reviews have been conducted to deliver a systematic overview on different subjects related to lakes and reservoirs, such as aquaculture (Verdegem and Bosma 2009; Weyl et al 2010; Belton and Azad 2012), fisheries and aquaculture (Minns 2014; Wurtsbaugh et al 2015; Matveyev and Samusenok 2015), pollution (Konstantinou et al 2006; Ho et al 2017), hydroelectricity (Demarty and Bastien 2011; Zarfl et al 2015; Lees et al 2016), ecosystem services (Sierszen et al 2012; Schallenberg et al 2013; Reynaud and Lanzanova 2017), greenhouse gas (Bastviken et al 2004; Stadmark and Leonardson 2005; Chen et al 2013), and climate change (Mooij et al 2005; Tranvik et al 2009; Zhang et al 2016) However, a complete picture of research hotspots and current challenges of lakes and reservoirs remains unclear To this end, a bibliometric analysis can be applied 13 Scientometrics to investigate systematically publication characteristics, such as authorship, sources, subjects, geographical origins, and citations, on a large scale Bibliometrics was first presented by Pritchard (1969), in which quantitative analyses and statistical measurements were applied on academic publications in order to gain a systematic, transparent, and reproducible review on the existing knowledge base, from that, allowed advancing research lines (Aria and Cuccurullo 2017; Yao et al 2018) In this study, a bibliometric analysis was used to first quantitatively and qualitatively assess the global trends of research related to lakes and reservoirs, focusing on the period of the last 10 years from 2010 to 2019 Furthermore, since two types of waterbodies are relatively different regarding their origin and functionality, neglecting these differences can cause serious problems in their management decisions Hence, in the second part, different search functions for each type were included to distinguish the studies on lakes and on reservoirs Proper text mining analyses were implemented for identifying their research hotspots and current challenges Particularly, four main research lines were made: (1) design and operation, (2) environment and ecology, (3) sanitation and human health, and (4) socioeconomics Within these research lines, 18 research topics were investigated to further explore their roles and contemporary issues Data collection and analysis Data collection Data were extracted from the online database of the Science Citation Index Expanded (SCI-Expanded) which covers 11,655 journals across 234 research categories and 80 countries according to Journal Citation Reports 2018 (JCR) The data were collected in the Web of Science (WoS) from Clarivate Analytics on 2020/02/09 All document types were collected from 1955 to 2019 Generally, common templates of the search query applied in WoS were for publications related to lakes and ponds as follows: TI = lake* and TI = reservoir* OR pond* However, to remove publications related to oil and gas reservoirs, we exclude publishers and research categories aiming at this topic Details can be found in Table S1 in Supplementary Materials Moreover, to assess the publication performance of countries, we add a term CU = **, in which ** was filled by the name of countries in the search queries Note that regarding the publication performance of the EU 28 region, this term was filled with the name of all 28 EU member states Furthermore, to investigate the research hotspots of lakes- and reservoirs-related publications, the common templates were combined with another search query of each research hotspot To define research hotspots that play important roles in the sustainable development of lakes and reservoirs, retrieved research topics in Jiang et al (2016) were taken into account In particular, the bibliometric analysis of Jiang et al (2016) focused on hydropower reservoir research which split research lines into four groups: (1) construction technology; (2) operational technology; (3) environmental issues; (4) socio-economic issues The topics of these four research lines were defined by using statistical algorithms to extract semantic information from the abstracts of articles that were collected by applying a search query with keywords related to hydropower in the SCI-Expanded databases By using Term Frequency-Inverse Document Frequencies transformation and Latent Dirichlet Allocation model, 29 topics were clustered, based on their similarities regarding terms and documents (Jiang et al 2016) Taking these 29 topics as a reference, we chose relevant research topics to this study by combining similar topics, 13 Scientometrics removing irrelevant topics, and adding absent, but relevant, topics Specifically, we grouped eleven topics in the research lines of construction technology and operational technology to five broad topics in the research line of design and operation (DO): (1) design and construction; (2) flood control; (3) hydroelectricity and dams; (4) operational and maintenance; (5) restoration Similarly, twelve topics of environmental issues were grouped into six topics in the research line of environmental and ecology (EE): (1) biodiversity; (2) climate change; (3) emerging contaminants; (4) eutrophication; (5) greenhouse gases; (6) heavy metals Furthermore, we added three new topics in the research line of Sanitation and Human Health (SH), i.e drinking water, waterborne diseases, and antimicrobial resistance, as numerous essential ecosystem services of lakes and reservoirs derive from direct use of their water resources, such as drinking water, irrigation, fisheries, recreation, and self-purification Regarding the research line of socio-economics (SO), we combined six topics on a list of Jiang et al (2016) into four topics: (1) economics; (2) fisheries and aquaculture; (3) policy development; (4) social development Note that several topics, i.e climate change, fisheries and aquaculture, and social development, were taken from recommended topics for bibliometric analysis of sustainable development by Hassan et al (2014) This list was generated, based on common topics in Sustainable Development taxonomy of the World Bank, OECD, and UN DESA The details of the chosen topics and their search query in the WoS can be found in Table S1 in Supplementary Materials We identified the keyword queries for each research topic based on our expertise and previous bibliometric analyses whose topics are comparable (Zhao et al 2018; Qian et al 2015; Wang et al 2014; Vincon-Leite and Casenave 2019; Tchounwou et al 2012; Yao et al 2018; Yang et al 2018; Liu et al 2011; Sweileh et al 2016; Brandt et al 2014; Nikolic et al 2011; Jaric et al 2012; Hassan et al 2014; Jiang et al 2016; Ding et al 2019; Emmer 2018; Long T Ho et al 2017) Data analysis An open-source statistical software R (R Core Team 2014) was used for data retrieval, bibliometric analysis, and text mining Particularly, after using rwos package for data retrieval, the bibliometrix package of Aria and Cuccurullo (2017) was used for performing the bibliometric analysis and text mining Collected publications were evaluated for the following features: document types and languages, publication outputs, research categories, authors, journals, countries, institutions, and keywords From these data, research hotspots and future trends of research on lakes and reservoirs were identified and analyzed Besides, to map network collaboration among the most productive countries, VOSviewer software was applied (van Eck and Waltman 2010) Specifically, the co-authorship of the collected publications, which shows how frequent researchers from one country share authorship in publications with other researchers from another country, was calculated To filter only top research collaboration, we set up a minimum number of publications that two countries must have shared their authorship in order to have only the top 20 most collaborative countries on a map Bibliographic analysis Document type and language of publication The total number of publications appeared in the WoS database via the common templates from 1955 to 2019 was 147,811, in which around 42.5% was produced from the 13 Scientometrics last 10 years From the distribution analysis, 38 different document types were found Scientific article was the most frequently published document type with 77%, followed by proceedings paper (13%), meeting abstract (4%) and book review (2%) This proportion has changed during the last 10 years, in which more article and editorial material were published (83% and 1.5%), while the portion of proceedings paper and meeting abstract reduced to 11% and 2%, respectively English was the predominant language in academic publications on lake and reservoir research Of 126,096 journal articles, 95.8% were published in English Russian articles comprised 1.1%, followed by French (1.0%) and German (0.7%) During the last 10 years, English became more prevalent with 97% of the number of articles published in this language In contrast to the slight increase in the portion of Chinese articles comprising 0.6%, non-English language articles became vanishing However, one should keep in mind a criticism towards the SCI-Expanded database for its heavy bias towards English-written publications, which has limited the popularity of publications in other languages (Kurmis 2003; Tardy 2004) Publication years From around 100 publications published per year during the 1950s, this figure increased to around 7800 publications related to lakes and reservoirs during the last 4 years (from 2016 to 2019) In fact, the number of publications started to rocket for the last 10 years when at least 4000 publications published per year (Fig. 1) From the previous data, two polynomial models were developed to describe the cumulative annual number of publications related to lakes and reservoirs Based on these models, the number of publications in 2030 were predicted to be one and a half times higher than the current number of publications, which can be tripled up by 2050 Details of these models can be found in Figures S1 and S2 in Supplementary Materials oteworthy is that from the 1950s to 1970s the number of publications per year with the topics on lakes was relatively even to the number of publications per year with the Fig A cumulative number of publications related to lakes and reservoirs from 1955 to 2019 13 Scientometrics topics on reservoirs, however, the difference became larger after the 1980s when the former number increased to around double the latter number This result may reflect previous research tendency which focused mostly on large lakes while the studies of small inland waterbodies were underemphasized However, during the last 10 years when the roles of small continental waters have been increasingly recognized (Downing et al 2006; Tranvik et al 2009; Downing 2010), the growing number of their publications comprised half of the total publications every year Especially, in the last 3 years more research interests were paid for their roles This research trend is expected to increase since recent analyses indicated that they cover as much or more inland areas as large lakes (Downing 2010) However, one should keep in mind the incomplete collection of early publications (pre-1990) in SCI-Expanded databases which were also one of the reasons why numerous bibliometric analyses only considered publications after 1990 (Qian et al 2015; Jiang et al 2016; McCallen et al 2019) Journals and research categories The 126,096 journal articles were published in 7825 journals in 35 research categories in SCI-Expanded databases from 1955 to 2019 Among these journals, 97% contained less than 100 articles, leaving only 242 frequent journals having publications related to lakes and reservoirs Particularly, Hydrobiologia published the most articles (3719; 4.0%) from 1955 to 2019, followed by the Journal of Great Lakes Research (2536, 2.7%), and Limnology and Oceanography (1790; 1.9%) However, during the last 10 years, lakes-related publications have been published the most in the Journal of Great Lakes Research occupying 3%, while the portion of Hydrobiologia has decreased to only 2.2% Interestingly, Science of The Total Environment has significantly increased their publications related to lakes and reservoirs for the last 10 years, which ranked third in lakes category and first in reservoirs category and occupied almost 1.6% of the total publications (Figure S3 in Supplementary Materials) Besides, some common popular journals can be found between the two research categories, namely PLOS One, Environmental Earth Sciences and International Association Of Theoretical And Applied Limnology Proceedings (SIL) Regarding research categories, high proportion of publications related to lakes focused on biology, limnology, and environment science, while reservoirs-related articles targeted more on water resources, water pollution, and engineering civil research As shown in Figure S4 in Supplementary Materials, multidisciplinary research occupied the first two positions on a list of the most research categories for the last 10 years On the other hand, water resources and engineering-related research occupied around 30% of the total research related to reservoirs This result implies that more interests in environmental perspectives have been paid for lake research while studies on reservoirs have covered largely reservoir engineering and water resources Publication performance Global performance Besides around 10% of the total publications without author address information, publications on lakes and reservoirs from 1955 to 2019 covered 202 countries, in which only 76 13 Scientometrics countries have involved in more than 10 articles In the top ten most productive countries, the USA demonstrated its dominance, occupying up to 30% of the total publications while China and Canada followed with 12% and 10%, respectively However, this domination has been challenged during the last 10 years when China has increased its investment in research and development (R&D) and become the second-largest spender for R&D in 2017 with around 21% of the world’s total expense, around 370.6 billion dollars, according to OECD (2017) This is indicated by the increase of their publication during the last 10 years in Fig. In fact, their contributions boosted to above 21% in both categories and were comparable with the USA regarding reservoir-related publications with around 20% Interestingly, the difference in the distribution of publications between lakes and reservoirs reflected their geographical distribution among the nations According to the Global Lakes and Wetlands Database (GLWD) in the study of B Lehner and Doll (2004), the dominating occupation of the North American Great Lakes among large lakes with surface area ≥ 50 km2 was revealed by the most prominent peak between 40° and 50° North in the latitudinal distribution of lake areas This dominating occupation correlated to the dominating contribution of the USA and Canada in publications related to lakes, around 40% of the total publications On the other hand, despite being a developing country, China and Brazil with their intensive application of hydropower dams ranked second and third in the top 10 productive countries producing publications related to reservoirs for the last 10 years According to the Global Reservoir and Dam (GRanD) database, 6862 records of reservoirs and dams mainly located in the USA, Russia, Canada, Brazil, and China (Lehner et al 2011a, b) The absence of publications from Russia could be caused by the heavy bias toward English-written publications, which has limited the popularity of non-English written publications (Kurmis 2003; Tardy 2004) Regional performance Five most productive regions/countries producing publications related to lakes and reservoirs are Brazil, Canada, China, EU 28 member states, and the United States The evolution of their publication performance over time can be seen in Figure S5 in Supplementary Materials, in which United States, China, and EU 28 show their dominance in both research fields Specifically, the United States and EU 28 have long been world leaders in research on lakes and reservoirs while China has tremendously boosted its publications within the past 20 years, from 50 to around 1500 publications per year, advancing China to the new world leader in 2019 in both categories The expansion of Chinese research has been in line with China’s ‘going-out’ strategy as China’s hydropower industry has increasingly extended its involvement in domestic and overseas markets (McDonald et al 2009) In fact, 45,000 large dams (dams over 15 m in height) were built within China’s borders at the turn of the twenty-first century (Fuggle and Smith 2000) and around 350 Chinesefunded and Chinese-built overseas hydropower reservoirs are currently under construction (Urban et al 2018) In contrast to the growth of reservoirs, lakes are disappearing in China as a loss of around 20% of total surface area of lakes in China over the past 60 years (Ma et al 2010; Yang and Lu 2014) Anthropogenic activities, such as land use changes, urbanization, water consumption, river regulation, were found to be major causes of these disappearances (Yang and Lu 2014) Besides, lake water quality has also been deteriorated in China due to intensive water exploitation and discharge from an increasing population in 13 Scientometrics 13 Scientometrics Fig Map of the countries producing publications related to lakes and reservoirs from 1955 to 2019 and for the last decade the surrounding watersheds (Zhao et al 2017) Hence, consideration attentions have been given by Chinese lake researchers together with Central government of China which introduced and enforced a wide range of strict laws and regulations, such as the Guidelines on Strengthening Water Environmental Protection for Critical Lakes in 2008 and the Water Pollution Control Action Plan in 2015 (Huang et al 2019) Thanks to these efforts, lake nutrient concentrations have declined in the most populated parts of China (Tong et al 2017) However, Huang et al (2019) found that eutrophication has not been eradicated and heavy metal pollution still displayed an increasing trend in China In fact, these two issues also appear in Fig. 3 as one of the most concerned topics of Chinese research on both lakes and reservoirs Certain lakes have been targeted for research in China, e.g Taihu Lake, Poyang Lake, Chaohu Lake, lakes in Tibetan Plateau, and Dongting Lake (in descending order of popularity of the keywords) Most attention has been paid on Taihu Lake, China’s third-largest freshwater lake, as a result of the drinking water crisis, which took place in Fig Most concerned keywords in publications related to lakes and reservoirs in five most productive regions/countries from 1955 to 2019 The size of the texts reflects the popularity of the keywords in publications within a country but not among the countries Red texts represent research on lakes while blue texts represent research on reservoirs (Color figure online) 13 ▸ Scientometrics Wuxi in 2007 due to the massive bloom of cyanobacteria Microcystis spp., leaving around two million people without drinking water for more than a week (Qin et al 2010) The other lakes have also gained great research interests due to their water pollution (Li et al 2013; Wu et al 2017; Huang et al 2018) On the other hand, climate change appears to be a key factor regulating the water level of lakes on the Tibetan Plateau Since the 1980s, abnormal warming in Tibet has caused ice and glacier melting, resulting in numerous new lakes and enlarged lakes in this area (Ma et al 2010) Except for Brazil, considerable attention on interactions between climate change and lakes are shared among Canada, US, and EU 28 as the term climate change has stayed on their list of top ten most popular keywords In fact, mutual interactions between lakes and climate change have recently been recognized as lakes are vulnerable for climate change impacts but they also play an important role in carbon storage, oxidation, and emission pathways (Berga 2016; Ho and Goethals 2019) This increasing recognition can be seen in Figure S6–S8 in Supplementary Materials These figures show the evolution of the most concerned keywords of lakes-related publications in the countries over the three periods, i.e 2010–2019, 2000–2009, and before 2000 Specifically, before 2000, climate change was barely mentioned as author keywords in any lakes-related publications while during the last decade, this term ranked in the top three most popular keywords in Canada, United States, and EU 28 Conversely, Brazilian and Chinese lake research has shared little attention on climate change as this term never reach the top ten most common keywords in Brazilian lake research while only reached the eighth most common keywords in Chinese lake research during the last decade Similarly, climate change has also received substantial concerns from reservoir researchers from Canada, United States, and EU 28 but much less from China and Brazil as this term was rarely used as keywords in their publications The role of hydropower in climate change mitigation has well recognized as it was concluded that this renewable energy technology can contribute substantially to the reductions of greenhouse gas (GHG) emissions in comparison with conventional coal power plants (IHA 2018) However, recent findings appear to be contradictory to this conclusion Higher emissions were found from 18 large hydroelectric reservoirs in the Amazon river basin compared to conventional fossil fuel power stations (Fearnside 2016) Likewise, it was concluded that hydropower in the Mekong Region cannot be considered categorically as low-emission energy due to their high variability of GHG emissions, from much lower to higher than the emissions of fossil fuel power stations (Rasanen et al 2018) Considerable concerns can also be found over the impacts of large dams on river hydromorphology, flow discharge, thermal dynamics, and inter-basin water transfers, which, consequently, exacerbates river ecological states with its habitat and species (Hauer et al 2017) In fact, fish, macroinvertebrates, zooplankton, phytoplankton, and vegetation are among the most popular keywords in reservoirsrelated publications As shown in Figures S10 and S11 in the Supplementary Materials, two neighboring countries, Canadian and the US, have been their closest partners and shared numerous common research interests A novel example is research on the Great Lakes, five largest lakes in the United States including Lake Superior, Lake Huron, Lake Michigan, Lake Erie, and Lake Ontario, located in the Northern Midwest along the border between the United States and Canada (Van Metre et al 2004) These lakes always appear on the list of the most concerned keywords of Canadian and US lake research over the three periods (Figures S6–S8 in the Supplementary Materials) Eutrophication and heavy metals were of great interest to lake research in both nations before 2010 while during the last decade, more focus has oriented towards climate change Strong preference towards lake research 13 Scientometrics Environment and ecology As indicated in Fig. 7, emerging contaminants, heavy metal, and eutrophication have been of the most interest of lake researchers while pollution has been increasingly studied in reservoirs Eutrophication and heavy metal contamination occurred in lakes since the 1930s as a result of industrialization and intensive agriculture, e.g the case of eutrophication in Lake Bourget (France) in 1933 (Giguet-Covex et al 2010; Jenny et al 2013) From the 1960s to 1980s, lake eutrophication was increasingly studied, which was indicated in several symposia devoted to these topics, e.g Madison Symposium (National Academy of Sciences 1969) or the Symposium of the American Society of Limnology and Oceanography (ASLO) in 1971 The concern was first focused more on deep lakes, located in the Northern hemisphere (United States, Canada, and Western Europe) and Australia Recently, the focus was directed towards the eutrophication of shallow lakes whose characteristics were high turbidity and low biodiversity due to the elimination of vegetation and dominance of bream in the fish community (Vincon-Leite and Casenave 2019) This can be explained by the rapid development of agricultural and industrial activities, and urbanization in developing countries, e.g China, Brazil, and India, which increased substantially wastewater discharge into receiving waterbodies (WHO 2006) Interestingly, lakes and reservoir studies have increasingly published on both climate change and GHG topics, especially during the last 5 years, reflecting the mutual relationships between inland waterbodies and climate change Specifically, for the mitigation of global warming, these waterbodies play a role as important sinks of CO2 and CH4, and hydropower reservoirs as renewable energy also reduce significantly the emissions Fig The number of publications related to lakes and reservoirs on major topics in the research line of environment and ecology during the last decade from 2010 to 2019 13 Scientometrics compared to fossil fuels, while climate change is likely to alter their water resources, habitat characteristics and ecosystems (Berga 2016) In contrast to limited concern about GHG emissions from lakes and reservoirs, their GHG emissions have recently become a research hotspot thanks to growing recognition of their roles in global processes and cycles (Bartosiewicz et al 2015; Wang et al 2017) Plus, it was found that, together with hydropower reservoirs, WSPs produced significantly GHGs as a result of their high inputs of organic matter and nutrients, leading to the elevated emissions (Hernandez-Paniagua et al 2014; Glaz et al 2016) Although the number of publications and the number of keyword occurrences in the EE research line are higher in lake studies compared to reservoir studies, large similarities can be found in the most concerned topics between two research categories (Fig. 8) For example, phytoplankton, zooplankton, macrophytes, macroinvertebrates, and fish are the most common keywords appearing in biodiversity topics of lakes and reservoirs as these flora and fauna are frequently used as ecological indicators of the inland waterbodies (Bartram et al 2016) Regarding GHG topics, carbon dioxide appears to be the most concerned GHG in both categories, followed by methane while less attention has been paid for the emission of nitrous oxide which has a global warming potential (GWP) of 265 times higher than CO2 for a 100-year timescale (Pachauri and Meyer 2015) This reflects the current limited understanding of N2O emission as its significance remains unclear due to the complexity of microbial metabolism in producing N2O (Musenze et al 2014; Porro et al 2017) In heavy metal topic, the concerns have been oriented towards mercury and sediment accumulation in both lakes and reservoirs High concerns over mercury were raised because it was found that people and wildlife that eat fish from lakes and reservoirs have an elevated risk of accumulating excessive amount of methyl mercury which is the main reason for fish consumption advisories worldwide (Mailman et al 2006; WHO/FAO 2010) Still, some differences can be found between lake and reservoir studies in the EE line Regarding emerging contaminants, much higher proportion of research has been conducted on the issues of antibiotics on reservoirs compared to that on lakes This distinctiveness comes from a growing concern about antimicrobial resistance in drinking water reservoirs as a result of its extensive use of this in many places (Okeke et al 2005a, b) On the other hand, being heavily exploited for intensive agriculture and industrialization, pesticides and heavy metals have been main targets of lake research in the emerging contaminant topic Molecular techniques are also mentioned more often in lake studies compared to reservoir studies as 16 s ribosomal RNA has been frequently applied in the former research Conversely, it appears that environmental and ecological models have not been often in use for environmental impact assessment in lakes and reservoirs This was in line with Nguyen et al (2018) which concluded that despite substantial impacts of hydropower reservoirs on water quality and habitat suitability for biological communities, a limited number of models has been applied Sanitation and human health As indicated in Fig. 9, sanitation and human health in reservoirs have received growing attention for the last 10 years while the number of studies on this topic in lake systems has remained relatively stable This can be a result of increasing outbreaks of waterborne diseases via water supply systems and recreational activities in reservoirs Around 900,000 cases of illnesses and 900 deaths occurred each year in the US due to microbial contamination of drinking water (Brookes et al 2004) Lack of sanitation systems directly discharged 13 Scientometrics Fig Most concerned keywords in publications related to lakes and reservoirs on major topics in the research line of environment and ecology from 1955 to 2019 The size of the texts reflects the popularity of the keywords in publications within a topic but not among the topics Red texts represent research on lakes while blue texts represent research on reservoirs (Color figure online) wastewater effluent, diffused sources from agriculture run-off are potential sources of high pathogen levels in receiving waterbodies Increasing recreational activities in these waterbodies also increase the risk of disease for participants According to epidemiological studies, e.g Seyfried et al (1985), Craun et al (2005), and Sinclair et al (2009), a higher 13 Scientometrics Fig The number of publications related to lakes and reservoirs on major topics in the research line of Sanitation and Human Health during the last decade from 2010 to 2019 chance for gastrointestinal, respiratory, ear, skin, and wound infections were found among those participating in water-based recreational activities In particular, E coli, diarrhea, cryptosporidiosis, fluoride, arsenic, cancer, chlorine, and H pylori were most frequently encountered keywords in title and abstract of articles related to drinking and recreational water-related diseases (Sweileh et al 2016) Growing concern about antimicrobial resistance (AR) in reservoirs can be associated with the increasing problem of AR in developing countries where antimicrobial misuse and shortfalls in infection control and public health have caused the spreading of AR to an unacceptable level (Okeke et al 2005a, b) Human activities, such as agriculture run-off, aquaculture, and wastewater discharge, caused a high level of AR in the surface drinking water systems (Xu et al 2016) As a result, the remaining AR in drinking water reservoirs can enter into drinking water supply system causing health problems to water consumers In fact, the number of publications related to drinking water reservoirs published during the last 5 years has been double that in lake systems, showing much higher concerns about sanitation and human health in these waterbodies As shown in Fig. 10, much higher concerns on the SH research line have been paid in reservoirs compared to its counterpart Specifically, the studies in antibiotic resistance (AR) in reservoirs have been focused on methicillin-resistant Staphylococcus aureus (MRSA) and extended-spectrum beta-lactamase (ESBL) Regarding MRSA, since its first appearance in 1960, the frequency of its infections has continued to grow and S aureus has become more resistant to currently available drugs (Boucher and Corey 2008) Besides, the production of ESBL is one of the major AR mechanisms in Enterobacteriaceae which has increasingly posed a serious threat to the public health due to a widespread dissemination of ESBL-carrying bacteria (Zhang et al 2015) Recent findings have recognized the role of WWTPs in promoting the widespread of antibiotic resistant bacteria (ARB) and their antimicrobial resistance genes (ARGs) in surface water which often cannot be completely eliminated by drinking water reservoirs (Turolla et al 2018) However, more research on this subject is needed as an estimate of 33,000 casualties each year in Europe was published as a direct consequence of an infection due to bacteria resistant to antibiotics (Cassini et al 2018) In fact, AR is not one of the most concerned subjects in drinking water topics in lakes and reservoirs which is in contrast to high scientific attention on microcytins and eutrophication These toxins are produced by cyanobacteria which can occur in drinking water reservoirs because of eutrophication (Falconer 2005) Microcytins are 13 Scientometrics Fig 10 Most concerned keywords in publications related to lakes and reservoirs on major topics in the research line of Sanitation and Human Health from 1955 to 2019 The size of the texts reflects the popularity of the keywords in publications within a topic but not among the topics Red texts represent research on lakes while blue texts represent research on reservoirs (Color figure online) responsible for numerous cases of injury and death of domestic animals, and human poisoning from drinking water (Falconer and Humpage 2005) Regarding waterborne diseases, while numerous most common keywords are locations, schistosomiasis appears to be the most concerned disease in lake research In fact, it was found that lakes are largely contributing to schistosomiasis transmission in developing countries (Ofoezie 2002) In fact, an estimated 12,000 to 200,000 people die from this disease every year with the vast majority in Africa (McCreesh et al 2015; McCreesh and Booth 2013) Also noteworthy is that Escherichia coli is one of the most popular keywords in both lakes- and reservoirs-related publications as this bacteria is a good fecal indicator microorganism for water quality and numerous publications were published on pathogen removal of WSPs (Nelson et al 2004) Many other diseases were mentioned in reservoirsrelated publications, such as Trypanosoma cruzi, Ulcerative colitis, Lyme disease, and white spot syndrome virus (WSSV) Socioeconomics Despite only 10% of the total number of lake and reservoir research devoted to socioeconomics, increasing concern of lake and reservoir researchers about this research line can be observed in Fig. 11 Higher attention on their roles in policy and social development can be seen as the number of publications on these research topics have doubled up during the last 13 Scientometrics Fig 11 The number of publications related to lakes and reservoirs on major topics in the research line of Socioeconomics during the last decade from 2010 to 2019 10 years Similarly, fisheries and aquaculture have become a research hotspot as the number of their research has triple up from only 20 to around 60 publications per year over the last decade In fact, from the 1970s to 2010s, global aquaculture production has boosted 40 times and is expected to quintuple in the next 50 years (Avnimelech et al 2008) According to FAO (2009), aquaculture and fisheries were the fastest growing sector in the food industry and its economic importance contributes increasingly to both financial and social sustainability Increasing needs for aquaculture products and technological progress expedited this expansion For example, the production of pangasius in the Mekong Delta went up three times, from four million tons to 12 million tons, between 2005 and 2008 due to the open of the markets in Europe and the US for this product from Vietnam As a result, the Vietnamese government provoked the creation of aquafarms and processing industries, leading to the bloom of the number of smallholder farmers (Bosma and Verdegem 2011) However, the increasing supply of produced pangasius leading to its lower selling prices, together with the oil price crisis in 2009, caused the bankruptcy of many farmers who eventually abandoned their reservoirs This unsustainable development of fisheries and aquaculture was the main reason for the demolishment of mangrove ecosystems worldwide In fact, around 50% of mangrove forests in Mekong delta in Vietnam were replaced by aquafarms despite the efforts of the Vietnamese government in intervening stricter laws and increasing enforcement for mangrove protection since 1999 (Thu and Populus 2007) 13 Scientometrics As indicated in Fig. 12, fisheries appear to be very important for lake economics while aquaculture plays a key role in reservoir development Particularly, lake studies have focused on fish stocking assessment while fish yield, density, and growth have been hotspots of reservoir studies in this research line Note that in contrast to the decline of natural fish resources due to overfishing in lakes, rapid development of pond fish culture accelerated since the 1960s (Jia et al 2013) Different most concerned fishes can also be found between two research categories, in which trout and whitefish have been more frequently used as keywords in lakes-related publications while pike perch and tilapia have been of great concern in reservoirs-related publications Also noteworthy is that, besides eutrophication, a lack of scientific concern has been paid to the negative impacts of fisheries and aquaculture on the ecosystems of lakes and reservoirs, such as reduced biodiversity, increasing alien species invasion, and degraded mangrove forests (Sampantamit et al 2020) Fig 12 Most concerned keywords in publications related to lakes and reservoirs on major topics in the research line of Socioeconomics from 1955 to 2019 The size of the texts reflects the popularity of the keywords in publications within a topic but not among the topics Red texts represent research on lakes while blue texts represent research on reservoirs (Color figure online) 13 Scientometrics Eutrophication and water quality are two major research subjects for policy development of lakes while reservoir researchers have targeted optimization and simulation subjects in reservoir policy development Climate change appears to be another key factor in lake policy development while models, such as genetic algorithm and dynamic programming, are a common tool in the development of reservoir policies to optimize its performance as these keywords have also frequently used in publications in reservoir OM research line Regarding social development, the impacts of urbanization on the two inland waterbodies have attracted major scientific concerns as eutrophication, water quality, heavy metal contamination appear in the list of the most concerned keywords Conclusion A rapidly growing number of publications related to lakes and reservoirs, from only 100 publications per year during the 1950s to around 7800 during the past few years, indicated their vital roles in the sustainable development of human society, environmental well-being, and economic welfare Their studies spread into 35 research categories, from Geosciences, Marine Freshwater Biology, Fisheries, to Engineering Environment, and Microbiology, within 7825 journals These figures, together with the boosted number of publications in most main research topics for the last 10 years, highlighted the important benefits of their ecosystem services Regarding publication performance, the United States and EU 28 have long been world leaders in lake and reservoir research while China has tremendously boosted its publications within the past 20 years, from 50 to around 1500 publications per year, advancing China to the new world leader in both categories in 2019 Geography appears to be a key factor in the distribution of publications related to lakes and reservoirs, Possessing large proportion of lakes in the world, the USA and Canada have contributed around 40% of the total publications related to lakes On the other hand, despite being developing countries, China and Brazil with extensive hydropower applications have ranked second and third in the most productive countries producing reservoirs-related publications over the last decade Increasing concern about climate change can be seen over the last decade as this term ranked in the top three most popular keywords in Canada, United States, and EU 28, however, it has rarely appeared in Brazilian and Chinese lake research By taking a closer look at 18 research topics in four research lines: (1) design and operation (DO), (2) environment and ecology (EE), (3) sanitation and human health (SH), and (4) socioeconomics (SO), differences in research hotspots on lakes and reservoirs were revealed Regarding the DO research line, lakes-related publications have increasingly focused on restoration while more studies on reservoir design and operation as well as hydropower dams were found This reflected the higher number of human interventions in design and operation in man-made reservoirs while the impacts of eutrophication, heavy metal, pollutions were much higher in natural lakes, leading to a high requirement for restoration This result was indicated by a much higher number of lakes studies on these topics compared to its counterpart in the EE research line Interestingly, both research categories have increasingly published on climate change and greenhouse gas (GHG) topics, reflecting the recognition of mutual relationships between inland waterbodies and climate change Specifically, for the mitigation of global warming, these waterbodies play a role as important sinks of CO2 and CH4, and hydropower reservoirs as renewable energy also 13 Scientometrics reduce significantly the emissions compared to fossil fuels, while climate change is likely to alter their water resources, habitat characteristics and ecosystems Regarding the SH research line, much higher concerns have been paid in reservoirs compared to its counterpart due to numerous reservoirs functionalities, such as drinking water, irrigation, fisheries, recreation, and self-purification Growing concerns about antimicrobial resistance and waterborne diseases in reservoirs are expected to continue as a result of increasing outbreaks in the US and Europe as well as emerging problems of antimicrobial resistance in developing countries where antimicrobial misuse and shortfalls in infection control and public health Accounting for only 10% of the total number of publications, however, studies on socioeconomics displayed the importance of freshwater bodies in economics, and policy and social development It appears that fisheries in lakes and aquaculture in reservoirs have produced tremendous benefits to human society, from smallholder farmers to international cooperation, but also expedited ecosystem destruction and environmental deterioration From these findings, clear distinctions in research hotspots and current challenges between two freshwater systems are expected to circumvent the misleading in management options between two types, hence, facilitating better control and supervision strategies for the two waterbodies Acknowledgements This research was performed in the context of the VLIR Ecuador Biodiversity Network project This project was funded by the Vlaamse Interuniversitaire Raad-Universitaire Ontwikkelingssamenwerking (VLIR-UOS), which supports partnerships between universities and university colleges in Flanders and the South Long Ho is supported by the special research fund (BOF) of Ghent University Compliance with ethical standards Conflict of interest The authors declare that they have no conflict of interest References Aria, M., & Cuccurullo, C (2017) bibliometrix: An R-tool for comprehensive science mapping analysis Journal of Informetrics, 11(4), 959–975 https://doi.org/10.1016/j.joi.2017.08.007 Avnimelech, Y., Verdegem, M., Kurup, M., & Keshavanath, P (2008) Sustainable land-based aquaculture: Rational utilization of water, land and feed resources Mediterranean 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