Research hotspots and current challenges

The United States and EU 28 have long been world leaders in lake and reser-voir research yet China has tremendously boosted its publications within the past 20 years, advancing this nati

Research hotspots and current challenges of lakes

and reservoirs: 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 functional-ity 147,811 publications from 1955 to 2019 were retrieved from the database of the Sci-ence Citation Index Expanded, and then, divided into four research lines, (1) design and operation; (2) environment and ecology; (3) sanitation and human health; (4) socioeco-nomics 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 reser-voir 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 socio-economics 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

1 Department of Animal Sciences and Aquatic Ecology, Ghent University, Ghent, Belgium

Keywords Lakes · Reservoirs · Research hotspots · Scientometrics · Text mining

Introduction

Occupying a small fraction of around five million km2, natural lakes and artificial voirs 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, oxida-tion, 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 (Ver-poorter et al 2012) According to Verpoorter et al (2014), the Earth has around 117 mil-lion 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 uncer-tainty in global estimates of the total lakes-occupied areas, their abundance and size distri-bution 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

reser-Equally important, the functions of lakes and reservoirs are very vital for human ety 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 wastewa-ter treatment plants (WWTPs) remains low because of the bottleneck of affordability (Ho

soci-et al 2019) Furthermore, lakes and reservoirs also serve as the water resource of tion, industrial applications, fishery-nourishment, and hydroelectricity generation In fact,

irriga-a number of hydropower dirriga-ams hirriga-ave irriga-accelerirriga-ated to 50,000 dirriga-ams (higher thirriga-an 15 m) cirriga-apirriga-able

of accumulating from 7000 to 8300 km3 of freshwater (Lehner et al 2011a, ) Apart from these services, the inland waterbodies also deliver substantial benefits with respect to cul-tural services including ceremonial practices, recreational and educational activities How-ever, 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 differ-ent 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

to investigate systematically publication characteristics, such as authorship, sources, jects, geographical origins, and citations, on a large scale Bibliometrics was first presented

sub-by Pritchard (1969), in which quantitative analyses and statistical measurements were applied on academic publications in order to gain a systematic, transparent, and reproduc-ible 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 function-ality, neglecting these differences can cause serious problems in their management deci-sions 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 imple-mented 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 coun-tries 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 lected from 1955 to 2019

col-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 Materi-als 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 tions, the common templates were combined with another search query of each research hot-spot 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

publica-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) opera-tional 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 Fre-quencies 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 ics as a reference, we chose relevant research topics to this study by combining similar topics,

top-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 con-taminants; (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-eco-nomics (SO), we combined six topics on a list of Jiang et al (2016) into four topics: (1) eco-nomics; (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 Has-san 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,

biblio-metric 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 ric 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

bibliomet-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 plates from 1955 to 2019 was 147,811, in which around 42.5% was produced from the

tem-last 10 years From the distribution analysis, 38 different document types were found entific 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.

Sci-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 lan-guage 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 publi-cations related to lakes and reservoirs Based on these models, the number of publica-tions 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 1 A cumulative number of publications related to lakes and reservoirs from 1955 to 2019

topics on reservoirs, however, the difference became larger after the 1980s when the mer number increased to around double the latter number This result may reflect previ-ous 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).

for-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 nology 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 occu-pying 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 Sup-plementary Materials) Besides, some common popular journals can be found between the two research categories, namely PLOS One, Environmental Earth Sciences and Inter-national Association Of Theoretical And Applied Limnology Proceedings (SIL)

Lim-Regarding research categories, high proportion of publications related to lakes focused on biology, limnology, and environment science, while reservoirs-related arti-cles 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 ronmental perspectives have been paid for lake research while studies on reservoirs have covered largely reservoir engineering and water resources

envi-Publication performance

Global performance

Besides around 10% of the total publications without author address information, tions on lakes and reservoirs from 1955 to 2019 covered 202 countries, in which only 76

publica-countries have involved in more than 10 articles In the top ten most productive publica-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. 2 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 reser-voirs 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 sur-face 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, ) 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 writ-ten publications (Kurmis 2003; Tardy 2004)

Regional performance

Five most productive regions/countries producing publications related to lakes and voirs are Brazil, Canada, China, EU 28 member states, and the United States The evolu-tion of their publication performance over time can be seen in Figure S5 in Supplemen-tary 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

reser-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 increas-ingly 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 funded and Chinese-built overseas hydropower reservoirs are currently under construction (Urban et al 2018)

Chinese-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, urbaniza-tion, water consumption, river regulation, were found to be major causes of these disap-pearances (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

the surrounding watersheds (Zhao et al 2017) Hence, consideration attentions have been given by Chinese lake researchers together with Central government of China which intro-duced 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 2 Map of the countries producing publications related to lakes and reservoirs from 1955 to 2019 and for the last decade

▸

Fig 3 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 publi- cations within a country but not among the countries Red texts represent research on lakes while blue texts represent research on reservoirs (Color figure online)

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 atten-tion on climate change as this term never reach the top ten most common keywords in Bra-zilian 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, zooplank-ton, phytoplankton, and vegetation are among the most popular keywords in reservoirs-related 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

compared to reservoir research can also be found in research of both nations Despite a large number of reservoirs, i.e 90,580 dams in the US (Federal Emergency Management Agency 2016) and over 14,000 dams in Canada (Canadian Dam Association 2019), the number of reservoirs-related publications has been only around one-third of the number

of lakes-related publications This is in contrast to the research priority on reservoirs in Brazil and relatively even ratio between lake and reservoir research in China Details of the collaboration in lake and reservoir research of Brazil, China, and EU 28 can be found in Figures S9, S12, and S13

Research hotspots and current challenges

To illustrate the whole picture of research hotspots supporting the sustainable ment of lakes and reservoirs, four research lines were made, including design and operation (DO), environment and ecology (EE), sanitation and human health (SH), and socioeco-nomics (SO) As shown in Fig. 4, DO and EE are two most attentive research lines that have published around 50,000 publications in total in which half of them were published within the last 10 years On the other hand, much less scientific attention was paid for SH and SO with less than 8000 publications published during the last 10 years Lakes-related publications focused the most on EE and DO, producing around 8000 and 6000 publica-tions, respectively, during the last 10 year This order is swapped in reservoir research as

develop-DO has been its most attentive research line with also around 8000 publications, followed

by EE with 5000 published during the last 10 year This trend between lakes- and voirs-research is also observed with the two other research lines, SH and SO, in which lake research has focused more on SO while its counterpart has more publications in SH research line The number of unique and shared publications between lake and reservoir research in each research line from 1955 to 2019 and for the last decade can be found in Tables S2 and S3 in Supplementary Materials As shown in these tables, the proportions of shared publications related to lakes and reservoirs are relatively small except for DO with

reser-Fig 4 Most concerned research topics on the sustainability of lakes and reservoirs for the last decade (from

2010 to 2019) The size of the text represents the number of publications in each topic Red texts represent research on reservoirs while blue texts represent research on lakes (Color figure online)

around 7.5% This result suggests that the applied search queries were adequate to separate the two research categories and/or researchers have tended to separate their studies into two individual categories.

Design and operation

Figure 5 demonstrates two opposing trends of research topics between lake and reservoir categories during the last 10  years Particularly, lakes-related publications have increas-ingly focused on restoration while studies on design and operation, and hydropower dams, can be found more on reservoirs This dominance of reservoirs-related publica-tions reflected the increasing number of human interventions in design and operation in man-made reservoirs Different types of reservoirs were constructed from ancient time to modern time for various purposes, such as flood control, hydropower supply, and water purification and storage (Lehner et al 2011b) These reservoirs have been key to the devel-opment of human societies by offering economic growth, increasing food production, and human health It was approximated by WCD (2000) that 12–16% of food production and 19% of global electricity supply were contributed by large dams Furthermore, to match the need for food and water of the rapidly increasing population, at least 3700 major dams are either planned or under construction in developing countries (Zarfl et al 2015) Besides, one major application of reservoir, waste stabilization ponds (WSPs) have become more popular due to many advantages of this natural system for wastewater treatment compared

to conventional mechanism-based systems, i.e inexpensive costs for operation and tenance, simplicity, and robustness (Long Ho et al 2018) Hence, these facts can explain

main-Fig 5 The number of publications related to lakes and reservoirs on major topics in the research line of design and operation during the last decade from 2010 to 2019

the much higher interest of researchers and engineers in reservoir design and operation compared to its counterpart Conversely, higher number of publications related to lake res-toration could be a result of increasing recognition of anthropogenic impacts on the water quality of these fresh waterbodies For example, eutrophication has emerged one of the most serious problems, which caused turbid and toxic water from cyanobacterial blooms, degradation of lake ecosystems and biodiversity in around 40% of the total number of lakes worldwide (Bartram et al 2016).

Figure 6 shows the top 20 most popular keywords in lakes- and reservoirs-related lications in the DO research line from 1955 to 2019 Regarding design and construction topic, two natural (waste)water treatment systems, i.e constructed wetlands (CWs) and waste stabilization ponds (WSPs), have been mainly focused to optimize their removal per-formance In fact, it is difficult to overstress the assets of the two natural systems for waste-water treatment, i.e high removal efficiency, simplicity, and low cost, especially when affordability remains a bottleneck for clean water and sanitation in developing countries (Ho et al 2018) These inland waterbodies can provide a high capacity of self-purification thanks to high complexity with an interconnected web of biochemical reactions and com-plex hydraulic processes, thereby resulting in their massive use (Ho et al 2017) As such, WSPs occupy more than 50% of wastewater treatment plants in the US (> 8000) and are also a major choice of most small remote communities in Northern Canada while 5000 constructed wetlands have been built in Europe (Ragush et al 2015; US EPA 2011) In Europe, WSPs contribute to 20% of the total number of urban WWTPs in France and one-third of the WWTPs in Germany (Mara 2009)

pub-Moreover, monitoring and modeling have been frequently employed in the studies

of both research lines Regarding water management, they are efficient tools for tative evaluations of the impacts on aquatic ecosystems as well as integrative and mul-tidisciplinary assessments of water quality (Jørgensen 2005) Especially, remote sensing has increasingly been applied to inventory the number of lakes (Lehner et al 2011a, ; Verpoorter et al 2014) while genetic algorithm is often used for multi-objective optimiza-tion in reservoirs (Reddy and Kumar 2006; Yu et al 2018; X Zhang et al 2019) These tools have been commonly applied in many places, such as Three Gorges Reservoir, Great Lakes, Lake Baikal, and Taihu Lake

quanti-Unlike the other research topics, differences between lake and reservoir research can

be found in the lists of most popular keywords in the research topics of flood control, and hydroelectricity and dams Particularly, while impacts of flood on fauna and flora and floodplain have been of the most concern in lakes, reservoir research has focused mainly

on stormwater management and retention ponds Besides, regarding hydroelectricity and dams, reservoir researchers have studied largely on optimization of design and operation, while lake researchers have concentrated more on impacts of dams on habitats and biodi-versity A lack of concerns on hydropower dams’ ecological impacts has recently eased

as researchers have increasingly recognized negative side effects of this renewable energy technology In fact, Stone (2011) concluded that the Three Gorges dam in China has become an environmental bane which might cost over $26.45 billion over the next 10 years due to the mitigation efforts for environment and ecosystems Additionally, studies have warned catastrophic impacts on fish productivity, food security and biodiversity from the planned dams in Amazonia and Mekong river basins (Ziv et al 2012; Lees et al 2016; Lauri et al 2012; Orr et al 2012)

Fig 6 Most concerned keywords in publications related to lakes and reservoirs on major topics in the research line of design and operation 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)