Challenges Regarding Water Quality of Eutrophic Reservoirs in Urban Landscapes: A Mapping Literature Review

Urbanized river basins usually suffer from anthropogenic pressure, compromising the quality of water. Unsafe water is a risk to public health, especially when there are occurrences of HABs (Harmful Algae Blooms) as in the case of cyanobacteria, which cause different human health problems. In this paper, we aimed to review the scientific literature documenting what has been studied in the scope of the stratified reservoirs of urbanized basins. The mapping review method was used to categorize existing literature on urbanized watersheds and eutrophic reservoirs. Using the keywords “Eutrophic Reservoir” and “Urban” and selecting all the years of open publication on the Science web page, we obtained 69 results, 53 of them meeting the requirements established for the search. Many of the studies mention as the most important determinant for eutrophication of reservoirs and the proliferation of algae, the anthropogenic influence through the diffuse load of streets, domestic and industrial sewage, and even drainage water from agricultural areas in the basin. The results of this study reinforce that informal settlements without sanitary infrastructure are aggravating the deterioration of water quality in urban water sources and therefore posing many risks to public health.


Introduction
Scarcity of water may be absolute lack of water, or may be due to inadequate exploitation and distribution, lack of access by part of population, and, in the case of drinking water, it can be related to its insufficient quality [1]. In this manuscript, we draw attention to deterioration of watersheds that may cause impediments to potable water access [1]. This is an issue from arid and distant backlands to humid environments where human life and technology abound: the cities.
Pressure of uncontrolled urban growth has been compromising existing water sources around the world. In low and middle income countries, the paucity of basic sanitation results in domestic sewage dumped into water bodies [2]. In addition, the shortage of adequate drainage and the increase of impervious soils by constructions and asphalt prevent soil recharge by the natural cycle. As consequence, contaminated runoffs, floods and torrents contribute to water quality loss [2].
As the city grows, the riparian vegetation might be substituted by improvised dwellings, compromising the health of the ecosystem and of the population living on the margins of reservoirs. The poorest and most vulnerable families often occupy those areas designated by law for preservation, because they are not part of the formal real estate market. There are also frequent irregular occupations

Materials and Methods
A mapping literature review was undertaken [6] in order to map out and categorize existing studies on urbanized eutrophic watersheds, with articles found at the Web of Science Platform. We have chosen this platform because it gives access to multiple databases that reference cross-disciplinary research, which allows for in-depth exploration of specialized sub-fields within an academic or scientific discipline. It allowed a wide picture of what is being done on the topic.
We first looked for the words "Eutrophic reservoirs" resulting in 1586 studies, so we refined for "Eutrophic reservoirs" and "Cyanobacteria", in order to know which of these contemplated the algae. This resulted in 357 studies, without limits of date or area. Among the indexed studies found in this initial phase, Brazil ranked as the first in numbers, with 72 publications; China followed with 38 publications; USA with 35; Poland with 26; and France with 23 publications. We found less than 5 manuscripts per country from Ethiopia, with 3 publications; Morocco, Nigeria, South Africa, Tunisia, Uganda, Vietnam and Singapore with 2 publications each.
From this starting point, we refined the search by adding the word urban, in order to know which of these studies were located in urbanized basins. We then chose to map out the eutrophic reservoirs found in urban basins using those results to explore further information. The aim was to collect the studies that mentioned eutrophic reservoirs in urban environments, because the eutrophic water body is already a threat for water quality, even if the study doesn´t mention HABs or Cyanobacteria, once the environment is ready for its frequent proliferation because of the water body´s trophic state and of rising temperatures.
This final search was done in November 2017, using the keywords "Eutrophic reservoirs" and "Urban", setting no time span.
From 69 articles found, 16 were discarded because they did not fulfill the study requirements: the study had to refer both to eutrophic reservoirs and to urban settings. Therefore 53 articles have been selected and then systematized and mapped.

Results
As shown in Figure 1, there has been a growing interest in the problem of eutrophication of urban reservoirs in the last decade. This interest certainly shows an increase in the problem. Since the search was done in November 2017, results for the year 2018 do not appear. Brazil passed through an acute water crisis during the period 2014-2016 and those years experienced large algae blooms. Since most of the studies were undertaken in Brazil, it might explain the higher number of studies in 2014, 2015 and 2017, following that crisis. From 69 articles found, 16 were discarded because they did not fulfill the study requirements: the study had to refer both to eutrophic reservoirs and to urban settings. Therefore 53 articles have been selected and then systematized and mapped.

Results
As shown in Figure 1, there has been a growing interest in the problem of eutrophication of urban reservoirs in the last decade. This interest certainly shows an increase in the problem. Since the search was done in November 2017, results for the year 2018 do not appear. Brazil passed through an acute water crisis during the period 2014-2016 and those years experienced large algae blooms. Since most of the studies were undertaken in Brazil, it might explain the higher number of studies in 2014, 2015 and 2017, following that crisis. Of the total, 32 of the studies were done to monitor the water quality of reservoirs, 11 studies aimed at the dynamics and peculiarities of living organisms in the limnological/aquatic environment. Ten (10) of the studies reported cases and techniques for reservoir recovery. Thus, this categorization was made to analyze the reviewed articles (Table 1).
Results exhibited in Table 1 with fields "Cy" (Cyanobacteria), "OA" (Other Algae) and "S/C" (Seasonality or Climate) refer to whether there was direct mention in the study of these terms and are indicated by "y" of yes, which means that the phenomenon was present and by "n" of "no" if the term was not contemplated. The "Country" field refers to the country in which the studied reservoir is located, regardless of the affiliation of the study researchers.  Of the total, 32 of the studies were done to monitor the water quality of reservoirs, 11 studies aimed at the dynamics and peculiarities of living organisms in the limnological/aquatic environment. Ten (10) of the studies reported cases and techniques for reservoir recovery. Thus, this categorization was made to analyze the reviewed articles (Table 1).
Results exhibited in Table 1 with fields "Cy" (Cyanobacteria), "OA" (Other Algae) and "S/C" (Seasonality or Climate) refer to whether there was direct mention in the study of these terms and are indicated by "y" of yes, which means that the phenomenon was present and by "n" of "no" if the term was not contemplated. The "Country" field refers to the country in which the studied reservoir is located, regardless of the affiliation of the study researchers.
Of the 53 studies selected in this literature mapping [6], 20 were made in Brazil, 9 in China, 7 in the United States and 6 in Poland, while Portugal and Canada hosted two studies each and the other countries only 1 each study, as can be seen in Figure 2.

China
The results of redundancy analysis and variation partitioning analysis showed that antibiotic and co-selection with heavy metals were the major factors driving the propagation of antibiotic resistance genes in six urban lakes.  Canada The blue-green genus Microcystis was detected in all ponds, and was the dominant taxon in most SPs. This raises concern that SPs may serve as reservoirs of toxin-producing algae. Multivariate analyses of metals, water quality variables, and algal species composition showed considerable dissimilarity among SPs, yet comparably high similarity among reference ponds.  Portugal Model estimates show that a 10% reduction in nutrient loads will suffice to change the state to mesotrophic, and should target primarily Waste Water Treatment Plants effluents, but also act on diffuse sources. The method applied in this study should provide a basis for water environmental management decision-making. USA Historical and current nutrient concentrations were elevated throughout the watershed, in most cases above suggested criteria, and indicated that water quality of the river was and continues to be nutrient rich. A future reservoir at recent levels of water quality will likely be highly eutrophic, and anthropogenic influences will further stress this ecosystem and its water quality as the urban region expands. n n y USA Streams and rivers located west of Hudson River (WOH) deliver good to very good water to most of the receiving reservoirs. The project confirmed the eutrophic condition of the WOH Cannonsville reservoir and further linked that condition to nutrient inputs from the West Branch Delaware River. The project also confirmed that many streams located east of Hudson River (EOH) had fair to poor water quality and that streams in the Croton and Kensico watersheds were biologically and functionally degraded. Anthropogenic changes in land use from forested to agricultural in the WOH region have affected water chemistry, macroinvertebrate community structure, and stream function, although the impact is less than that caused by changes in land use from forested to urban in the EOH region.  Turkey Significance of site selection for urban activities, construction, settlements, and industry to sustain acceptable water quality in the drinking water reservoirs, which in turn will provide a higher quality of urban life for the inhabitants of the megacity. Canada Nutrient control is the primary issue, and one of the reservoirs has already exceeded the limits of being eutrophic, one is at mesotrophic conditions, and the remaining four are at the limit of being eutrophic, indicating the significance of making the correct decision and taking pertinent measures for management and control. The only mesotrophic resource has no industry and a very low population density, whereas the one that is already eutrophic has the highest population density, and the greatest percentage of urban land use within its watershed. y y y   Brazil The initial community mainly represented by R-and S-strategists (Planktothrix, Cylindrospermopsis and Microcystis) were gradually replaced by C-strategists (Cryptomonas spp., Chloroccoccales in general). Characteristics of the initial succession phases were observed in all treatments. The community was first inhabited by fast growing species but no important biomass contribution of size fractions was observed. Space liberated by blue-green species that did not adapt to the new nutrient-impoverished conditions was gradually occupied by other algal species, which together contributed to most of the total biomass registered in all treatments. In the 53 manuscripts, 39 reservoirs were studied as some merited more than one article, for they were analyzed in different aspects. The world spatial distribution of the studied reservoirs revealed locations in latitudes that span past the tropical and temperate zones, reaching cooler territories in the northern hemisphere. It was not possible to establish a distribution pattern of the studies according to the climatic classification of each area. This may indicate that aspects related to land use and water pollution play a more important role than climatic factors. However, the seasonality of time was presented in 28 of the studies (53%), indicating that variations in temperature, radiation, and precipitation are relevant factors for the proliferation of algae. The researches indicated as contributors to blooms: increased air temperatures [8], water temperature [21], stability of water column [7] reflecting weak winds, light penetration [44]. Regarding season of the year favorable for the proliferation, some studies refer to autumn [19,32,45], other mid summer [49], or refer to dry season [12] or to rainy season [44]. Thus, there is not a definite pattern regarding climate variables around the world. On the other hand, some studies reinforce that those undergoing sedimentation processes, and shallow rivers and reservoirs are more prone to eutrofization [15]. Major run offs [32,44] in rainy season are largely responsible for the process.
More than 50% of the studies are in zones of continental humid climate or continental climates, characterized by rains in the summer or throughout the year. The climatic classification of Köppen- In the 53 manuscripts, 39 reservoirs were studied as some merited more than one article, for they were analyzed in different aspects. The world spatial distribution of the studied reservoirs revealed locations in latitudes that span past the tropical and temperate zones, reaching cooler territories in the northern hemisphere. It was not possible to establish a distribution pattern of the studies according to the climatic classification of each area. This may indicate that aspects related to land use and water pollution play a more important role than climatic factors. However, the seasonality of time was presented in 28 of the studies (53%), indicating that variations in temperature, radiation, and precipitation are relevant factors for the proliferation of algae. The researches indicated as contributors to blooms: increased air temperatures [8], water temperature [21], stability of water column [7] reflecting weak winds, light penetration [44]. Regarding season of the year favorable for the proliferation, some studies refer to autumn [19,32,45], other mid summer [49], or refer to dry season [12] or to rainy season [44]. Thus, there is not a definite pattern regarding climate variables around the world. On the other hand, some studies reinforce that those undergoing sedimentation processes, and shallow rivers and reservoirs are more prone to eutrofization [15]. Major run offs [32,44] in rainy season are largely responsible for the process.
More than 50% of the studies are in zones of continental humid climate or continental climates, characterized by rains in the summer or throughout the year. The climatic classification of Köppen-Geiger is represented by the abbreviations Dfa, Dfb, Dwa and Dwb as in Figure 3. Other reservoirs, although smaller in number, are located in warmer climates that are rainy with dry winter, in the climatic zone Aw.
However, only one of them mentioned "climate change" as important in the investigation approach [7]. It dates back to 2017, one of the most recent of the bibliographical research, indicating that this theme has only recently been receiving interest in the field of limnology. Investigations highlight the contribution of phosphorus [35] and nitrogen [21,24] in the water for the outbreak of algae blooms. Of the total manuscripts analyzed, regardless of the category in which they were classified, 25 described algal blooms in general; 20 mentioned cyanobacteria blooms; and 15 reported the occurrence of several species of algae, including cyanobacteria. Thus, the presence of algae represents a constant problem of urban reservoirs and, as mentioned, it may pose a serious threat to public health since many of these reservoirs are, or have the potential to be, used for water supply due to the proximity of populations. However none of the articles presented the health risks this poses, although 2 of them [24,36] mention "drinking water", showing concern about quality for human consumption. Other articles show indirect concern for "environmental health" [9,22], "sustainable water resources" [22] or "propagation of antibiotic resistance genes in urban lakes" [9].
Even though the origin of the reservoirs was not the main object of the research, it allowed verifying its variability. Studies in Poland indicate that most of the reservoirs originated from mining caves and many were in the process of remediation. In China and Brazil, the reservoirs are mostly dammed rivers and streams. In temperate countries, reservoirs could be formed from snowmelt.
Among the 10 studies describing techniques used for reservoir remediation, 9 showed positive results and a decrease in the presence of toxic algae. Investigations highlight the contribution of phosphorus [35] and nitrogen [21,24] in the water for the outbreak of algae blooms. Of the total manuscripts analyzed, regardless of the category in which they were classified, 25 described algal blooms in general; 20 mentioned cyanobacteria blooms; and 15 reported the occurrence of several species of algae, including cyanobacteria. Thus, the presence of algae represents a constant problem of urban reservoirs and, as mentioned, it may pose a serious threat to public health since many of these reservoirs are, or have the potential to be, used for water supply due to the proximity of populations. However none of the articles presented the health risks this poses, although 2 of them [24,36] mention "drinking water", showing concern about quality for human consumption. Other articles show indirect concern for "environmental health" [9,22], "sustainable water resources" [22] or "propagation of antibiotic resistance genes in urban lakes" [9].

Discussion
Even though the origin of the reservoirs was not the main object of the research, it allowed verifying its variability. Studies in Poland indicate that most of the reservoirs originated from mining caves and many were in the process of remediation. In China and Brazil, the reservoirs are mostly dammed rivers and streams. In temperate countries, reservoirs could be formed from snowmelt.
Among the 10 studies describing techniques used for reservoir remediation, 9 showed positive results and a decrease in the presence of toxic algae.

Discussion
The results of this research, done without time or space limits, indicated reduced numbers of investigated reservoirs considering that this is such an important topic in a context of great worldwide concern with water resources. It is a limited number of papers that consider the approach of the theme identifying the reservoir as urban, recognizing this environmental characteristic as an important factor. Another limitation of the research may be the fact that the database used searches only in indexed journals that have digitized collections.
Although literature indicates studies on these algae since 1992, the pioneering study was digitized in 2006. Other studies on the subject may have been done previously, although they have not been indexed in a bibliographic database. Our literature review shows that concerns and studies are growing in time. This may reflect an increase in the problem of reservoir eutrophication, which has been concentrated in large, less recent industrial urban centers, such as in the Great Lakes region of North America, the site of a significant number of studies on the subject. Nowadays, with the acceleration of urbanization in low-and middle-income countries, this problem reaches urban centers in these nations more intensely, mainly due to the deficits in sanitation they present, thus prompting further research. This is the case of Brazil, which has seen some of its water supply reservoirs, such as the Guarapiranga dam, threatened by the proliferation of cyanobacteria [5].
Many of the studies mention as the most important determinant for eutrophication of reservoirs and the proliferation of algae the anthropogenic influence through diffuse loads of streets, domestic and industrial sewage, and even runoffs from agricultural areas in the drainage basin. The sediments in the reservoirs and the nutrient loads of phosphorus and nitrogen are directly related to the soil occupation for urban settlement, and to the removal of natural vegetation [22,31]. However, those are not the only drivers: high values of coliforms due to untreated sewage, low values for diversity of epibenthic macrofauna and fish [23], air dried organic matter in shallow lake sediment [27], excretion by larvae at high temperatures area also important contributors, according to the studies cited in this review. Thus, there are many challenges regarding the control of eutrophication in cities reservoirs, streams and rivers.
The outbreak of cyanobacteria changes the functional phytoplankton group and zooplankton communities in urban eutrophic lakes [33,40], which are further altered by copper sulphate treatment [41] that might also alter the morphology of fish gills [42].
Even though the restoration studies, mentioned in this review, report good results in the control of nutrient load by wastewater treatment and prevention of diffuse sources to enter the water bodies, sewerage and adequate controls need also to be placed in regions upstream [21,35] and a selection of activities must be made by city government and residents. However, high costs, complexity of actions, and the long term seem to discourage decision makers.
The relationship found in some papers between climate variables and cyanobacteria blooms has raised concern that global climate change, by reinforcing favorable flowering time periods, could aggravate the frequency of the phenomenon [5], turning climate control another major challenge.
These studies cited should base programs and actions that aim to limit water pollution, and reinforce arguments for the relevance of facing this problem urgently, often by giving economic value to reservoirs, but above all to protect human and animal life. Reducing water pollution can be done by government policies such as zoning or land use regulations, even though extensive basic sanitation programs in cities of middle and low-income countries may chiefly achieve this reduction. The conservation of water sources can be an important strategy to safeguard water supplies for coming decades.

Conclusions
The quality of human water supply is, at the same time, a very old and a very recent topic. It is an area of interdisciplinary and multidisciplinary research and action that is gaining importance given the water scarcity that has affected populations in various parts of the globe, and because of large-scale climatic oscillations prompted increased attention to the issue. The proliferation of cyanobacteria, as well as the contamination of urban reservoirs, once considered topics of limnology, must also be considered a problem of urban planning and governance.
The number of articles found in this mapping literature search does not reflect the real environmental issue of eutrophic reservoirs in urban areas of the world. However, there might be more abundant studies on the topic in specific scientific areas exclusively. The importance of our study was to highlight the relation between urban land and eutrophication of reservoirs, and the risk algae blooms pose to growing urban populations. We verified that little literature has been published bridging these knowledge areas.
As it is such a widespread problem and few countries are studying it as an approach of urban environment, which makes it a true Challenge for Public Health, and we recommend further investigations in more countries and hydrographic basins. As mentioned, because these algae are resistant to most water treatment technologies, they pose health risks or they might cause the abandonment of water sources closer to cities, consequently increasing the costs for providing safe water to their inhabitants.
Finally, we call for joint studies and cooperation of laboratories and study groups in limnology, hydrology and environmental health, but also in urban planning, and urban climatology.