From the 53 studies analyzed, a total of 238 sampling stations were geo-referenced. These were superimposed on the map of life zones of Peru; from it, all studies are located in 25 life zones. Figure 2 shows the number of sampling stations included in all studies and distributed in the 25 life zones. The life zone most frequently evaluated at the national level, with a total of 90 stations, was the subtropical dry desert located mostly in the department of Lima. Second, the life zones with the highest number of stations were the tropical premontane dry forest, located in the departments of La Libertad and Cajamarca, and the tropical low montane thorny steppe, in the departments of La Libertad and Lima; each with 21 stations. The other life zones have less than 15 stations each.

Table 1 shows the number of life zones monitored in each department. It is evident that, although the department of Lima has the largest number of sampling stations, La Libertad has the greatest diversity of life zones, with a total of 12. As the department of La Libertad is located to the north and close to Ecuador, it shows a greater diversification of arid and semi-arid ecosystems. The sampling station evaluated at the highest altitude was located at 3,831 m above sea level; this zone comprises a tropical montane steppe; according to its physiographic characteristics, it is a high mountain corresponding to the high-montane altitudinal belt (MINAM, 2019a). The stations evaluated at the lowest altitude were located from 0 to 5 m a.s.l. in the department of Lima, corresponding to the subtropical superarid desert life zone. The ecosystems evaluated in these regions are coastal wetlands belonging to the complex coastal fringe physiography (MINAM, 2019a).

Peru includes three macro hydrographic slopes that host all rivers, namely the Pacific slope, the Atlantic (Amazonas) slope, and Lake Titicaca slope (MINAM, 2015b). As shown in .Supplementary Figure S1 Map of Peruvian slopes showing the sampling stations in the 53 studies (1:200,000 scale), when locating the sampling stations of the studies considered, it is apparent that most of the freshwater ecosystems evaluated flow into the Pacific slope, except for the stations in the departments of Cajamarca and Amazonas, which flow into the Atlantic slope. None of the stations flow into Lake Titicaca. To note, the present study represents the first report on the ecosystemic distribution of this group of organisms in Peru.

Of a total of 53 sources of information covering the years 1992–2020 and addressing aquatic macroinvertebrates in arid and semi-arid ecosystems, 71.7% were academic theses; of these, 58.5% (31) were undergraduate theses and 13.2% (7) were post-graduate theses; 28.3% (15) corresponded to scientific articles (Figure 3).

To note, theses are generally considered to be gray scientific literature, in the sense that the information is intended only to be awarded a professional degree, allowing the author to obtain a license to conduct professional activities, but not published in a scientific article. As this is a widespread reality for many Peruvian universities, there is a large number of academic theses on macroinvertebrates relative to scientific articles (Ferreras-Fernández et al., 2015). From the information gathered in the present study, we observed that there are few theses (6) that have been published as scientific articles; thus, a huge amount of valuable information is hidden or difficult to access.

Figure 4 shows that the departments with the largest number of studies conducted on aquatic macroinvertebrates in arid and semi-arid ecosystems are Lima and La Libertad, with 37.7 and 18.9% of publications, respectively, while Cajamarca (9.4%), Callao (7.5%), and Piura (5.7%) contributed 10% or less, and Ica, Arequipa, Ancash, Arequipa-Ayacucho, La Libertad-Cajamarca, Lima-Callao, and Amazonas have very few studies. The larger number of studies have been conducted in Lima and La Libertad because a large portion of the territory comprises arid and semi-arid areas (Guerrero et al., 2019). It should be noted that some of the works were carried out in two departments (Lima-Callao, Arequipa-Ayacucho, and La Libertad-Cajamarca). On the other hand, it is worth mentioning that Callao is the only constitutional province of Peru that has its own autonomous government (Municipalidad Provincial del Callao, 2013); therefore, the results of the studies carried out in this province are considered as corresponding to a department. In the case of the description of life zones, the department of Lima comprises the studies conducted in Lima, Callao, and Lima-Callao, since geographically they belong to the same ecological regions; on the other hand, the studies conducted for Arequipa-Ayacucho and La Libertad-Cajamarca were processed separately, i.e., by department (Figure 4).

Aquatic macroinvertebrates are important in arid ecosystems (Scheibler, 2008), which are confined to a small part of the Peruvian territory. The higher frequency of studies conducted in Lima reflects the existence of a larger number of universities with professional degrees compatible with the study of macroinvertebrates, in addition to the specialization of advisors, who encourage the elaboration of theses addressing subjects on related areas and lines of research.

The topics most frequently addressed in relation to aquatic macroinvertebrates in arid and semi-arid ecosystems of Peru are bioindication (49.1%) and biodiversity issues (34%), whereas taxonomy, distribution, biological control, and toxicity are less frequent, accounting for less than 17.5% (Figure 5).

Bioindication and biomonitoring have received growing attention in recent years, a trend likely to continue in the future. To a certain extent, the focus on these topics derives from the interest in objectively measuring the effect of human activity on water resources as a result of increased pressure when managed as sources of goods and services for the growing human population. This is particularly evident for urban areas, the number of which has increased dramatically in recent decades (Montgomery and Elimelech, 2007; McDonald et al., 2011). Rivers have been the most heavily impacted aquatic environments, given the extraction of large volumes of water and the disposal of untreated wastewater into them, both of which have led to their deterioration (Da Cruz and Ríos-Touma, 2018; Ríos-Touma and Ramírez, 2019). The value of macroinvertebrates to estimate the ecological quality of aquatic ecosystems to quantify the effect of pollution has been demonstrated (Roldán-Pérez, 2016; Domínguez et al., 2020). Therefore, a high percentage of the studies reviewed uses indices such as the Biological Monitoring Working Party (BMWP) adapted to countries such as Colombia, and the Andean Biotic Index (ABI), used for rivers above 2,000 m a.s.l. (Ríos-Touma et al., 2014). Few investigations have adapted these indices to the particular conditions of the water resources studied, which is necessary given the heterogeneous environmental conditions involved.

On the other hand, the biodiversity topic, which is closely related to taxonomy, has been addressed because of the need to generate additional scientific information on macroinvertebrates in Latin America, Peru included, as this information is scarce relative to data for other continents (Ramírez and Gutiérrez-Fonseca, 2014). In our view, this topic will also be addressed more frequently in the coming years, aiming to determine the species composition, richness, and abundance, which will set a solid base for the development of adaptations and calibration of biological quality indices suitable for the Peruvian rivers.

The studies reviewed have addressed mainly macroinvertebrates as components of rivers, accounting for 70.4%, contrasting with 29.6% for coastal lagoons and wetlands (Figure 6). Peru is home to 12,201 lagoons, 31.9% in the Pacific slope that includes mostly arid and semi-arid ecosystems, 70% in the Atlantic slope, and only 6.9% in the Titicaca basin (Masson, 2002). As regards rivers, more than one thousand are reported flowing across 53 basins, mainly on the Pacific slope and only during the rainy season in most cases (Brack and Mendiola, 2000). The greatest research effort focused on macroinvertebrates in lotic systems, probably because these are easier to access for characterization. Also, the negative impacts of urban centers are most frequently observed in rivers, hence the interest in determining their ecological status aiming to set conservation measures, which has yet to be achieved in most cases. The concern about rivers affected by human populations has led to concepts describing the changes associated with what is known as the “urban river syndrome” (Meyer et al., 2005; Walsh et al., 2005; Roy et al., 2016), describing changes in hydrogeology, increased nutrient loads, and loss of native fauna. Studies addressing macroinvertebrates in coastal lagoons and wetlands are less frequent. Therefore, these studies should be promoted to get detailed knowledge about the characteristics of macroinvertebrates and the functioning of these ecosystems, as well as the adaptation and calibration of indices to conduct biomonitoring with tangible results, as in the case of lotic systems.

According to the online data reviewed, early research on aquatic macroinvertebrates in the arid and semi-arid ecosystems of Peru started in the 1990’s. Some theses works were probably conducted before this decade, which may be available at college libraries, thus hampering their analysis and inclusion in the present revision. Today, eleven Peruvian universities are conducting this type of research: Universidad Ricardo Palma (URP); Universidad Privada Antonio Guillermo Urrelo (UPAGU); Universidad Nacional de Trujillo (UNT); Universidad Nacional de San Agustín de Arequipa (UNSA); Universidad Nacional del Santa (UNS); Universidad Nacional de Piura (UNP); Universidad Nacional Mayor de San Marcos (UNMSM); Universidad Nacional Federico Villarreal (UNFV); Universidad Nacional de Cajamarca (UNC); Universidad Nacional Agraria la Molina (UNALM) and Universidad Nacional de Jaén (UNJ). Besides these institutions, the University of Barcelona (UB), Spain, also carried out a study on these ecosystems (Figure 7). Most studies (71.7% being theses) were conducted by researchers affiliated to Universidad Nacional de Trujillo; Universidad Nacional Mayor de San Marcos, and Universidad Nacional Agraria La Molina, these being the institutions that contributed the most to the current knowledge. These are followed by Universidad Nacional Cajamarca and Universidad Nacional de San Agustín de Arequipa (10.6% of total theses). The participation of the rest of the universities was scarce, with a single research study (thesis) on macroinvertebrates recorded for the period analyzed (Figure 7). These results show the research activities of Peruvian universities in these ecosystems. The few studies on the subject of interest at most universities face serious limitations that restrain scientific productivity (Ramírez and Gutiérrez-Fonseca, 2014). Besides, in the Peruvian universities there is little communication between researchers of the different universities; this is likely one of the obstacles for conducting research in universities with a small contribution to the training and professional development of scientists, added to the scarce availability of economic funds for research. Thus, a greater collaboration between them will certainly improve the communication and collaboration between universities (Ramírez and Gutiérrez-Fonseca, 2020). In this paper, the inclusion of the information by department and university will make possible seeking national and international funds for the conduct of studies with international partnership, such as the Universidad Nacional de Cajamarca (National University of Cajamarca; Programa de Cooperación Interuniversitaria e Investigación Científica entre España e Iberoamérica, 2011; VLIR-UOS, 2018).

Regarding the number of publications (theses and scientific articles), a significant increase occurred in the past decade (Figure 8). In the 1990’s, little work was conducted (5.7% of the total recorded), which increased in the period 2001–2010 (18.9%); the past decade was the period when most studies were carried out (75.5% of the total), mainly undergraduate theses. This boost in research productivity over the past 10 years probably reflects the effort made by some of the universities to finance research and training of human resources, not to mention the increased access to the information in some universities by acquiring the Web of Science database service, which is available to the university community. Also important is the development of Limnology, mainly in Colombia, which paved the way for other Latin American countries (Roldán, 2020). These 53 research studies have contributed valuable information about macroinvertebrate communities of arid and semi-arid ecosystems of Peru, as research projects to advance the knowledge of these aquatic ecosystems have become a priority (Hankel et al., 2018), facilitating the identification and classification of freshwater species, as well as a deeper insight on their life histories (Contador et al., 2012).

The information sources reviewed resulted in a total of seven phyla, 10 classes, 39 orders, and 118 families of aquatic macroinvertebrates recorded in arid and semi-arid ecosystems of Peru, with the most representative groups belonging to the class Insecta; these findings are consistent with previous investigations conducted in Peru (Ortega et al., 2010; Lujan et al., 2013; Arana and Cabrera, 2017; Carrasco et al., 2020; Sweeney et al., 2020). Geographically, the western hydrographic slope recorded the highest richness of the different taxonomic categories, probably because most research studies (theses and scientific articles) were conducted in this area (Figure 3).

The phyla Cnidaria, Nematoda, and Nematomorpha were not recorded in the eastern slope (Table 2); also, a smaller number of classes (9) (Table 3) and orders (16) were reported in the eastern versus the western slope (10 and 39, respectively). A similar trend was also observed at the family level; of the 118 families recorded, 59 were reported for the eastern slope. According to the type of aquatic ecosystem, rivers showed the highest number of records for the categories phylum (7), class (10), order (34), and family (110), followed by coastal wetlands with five phyla, seven classes, 21 orders, and 57 families (Table 4).

At the order level, Diptera showed the highest species richness (Table 5), similar to arid zones of northern Chile (Ferru and Fierro, 2015), the Andean centerwest of Argentina (Scheibler, 2008), and the northwest of Argentina (Hankel et al., 2018; Domínguez et al., 2020). Also, the data on the richness of aquatic macroinvertebrates reported to family were arranged systematically, which will facilitate comparisons with studies carried out in similar areas of South America (Palma et al., 2013). These figures may be higher because several studies have been conducted in these environments in the past decade, mainly located in the central coast of Peru, aiming to explore the effects of climate change on the coast of Lima (Aponte, 2017).

The families recorded most frequently in both slopes were Chironomidae, Baetidae, Simuliidae, Elmidae, Hydrophilidae, Libellulidae, Physidae, Dytiscidae, Ceratopogonidae, Coenagrionidae, Hydroptilidae, Hydropsychidae, and Tipulidae. On the other hand, the most frequent families considering all types of aquatic ecosystems evaluated were Chironomidae, Baetidae, and Dytiscidae (Table 6).

To note, the cosmopolitan family Chironomidae (Andersen et al., 2000; Sæther and Ekrem, 2003) showed the highest frequency in the study area, consistent with previous studies of arid and semi-arid environments in South America (Scheibler, 2008; Colla et al., 2013; Zelarayán and Salas, 2014; Ferru and Fierro, 2015; Domínguez et al., 2020).

Universidad Nacional Mayor de San Marcos is supervising a number of theses on coastal wetlands. One study has focused on the aquatic macroinvertebrate community in the Regional Conservation Area of Ventanilla Wetlands (Callao), during 2018. In addition, two theses are ongoing to evaluate the conservation status of the El Paraíso Coastal Wetland (Lima) considering aquatic communities, macroinvertebrates included. On the other hand, for lotic ecosystems, there is a thesis on biodiversity and bioindication of macroinvertebrates in the Chancay River, Province of Santa Cruz, Cajamarca, which is in the taxonomic identification stage, and another on the altitudinal distribution in the Chuschama gorge, Sucre Province, Ayacucho.

Since 2016, the Universidad Nacional de San Agustín de Arequipa has strongly supported scientific research in various lines of research, e.g., Limnology. Four projects are ongoing: “Ecological status and environmental variability of the Tambo Basin through the study of aquatic communities as part of integrated management”; “Selection and adaptation of biotic indices for the assessment of the ecological status of the Sihuas and Vitor rivers in the southwestern slope of Peru”; and another two addressing the pollution of lotic systems and the physicochemical characterization of metals and agrochemicals in water, sediments, and the trophic chain of Criphiops caementarius in the Tambo, Camaná-Majes, and Ocoña rivers.

To continue advancing the scientific knowledge on aquatic macroinvertebrates, research work should focus on taxonomy and identification of these organisms, particularly on immature stages of aquatic insects, as well as platyhelminthes, molluscs, and annelids; this requires providing proper training to future taxonomy specialists. Besides, taxonomic keys are needed for the various macroinvertebrate groups inhabiting rives and wetlands along the Peruvian coast; these will facilitate conducting basic research on systematics and biodiversity, which in turn contribute to the identification of biomarkers.

Also needed are studies addressing the functional structure of macroinvertebrates to gain a deeper understanding of ecological processes and the participation of these organisms in the functioning of the Peruvian arid and semi-arid ecosystems. As regards bioindicators, research is needed on the calibration of the indices currently available (ABI, BMWP, etc.) for lotic and lentic environments of the hydrographic basins in arid and semi-arid areas. The geological, geographic, and climatic conditions that characterize these ecosystems are unique, subject to highly variable large-scale weather events such as El Niño and La Niña. For this reason, future studies should also explore the relationships between environmental variability and the patterns of abundance, richness, and distribution of aquatic macroinvertebrates. The development of networks of scientists working on these topics should be encouraged, including scientific meetings of Peruvian specialists to address future studies in the northern and southern areas of Peru, aiming to obtain further information on the ecology of aquatic macroinvertebrates in these areas.

Further support should be provided to studies addressing taxonomic and ecological aspects of macroinvertebrates in the eastern slope and to the departments with the lowest number of research studies as there are limited studies currently available, probably for the lack of specialists and economic resources in research institutions, as well as access to potential study areas, broadening the thematic approach to an integral ecosystem-oriented perspective, and calibrating and validating biotic indexes for the major hydrographic basins.

Last but not least, we propose the elaboration of a national database chartered by a Peruvian agency such as the Ministry of the Environment to gather information on the diversity of the taxonomic groups of aquatic macroinvertebrates, in partnership with academic institutions conducting research on this field, as indicated in Figure 7.

This work represents an early effort to review, systematize, analyze, and gather the results of the various studies on aquatic macroinvertebrates in Peru, particularly in arid and semi-arid ecosystems. Likewise, it contributes valuable information to the knowledge of aquatic biodiversity and its distribution in these types of ecosystems for Latin America, in addition to the studies recently conducted in Argentina (Hankel et al., 2018; Domínguez et al., 2020), Venezuela (Rodríguez, 2020), Colombia (González and Moreno, 2017; Roldán, 2020), Mexico (López-López et al., 2019), Ecuador (Valdiviezo-Rivera et al., 2018), Brazil (Barbosa et al., 2012), Chile (Valdovinos, 2008), and Bolivia (Rocabado, 2003).