We found a high number of species of aquatic macrophytes (689) for the southern Amazon region, comparable to extensive inventories in larger areas in southern, northeastern, and northern regions of Brazil. We present new records of aquatic macrophyte species, together with their degree of endemism and occurrence restricted to the region. The Neotropical region is recognized for the high diversity of aquatic macrophytes, with 3457 species (Murphy et al. 2019), and its known richness continues to grow with new studies and records, chiefly in Amazonia and ecotones of vegetation types. The flora of this region shows a high degree of endemism, mainly in the northern part of the Amazon domain (Moura-Júnior et al. 2015; Murphy et al. 2019). The richness of aquatic macrophytes is related to several factors, with area size often noted (Ferreira et al. 2011; Murphy et al. 2003; Moura-Júnior et al. 2015; Moura-Júnior and Cotarelli 2019; Oliveira et al. 2019). Thus, the large area occupied by southern Amazonia (Amazonia of Mato Grosso), with 500,000 km2 and a variety of wetlands containing rivers of the main Amazon basins, favors this high diversity of aquatic macrophytes. The bioclimatic conditions related to the low latitude of southern Amazonia, including the high temperature and humidity, variety of water-bodies, and plant cover lead to the high richness of aquatic macrophytes in this region (Murphy et al. 2019). Many studies have documented the increased richness of plant species in lower-latitude regions (Signor 1990; Cox and Moore 2006).
The high species richness recorded here may also be related to the location of the study area in southern Amazonia, in the Cerrado-Amazonia transition. This transition area tends to accumulate a high and often unique floral diversity (Marimon et al. 2006; Maracahipes-Santos et al. 2015; Marques et al. 2020). The meeting of the two largest biogeographic domains in South America, the Amazon and the Cerrado, provides a wide variety of habitats for the permanent or temporary occurrence of many aquatic macrophytes. Furthermore, the main rivers of southern Amazonia, the Juruena, Xingu, and Teles Pires, formed by the confluence of rivers that arise in the Cerrado, such as the Verde and Ronuro rivers, may facilitate the distribution and colonization of species in the transition to the Amazon region. This transition zone harbors a high phylogenetic diversity of plants (Silva-Pereira et al. 2020).
The species richness found here exceeds that reported in similar studies, where the number of species did not exceed 300 ( Pott and Pott 1997; Costa et al. 2016; Oliveira and Bove 2016). However, comparison among inventories must consider the criteria for inclusion, coverage area, and collecting effort. Nevertheless, this highest number of species records for northern Brazil (515 species) may be related to the larger extent and higher diversity of the areas sampled and the periods when the surveys were taken (Moura-Júnior et al. 2015). For northeastern and southern Brazil, long-term studies with sampling in widely distributed areas have reported high species richness (637 and 760 species, respectively) (Moura-Júnior and Cotarelli 2019; Oliveira et al. 2019). Most surveys in the Amazon region were done before 2016. However, we found the largest number of records between the years 2012 and 2019, derived from collections in aquatic-macrophyte monitoring programs for hydroelectric power plants (UHEs) in the study area, mainly in the Teles Pires River (UHEs Teles Pires, Colíder, and Sinop). Inclusion of this sampling effort in our study contributed to the increase of 343 species records for Amazonia, 29% of which are new occurrences for this phytogeographic domain. In addition, our inventory revealed four species (Myriophyllum mattogrossensis Hoehne, Apinagia fluitans P. Royen, Lophogyne aripuanensis (A.S. Tav.) C.T. Philbrick & C.P. Bove, and Borreria flexuosa E.L. Cabral) that occur only in Mato Grosso, and one, L. aripuanensis, that is exclusive to southern Amazonia.
The increase in species records for the region may also be due to failure to include many amphibious/emergent and epiphyte/climber species in the older lists. Labels from collections and herbarium records in Specieslink and GBIF do not adequately specify or describe the life form and/or habitat, hindering the characterization of species as aquatic macrophytes. We included many terrestrial plants that are tolerant of rapid inundation or running water in the sorting criteria. Correct recording of life forms is a key aspect in the study of aquatic macrophytes. We found many amphibious and emergent species, which results in a pattern similar to those found in other floristic studies of aquatic macrophytes, as mentioned above. The high number of amphibious and emergent species is attributable to habitat types such as riparian forests, where these species are frequent and often abundant in these temporarily flooded areas (Pott et al. 2011). The seasonality of other life forms, such as floating and submerged plants, may prevent them from being recorded in the field, since many appear in wetlands for only a short time; this is reflected in the lower number of species and survey records. Accuracy in determination and description of life forms often requires that studies be conducted for extended periods and over several hydrological cycles (Pivari et al. 2019).
The similarity of the macrophytes of southern Amazonia to the species recorded for northern areas of the Amazon domain was high, especially in the Xingu River region, mainly because of its proximity to the eastern boundary of southern Amazonia. Furthermore, the community of aquatic macrophytes showed differences within the Amazon domain, i.e., between central and southern Amazonia, as supported by the low similarity to inventories in the states of Roraima and Amazonas. This difference may be due to the influence of the Cerrado domain on southern Amazonia, as described for the tree flora of these regions by Morandi et al. (2016b). The low floristic similarity of southern Amazonia to the northern Amazon region (Viruá National Park), the Amazon igapó habitats, and the Araguaia River basin may have been influenced by the fewer species recorded in these situations, which although poorest in terms of species numbers, also possess a unique flora with few species shared with the other areas. Studies with greater temporal and spatial coverage in these areas are needed to explore their potential richness.
The richness of herbaceous aquatic macrophytes was determinant for the similarity between the study sites, particularly regarding families such as Poaceae and Cyperaceae. These families are the richest in wetlands worldwide (Murphy et al. 2019) and are very common in Brazilian wetlands, such as the Pantanal (Pott and Pott 1997) and the Paraná River basin (Murphy et al. 2003; Ferreira et al. 2011). The Lista de Espécies da Flora do Brasil (2020) includes 1653 species of Poaceae. Their efficient vegetative propagation (rhizomes and/or stolons) allows species of Cyperaceae and Poaceae to adapt to temporarily or permanently flooded areas, which explains their occurrence in all the habitats analyzed. The richness of Fabaceae in southern Amazonia coincides with studies of aquatic macrophytes in other Brazilian regions such as the Northeast (Moura-Júnior et al. 2019), North (Moura-Júnior et al. 2015), and South (Oliveira et al. 2019), where it is among the five richest families. Also notable is the relative importance of Onagraceae, which in most studies is not among the richest families despite its wide distribution in Brazil (Wagner et al. 2007). However, in our study it had the fifth highest species richness, with the genus Ludwigia.
The high species diversity in the region emphasizes the importance of conserving wetlands and vegetation types where they grow; many species have restricted habitats, as shown, for example, in the endemism levels. Conservation of the vegetation associated with watercourses is directly related to conservation of water resources, mainly in the Amazon region (Zaiatz et al. 2018; Lopes et al. 2020b). Despite this, over time, wetlands in Brazil have not received the economic, ecological and/ or social value that they deserve. On the contrary, many people favor transforming these areas for agriculture and cattle ranching, or for engineering projects such as hydroelectric power stations and roads (Junk et al. 2020), mainly in the so-called deforestation arc in southern Amazonia (Fearnside 2019). A frequent motive for transformation in the ruderal many aquatic macrophytes,which grow rapidly and can become invasiveand colonize impacted habitats (Barbieri and Carreiro 2017; Demarchi et al. 2018). This reinforces the importance of and need for maintenance and conservation of wetlands.
Floristic studies in wetlands in Brazil have continuously increased in sampling areas and number of species (Pivari et al. 2019). The present findings expand the available information on Amazon areas and habitats, and also increase the knowledge of species of aquatic macrophytes that occur in transitional areas such as southern Amazonia. This new information will help to clarify the classification of these areas, which are often relicts of many communities and populations (Márquez et al. 2020). In summary, we recorded a large number of species, families, and life forms of aquatic macrophytes, with a high diversity, high degree of endemism, and several species with restricted occurrence in southern Amazonia, exceeding expectations and suggesting the desirability of further floristic studies in the region.