Aquatic Macrophytes in Southern Amazonia, Brazil: Richness, Endemism, and Comparative Floristics

Southern Amazonia harbors a wide diversity of aquatic macrophyte species because of its diverse wetland habitats and location in the Amazon-Cerrado transition zone, which spans the two largest biogeographic domains in South America. We investigated the taxonomic diversity of aquatic macrophytes in the region with a focus on endemism, species richness, and life forms. We present new records of aquatic macrophyte species and compare our results with other Brazilian phytogeographic domains. We found a high number of species of aquatic macrophytes for the southern Amazon region, comparable to extensive inventories in southern, northeastern, and northern regions of Brazil. We recorded 709 species of aquatic macrophytes in 313 genera and 97 families, which includes 90 species endemic to Brazil and five species endemic to the Brazilian Amazonia. The macrophyte species list of southern Amazonia showed < 25% similarity to inventories in Amazonia and Cerrado. This high diversity of aquatic macrophytes in southern Amazonia, with endemic species and others with restricted ranges, emphasizes the need for conserving these wetlands and vegetation types.


Introduction
Aquatic macrophytes grow actively while permanently or periodically submerged, floating, or emerging from the water surface (Chambers et al. 2008) and colonize most aquatic ecosystems to different degrees. The composition, richness, and cover of aquatic macrophytes may be determined by abiotic factors such as climate, isolation, diversity of habitats, nutrients, and hydroperiod (Sousa et al. 2011;Schneider et al. 2019;Yang et al. 2020). The richness of aquatic macrophytes depends mainly on the size of a wetland, although their composition may not follow this pattern (Maltchik et al. 2007). Neotropical aquatic environments, with generally high temperatures, extreme water-level fluctuations, and a wide variety of habitats, harbor the highest richness and endemism of aquatic macrophytes (Murphy et al. 2003;Fortney et al. 2004;Murphy et al. 2019;Moura-Junior et al. 2021).
Brazil is a continental country with several phytogeographic domains (Amazonia, Caatinga, Cerrado, Atlantic Forest, Pampas, and Pantanal) with well-defined morphoclimatic characteristics. However, extensive rivers and watersheds connect these domains. For example, many Amazonia and Caatinga hydrographic basins originate in the Cerrado (Marcuzzo 2017). Due to these hydrologic connections, some aquatic macrophytes are widely distributed across the country, but there are some isolated distributions that are endemic to certain Brazilian phytogeographic domains. (Pivari et al. 2019;Moura-Junior et al. 2021).
The Amazon Forest, the largest freshwater reserve globally (except Antarctica), supports one of the highest known levels of biological diversity, including over 50,000 terrestrial vascular plant species (Hubbell et al. 2009). The Amazon Basin, situated between the Guianas Plateau to the north, the Central Plateau to the south, the Andes Mountains to the west, and the Atlantic Ocean to the east, drains parts of eight South American countries besides Brazil (Hess et al. 2003). This phytogeographic domain covers an area of 6,500,000 km 2 , with 60% or about 4.2 million km 2 in Brazil, representing 49% of the country (IBGE 2010). Southern Amazonia is a large region with a width of approximately 6000 km that includes a transition zone between the Amazon Rainforest and the Cerrado (Brazilian savanna). This ecotone region contains a diversity of habitats and vegetation types, from dense deciduous or evergreen forests to savanna formations (Marimon et al. 2006;Torello-Raventos et al. 2013). The diverse wetland habitats include permanently or seasonally flooded areas such as streams, rivers, lakes, floodplains, marshes, and swamps, plus several large reservoirs of hydroelectric power plants (Junk et al. 2020). Within this environmental complexity, the Cerrado-Amazonia transition zone is rich in typical Amazonian tree species, also shared with the Atlantic Forest and the Cerrado (Marimon et al. 2006;Oliveira-Filho 2017). The woody flora of this transitional region is well known, but the aquatic macrophyte flora has not been thoroughly studied, which represents a knowledge gap regarding floristic relationships between different regions of the Amazon domain and other Brazilian phytogeographic domains.
Because of heavy deforestation pressures from agricultural, timber, and hydroelectric development, this ecotonal region is also known as the "Amazon deforestation arc" (Fearnside 2005). Deforestation may destroy immense genetic stocks that are not well known, documented in herbaria, or conserved in germplasm banks (Morandi et al. 2016b). Such genetic erosion can result in the loss of valuable information, which may be useful for agriculture, medicine, and industry (Fearnside 2019). Thus, there is an urgent need to investigate and document the diversity of aquatic macrophyte species in the southern Amazonia region (Murphy et al. 2019). Our study investigates the taxonomic diversity of aquatic macrophytes in southern Amazonia with a focus on endemism, species richness, and life forms. We present new records of aquatic macrophyte species and compare our results with other Brazilian regions.

Study Area
In central-western Brazil, the State of Mato Grosso contains parts of three phytogeographic domains (Amazon, Cerrado, and Pantanal) and diverse flora with high socio-economic and environmental potential. Vegetation types in the southern Amazon part of northern Mato Grosso include Transitional Forest, Evergreen Seasonal Forest, and Seasonal Forest of the southern Amazon border (Ivanauskas et al. 2004;Kunz et al. 2008). This part of southern Amazonia is an area of ecological tension (Veloso et al. 1991). The flux of biodiversity in the Cerrado-Amazonia transition and the complex mosaic of vegetation landscapes make it difficult to delimit the phytogeographic domains, especially in the transition zone (Marimon et al. 2006;Morandi et al. 2016a). The Amazon Forest covers approximately 500,000 km 2 in Mato Grosso, with areas of dense forest containing trees that can reach 50 m in height. The forest is divided by large rivers and includes preservation areas like Cristalino State Park, Rio Ronuro Ecological Station, and Xingu State Park. In the northern region, the Teles Pires and Juruena rivers join to form the large Tapajós River, and together with the Xingu River, they have a highly diverse flora along the banks (Fig. 1).

Survey of Aquatic Macrophyte Species Recorded in Databases
To develop the species list for the southern Amazonia portion in mid-northern Mato Grosso, we included georeferenced records of herbaria in the region available in the spe-ciesLink (https:// splink. cria. org. br) and Global Biodiversity Information Facility (GBIF -www. GBIF. org) databases. We checked all records individually and added geographic coordinates of records with detailed information on the site location. Our selection of aquatic species followed the Repository of Aquatic Plants of Brazil (https:// sites. icb. ufmg. br/ plant asaqu atica sbras il) and the Brazilian Flora database (Flora do Brasil 2020), using the search filters "State: Mato Grosso", "Phytogeographic domain: Amazonia", "Substrate: Aquatic" and "Formation: Aquatic Vegetation". Furthermore, they were considered reports from the state (Pott and Pott 1997)

Field Collections
We also incorporated species we collected during five years of field research (2015)(2016)(2017)(2018)(2019)(2020) in the Tapajós (Teles Pires, Cristalino, Juruena) and Xingu (Xingu and Ronuro) subbasins of the Amazon River, at different times and hydrological periods (drought, rising water, flood, and drawdown). Fertile specimens were recorded using random sampling, following the procedure described by Filgueiras et al. (1994). We identified the species by consulting literature, taxonomists, and herbarium collections. The specimens are kept in the Centro-Norte-Mato-Grossense Herbarium (CNMT) of the Acervo Biológico da Amazônia Meridional (ABAM) at the Universidade Federal de Mato Grosso (Sinop Campus), and established several unique records for the State of Mato Grosso. We used the concept of aquatic macrophytes suggested by Cook (1996), including plants with photosynthetically active organs that are totally or partially submerged in freshwater or floating in aquatic habitats, either permanently or for several months of the year.

Data Processing, Life Forms, and Habitats
We used geographical coordinates to circumscribe the databases and field records in the Amazon part of Mato Grosso with the help of the software QGIS (Q-Gis. org 2020), according to the delimitation of Brazilian phytogeographic domains (IBGE 2010), excluding records from other domains in the state (Cerrado and Pantanal). The habitat description or the collection habitat was useful for selecting relevant records. Target habitats included riparian forests, lakes, ponds, rivers, streams, palm swamps (veredas), floodable grasslands, marshes, swamps, reservoirs, headwaters, igapós (seasonally flooded forests), and igarapés (small streams or channels). We considered records with identification confirmed at the epithet level by taxonomists. The remaining records were verified by consulting herbarium sheets and available or requested images from various herbaria. We obtained information on phytogeographic domains and growth habits from the Brazilian Flora database (Flora do Brasil 2020). The species list followed the classification of families proposed by the Angiosperm Phylogeny Group -APG IV (Chase et al. 2016) for angiosperms, the Pteridophyte Phylogeny Group (PPG I 2016) for ferns and lycophytes, and Goffinet and Buck (2004) for bryophytes.
Classification of the aquatic species life forms followed Irgang and Gastal (1996), which is a classification scheme that incorporates water depth and species' horizontal zonation within the ecosystem. Life forms included: (1) amphibious species, which colonize the interface between aquatic and terrestrial habitats; (2) epiphytes/climbers: plants that root and develop on emergent and/or floating plants; (3) emergent species, which are rooted plants with emerging leaves and flowers occurring in shallow areas and close to the shore; and (4) submerged and floating species, which are present in comparatively deep-water bodies, occurring beneath or on the water surface, respectively. We obtained species life forms from record collections and the literature (Pott and Pott 1997;Moura-Júnior et al. 2015;Costa et al. 2016).
We categorized the habitat types as follows: lotic (rivers or streams), lentic (lakes and ponds), and intermediate (reservoirs or ecosystems with abiotic characteristics similar to lotic or lentic habitats) (Esteves 2011). We categorized the habitats according to the information in herbarium records. The extinction risk of species was categorized using Red List of the Brazilian Flora (Martinelli and Davilla 2012). We obtained the species' characteristics such as endemism and the phytogeographic domain (Amazon and/or Cerrado) from the Brazilian Flora database (Flora do Brasil 2020).

Data Analyses
To obtain a general picture of the potential taxonomic richness in the region, we performed a rarefaction analysis of species to compare the observed (from the records) and the estimated richness (Jackknife 1 estimator), considering years with herbarium records of aquatic macrophytes as samples, accounting for 72 samples (years) between 1903 and 2020. We used the function alpha.accum of the bat package (R version 3.6.2, Cardoso et al. 2021). We analyzed the floristic similarity between the southern Amazon aquatic macrophyte flora and other Brazilian phytogeographic domains (Table 1, Supplementary material 1), considering the wide geographical distribution of this group of plants in the Neotropical region (Murphy et al. 2019). Species that were identified to only the genus or family level, and species requiring confirmation, were excluded from the list. We performed a cluster analysis (UPGMA) based on a presence/absence matrix, using the Simpson index (Magurran 2004). The analysis was conducted in the platform R using the package recluster, function recluster.cons, and recluster.boot (R version 3.6.2, Dapporto et al. 2020). We updated all species names according to the Brazilian Flora database Flora do Brasil 2020) through the flora package (R version 3.6.2, Carvalho 2020).

Results
We cataloged 709 species of aquatic macrophytes (5 bryophytes, 29 ferns and lycophytes, and 675 angiosperms) in 313 genera (2 bryophytes, 19 ferns and lycophytes, and 292 angiosperms) and 97 families (2 bryophytes, 13 ferns and lycophytes, and 82 angiosperms). Of this total, 521 species (73%) were classified as aquatic macrophytes according to the Repository of Aquatic Plants of Brazil and the Brazilian Flora database, and 145 species (20%) were classified based on other surveys in the Amazon domain and other domains in Mato Grosso. We collected 334 species (47% of our study total) (Supplementary material 2; Fig. 2).
It is worth highlighting that 15% (100 species) had not been recorded in the Amazon domain, and 12% (79 species) did not have occurrence records for the State of Mato Grosso (Brazilian Flora database). Of the aquatic macrophytes recorded in this study, 13% (90 species belonging to 35 families) are endemic to Brazil, particularly the families Podostemaceae (11 species) and Fabaceae (9 species). We found two species in the Vulnerable category (Mourera weddeliana Tul., Podostemaceae and Ipomoea subrevoluta Choisy, Convolvulaceae), one in Nearly Threatened (Ottelia brasiliensis (Planch.) Walp., Hydrocharitaceae), 47 in the Least Concern, and six in Deficient Data (Table 2).
According to the Brazilian Flora database, only 369 species of this group of plants have been recorded in the State of Mato Grosso. In this state, in the Amazon Basin (southern Amazonia), the 331 species recorded exceed other phytogeographic domains, the Pantanal (254) and Cerrado (269). The same database lists only 453 species of aquatic macrophytes for the entire Brazilian Amazonia ( Table 2).
The year-based rarefaction curve of species richness did not reach an asymptote (Fig. 3). Therefore, southern Amazonia may harbor an even higher number of species than we present here. Southern Amazonia showed low floristic similarity with the Brazilian phytogeographic domains. The similarities with Amazonia and Cerrado were 35% and 25%, respectively, clearly suggesting that the diversity of aquatic macrophytes in our study area was influenced by the adjacent large domains (Fig. 4). Southern Amazonia flora had low similarity (20%) with the Pantanal, Caatinga, Pampas, and Atlantic Forest (Fig. 4).

Discussion
We found a high number of species of aquatic macrophytes (709) for the southern Amazon region, comparable to extensive inventories in Caatinga, Cerrado, Pantanal and Pampa phytogeographic domains and other Amazon parts. We present new records of aquatic macrophyte species and assess their degree of endemism and occurrence restricted to the region. The Neotropical region is recognized for the high diversity of aquatic macrophytes with 1619 species ( . Thus, the large area occupied by southern Amazonia (Amazonia of Mato Grosso), with 500,000 km 2 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 high temperatures 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 plant diversity (Marimon et al. 2006;Maracahipes-Santos et al. 2015;Marques et al. 2020). The meeting of the two main 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, 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 investigating several types of aquatic ecosystems on large spatial scales, 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. The highest number of species records of aquatic macrophytes in the North Amazon (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). Most surveys in the Amazon region were done before 2016. However, we found many records between 2012 and 2019, derived from collections in aquatic-macrophyte monitoring programs for hydroelectric power plants (mainly Teles Pires, Colíder, and Sinop) in the Teles Pires River. The 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  cuzzo 2017). The increase in species records for the southern Amazonia 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 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 (Moura-Júnior and Cotarelli 2019). As mentioned above, we found many amphibious and emergent species, which results in a pattern similar to those found in other floristic studies of aquatic macrophytes. 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 . 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 the determination and description of life forms often requires studies for extended periods and over several hydrological cycles (Moura-Júnior and Cotarelli 2019; Pivari et al. 2019). The similarity of the macrophytes of southern Amazonia to the species recorded for the Amazon and Cerrado domains were higher, mainly because of their proximity to southern Amazonia. Nevertheless, the community of aquatic macrophytes showed differences within the Amazon domain, i.e., between central and southern Amazonia, as supported by the species inclusion from inventories in the states of Roraima (Costa et al. 2016) and Amazonas (Lopes et al. 2020a). The environmental traits, such as the white, black and clearwater rivers of the Amazon Basin may have been related to their similarity. White waters are nutrient-rich from the Andes and are found in much of the central Amazon (Furch and Junk 1997), and clear and black waters are found in the Cerrado and other domains (Mounier et al. 1998). In this sense, clear and white waters support a higher diversity of herbaceous aquatic macrophytes than black waters (Piedade et al. 2010;Lopes et al. 2020a). In southern Amazonia, all these types of water favored the similarity with Amazonia and Cerrado domains, increasing species richness. This similarity may also 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). However, this similarity between southern Amazonia and the Cerrado may change if there were more studies or reviews with all aquatic macrophyte groups (angiosperms, lycophytes and ferns, and bryophytes) carried out in this domain. The absence of such data makes it challenging to generate a more extensive list where similarities could have been more evident. Studies with broader temporal and spatial coverage in these areas are needed to explore their potential richness. The richness of herbaceous aquatic macrophytes influences the similarity between the study sites, particularly regarding 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 Brazilian Flora Database (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 analyzed habitats ). In addition, Fabaceae is the third species-rich family in southern Amazonia and ranks second in the number of endemic species, which agrees with studies on aquatic macrophytes in other Brazilian phytogeographic domains such a Caatinga (Moura-Júnior and Cotarelli 2019) and Pampa ). The endemism found and represented mainly by two families (Podostemaceae and Fabaceae) shows the importance of southern Amazonia as a refuge for species of restricted occurrence and threatened with extinction. We highlight the yet unreported high richness of Podostemaceae (16 species) in our study, of which 11 are endemic, in line with that found in northern Amazonia (Moura-Junior et al. 2015). We call attention to the occurring Sphagnaceae (Bryophyta), which are highly endemic in Brazil but not commonly reported for Amazonia since most records and the highest richness were found in the Atlantic Forest and Cerrado domains (Costa and Peralta 2015). 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 conserving water resources, mainly in the Amazon region (Zaiatz et al. 2018;Lopes et al. 2020b).
Despite this, 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, cattle ranching, and engineering projects, such as hydroelectric power stations and roads (Junk et al. 2020), primarily in the so-called deforestation arc in southern Amazonia (Fearnside 2019). These changes in the landscape, mainly in water management, can lead to changes in aquatic macrophyte communities, for example favoring fast-growing and rapidly developing invasive species (Fares et al. 2020). Our results reinforces the need for maintenance and conservation of wetlands, especially in ecotone areas such as southern Amazonia.
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 increase the knowledge of species of aquatic macrophytes that occur in transitional areas such as southern Amazonia. This new information will help clarify the classification of these areas, which are often relicts of many communities and populations (Márques et al. 2020). In summary, we recorded a high number of species, families, and life forms of aquatic macrophytes, with high diversity, high degree of endemism, and several species with restricted occurrence in southern Amazonia. Our results exceeded expectations and indicate that further floristic studies are needed in the southern Amazonia region.