Composition and richness of monogonont rotifers from La Plata River Basin, South America

: We present here the first study that analyzed the composition and richness of rotifers of the entire La Plata River basin, the second largest in South America, based on simultaneous and standardized sampling. Fifteen large reservoirs and eight river stretches were selected in the upper, middle, and lower portions of the Paraná, Paraguay, and Uruguay Rivers, which are the major rivers of the La Plata basin. We took a total of 86 samples (open water habitats) in 2010. A mean of 27±11 species per sub-basin was found, with the highest richness in the Lower Paraná (41 species), followed by the Paranapanema (40 species) and Lower Uruguay (38 species). Low richness was observed in the Middle Uruguay and Middle Paraná. We found 106 species belonging to 21 families and two orders. The family with the highest number of species was Lecanidae (21), followed by Brachionidae (20), Trichocercidae (9), and Synchaetidae (8). The species with higher occurrences were Conochilus dossuarius , Kellicottia bostoniensis , Keratella americana , Keratella cochlearis and Hexarthra mira . New occurrences of rotifers were registered for Brazil ( Colurella adriatica ), São Paulo State ( Enteroplea lacustris ), and Argentina ( Gastropus hyptopus , Harringia rousseleti and Lecane thienemanni ). Spearman correlation between the number of species and physical and chemical variables demonstrated positive correlation with chlorophyll and temperature, and negative correlation with dissolved oxygen. We extend the distribution list for some native ( Lecane ludwigii ) and non-native species of rotifers ( K. bostoniensis ). We also list the monogonont rotifer species found at the sampling stations.


Introduction
Species inventories are important tools for conservation measures and management, especially in areas imperiled by human actions. It is also useful to show gaps in the scientific knowledge about zooplankton diversity and directions for future research.
There have been surveys of Rotifera diversity in the La Plata River basin, the second largest in South America. However, these surveys have focused on regions such as in the Upper Paraná floodplain (Lansac-Tôha et al. 2009), waterbodies of São Paulo State (Souza-Soares et al. 2011), and a few tributaries (Neschuk et al. 2002, Kuczynski 2017. There have been no basin-wide surveys that included all the countries drained by the basin. The La Plata River basin has very distinct environments, with extensively dammed and undammed reaches. For example, there are reservoirs in more than half of the upper reaches in the Paraná River basin, leaving few truly lotic reaches; the opposite occurs in its middle and lower reaches (Agostinho et al. 2007). The situation is very similar for the Uruguay River. However, there are no reservoirs in the Paraguay River (Perbiche-Neves et al. 2016). This results in different habitats with distinct limnological features, which may favor differences in rotifer species composition among lotic and lentic regions.
There have been multiple studies of rotifer richness and distribution in Brazilian and Argentinian waters of the La Plata River basin. For example, Garraffoni & Lourenço (2012) surveyed rotifer species throughout Brazil. Other rotifer surveys were less extensive, such in Mato Grosso do Sul (Roche & Silva 2017), São Paulo (Souza-Soares et al. 2011), the Upper Tietê River basin (Lucinda et al. 2004), and Paranoá Reservoir (Padovesi & Andreoni 2011). Despite those surveys, the number of rotifer surveys are underrepresented , when compared to other groups of zooplankton such as copepods (Silva et al. 2009, Perbiche-Neves et al. 2014.
Therefore, we provide for the first time a spatially extensive survey of Rotifera species found in the lentic and lotic stretches of the La Plata River basin to characterize its species diversity patterns. In addition, we have expanded the distribution of some Rotifera species not yet reported in the literature, thus contributing to the general knowledge of the diversity of the group in the region.

Study area
The La Plata River basin has an area of 3.1 million km 2 (Cuya et al. 2013) and drains portions of five countries: Brazil, Paraguay, Uruguay, Argentina, and Bolivia. The main sub-basins are the Paraná, Paraguay, and Uruguay River basins. The Paraná basin is the largest, covering 48.7% of the basin, followed by the Paraguay (35.3%) and Uruguay (11.8%) basins (Cuya et al. 2013).

Sampling
A total of 86 samples were collected at 43 stations, including 15 reservoirs (in dam and upriver zones) and 13 lotic stretches distributed in the three main sub-basins of La Plata River ( Figure 1, Table 1). Sites (open water -littoral habitats were not included) were sampled in January (summer -wet season) and July (winter-dry season) 2010. Ten water quality variables were measured at each sampling station during each visit following Perbiche-Neves et al. (2016) and Pessotto & Nogueira (2018): total phosphorus and nitrogen, temperature, transparency, turbidity, conductivity, pH, dissolved oxygen, depth, and total chlorophyll.  We sampled rotifers through vertical hauls by using a 50 μm mesh conical plankton net. In deep sites, the maximum depth hauled was 40 m (Perbiche-Neves et al. 2019). The sampled rotifers were subsequently packed, labeled, and fixed with 4% formalin solution. Identifications were conducted with an optical microscope (Zeiss Axio Imager.A2m) and by using species keys (Edmondson 1959, Koste 1978, Nogrady et al. 1995, Segers & Dumont 1995, Smet & Pourriot 1997, Nogrady 2002, Wallace et al. 2019. Voucher specimens were deposited in the Laboratory of Continental Waters Ecology, Institute of Biosciences of Botucatu at the Universidade Estadual Paulista Júlio de Mesquita Filho (UNESP), Brazil. The number of species was correlated with water quality variables by using non-parametric Spearman correlation and a logarithmic transformation in R Cran Project 3.3.0 (2016) using the Hmisc package of R.

Results
The mean rotifer richness was 27±11 species. The sub-basins with higher richness were the Lower Paraná (41 species), followed by the Paranapanema (40 species) and Tietê (35 species). The basins with lower richness were the Middle Paraná and Lower Uruguay ( Figure 2A).
The mean ± standard deviations of water quality variables (Table 3) stratified by sub-basin reveals that the Tietê River has higher levels of total nitrogen, phosphorus, chlorophyll, and electrical conductivity.
The Lower Paraná River also demonstrates high values for these variables except for nitrogen. Higher temperatures were found in the Paraguay and Iguaçu Rivers. The lowest levels of dissolved oxygen occurred in the Paraguay River. Spearman correlations indicated that total chlorophyll and water temperature were positively correlated with species richness; dissolved oxygen demonstrated a negative correlation (Table 4).

Discussion
We found a total of 106 rotifer species in the La Plata basin. Our data represent 14% of the rotifer species richness known to Brazil (Garraffoni & Lourenço 2012), 37% of that for São Paulo State (Souza-Soares et al. 2011), 30% of that for the Upper Paraná (Lansac-Tôha et al. 2009), and 40% for the Upper Paraguay River (Branco et al. 2018). Data from other inventories show that the rotifer fauna in the La Plata River basin is richer than what was demonstrated in our study, possibly because we sampled in few Uruguay and Paraguay River stretches, and exclusively in open water habitats, not in littoral. Therefore, as recommended by Ferrando & Claps (2016), further investigations should be carried out to expand the distribution list of species in the La Plata River Basin.
The higher summer (wet season) rotifer richness may be associated with the concentrated rainfall events that occur during this season. Summer rains can carry nutrients and organic matter from the margins of aquatic environments resulting in increased food concentration and a reduction in competition for resources. The same tendency was observed for the rainy season in a study performed on a tropical lake in Mexico (Jiménez-Contreras et al. 2018). Richness may also be related to the sediment mixture caused by intense rains. This process provides a favorable condition for hatching of dormant stages (i.e., resting eggs), resulting in an increase in rotifer species richness. Greater rotifer species richness was observed in the Lower Paraná sub-basin. Rotifers have low locomotion capacity and are carried by drifting through the central channel of the river and consequently the species richness increase towards downstream.
Barra Bonita Reservoir in the Tietê River sub-basin was the site with the greatest richness. Despite being a reservoir with a high degree of anthropogenic disturbance, including eutrophication (Tundisi et al. 2008), many studies have shown high biodiversity for other groups, which include rotifers (Matsumura- Tundisi & Tundisi 2005, Rocha et al. 2006). In the Barra Bonita Reservoir, Matsumura- Tundisi & Tundisi (2005) found 32 species of rotifers. However, in our work we found 22 species. The Spearman correlation suggested a positive relation between richness and chlorophyll levels, with Barra Bonita Reservoir demonstrating the highest values of observed chlorophyll. Presumably, this higher richness is a result of greater numbers of tolerant rotifer species (Allen et al. 1999).
The commonest species in the La Plata basin were Keratella americana, K. cochlearis, and Hexarthra mira. Others have reported the occurrence these species in the Uruguay (Di Persia & Neiff 1986), Paraguay (Frutos et al. 2006, Branco et al. 2018 and Upper Paraná Rivers (Bonetto & Wais 2006), indicating the wide distribution of these rotifers in the study area.
Colurella has been found in several inland waters (Arroyo-Castro et al. 2019, Tasevska et al. 2019, Wei et al. 2019). In the La Plata Basin we found two species of this genus: Colurella adriatica Ehrenberg, 1831 and C. obtusa (Gosse, 1886). Colurella adriatica originates in the Adriatic Sea and has been described as endemic (Ehrenberg 1831), but it is now widely distributed, including in Neotropical regions (Segers 2007). We found it in Foz do Areia Reservoir, in the Iguaçu sub-basin, Paraná State, which is its first record in Brazil.
Enteroplea lacustris is widely distributed in the Australasia, Neoarctic, Neotropical, Oriental, and Paleoarctic regions (Segers 2007). In Brazil, it occurs in Mato Grosso do Sul (Roche & Silva 2017) and Paraná States in the Paranapanema River basin (Dias et al. 2011, Roche & Silva 2017. We found it in Três Irmãos Reservoir, Tietê sub-basin, São Paulo State, near the Paraná River, indicating a gap in previous studies of this region. For Argentina, Ferrando & Claps (2016) recorded 351 species of monogonont rotifers from lotic and lentic environments. Among the species they recorded, we found 43 (12.2%). Three other species of rotifers (Gastropus hyptopus, Harringia rousseleti, and Lecane thienemanni) found in our study are new records for Argentina. Gastropus hyptopus was found in the La Plata River Basin, in Rosario, Argentina. In Brazil, it had been registered in several regions (Serafim Jr. et al. 2003, Bonecker et al. 2005, Serafim-Júnior et al. 2010, Souza-Soares et al. 2011. Harringia rousseleti and L. thienemanni were recorded for the first time in Argentine reaches of the Paraná River, in the Bella Vista municipality. A new locality was found for L. ludwigii, which had been recorded in Corrientes Province (José de Paggi 1996); however, there is no previous record in the La Plata River estuary where we collected it.
We found a non-native species in the La Plata River basin, Kellicottia bostoniensis (Rousselet, 1908), which is native to North America (Edmondson 1959). For Argentina, José de Paggi (2002) first recorded the species in the Iguaçu River and Salto Grande Reservoir. We found the species in the La Plata River (Uruguay and Argentina reach), where there were no prior records of it. We thus extended the known distribution of K. bostoniensis. It is possible that its occurrence in the La Plata basin is related to aquaculture activities as has occurred in other regions (Coelho & Henry 2017). In many reservoirs of the La Plata basin, there are aquaculture activities, mainly with non-native fish species (Azevedo-Santos et al. 2011, Nobile et al. 2018). This rotifer may be introduced from cage aquaculture in upstream rivers (e.g., Grande and Paranapanema Rivers) and reached downstream areas where we captured it.
Seven Neotropical endemic species (sensu José de Paggi 1996) were found in the La Plata River basin. Their presence highlights the importance of preserving the condition of these ecosystems. However, anthropogenic stressors imperil many areas where these seven species occur. For example, in the Barra Bonita and Três Irmãos Reservoirs, where Brachionus dolobratus, B. mirus, and L. proiecta were captured, waters are polluted (Rodgher et al. 2005, Favaro et al. 2018. Similarly, eutrophic tributaries in the Grande River sub-basin (Melo et al. 2017) may affect endemic rotifer species. Another example is the occurrence of L. amazonica in the La Plata River; which also receives water from these polluted river basins. Conservation policies must be discussed for the entire La Plata system because of fluvial connectivity (Azevedo-Santos et al. 2019).
In conclusion, surveys covering wide spatial extents, such as in our study, are important for increasing the knowledge of species diversity and distribution. Our findings may contribute to future monitoring studies as well as management and conservation programs for the La Plata River basin. Finally, we recommend that future rotifer surveys should be concentrated in Paraguay and Uruguay River reaches because of the scarcity of data from them.