Desmids ( Zygnematophyceae ) from the littoral zone of an urban artificial lake : taxonomic aspects and geographical distribution

Aim: The aim of this study was to assess the taxonomic aspects and geographical distribution of the periphytic desmids from two different substrates in the littoral zone of an urban artificial lake. Methods: Samples of epiphytic and epilithic desmids were collected by grass and rock scraping, respectively, in two stations in the Cascavel municipal lake, Paraná, during April 2015, and March 2016 and 2017. In the laboratory, the taxa obtained from the samples were photographed and their morphometric characteristics recorded. The species composition of the desmid was characterized, and the frequency of occurrence as well as the geographic distribution of each taxon in the Paraná State were recorded. Results: We identified 35 taxa, distributed in 10 genera. The most representative were Closterium Nitzsch ex Ralfs and Cosmarium Corda ex Ralfs. A high occurrence of sporadic taxa (60%) was observed and among the recorded taxa, the most frequent were Cosmarium pseudoconnatum Nordstedt var. pseudoconnatum and Desmidium grevillei (Kützing ex Ralfs) De Bary. Nine taxa were exclusive to the epilithic substrate and only Netrium digitus (Ralfs) Itzigsohn & Rothe var. digitus was exclusive to the epiphytic substrate, while the remaining 25 taxa occurred in both substrates. Seven taxa are new records for the Paraná State. Conclusion: Although our results are of qualitative nature, they provide baseline data on the biogeographic distribution of taxa, which are crucial for future taxonomic and ecological studies that might contribute to the preservation of the biodiversity.

studied group of algae due to their often appealing appearance (Coesel & Krienitz, 2008).Desmids have a few truly planktonic representatives, which are mostly associated with periphyton, in mesotrophic to oligotrophic environments of slightly acid pH, high temperatures and low electrical conductivity (Coesel, 1982(Coesel, , 1996;;Černá & Neustupa, 2010;Stamenkovič & Hanelt, 2017).In addition, desmids are ecologically sensitive organisms to environmental changes and thus can be useful tools as bioindicators for the monitoring and management of aquatic environments (Coesel, 2001).For these reasons, taxonomic and floristic inventories, often ignored, become relevant for understanding the complex association of these organisms with the environment, and can contribute to the better knowledge of the biodiversity and the consequences of its changes (Meester & Declerck, 2005).
The aim of this study was to assess the epilithic and epiphytic composition of desmids in the littoral zone of an urban artificial lake, as well as the geographic distribution of the taxa recorded, contributing to the knowledge on the desmid diversity of the Paraná State.

Study area
The Cascavel River belongs to the Cascavel River basin and is the main water source of the municipality of Cascavel, Paraná.Its principal springs are located in the municipal lake region (24°32' and 25°17' S and 53°05' and 53°50' W), which is placed in the Paulo Gorski Ecological Park (Casagrande, 1996).The Cascavel municipal lake (Figure 1) is supplied by several streams and has a drainage area of 117.5 km 2 .This lake is an

Introduction
Landscapes are composed of different aquatic ecosystems.In these ecosystems, conditions of light and nutrients are appropriate for the development and establishment of microorganisms that are key elements for their functioning.Artificial lakes are remarkable components of the local and regional landscapes, in where a wide, but a still little-known diversity of microalgae can be found (Menezes et al., 2011a).
The periphyton is a complex assemblage of different microorganisms, including microalgae, cyanobacteria, microinvertebrates, and detritus attached to organic or inorganic substrates (Wetzel, 1983).Depending on the type of substrate to which they are attached, they are known as epilithon, if attached to rocky substrates, and epiphyton, if attached to aquatic plants (Wetzel, 1981;Fransceschini et al., 2010).Periphytic microalgae play key roles in the energy and matter cycles in the littoral zones (Vercellino & Bicudo, 2006), being their temporal and spatial dynamics influenced by environmental conditions (Moschini-Carlos, 1999;Goldsborough & Robinson, 1996).
T h e d e s m i d s b e l o n g t o t h e c l a s s Zygnematophyceae (Phylum Charophyta) and are considered the greatest and most diverse group of the Infrakingdom Streptophyta (Gontcharov & Melkonian, 2005;Ruggiero et al., 2015).They are important components of the structure of the periphyton community, due to the high diversity of taxa and of functional strategies (Felisberto et al., 2014).

Sampling and laboratory analysis
Collection of samples and measurement of environmental variables at each sampling station were performed during April 2015, and March 2016and 2017 (n = 12).Biological samples were collected from epiphytic and epilithic substrates in the littoral zone of the lake, by grass and rock scraping, respectively.Samples were properly stored and preserved in 1:1 Transeau solution (Bicudo & Menezes, 2006).For qualitative analyses, an average of 10 temporary slides per sample was prepared, and the microalgae photographed and characterized morphometrically under 400 or 1000x magnification with an Olympus trinocular microscope model CX31 with camera attached.
were used for other genera.Taxa were characterized morphometrically and their geographic distribution in the Paraná State recorded.Measurements were expressed in µm and represented by the symbols L: (length), W: (width), and I: (isthmus).The samples were deposited in the Herbarium of the State University of West Paraná (UNOP-Algae), Cascavel campus (Table 2).
The frequency of occurrence of the taxa in the samples (constancy = C) was calculated according to Dajoz (2005).Taxa were classified as constant (C ≥ 70%), common (30% ≥ C ≤ 70%), sporadic (10% ≥ C ≤ 30%) or rare (C ≤ 10%).Artificial keys to identify the taxa in each genus were constructed for those genera more than two taxa.
Geographic distribution in the Paraná State: First record of the taxon.
Cell 6 times longer than wide.Semicells cylindrical and elongate.Apex truncated-rounded.Cell wall brownish, ornamented with longitudinal spiral striae of uneven spaced granules; presence of several constrictions along the cell.Median constriction shallow or little apparent.Chloroplast 2-4 per cell, each one with 5-7 longitudinal rings.Pyrenoids central, spherical to slightly elongate in each branch.L: 125.7 µm, W: 18.9 µm.Figure 50.
Geographic distribution in the Paraná State: First record of the taxon.
Frequency of occurrence: Common In this study, even though we did not attempt to make any quantitative estimation, a decrease in the number of taxa was recognized throughout the analyzed period.The greatest number of taxa was recorded in 2015, when 33 taxa occurred in the epilithic substrate and 18 taxa in the epiphytic substrate, while in 2016 and 2017 the number of taxa recorded in both substrates was smaller (17 and 5, respectively).Among the taxa recorded, nine were exclusive to the epilithic substrate and only Netrium digitus (Ralfs) Itzigsohn & Rothe var.digitus was recovered from the epiphytic substrate, whereas the other 25 taxa occurred on both substrates (Table 3).The presence of natural substrates potentially influences the desmids, whose development depends on the type of substrates available (Coesel, 1982;Barbosa et al., 2013) The genera Cosmarium (12 taxa) and Closterium (10 taxa) presented the greatest number of taxa.These genera are cosmopolitan and exhibit high diversity in different environmental scenarios (Coesel, 1996), being common in oligotrophic and mesotrophic aquatic systems (Graham & Wilcox, 2000).Members of Cosmarium are ubiquitous and have developed numerous protective strategies (e.g.low number of thylakoids per granum, long and exposed stroma lamellae, numerous and moderately large plastoglobules and large starch grains) to survive higher temperature conditions at various latitudes and altitudes (Stamenkovič & Hanelt, 2017).Regarding the frequency of occurrence, most taxa were sporadic, followed by common and rare taxa, while no constant taxa were recorded (Table 3).Cosmarium pseudoconnatum Nordstedt var.pseudoconnatum and Desmidium grevillei (Kützing ex Ralfs) De Bary were the taxa with the greatest occurrence (58% in both cases).
Although the results obtained in the present study are of qualitative nature, they provide baseline data on the biogeographic distribution of taxa in continental aquatic environments.Desmids are considered as excellent indicators of water quality  because they are highly sensitive to changes in the environmental conditions.Therefore, studies of this nature are of great importance for the monitoring of aquatic ecosystems, once the local and regional biodiversity knowledge provides the base to propose measures of preservation and conservation of the biodiversity.

Table 1 .
Characterization of the sampling stations in the Cascavel municipal lake, Paraná, Brazil.Values of the abiotic variables are expressed as means and coefficients of variation (%).