Taxonomical and ecological characteristics of the desmids placoderms in reservoir: analyzing the spatial and temporal distribution

Aim: This study aimed to evaluate the influence of river-dam axis and abiotic factors on the composition of Closteriaceae, Gonatozygaceae, Mesotaeniaceae and Peniaceae in a tropical reservoir. Methods: Water samples for physical, chemical and periphyton analysis were collected in April and August 2002 in different regions along the axis of the river-dam of Rosana Reservoir, River Basin Paranapanema. The substrates collected, always in the litoranea region, were petioles of Eichhornia azurea (Swartz) Kunth. To examine the relationship of abiotic variables with reservoir zones and between the floristic composition of desmids, we used principal component analysis (PCA) and canonical correspondence analysis (CCA). Results: The results of the PCA explained 81.3% of the total variability in the first two axes. In the first axis, the variables of conductivity, water temperature and the pH were related to the sampling regions of April with higher values, while for the month of August, nitrate, total phosphorus and dissolved oxygen showed higher values. We identified 20 taxa, distributed in the genera Closterium (14), Gonatozygon (4), Netrium (1) and Penium (1). Spatially, the higher taxa were recorded in the lacustrine region for both collection periods. The canonical correspondence analysis (CCA) summarized 62.2% of total data variability of taxa in the first two axes, and in August, Closterium incurvum Brébisson, C. cornu Ehrenberg ex Ralfs and Gonatozygon monotaenium De Bary, were related to higher values of turbidity and nitrate to the lacustrine and intermediate regions. Conclusion: Thus, the formation of groups was due to the regions along the longitudinal axis, then the seasonal period, which must be related to the low current velocity, the higher values of temperature and the water transparency, especially in late summer.


Study area description
The Rosana Reservoir, whose hydroelectric was inaugurated in 1987, is situated in the basin of the Paranapanema river, which forms the border between the states of São Paulo and Paraná, between the coordinates, 22° 36' S and 52° 50' W (Figure 1).This reservoir represent 220 km 2 of surface area, 116 km of long; 12 m of depth in the intermediate region and riverine, and 30 m in the lacustrine region.
The predominance of the uses of the watershed associated with rural environment (agriculture, pasture, reforestation and own settlements and farms), is significant, accounting for almost 80% of the entire watershed area (Nogueira et al., 2001).

Sampling data
For this work, samples were collected in 09-11 April, and 27-29 August 2002.In relation to the longitudinal axis of the reservoir three sampling stations were established, located in the municipalities of Teodoro Sampaio (riverine), Euclides da Cunha (intermediate) and Rosana (lacustrine).The substrates collected, always in the litoranea region and in all sampling site were petioles of Eichhornia azurea (Swartz) Kunth.(a rooted macrophyte with several stands in the arms of Rosana Reservoir) and always in the adult stage.Afterwards, the periphyton was removed from the substrate with a razor blade and water jets distilled, transferred to 150 mL flasks, fixed and preserved with Transeau solution.Each petiole was measured using a caliper, converted in cm 2 , to obtain the area of the cylinder.
Abiotic data evaluated in the study were furnished by Limnology Laboratory, from Núcleo de Pesquisas em Limnologia, Ictiologia e Aquicultura -Nupélia.Water temperature and dissolved oxygen (YSI 55 portable oximeter), turbidity (LaMotte portable turbidimeter) and secchi disk were measured during the samplings.The concentrations of total nitrogen and total phosphorus were determined based on Valderrama

Introduction
Reservoirs can be described as a complex network between organisms and their physical and chemical environment, resulting from permanent responses to climatic forces functions and effects produced by manipulation of the system at the dam (Tundisi, 1999).Changing the status of the river conditions to the lake conditions, causes instability that can generate eutrophication due to the decomposition of plant material and nutrient input from an external source (Tundisi, 1999).
Studies show that the changes of physical and chemical parameters, associated, especially the change in the hydrological cycle, have a dominance of control between different species, and the rapid disappearance of phycoperiphyton in certain seasons (O'Reilly, 2006), especially during the filling phase of the reservoir, when the river is going to be altered (Felisberto et al., 2001;Felisberto and Rodrigues, 2002).
Among the periphytic algal, the desmids (Zygnemphyceae) constitute a representative group in number of genera and species.The desmids consist of microscopic green algae, which tend to have a cosmopolitan distribution, with most taxa confined in the oligotrophic environments to mesotrophic (Coesel, 1996).The Zygnemaphyceae, a relevant group in aquatic environments, with high morphological diversity compared to the other groups of green algae, comprise a large proportion of taxa in periphytic algal community.
The absences of structures to be fixed in the desmids substrates remain loosely attached to the substrate, but are part of the biofilm periphytic habitats.Thus, in this work, we hypothesized that due to these conditions of desmids in lake regions should have greater diversity and abundance of species.So, this study aimed to evaluate the influence of river-dam axis and abiotic factors on the composition and abundance of Closteriaceae, Gonatozygaceae, Mesotaeniaceae and Peniaceae in a tropical reservoir.
densities greater than 50% of the total density of sample and, the abundance with densities greater than the mean densities of each sample (Lobo and Leighton, 1986).
The Principal Component Analysis (PCA) used to examine the longitudinal and seasonal variation relative to abiotic variables, was performed with nine variables (pH, secchi disk, conductivity, turbidity, dissolved oxygen, water temperature, total nitrogen, nitrate, total phosphorus).For interpreting the results, we used the axes with eigenvalues higher than of the Broken-Stick model (suggested by Jackson, 1993), as a consistent assessment to determine the adequate number of components for interpretation.
The influence from the five abiotic variables (total nitrogen, nitrate, total phosphorus, turbidity and conductivity) on the periphytic desmids community structure was evaluated by a canonical correspondence analysis (CCA) with the significance by the Monte Carlo test (p < 0.05), with 999 randomizations.For PCA and CCA, the variables were in log transformed and analyses were run with the software PC-ORD 5.15 (McCune and Mefford, 2006).

Presentation and data analysis
All organisms were quantified using inverted microscope at 400X, according to Utermöhl (1958).Only live algae were counted for the densities calculation expressed in ind.cm -2 .The density was calculated according to Ros (1979).
The taxonomical study of periphytic algae was accomplished using approximately 15 temporary slides, by samples.For this procedure, we used optical microscope with micrometric ocular.The richness species, expressed in number of taxa for region, was obtained from qualitative samples complemented with quantitative samples.Dominant species were considered the species with Taxonomical and ecological characteristics of the desmids… lacustrine regions.However, during August there was a separation between sampling sites from upstream of the reservoir (riverine) and from intermediate region of the reservoir, where they were more influenced by chemical variables, i.e. total phosphorus and turbidity (Figure 2).
The reservoirs of Paranapanema Basin show a pattern of variation defined by alternating between rainy season in summer and dry in winter (Nogueira et al., 2001).Thus, the rains influenced the differentiation of limnological variables in early April (Felisberto and Rodrigues, 2005b), and with the elevation of the water level, the intermediate region acquired more lentic conditions, resembling the lacustrine region.
This differentiation between regions within the reservoir also is related to the operation of the dam to generate electricity.Still, the longitudinal processes in reservoirs, usually associated with the residence time of water and allochthonous material input, promote the emergence of different regions along the axis-river dam, called lotic region, lentic and transition region (Thornton, 1990;Thomaz et al., 1997).So, the construction of reservoirs for various purposes has changed the natural ecosystems, and thus altered the physical and chemical conditions of lotic systems upstream, which can disturb the longitudinal gradients along river courses (Ward and Stanford, 1983;Straškraba et al., 1993;Silva et al., 2010).

Spatial and temporal characterization
The variable values accounted for 81.3% of the variance accumulated on the first two axes (62.3 and 19%, respectively) (Table 1).The first axis variables conductivity, water temperature and pH were related sampling regions (intermediate and lacustrine) from April with higher values , while for the lacustrine from August, total phosphorus, nitrate and dissolved oxygen showed higher values (Figure 2).
Although it had a low value of explicability, axis 2 revealed a clear separation among seasonal periods (April and August), showing more homogeneous groups, especially in April for intermediate and  Paulo: Bicudo and Castro (1994); Sormus and Bicudo (1994).
Among the 35 species with abundance in April, Closterium moniliferum and Gonatozygon monotaenium (two and three individuals; cm -2 , respectively) were plenty found in the riverine region, while G. aculeatum (49) and G. brebissonii (48) were abundant in lacustrine region.Among the 19 species with abundance in August, Closterium moniliferum (19) and Gonatozygon monotaenium (64) were plenty found in the riverine and lacustrine regions, respectively (Felisberto and Rodrigues, 2005b).
Comparing the two samples analyzed in different regions of the Rosana Reservoir, the most taxa were recorded in the lacustrine region for both sampling periods (Table 2).Closterium incurvum was more frequent, being present in all regions (100%), Gonatozygon monotaenium ocorred in 83.3% of regions, while Gonatozygon aculeatum, Closterium the first two axes.In August, Closterium incurvum, C. Cornu, C. navicula, Gonatozygon monotaenium and Closterium ehrenbergii var.immane were related to higher values of total phosphorus and nitrate to the lacustrine and intermediate regions (Figure 4).Still in the first axes, in April, a group of species formed for Closterium dianae var.minus, C. exiguum, C. laterale, C. venus, Gonatozygon aculeatum and Penium margaritaceum were related to lacustrine region, with higher values of conductivity (Figure 4).
In the second axes, in April, a group of species formed for Closterium calosporum, Gonatozygon pilosum, C. tumidum var.nylandicum and Netrium digitus were related to intermediate region, with higher values of conductivity.While, in August the species Closterium moniliferum, C. venus var.westii and Gonatozygon brebissonii were related to riverine cornu, C. exiguum, C. dianae var.minus, and C. laterale showed only 16.6% of regions (Table 2).
The greater richness and relative abundance of species recorded in the lacustrine region of the Rosana Reservoir for both collection periods, parts must be related to the lentic water conditions, higher nutrient s values, and the greater amount of substrate and water transparency (Felisberto andRodrigues, 2005a, 2005b).
Furthermore, near of the tropics, the water temperature has values close to the optimum values for the development of desmids, i.e. around 25 -30 °C (Coesel and Wardenaar, 1990), this explains the diversity of flora, both in species and in genera.This fact was observed in the lacustrine region of Rosana Reservoir (26.8 °C, seen Felisberto andRodrigues 2005a, 2005b).
The canonical correspondence analysis (CCA) summarized 62.2% of total data variability of taxa in region, with higher values of total phosphorus (Figure 4).
Thus, besides the conditions evidenced in the CCA, the largest water transparency, lentic waters coupled with higher amount of substrate available for colonization of algae in the lacustrine and intermediate region benefited certain species, such as Closterium cornu, C. dianae var.minus, C. ehrenbergii var.immane.C. exiguum, C. incurvum, C. laterale, C. navicula, C. venus, Gonatozygon aculeatum, G. monotaenium and Penium margaritaceum.Therefore, the formation of groups evidenced PCA and the CCA is given depending on the region along the longitudinal axis, then the seasonal period and relative abundance of species, which was related to increased availability of nutrients, turbidity, conductivity and diversity of substrate, lentic waters and higher water transparency mainly for the month of April (late summer and early fall).Closterium venus var.westii is first register for Brazil.

Figure 1 .
Figure 1.Map of the localization of Rosana Reservoir (River Paranapanema basin).

Figure 2 .
Figure 2. Principal Component Analysis (PCA) for physi cal and chemical parameters analyzed for the six sampling sites along the Rosana Reservoir, from April/2002 and August/2002.

Figure 4 .
Figure 4. Ordination by the Canonical correspondence analysis (CCA) of abiotic variables and species, analyzed for the six sampling sites along the Rosana Reservoir, from April/2002 and August/2002.

Table 1 .
Correlation and performance values obtained through Principal Component Analysis (PCA) to Rosana Reservoir from April to August/2002.
Taxonomical and ecological characteristics of the desmids…