Periphytic algal community in artificial and natural substratum in a tributary of the Rosana reservoir ( Corvo Stream , Paraná State , Brazil )

Periphytic algal community in artificial and natural substratum in a tributary of the Rosana reservoir (Corvo Stream, Paraná State, Brazil). This study evaluated the heterogeneity in periphytic algal community, under the influence of time colonization on artificial substratum. We also examined which abiotic variables most influenced the community in natural and artificial substratum. Egeria najas Planchon was used as natural substratum, and a plastic plant, as artificial. This experiment was carried out in a lateral arm from Rosana Reservoir, formed by Corvo Stream (Paranapanema river basin), in the period from November 21 to December 12, 2003, characterized as a warm and rainy period. Changes in species composition were assessed using the similarity indices. 495 taxa were registered in the phycoperiphytic community, distributed in 133 genera and 11 classes. Zygnemaphyceae, Bacillariophyceae, Chlorophyceae and Cyanophyceae presented higher species number, in both substrata. Staurodesmus, Closterium, Staurastrum and Cosmarium (Desmidiales); Gomphonema and Eunotia (Pennales); Characium, Scenedesmus and Desmodesmus (Chlorococcales); Anabaena and Aphanocapsa (Nostocales and Chroococcales, respectively) were the most species-rich genera. The epiphytic community reached the highest species richness in the 15 successional day. Regardless the substratum type, the number of species was probably related to the high concentrations of nutrients (phosphorus and nitrogen).


Introduction
The species diversity plays an important role in the processes of production, consumption, respiration and decaying, mainly at littoral regions, since the aquatic vegetation is generally associated to different communities.The high availability of habitats provided by the diversity of aquatic vegetation favors the establishment and development of several organisms, mostly the periphyton (WETZEL, 1981;WETZEL;LIKENS, 1991).
The periphyton is a sessile community that responds punctually to environmental conditions.This community presents short life cycle (3 -10 days) which allows quick answers to environmental changes (HAMBROOK, 2002;RODRIGUES et al., 2003;WU et al., 2009).The heterogeneity in the Acta Scientiarum.Biological Sciences Maringá, v. 32, n. 4, p. 373-385, 2010 structure of phycoperiphytic community may be understood through the species composition in different successional stages.The studies about the distribution and mobility of algae from this community, when analyzed in short time scale, are more suitable due to population changes (WETZEL, 1983).This distribution extremely homogeneous of periphyton and its interaction with natural substrata, coupled to the need to standardize the community development time and the substratum nature, have motivated the use of artificial substrata.The review made by Cattaneo and Amireault (1992) indicated that 60% from published papers had used different types of artificial substrata.
In Brazil, among the published researches with artificial substratum reproducing a natural one are those using glass tubes (MOSCHINI-CARLOS et al., 2000), glass slides (RODRIGUES; BICUDO, 2001BICUDO, , 2004) ) and plastic tubes (FERNANDES;ESTEVES, 2003) to compare with petioles of emerging macrophytes, whereas other studies had used polyamide strings (OLIVEIRA et al., 2001) and glass slides (VERCELLINO; BICUDO, 2006) to analyze the structure and dynamic of periphytic algae community.Although Brazil presents one of the major hydroelectric parks of the world, the knowledge about periphyton in these environments is still scarce.
This study hypothesized that the composition of periphytic algae varies in function to the type of substratum (artificial and natural) and/or due to abiotic conditions of the system.The prediction is that in the course of time the phycoperiphytic community from an artificial substratum will be similar to that found in natural one, considering the species composition, and that both communities will be mainly influenced by nutrient concentrations.Therefore, we analyzed (a) the floristic composition from the community of periphytic algae in both substrata of one tributary of Rosana reservoir, Corvo Stream, during a warm and rainy period; (b) the algae similarity, between artificial and natural substratum; and (c) the principal abiotic factors influencing the species composition and richness of phycoperiphytic community, in both substrata.

Study area
The samplings location is one tributary that flows into the lacustrine region from Rosana reservoir (Corvo Stream, 22°39'S; 052°46'W, Figure 1), near the dam.This river is situated in Paraná State, between the counties of Diamante do Norte and Terra Rica.The sampling station was 4.95 km far from the lacustrine region of the reservoir.

Samplings and analyses
Periphyton samplings were performed from November 21 st to December 12 nd , 2003, a period characterized as warm and rainy.Two types of substrata were chosen for the samplings: Egeria najas (natural substratum) and a plastic plant (artificial substratum), similar to natural substratum (Figure 2).
For the natural substratum, a fragment of Egeria najas was removed from the environment, and a median part was detached (about 7 to 8 cm of length), excluding the apex and base.Afterwards, this median part was transfered to previously moistened flasks, and kept in ice, and later the periphyton was removed for analysis.The samplings of both substrata were carried out in the lacustrine region of the environment.For Acta Scientiarum.Biological Sciences Maringá, v. 32, n. 4, p. 373-385, 2010 the artificial substratum, each one was washed in running water and after this; each branch was attached to a weight (pebbles wrapped in plastic) using a nylon line, and Styrofoam pieces as buoys.These sets were put in Corvo Stream, close to the left bank, about 60 cm of depth, in November 21 st .Samplings of artificial substratum occurred every three days, between November 24 th (1 st sampling and 3 rd successional days) and December 12 nd , 2003 (last sampling and 21 st day), while for the natural substratum, the samplings began at the first day of installation of artificial substratum (11/21), and during alternate days (11/26; 01, 06, 09 and 12/12), totaling 13 samples (six for the natural substratum, and seven for the artificial one).The periphyton was washed from the substrata using brush and distilled water, and then transferred to transparent flasks.The samples were preserved with Transeau solution (BICUDO; MENEZES, 2006).
The taxonomical study of periphytic algae was accomplished using approximately 15 temporary slides, by sample.For this procedure, we used optical microscope with micrometric ocular.Some genera of green filamentous algae, as Bulbochaete, Oedogonium, Mougeotia and Spirogyra, were distinguished only in vegetative groups, based on the cells diameter and length, due to the absence of reproductive structures in the analyzed individuals.
The species richness, expressed in number of taxa for both substrata, was obtained from qualitative samples complemented with quantitative samples.
The similarity of periphytic algae from artificial and natural substratum, between the sampled periods, was analyzed using the Jaccard similarity index (species presence/absence data), using NTSYS software, version 2.1 (ROHLF, 2000) and unweighted average (UPGMA).
The values of total richness of algae classes from both substrata were correlated with the abiotic variables, through Pearson correlation analysis, using Statistica software, version 7.1 (STATSOFT, 2005).

Floristic composition
The community of periphytic algae comprised 495 taxa distributed in 133 genera and 11 classes, considering both substrata (natural and artificial) (Table 1).Among these taxa, 403 occurred in the natural substratum, and 401 in the artificial one.In the table 1, the complete list of species is presented, and, from this total, 86 taxa occurred exclusively in the natural substratum, and 98 in the artificial.Therefore, independently of substratum type, we verified the predominance of Zygnemaphyceae (desmids), followed by Bacillariophyceae (diatoms), Chlorophyceae and Cyanophyceae (Table 1, Figure 4).
The number of species, in natural substratum, ranged from 156, in December 12 nd (final of the experiment, 6 th sampling) to 261 taxa, in November 26 th (beginning of the experiment, 2 nd sampling), thus, we observed a sudden decrease in the number of species during the final phase (Figure 3).In relation to artificial substratum, this number varied between 169, in December 9 th (18 th day), and 259 taxa, in December 6 th (15 th day).In the natural substratum, there was a change in the number of desmids taxa, in December 6 th (4 th sampling), when we registered an expressive contribution of diatoms.The same was observed for the artificial substratum, however, in the last two samplings (December 9 th and 12 th , 18 th and 21 st successional days, Figure 4).

Taxonomic similarity in periphytic community
In the diagram from the grouping analysis performed with periphytic algae from both substrata, there was a separation of groups, for the distinct substrata.Two groups were distinguished for artificial and natural substratum (Figure 5).For the artificial substratum, the assemblages of species referring to 3 rd and 6 th successional days (1 st and 2 nd samplings) were distinguished amongst themselves, and from the other days; a second group was formed, between the 9 th and 12 nd days (3 rd and 4 th samplings).The 15 th successional day (5 th sampling) was distinct from the others, however presented higher similarity with the final phase of the experiment, between the 18 th and 21 st days (Figure 5).
For the natural substratum, the results pointed higher similarity among the samples from the three Acta Scientiarum.Biological Sciences Maringá, v. 32, n. 4, p. 373-385, 2010 last samplings (December 6 th , 9 th and 12 th ), otherwise, the sample of the beginning of the experiment (November, 21 st ) was grouped with intermediary experimental phase (November 26 th , and December 1 st ; Figure 5).The values referring to physical and chemical parameters of the water, electric conductivity, dissolved oxygen, water temperature and pH presented more constant values, while the other variables as nutrient concentrations and turbidity presented higher variation (Table 2).

Relationship between abiotic variables and phycoperiphytic community
In Paranapanema river watershed, as well as in Corvo Stream (lateral arm from Rosana reservoir), here is a precipitation regime, with concentration of rainfall between November to February, period characterized as warm and rainy.In the study period (November-December), there was higher volume of precipitation at the end of November (11/28, 1 st study week) and beginning of December (12/01, Figure 6), two days before the 4 th sampling, 12 nd successional day at artificial substratum.This high precipitation along with the strong winds during the study period was responsible for the high quantity of material carried into the reservoir, contributing to increase the turbidity and nutrients concentration, which influenced the species composition of periphytic algae.In this way, the number of taxa recorded in the natural substratum was positively correlated to NT (r = 0.95), PT (r = 0.88), PO 4 -3 (r = 0.93), PDT (r = 0.84) during the 1 st week.This community attribute was also positively correlated to PO 4 -3 (r = 0.92) during the 2 nd week, and also with the turbidity (r = 0.93) in the 3 rd week.

Discussion
The increase in species richness observed during the first days of colonization in the artificial substratum, was possibly due to the propagules present in the environment, since the habitat was not yet colonized, the migration processes favor the colonization and succession of species, allowing the composition of not only colonizing and opportunistic species of diatoms, as well as Chlorococcales, Desmidiales and filamentous algae, as Bulbochaete and Oedogonium.The immigration is an important process in the composition, colonization and increase of periphytic algae in artificial substratum (HILLEBRAND; SOMMER, 2000;PETERSON, 1996;STEVENSON;PETERSON, 1989).
The decrease in the number of species in the natural substratum over the experiment occurred during the days with higher pluviometric intensity.On the other hand, in artificial substratum, there was a different result, possibly, because this substratum is under succession process, since the increase in the number of species was gradate until November 30 th (9 th successional day and 3 rd sampling), with a decrease in December 3 rd (12 nd day and 4 th sampling).This reduction during the 4 th sampling for both substrata may be related to the high precipitation that occurred in November 28 th and mainly during the night of December 1 st , i.e., two days before the sampling.From this period, the environmental conditions stabilized and hence, the number of species reached the maximum values during the 5 th sampling (December 6 th , 15 th successional day), decreasing drastically again from the 6 th sampling (December 9 th , 18 th day).
The oscillation in the species heterogeneity may be explained by the predominance of resilient species in phycoperiphytic community.These species can recover from disturbing events, as fluctuations in water level and large movement of the water column, caused by rainfall and constant winds.Based on studies about the succession of periphytic algae, the assemblages would be more resilient in more heterogeneous environments (more diversified environmental conditions in face of disturbing events, e.g., rainfall, wind), than in homogeneous ones, because the species diversity in one habitat, would increase the efficient use of available resources (STEVENSON, 1997).The high resistance of many diatoms to wave's effect, even flooding, shows competitive advantages (BIGGS;THOMSEN, 1995;PETERSON, 1996;SABATER et al., 1998;STEVENSON, 1996b), and allows greater representativeness in rivers with high frequency of flooding.A. minutissimum, e.g., (species present in all samplings from both substrata), is widely recorded as intermediary colonizer, and highly resistant to disturbances (BIGGS;THOMSEN, 1995;PETERSON, 1996).
The differentiation of phycoperiphytic community in artificial substratum, was evidenced by the grouping analysis, in three phases: the initial, mainly formed when the community receives a gradate increase of species; the intermediate, when there is still an increase in the number of species, reaching a maximum; and the final phase, when there is a severe decrease in number of species, probably due to the replacement and addition of species in different successional stages.The exposure time necessary to periphyton community reaches the maturity stage (maximum in the number and density of species) may vary from two weeks to longer periods, depending on the type of environment, water temperature and the type substratum (LOBO; BUSELATO-TONIOLL, 1985;PATRICK;REIMER, 1975;SABATER et al., 1998).In Corvo Stream, this phase was achieved until the 15 th colonization day, as also verified in other studies (CATTANEO et al., 1975;LAM;LEI, 1999;RODRIGUES;BICUDO, 2001).
Moreover, the environmental processes of each habitat, exert a direct influence on the development, composition, and distribution of phycoperiphytic community, specifically concerning the algal flora of each location (CASCO; TOJA, 1994; STEVENSON, 1996a,).Consequently, the high variation in nutrient concentrations (especially phosphorus and nitrogen), mainly from rainfall and constant winds that cause greater displacement of allochthonous material into the environment, besides the resuspension of sediment for the water column, elevating the turbidity, indicate a possible influence on the number and composition of species.On the one hand, when these events are more intense (high precipitation, strong winds) there may be a negative influence, as seen during the 4 th sampling, both in natural and artificial substrata, when a sharp decrease in species number was detected.Otherwise, the positive effects of environmental variables previously mentioned may Acta Scientiarum.Biological Sciences Maringá, v. 32, n. 4, p. 373-385, 2010 be corroborated by the correlations between the species richness with nutrients (especially phosphorus and nitrogen), turbidity, wind and rainfall.
The highest number of Zygnemaphyceae species, followed by Bacillariophyceae, in both substrata, leads to a greater representativeness of these unicellular algae.Unicellular organisms are fast colonizers, due to their high reproductive rate; while the colonial or filamentous organisms are slower in propagation process (HILLEBRAND;SOMMER, 2000).These traits should influence the phycoperiphytic communities in the upper Paraná river floodplain, considering the high predominance of these non-flagellate unicellular forms, in the studied environments (RODRIGUES; BICUDO, 2004).
The contribution of diatoms, with several species of Eunotia (higher number of species in Penales), forming large filaments of cells aggregate in chains (E.pectinalis, E. sudetica and E. camelus), or composing extensive 'arborescent colonies' connected by the edges (E.flexuosa and E. lineolata), among others, may be associated to morphological and adaptive traits from each taxa, since these algae are better adapted to periphytic habit, besides being grouped and involved in mucilage sheaths that favor the better attachment to the substrata.
The great representativeness of Zygnemaphyceae (desmids) may be related in part to the amount of available substratum for colonization (diversity of aquatic vegetation), and the high values of nutrients and to the water temperature; and also due to the unicellular condition with mucilage sheaths around the cells.In a study of colonization by periphytic algae in glass slides close to macrophyte stands, Rodrigues and Bicudo (2001) related the high richness of desmids in the periphytic community to the presence of macrophyte stands.Diversified flora of desmids presents faster growth at warmer temperatures, with optimum ranging from 25ºC to 30ºC (COESEL, 1996;COESEL;WARDENAAR, 1990;FELISBERTO;RODRIGUES, 2005a and b).In this way, Corvo Stream, tropical environment, with temperatures varying between 26 and 28°C, with high nutrient concentrations and diversity of aquatic vegetation, is a favorable local to the development of this algal flora.
Regarding the Chlorococcales, Desmodesmus and Scenedesmus are exceptionally common in any environment, oligo, meso or eutrophic (BICUDO;MENEZES, 2006;LÜRLING, 2003), and are among the first to colonize the environment (BICUDO; MENEZES, 2006).The high variation in nutrient concentrations of the studied environment allowed the development of species from this Order.Additionally, Characium species, possibly due to the development stage of periphytic biofilm, were favored by the increase of filamentous green algae (as Oedogonium and Bulbochaete) and also by the quantity of species from the Eunotia genus, to which they were fixed through attaching discs.
In summary, the results of taxonomical composition of periphytic algae in natural (Egeria najas Planchon) and artificial (plastic plant) substratum were similar, revealing that the development of these algae were also related to environmental conditions, over time.The grouping analysis enabled to register three growth phases of the community, with a maximum peak at 15º successional day, which allow suggesting that the community achieved the maturity stage.Thus, we may state that the heterogeneity in the structure of phycoperiphytic community may be understood through species composition in different succession stages.Furthermore, the number of species, regardless the substratum type, was positively related to the higher nutrient availability, higher values of turbidity, especially during the first week; and negatively related to the turbidity, during the second and third weeks.

Figure 2 .
Figure 2. Types of substrata, Egeria najas Planchon (natural) and a plastic plant, sampled in Corvo Stream (tributary of Rosana reservoir), during the study period.