Abundance and diversity of phytoplankton in the Paraná River ( Argentina ) 220 km downstream of the Yacyretá reservoir

Patterns in the temporal composition, abundance and diversity of the phytoplankton community of the Paraná river prior to and after the initial filling phase of the Yacyretá reservoir are analyzed. The study site is located 220 km downstream from the Yacyretá reservoir and 30 km downstream from the confluence with the Paraguay river. Because both rivers remain separate and unmixed at the study site, we compared the possible effects of the impoundment on both river banks (left and right banks) in hydrological periods with similar duration and magnitude of the low and high water phases. Physical and chemical conditions measured on the right bank (water from the Paraguay river) were similar at both periods (pre and post-impoundment) whereas conductivity, pH and orthophosphate concentration increased on the left bank (water from the High Paraná river and Yacyretá reservoir) after the impoundment. Changes in phytoplankton density and diversity were observed only in samples collected from water flowing from the reservoir (left bank). The density of Chlorophyceae (Chloromonas acidophila, Chlamydomonas leptobasis, Choricystis minor, Chlorella vulgaris, Scenedesmus ecornis, Monoraphidium minutum, M. contortum and M. pusillum) and Cryptophyceae (Rhodomonas minuta, Cryptomonas marssonii and C. ovata) increased while Cyanophyceae (Cylindrospermopsis raciborskii, Raphidiopsis mediterranea and Planktolyngbya subtilis) and Bacillariophyceae (Aulacoseira granulata and its bioforms) decreased compared to previous studies conducted on the left bank of the Paraná river. Phytoplankton collected from the right bank of the river did not differ in pre and postimpoundment samples because they originate from the Paraguay river, which remains relatively unaffected by human activities.


Introduction
The impoundment of a river determines a series of modifications in its physical, chemical and biological characteristics observed from the filling stage until reaching a relative stability.This period is very variable and can last up to twenty years in some tropical reservoirs (Balon and Coche, 1974).Differences in the water residence time, temperature, transparency and nutrient level produce changes in the quali-quantitative phytoplankton composition (Tundisi, 1993), which can be seen up to 350 km downstream from the dam (Petts, 1984).
The Paraná river basin, with a drainage area of 2.6 x 10 6 km 2 and 3,965 km length is the second largest catchment of South America, shared by Brazil, Bolivia, Paraguay and Argentina (Orfeo and Stevaux, 2002).Several reservoirs (approximately 23), situated in cascade (Bonetto et al., 1989), occupy its main channel, mainly in Brazil (Upper Paraná).The construction of Yacyretá in the High Paraná, one of the largest impoundments built in Argentina, was finished in April 1990, although the first turbine did not start operating until September 1 st , 1994.This had an important impact on aquatic communities of the reservoir (Garrido, 1999;Meichtry de Zaburlín, 1999;Peso and Bechara, 1999;Neiff et al., 2000) and downstream from it (Domitrovic et al., 1994).
The aim of this study is to analyze the variations in abundance and diversity of the phytoplankton on both banks of the Paraná river, 30 km downstream from the confluence with the Paraguay and 220 km downstream from the reservoir.We also analyzed pre-and post-impoundment phytoplankton for both banks (1980-1981 and 1995-1996) in relation to environmental variables.
Our hypothesis is that phytoplankton abundance and diversity on the left bank have changed following the impoundment of the Yacyretá reservoir.

Study site
The Yacyretá reservoir is located on the High Paraná river (27° 28' S and 56° 44' W) near Ituzaingó (Argentina) and Ayolas (Paraguay).During the initial filling phase (water level = 76 m a.s.l), the reservoir had a retention time of less than 20 days.Its main morphometric characteristics are: length of 66.5 km; surface area of 1,600 km 2 at 82 m above sea level; volume of 21,000 hm 3 ; maximum depth of 35 m and maximum width of 21 km.
The study site (Figure 1) is located 220 km downstream from the reservoir near Corrientes city (27° 30 ' S and 58° 55' W), where the river is approximately 1,200 m wide.At this site, the Paraná river flows as a typical floodplain river with an irregular hydrological regime.Thirty kilometers upstream from this site, the Paraguay river merges with the High Paraná.Both rivers remain unmixed until they reach 200-300 km downstream.The Paraguay river drains the Pantanal and 100 km before the study site receives the high suspended load of the Bermejo river (3 to 10 g litter) which drains Andean sedimentary rocks rich in carbonates.The Upper Paraná (Brazil) flows over basalts and sandstones covered by lateritic soils rich in iron and aluminum oxides.These different watershed lithologies are reflected in the contrasting chemical and physical composition of both rivers.At the confluence, the annual discharge of the Paraná river increases from 11,983 to 16,941 m 3 s -1 , and the discharge of suspended sediment increases from 5.1 x 10 6 to 118.7 x 10 6 tons.year - (Orfeo and Stevaux, 2002).
The water level of the Paraná river at Puerto Corrientes fluctuated between 2.04 (44.33 m a.s.l.) and 5.79 m (48.18 m a.s.l.) between 1995-1996 and between 2.35 (44.74 m a.s.l.) and 6.88 m (49.27 m a.s.l.) prior to the Yacyretá impoundment (1980)(1981).There were no significant differences in the water level of the Paraná river at Puerto Corrientes between both periods.
As comparative data, a limnological study (including information on phytoplankton) was carried out in the current dam area and 220 km downstream, near Corrientes city from 1980 to 1981, i.e., before the construction of the dam (Bonetto et al., 1982).

Methodology
Monthly (exceptionally fortnightly) samplings were carried out at two stations located near both banks of the Paraná river from April 1995 (7 months after the Yacyretá started operating) to May, 1996.Single subsurface samples were taken with a Van Dorn sampler approximately 150 m from each bank and fixed with Lugol's solution for phytoplankton counts.Counts were carried out using an inverted microscope with the same methods (Lund et al., 1958;Utermöhl, 1958) and by the same operators as the 1980-1981 pre-impoundment work.Specific diversity (H') was estimated using the Shannon-Wiener index (Shannon and Weaver, 1963) from the density data (bits ind. - ).Specific richness (SR) refers to the number of algal taxa registered in each sample in the Utermöhl counting chambers.
At each sampling station, in situ measurements of water temperature, transparency (25 cm diameter Secchi disk), pH (digital pH meter Metrohm A G Herisau), electrical conductivity (conductometer YSI 33 SCT) and dissolved oxygen (oxygen meter YSI 54 A) were carried out.Analyses of dissolved nutrients (NO 3 + NO 2 and orthophosphate) were performed according to APHA (1981) on filtered (Whatman GF/C) samples by the staff of the Chemical Laboratories at the Centro de Ecología Aplicada del Litoral.
Results obtained near both banks of the river (Station 1 vs. Station 2) between 1995-1996 and those obtained before the dam operation (1980)(1981) were compared using the Wilcoxon matched pairs test (Steel and Torrie, 1988).The ordination of environmental variables was performed by Principal Component Analysis (PCA).Before computation, the data, except pH, were log transformed to stabilize the variance.
The pre and post-impoundment phytoplankton densities of both banks were analyzed using Detrended Correspondence Analysis (DCA).Subsequently, a forward stepwise multiple regression analysis was used to examine the relationship between species abundance and the DCA axes.For this, the sample scores which are weighed mean species scores of the two first axes of DCA (dependents variables) and eight abiotic variables (independents variables): water level, temperature, Secchi disk, dissolved oxygen, pH, conductivity, nitrite+nitrate concentration and orthophosphate concentration were analyzed.Twenty three species, reaching a relative abundance greater than 10% and/or samples frequency greater than 30%, were included in the analysis (Legendre and Legendre, 1983;Zalocar de Domitrovic, 1999).

Environmental variables
In both periods (pre and post-impoundment), mean annual values of transparency (Secchi disk) and dissolved oxygen were significantly higher at Station 1 (left bank, water from the High Paraná river).On the other hand, mean values of conductivity and orthophosphate concentration were significantly higher at Station 2 (right bank, water from the Paraguay river) (Table 1).
The first three axes of a Principal Component Analysis (PCA) of abiotic variables accounted for 75% of the total variance.The first axis (37%) was negatively correlated with conductivity (Cond) and positively correlated with transparency (Sec) and dissolved oxygen (Ox).Axis 2 was negatively correlated with nitrite+nitrate concentration (N).The pH was positively correlated with axis 3. The first principal component scores contrasted Station 1 (left bank, water from the High Paraná river) with Station 2 (right bank, water from the Paraguay river), the former having relatively high values of transparency and dissolved oxygen.Station 2 (right bank) was characterized by more conductivity and concentration of nutrients (Figure 2).
Physical and chemical conditions measured at Station 2 (right bank) in the pre-impoundment period were, in general, similar to those registered post-impoundment.On the other hand, conductivity, pH and orthophosphate concentration at Station 1 (left bank, water from the High Paraná and Yacyretá dam) increased significantly (p <0.05) after the impoundment (Table 1).
Species richness (number of taxa) ranged between 19 and 47 taxa per sample on Station 1 and between 3 and 18 taxa per sample on Station 2. These values were significantly different (Z = 3.296, p = 0.001).Between 1995Between -1996 (post-impoundment) (post-impoundment), the species composition was not homogeneous between the two river banks (Table 2).Out of a total taxa of 143, there were 69 exceeding a relative abundance of 1%.Fifty seven taxa corresponded to Station 1 (left bank) and 36 to Station 2 (right bank), with 26 species in common between both river banks.
Differences registered between the two banks in the physical and chemical variables, as well as in the phytoplanktonic community, are likely related to the influence exerted by the Bermejo river (Andean origin, mineralized waters and high content of suspended solids) on the lower reach of the Paraguay river, which flows along the right bank of the Paraná river at our study site (Bonetto et al., 1982;Bonetto and Orfeo, 1984).The tendency for higher phytoplankton densities at Station 1 (left bank) was similar to what was reported in preimpoundment studies.Although some common algal species were observed on both banks, in general, taxa from the right bank corresponded to those brought by the Paraguay river, while those from the left bank corresponded to those brought from the High Paraná river (Zalocar de Domitrovic and Vallejos, 1982;Zalocar de Domitrovic, 1999;2002).

Comparison between phytoplankton abundance before and after the impoundment of Yacyretá
At Station 1 (left bank), Chlorophyceae were the dominant group in 1995-1996, followed by Cryptophyceae, showing a statistically significant increase in density in relation to studies carried out previously to the construction of the dam (Z = 2.731; p = 0.006 and Z = 3.233; p = 0.001, respectively).Before Yacyretá, dominant groups were Chlorophyceae and/or Cyanophyceae and Bacillariophyceae (Bonetto et al., 1982).There was a The Detrended Correspondence Analysis (DCA) of phytoplankton species and samples, before and after impoundment is shown in Figure 4. Eigenvalues were 0.79 and 0.23 for axis 1 and 2 respectively and lengths of gradients were 3.56 and 2.06 S.D. Total variance ("inertia") in the species data was 2.62.Factor 1 shows the order of the species between the periods of the pre and post-impoundment.The first two groups (Groups 1 and 2) show samples taken during the pre-impoundment period, and Groups 3 and 4 represent samples collected during the post-impoundment period.In the first axis, samples from Station 1 (waters from High Paraná) collected during preimpoundment period are located to the right of the gradient and are distinguished by species: Cylindrospermopsis raciborskii, Raphidiopsis mediterranea, Planktolyngbya subtilis, Aulacoseira granulata and A. alpigena.The two last species of diatoms, especially A. granulata, was predominant in Station 2 (water from Paraguay river), in all the samples of the pre and post-impoundment periods (Groups 2 and 4).Samples from Group 3 correspond to Station 1 (left bank) of the post-impoundment period and are allocated in the negative region of Factor 1, which is related to many species of Chlorophyceae (Chlorella vulgaris, Chlamydomonas acidophila, Choricystis minor), Cryptophyceae (Cryptomonas marssonii, C. ovata), Chrysophyceae, diatoms and Euglenophyceae (see Table 2).Some common species of both periods (pre and post-impoundment) occupy an intermediate position in the gradient.
Multiple regression reveals that three variables (pH, Secchi disk and conductivity) explain 39% of the variability of species for axis 1 and pH explain 6% for Axis 2 (Table 3).
When comparing our results for Station 1 (left bank) with those of Meichtry de Zaburlín (1999) on the left side of the Yacyretá reservoir, Chlorophyceae and Cryptophyceae densities show significant differences (Z = 3.296; p = 0.001 and Z = 2.542; p = 0.011, respectively).Lentic Desmidiaceae and Euglenophyceae were not detected downstream from the reservoir probably because of selection mechanisms typical of lotic waters (Reynolds, 1999).
The replacement of lentic plankton associations by lotic ones downstream from reservoirs was observed in small rivers like the Loire in France (Lair and Reyes-Marchant, 1997).In large rivers, like the Paraná, they are probably detected to short distances downstream from the reservoir.A high percentage of algae cannot be detected by count methods in the study area, but downstream they could act as "memory species" (sensu Padisák, 1992), being a potential inocula for the colonization of lentic floodplain environments (Zalocar de Domitrovic, 2002;2003).
In the Salto Grande reservoir of the Uruguay river, Quirós andLuchini (1982) andO'Farrel andIzaguirre (1994) observed the reduction in the number of plankton species downstream from the reservoir.Petts (1984) indicated that reservoirs are an important source of plankton for a river.The high phytoplankton density 220 km downstream from Yacyretá corresponds to what was pointed out by several authors who registered a high algal density downstream from some dams, as far as 350 km from them (Talling and Rzóska, 1967;Hynes, 1970;Hammerton, 1972;Literáthy et al., 2002).
Although nutrient enrichment usually favours the development of Cyanophyceae during the first year of resevoir operation (Petts, 1984), there was a quantitative reduction of this group in Yacyretá, as well as down-   et al., 1999;Ludwig et al., 1997).
Marked changes in the dominant algal species were also observed in other reservoirs (Magrin and Matsumura-Tundisi, 1997;Marinho et al., 1993).
The most important factor controlling the development of reservoir-phytoplankton would be the residence time of the reservoir water which, during these studies, was approximately 6 days.
In the Ohio river (the largest tributary of the Mississippi river), Sellers and Bukaveckas (2003) suggest that the presence of water regulation structures substantially alters spatial and temporal patterns in light intensity and that these effects have important consequences for phytoplankton production.Short residence times are a parameter that in fact summarizes the effect of river circulation through the reservoir (Armengol et al., 1999).
Conditions observed in the reservoir by Meichtry de Zaburlín (1999) such as the increase in suspended matter and the decrease in water transparency would directly affect the generation time necessary for a complete development of algal populations (Wetzel, 1973).Cryptophyceae (of small size, high surface/volume relationship and good nutrient competitors) with a high generation rate (C-strategists sensu Reynolds, 1988) were predominant in such conditions, which are compatible with what was observed by various authors for reservoirs with a high renovation rate (Calijuri et al., 1999;Hynes, 1970;Petts, 1984;Tundisi, 1990).In this algae group, Cryptomonas is one of the species that tolerates vertical high-frequency vertical disturbances well (Reynolds, 1987).In tropical reservoirs, very short residence times can favour the development of picophytoplankton (Tundisi, 1993).In oligotrophic reservoirs, the dominance of Chlorophyceae and Bacillariophyceae, in relation to Cyanophyceae, which are usually abundant in eutrophic reservoirs, was observed (Tundisi et al., 1993).Cylindrospermopsis raciborskii and Microcystis aeruginosa (Cyanophyceae) were the two common species in the Itaipú reservoir (Andrade et al., 1988) and High Paraná river course in previous studies (Bonetto et al., 1982;Zalocar de Domitrovic and Vallejos, 1982); however, they were scarcely represented during the postimpoundment period.These species were characteristics of eutrophic reservoirs in the Tieté river, a tributary of the Upper Paraná (Barbosa et al., 1999).Calijuri et al. (1999) indicated the dominance of Cyanophyceae (Microcystis spp.and Pseudanabaena spp.) in Salto Grande (Americana, SP, Brazil), an eutrophic reservoir of the Atibaia river, where high urban and industrial densities produce an increasing deterioration of the water quality.Low nutrients concentration in the Yacyretá reservoir, even in the filling stage (Neiff et al., 2000), and the short residence time, may explain the scarcity of Cyanophyceae in the studied sector.
Dominant algal groups, in the High Paraná river (Brazil-Argentina) before the Yacyretá dam, were Bacillariophyceae, Chlorophyceae and/or Cyanophyceae (CECOAL, 1977;EBY, 1979;Zalocar de Domitrovic and Vallejos, 1982;Andrade et al., 1988).When comparing studies conducted near Ituzaingó (High Paraná) with those carried out after by Meichtry de Zaburlín (1999) in the same sampling area, there was an increase in phytoplankton density.The changes occurring in the phytoplankton community downstream from the reservoir in the period of maturity are very variable and depend on several factors including morphometry, climate, physical and chemical properties of water, residence time, biological interactions and their location in the river gradient.The existence of numerous reservoirs in the Paraná have caused great discontinuities in the potamoplankton, interrupting the natural continuum (Vannote et al., 1980).By regulating river flow, reservoirs may have cascading effects on downstream lakes (Barbosa et al., 1999;Dumont, 1999) and the ecological changes that happen downstream from the reservoir depend on its location in the continuum according to the serial discontinuity concept (SDC) introduced by Ward and Stanford (in Petts, 1989).

Conclusions
Physical and chemical conditions measured on the right bank (water from Paraguay river) were similar at both periods (pre and post-impoundment) whereas conductivity, pH and orthophosphate concentration increased on the left bank (water from High Paraná river and Yacyretá reservoir) after the impoundment.
The density, structure and specific diversity of phytoplankton of the Paraná river showed statistically significant differences between water under the influence of the High Paraná river (Station 1) and that under the influence of the Paraguay river (Station 2).
The phytoplankton of Station 2 (right bank) did not show significant differences in relation to pre-impoundment studies (Zalocar de Domitrovic and Vallejos, 1982;Bonetto et al., 1982), as expected since the Paraguay river (which flows along this bank) remains little affected by human activities and originates in the Pantanal, one of the largest wetlands of the world.In contrast, phytoplankton abundance and structure at Station 1 (water from the High Paraná river) showed marked changes after the creation of the Yacyretá dam.There were significant increases in Chlorophyceae and Cryptophyceae densities, whereas Cyanophyceae and Bacillariophyceae decreased in relation to their pre-impoundment levels (Zalocar de Domitrovic and Vallejos, 1982;Bonetto et al., 1982).
The presence of the Yacyretá reservoir in the High Paraná river causes considerable changes in the density and structure of phytoplankton communities.Although phytoplankton variations would be influenced by the hydrological regime of the river and to associated factors, the increase in phytoplankton density appears to be related to the short residence time of the water in the reservoir and to favorable environmental conditions for observed 220 km downstream from Yacyretá.

Figure 1 .
Figure 1.Location map of the study site showing the two sampled stations in the Paraná river.Station 1: left bank, associated with water from the High Paraná river and the Yacyretá dam.Station 2: right bank, associated with water from the Paraguay river.

Figure 3 .Figure 4 .
Figure 3. Variations in density (D) and diversity (H'= Shannon-Wiener index) of phytoplankton in the two stations sampled in the Paraná river.S1: Station 1 (left bank, associated with water from High Paraná river and the Yacyretá dam); S2: Station 2 (right bank, associated with water from the Paraguay river).

Table 1 .
Mean annual values, standard deviation (S.D.)and the results of the Wilcoxon test (Z) evaluating the difference between Station 1 (left bank) and Station 2 (right bank) for each environmental variable.p <0.05 are in bold; n = number of samples.

Table 3 .
Results of fitting a multiple regression model to describe the relationship between two first axes of DCA (sample scores which are weighed mean species scores) and eight independent variables: water level, temperature, Secchi disk, disolved oxygen, conductivity, nitrite+nitrate concentration and orthophosphate concentration.