Aquatic macrophytes of Itaipu Reservoir, Brazil: survey of species and ecological considerations

Abstracts

In a survey of the aquatic macrophytes of the Itaipu Reservoir, we identified 62 taxa in 25 families and 42 genera. The highest number taxa was observed for the emergent macrophytes (40 taxa). Reduced fluctuation in water level, increased light penetration, and sediment enrichment by nutrients and organic matter following the formation of the reservoir favored the appearance of a species-rich community of submerged macrophytes (23% of the taxa identified). The aquatic macrophytes were found mainly near the mouths of the main tributaries of the reservoir, in shallow area of depth less than 2 meters. In addition to the shallow depth, the greater nutrient input from the tributaries and relative protection from wind explained this distribution. Among the species found, Egeria najas merits mention for its occurrence in all localities sampled, with biomass values varying between 98 and 186 gDW/m². Some potential nuisance species such as Eichhornia crassipes, Salvinia auriculata, and Pistia stratiotes also deserve attention, since they were also observed to be covering large areas of Itaipu Reservoir.

Nuisance species; aquatic macrophytes; Itaipu reservoir; Egeria najas; Egeria densa

Em um levantamento de macrófitas aquáticas realizado no reservatório de Itaipu foram identificados 62 táxons pertencentes a 25 famílias e 42 gêneros. O maior número de táxons foi representado pelas macrófitas emergentes (40 táxons). O surgimento de uma rica comunidade de espécies submersas (23% dos táxons identificados) parece ter sido favorecido pelas alterações decorrentes da formação dor reservatório, tais como redução da flutuação dos níveis de água, aumento da penetração de luz e enriquecimento do sedimento em nutrientes e matéria orgânica. As macrófitas aquáticas foram registradas principalmente próximo à foz dos tributários do reservatório, em áreas rasas (profundidade inferior a 2 metros). Além da reduzida profundidade, o maior aporte de nutrientes associado à proteção do vento explicam essa distribuição. Entre as espécies registradas, Egeria najas merece destaque por ocorrer em todas as localidades visitadas com valores de biomassa entre 98 e 168 gPS/m². Algumas espécies potencialmente daninhas como Eichhornia crassipes, Salvinia auriculata e Pistia straiotes também merecem atenção, pois foram encontradas cobrindo amplas áreas de alguns braços do reservatório.

Aquatic macrophytes of Itaipu Reservoir, Brazil: survey of species and ecological considerations

Sidinei Magela Thomaz^*I; Luis Maurício Bini^II; Maria Conceição de Souza^I; Kazue Kawakita Kita^I; Antonio Fernando Monteiro Camargo^III

^IUniversidade Estadual de Maringá, Departamento de Biologia-Nupélia, Maringá, PR, 87020-900

^IIUniversidade Federal de Goiás, ICB, DBG, CP 131, Goiânia, GO, 74001-970

^IIIUniversdiade Estadual Paulista, Departamento de Ecologia, Av. 24A, 1515, Rio Claro, SP, 13506-900

ABSTRACT

In a survey of the aquatic macrophytes of the Itaipu Reservoir, we identified 62 taxa in 25 families and 42 genera. The highest number taxa was observed for the emergent macrophytes (40 taxa). Reduced fluctuation in water level, increased light penetration, and sediment enrichment by nutrients and organic matter following the formation of the reservoir favored the appearance of a species-rich community of submerged macrophytes (23% of the taxa identified). The aquatic macrophytes were found mainly near the mouths of the main tributaries of the reservoir, in shallow area of depth less than 2 meters. In addition to the shallow depth, the greater nutrient input from the tributaries and relative protection from wind explained this distribution. Among the species found, Egeria najas merits mention for its occurrence in all localities sampled, with biomass values varying between 98 and 186 gDW/m². Some potential nuisance species such as Eichhornia crassipes, Salvinia auriculata, and Pistia stratiotes also deserve attention, since they were also observed to be covering large areas of Itaipu Reservoir.

Key words: Nuisance species, aquatic macrophytes, Itaipu reservoir, Egeria najas, Egeria densa

RESUMO

Em um levantamento de macrófitas aquáticas realizado no reservatório de Itaipu foram identificados 62 táxons pertencentes a 25 famílias e 42 gêneros. O maior número de táxons foi representado pelas macrófitas emergentes (40 táxons). O surgimento de uma rica comunidade de espécies submersas (23% dos táxons identificados) parece ter sido favorecido pelas alterações decorrentes da formação dor reservatório, tais como redução da flutuação dos níveis de água, aumento da penetração de luz e enriquecimento do sedimento em nutrientes e matéria orgânica. As macrófitas aquáticas foram registradas principalmente próximo à foz dos tributários do reservatório, em áreas rasas (profundidade inferior a 2 metros). Além da reduzida profundidade, o maior aporte de nutrientes associado à proteção do vento explicam essa distribuição. Entre as espécies registradas, Egeria najas merece destaque por ocorrer em todas as localidades visitadas com valores de biomassa entre 98 e 168 gPS/m². Algumas espécies potencialmente daninhas como Eichhornia crassipes, Salvinia auriculata e Pistia straiotes também merecem atenção, pois foram encontradas cobrindo amplas áreas de alguns braços do reservatório.

INTRODUCTION

Reservoir construction causes changes in bodies of water, most obviously reduction in current velocity and increases in shoreline development, water transparency, and sediment stability (Tundisi et al., 1993). Moreover, depending on the limnological characteristics of the river, the reservoir may undergo rapid eutrophication, which is most accentuated in the first reservoirs of a series (Bini, 1995). These changes can lead to the invasion and rapid development of different species and ecological types of aquatic macrophytes (Pieterse & Murphy, 1990).

Floristic surveys represent the first phase of studies envisioning the identification of potential nuisance species and management of the aquatic macrophyte community (Wade, 1990). In spite of the large number of reservoirs constructed in Brazil (130 reservoirs more than 100 km² in the southeast alone; Agostinho et al., 1995) and the ecological importance of aquatic macrophytes (Esteves, 1988), studies on this community in reservoirs have just began in Brazil. Accordingly, floristic surveys are important in view of the fact that excessive growth of these plants can generate problems for multiple uses of reservoirs such as recreation, fishing, water supply, and generation of electrical power.

In the present study, a survey of species composition of aquatic macrophytes along the left shore of the Itaipu Reservoir was undertaken, with the objective of identifying the species with greatest potential to cause problems for multiple uses of the reservoir. The biomass, distribution, and influence of the principal macrophyte species on some physical and chemical characteristics of the water were also considered.

STUDY AREA

Itaipu Reservoir (24º05' to 25º33'S and 50º00' to 50º30'W) is 1350 km² in area, 170 km long, with a mean width of 7 km (Fig. 1). The mean depth is 21.5 m and the theoretical residence time of the water is 40 days. The water level fluctuates less than 1 m annually. The central body of the reservoir can be considered mesotrophic, while the arms along the left (Brazilian) shore may be mesotrophic or eutrophic, depending on the time of year (Andrade et al., 1988).

Water characteristics of the 9 surveyed arms shows marked differences between tributary mouths (regions more frequently colonized by aquatic macrophytes) and open waters.

The reservoir has several environmental characteristics that favour the development of different species of aquatic macrophytes. Primarily, the annual mean temperature of 21ºC recorded in the area influenced by the reservoir does not strongly restrict the development of these plants. On a regional scale, the floodplain of the Upper Paraná River upstream from the reservoir is a continuous source of propagules of many species.

METHODS

Subsurface water samples were collected from two areas of the 8 principal arms along the left shore of the reservoir in the open water of the arms (near the main body of the reservoir), and the other near the mouths of the tributaries (Fig. 1). Macrophytes were also collected in these same arms. The collections were made during 3-14 December 1995 and 15-28 September 1996. The monitoring carried out monthly since April 1997 until April 1998 did not add new species.

Because of the large size of the reservoir, an initial aerial survey, with helicopter was carried out to locate the areas most densely colonized by aquatic macrophytes and to maximize efficiency of the fieldwork. Emergent and free-floating macrophytes and species with floating leaves were collected manually. Submerged plants were collected with the aid of a Petersen grab and hooks.

In the arms of the São João, Ocoí, São Francisco Falso, and São Francisco Verdadeiro Rivers, we evaluated the biomass of Egeria najas, Potamogeton pusillus, Salvinia auriculata, Eichhornia crassipes, and Pistia stratiotes. For the first two (submerged) species, we collected all plant matter (leaves, petioles, and flowers) contained within 0.25 m² quadrats. For the other (free-floating) species, we collected all plant parts (leaves, petioles, flowers, and roots) contained within 0.50 m² quadrats. The samples were dried to constant weight (DW) at 105 ºC.

The frequency of occurrence of E. najas, the species, which Itaipu managers are more concerned about, was evaluated in the arms of the São João, São Vicente, and São Francisco Falso Rivers. In these localities, we established 25, 11, and 26 collection points respectively, located in the littoral zone demarcated in the map. The number of localities was chosen according to arm heterogeneity and size. At each point the presence or absence of E. najas was recorded. The results are presented as percentage of points in which this species was registered.

RESULTS AND DISCUSSION

List of taxa and ecological groups of aquatic macrophytes

A total of 62 taxa belonging to 25 families and 42 genera were identified. Of these, 42 taxa were identified to species level (Table 2). The family Poaceae was best represented, with 14 taxa, next the Characeae with 7 taxa, and the Polygonaceae with 4 taxa. The families Pontederiaceae, Potamogetonaceae, and Hydrocharitaceae were each represented by 3 taxa. Only 1 or 2 taxa of each of the remaining families were identified.

The number of species (62 taxa) contrasted with other surveys in large reservoirs in South America. In Guri Reservoir (Venezuela), an oligotrophic, blackwater environment with an area (4,500 km²) three times greater than the Itaipu, only 27 taxa of aquatic macrophytes were recorded (Vilarrubia & Cora, 1993). The higher number of species in the Itaipu may be associated with input of nutrients from its tributaries. According to Andrade et al. (1988), the Itaipu backwaters are meso-eutrophic. Additionally, the moderate water levels variations, which oscillate aproximately one meter per year in the Itaipu, may also favour colonization by different species. It has been shown for other reservoirs that water fluctuations of up to three meters increase the aquatic macrophyte species richness, but higher fluctuations cause its decrease (RØrslett, 1991).

The majority (64%) of the taxa identified in Itaipu Reservoir were emergent macrophytes. Nevertheless, Itaipu is rich in submerged rooted taxa (14 taxa, 23% of the total) compared to undammed environments of the Paraná River, where there are still floodplains. In the Argentine stretch, for example, none of the environments that connect with the annual flood pulse of the Paraná River have submerged macrophytes (J. J. Neiff, personal communication). A similar situation is observed in the floodplain of the Upper Paraná upstream from Itaipu, where only five taxa of submerged macrophytes have been recorded (S.M.Thomaz, personal observation).

The changes caused by construction of the reservoir therefore, created a favorable environment for the development of submerged rooted macrophytes. This statement is supported when we consider the data from a survey of the Itaipu area before the reservoir was constructed (Surehma, 1980), which documented only five species of submerged macrophytes. Although these results deserve caution, since only the main tributaries were surveyed, E. najas, a common species in Itaipu nowadays, was not found at that time. In this way, it is postulated that the factors contributing to the development of the submerged macrophytes following reservoir construction probably include the reduction in water velocity, increase in system stability when compared to the natural floodplains (the fluctuation in water level being usually lower than 1 meter per year at Itaipu), increased light penetration, and enrichment of sediment by nutrients and organic matter.

Spatial distribution and biomass

The aerial survey established that the aquatic macrophytes were distributed mainly in the upper parts of the reservoir arms, i.e in shallow areas of depth less than 2 meters, near the mouths of the tributaries. These locations have several characteristics that together account for the presence of macrophytes: greater nutrient input, greater sedimentation rate, which reduces the depth and increases the area available for colonization, and protection from wind.

Of all the species of aquatic macrophytes found in the reservoir, E. najas required attention since it was recorded in all the arms investigated. Its wide distribution in the reservoir contrasted with its restricted distribution in the Paraná River floodplain. In the Argentine stretch, this species colonized only the most stable habitats such as small, highly transparent ponds with limited influence from the river (Neiff, 1986). E. najas was not only widely distributed, but occurs frequently and developed high biomass in the Itaipu Reservoir. This high frequency of occurrence was indicated by its presence at previously established sampling points in the arms of the São João (16% of points), São Vicente (36%), and São Francisco Falso (54%) Rivers. Biomass values for E. najas varied between 98 and 186 gDW/m² (Table 3). These results are high comparing to the biomass of P. pusillus, another submerged species found in Itaipu (Table 3).

In some localities large mixed stands of floating macrophytes were observed. The Ocoí River debouches into the reservoir we observed a large area covered by S. auriculata, P. stratiotes, and E. crassipes. In spite of the extensive coverage, the biomass values (Table 4) were lower than some values recorded for reservoirs in the State of São Paulo: 689 to 1,638 gDW/m² for E. crassipes (Esteves, 1982). It is appropriate to emphasize that free-floating species have a high growth rate. Cook (1990) noted that E. crassipes required only 13 days to double the area occupied, while Salvinia spp. required between 7 - 17 days. Thus, under favorable conditions, such as high nutrient input, extensive areas may be colonized in a short period of time.

Potential nuisance species of aquatic macrophytes

According to Fernández et al. (1990), submerged aquatic macrophytes caused most problems in temperate aquatic ecosystems, while floating species were more troublesome in the tropics. Of the species recorded in Itaipu, several are potential nuisance species: the floating E. crassipes, S. auriculata, and P. stratiotes, and the submerged E. najas, E. densa, and Chara spp.

E. crassipes and P. stratiotes have been considered the main weed species in several countries of Central and South America (Fernández et al., 1990). These authors also documented problems, though to a lesser degree, caused in several countries by Eichhornia azurea, Typha domingensis, Myriophyllum brasiliensis and Salvinia auriculata, all species found in Itaipu. Egeria was cited as one of the genera that rarely caused problems in waterbodies of the Americas, except in Chile. Egeria najas, however, was widely distributed in Itaipu and could be considered as the submerged species with greatest potential to interfere significantly with the multiple uses of the reservoir.

CONCLUSIONS

Construction of the Itaipu Reservoir favoured the development of a large number of aquatic macrophyte species, largely dominated by a few species. Those with the highest potential as nuisance species, however, belong to the groups of submerged and floating species, mainly E. najas, E. densa, P. stratiotes, S. auriculata, and E. crassipes.

Of the submerged species, E. najas was found in all the arms along the left shore of the reservoir, and was recorded with high frequency and high biomass values.

The aquatic macrophytes of Itaipu Reservoir were still in the initial phase of colonization, and remain restricted to the backwaters, specifically near the mouths of the tributaries. The great potential for development of floating macrophytes, however, which in this reservoir was associated with nutrient input (FUEM/Itaipu Binacional, 1997), requires caution. On the other hand, the great depth and low water transparency values of most of the reservoir indicated that massive development of submerged species such as E. najas would be restricted to certain backwater areas.

ACKNOWLEDGEMENTS

Dr. Juan José Neiff (CECOAL - Centro de Ecologia Aplicada do Litoral, Corrientes, Argentina), Dr. Kevin J. Murphy (University of Glasgow, Scotland) and Bch. Débora Cristina de Souza (Universidade Estadual de Maringá) lent their assistance in identifying the plant taxa. The Charophyta were identified by Dr. Carlos Eduardo Bicudo and Dr. Norma Catarina Bueno (Instituto de Botânica de São Paulo). Dr. Janet W. Reid (Smithsonian Institute, Washington DC) translated the manuscript into English. We thank also the boatman Sandro Alves Heil (Itaipu Binacional) for field assistance. This project was financed by Itaipu Binacional.

Received: December 01, 1997

Revised: May 26, 1998

Accepted: November 23, 1998

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*

Author for correspondence

Publication Dates

• Publication in this collection
  03 June 2011
• Date of issue
  1999

History

• Received
  01 Dec 1997
• Reviewed
  26 May 1998
• Accepted
  23 Nov 1998