Hydrobiologia 415: 147–154, 1999. J. M. Caffrey, P. R. F. Barrett, M. T. Ferreira, I.S. Moreira, K. J. Murphy & P. M. Wade (eds), Biology, Ecology and Management of Aquatic Plants. © 1999 Kluwer Academic Publishers. Printed in the Netherlands.
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Aquatic macrophyte distribution in relation to water and sediment conditions in the Itaipu Reservoir, Brazil L. Mauricio Bini1,∗ , Sidinei M. Thomaz2 , Kevin J. Murphy3 & Antonio F. M. Camargo 4 1 Universidade
Federal de Goi´as, ICB, DBG, Caixa Postal 131, CEP 74001-970, Goiânia, GO, Brazil E-mail:
[email protected] 2 NUPELIA (N´ ucleo de Pesquisas em Limnologia, Ictiologia e Aquicultura), Universidade Estadual de Maring´a, Avenida Colombo 5790, CEP 87020-900, Maring´a, Brazil E-mail:
[email protected] 3 Institute of Biomedical and Life Sciences, Division of Environmental and Evolutionary Biology, Graham Kerr Building, University of Glasgow, Glasgow G12 8QQ, Scotland E-mail:
[email protected] 4 Departamento de Ecologia, Instituto de Biociências, UNESP, Rio Claro, SP, Brazil E-mail:
[email protected] Key words: freshwater macrophytes, reservoirs, South America, aquatic weeds
Abstract Aquatic macrophyte community distribution along the eastern shoreline of the Itaipu Reservoir (one of the South America’s largest impoundments) is described in relation to limnological and sedimentological factors. The central body of the reservoir is mesotrophic, while the arms (flooded influent river valleys) along the eastern shore may be oligo-mesotrophic to eutrophic, depending on time of year and sub-catchment characteristics. Macrophyte community composition and species cover were surveyed at 30 sites in four arms, in relation to sediment total P and organic matter; underwater light regime; and water total P and Kjeldahl N concentration, alkalinity, conductivity, depth and pH. Seventeen euhydrophyte and six emergent macrophyte species were recorded. Large stands of Egeria najas dominated the euhydrophyte vegetation, together with free-floating weed species (Pistia stratiotes Linn., Salvinia auriculata Aublet and Eichhornia crassipes (Mart.) Solms.). Canonical Correspondence Analysis of the data showed that two sets of variables were important predictors of aquatic macrophyte community structure. Floating macrophyte assemblage was closely related to concentration of nutrients in both water and sediment, while light penetration was the strongest predictor of submerged species occurrence. Although a large number of potential nuisance species were present, dense growths were restricted to shallower areas of the Itaipu Reservoir, causing localised problems. The possibility of increasing interference by these plants with fisheries, recreational use, transport and hydroelectricity generation suggests a need for continued monitoring of weed distribution and abundance, and investigation of appropriate management measures.
Introduction Abundant growth of aquatic macrophytes is a common feature of large reservoir systems in sub-tropical and tropical regions, especially in impoundments which have complex shapes with extensive areas of sheltered, shallow water ideal for macrophyte growth ∗
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(e.g. Mitchell et al., 1990; Ali et al., 1995). If unchecked by appropriate management, aquatic weeds (especially free-floating plants such as E. crassipes, S. auritulata and P. stratiotes, and submerged species of Hydrocharitaceae, such as Egeria densa Planch. and Egeria najas Planch.) may cause serious problems in reservoirs with suitable hydrogeomorphological, climatic and trophic characteristics. Weed species of Hydrocharitaceae for example, are notorious for their
148 tendency to form large detached masses of vegetation which interfere with the utilization of water resources (Pieterse & Murphy, 1990). Such problems are increasingly common in South America. For example, Tundisi et al. (1993) reported problems of ‘excessive growth’ of E. crassipes and Eichhornia azurea (Swartz) Kunth in reservoirs of the Tietê River in São Paulo State, Brazil. It is a notable feature of South American reservoir systems that aquatic weed problems are usually caused by native Neotropical species (Fernández et al., 1990) unlike reservoirs elsewhere in the subtropical - tropical world where introduced species are commonly the main cause of problems. E. najas, E. densa, E. crassipes and P. stratiotes are all native to the Paraná system (Hoehne, 1948, Neiff, 1986). E. najas typically occurs in high transparent ponds with remote influence of the High Paraná River in Argentina (Neiff, 1986) and in backwaters of the Paraná in Brazil, upstream to Itaipu reservoir (S.M.Thomaz, personal observation). This species has recently been widespread in Itaipu and in several Brazilian reservoirs (Thomaz et al., 1999). The objective of this work was to identify the structure of euhydrophyte assemblages present in the Itaipu Reservoir and to establish the relative importance of sets of abiotic factors as predictors of vegetation type. This study arose from concerns of the Itaipu Reservoir managers about the development of free-floating and submerged aquatic weed problems in the lake.
Materials and methods The Itaipu Reservoir (24◦ 050 – 25◦ 330 S; 50◦ 000 – 50◦ 300 W) is a large impoundment (mean area 1350 km2 ; mean depth 21.5 m; length 170 km) formed since 1982 behind the Itaipu Dam on the Rio Paraná, itself one of the largest rivers in the world, draining an area of 2.8 × 106 km2 of South America. The reservoir has an irregular shape (Figure 1) with a large number of shallow arms representing the flooded lower valleys of small to medium-sized tributaries draining sub-catchments having spatially and seasonally varied characteristics. A preliminary survey of the presence of aquatic macrophytes along the Brazilian shore of the Itaipu Reservoir conducted during 1995 showed substantial variation in both abundance and species composition of the macrophyte community (Thomaz et al., 1999). Results from this study were used to select four reser-
voir arms for further study during August 1996. This intensive survey of macrophyte abundance and ambient conditions in water and sediment was undertaken at 90 individual stations, located at 30 sites in the selected arms (Figure 1). At each site, three stations were selected at random along the 0.5, 1.0 and 1.5 m isobaths for sampling from a boat. Water levels in the reservoir were low (some 0.7 m below average) but did not vary during the sampling period. Submerged plants were sampled in deeper water using a Petersen grab (0.12 × 0.30 m), which worked well in the soft sediments of the reservoir. A floating quadrat (0.5 × 0.5 m) was used in shallow water stations. In both cases, plant species present within the sample were visually scored on a Domin-Krajina abundance scale (1 =
