The Lower Paraguay river-floodplain habitats in the context of the Fluvial Hydrosystem Approach Edmundo C. Drago1, Karl M. Wantzen2, Aldo R. Paira1 1Instituto Nacional de Limnología (CONICET-UNL), J. Macia 1933, 3016 Santo Tomé, Santa Fe, Argentina. e-mails: [email protected]; [email protected] 2Max-Planck-Institute for Limnology, Tropical Ecology Work Group, 24302 Plön, Germany and University of Konstanz, 78467 Konstanz, Germany. e-mail: wantzen@mpil-ploen mpg.de Abstract We report herein the first description of the physical structure of the aquatic habitats of the Lower Paraguay River along 390 km from Asunción city (Paraguay) to the confluence with the Paraná River. The hierarchical ordination of the Fluvial Hydrosystem Approach (FHA) allowed us to classify the Lower Paraguay as a mean- dering functional sector where five functional sets were identified: (a) main channel, (b) floodplain channel, (c) floodplain lentic environment, (d) tributary, and (f) aquatic-terrestrial transition zone. These functional sets encompassed twenty one functional units and sixty one major mesohabitats. We attribute the riverine habitat diversity to the changes in the channel-floodplain morphology and in the strength, duration and frequency of their hydrological connectivity. The variable river-flood- plain-tributary complex developed several types of fluvial-lacustrine boundaries and riverine ecotones. Key words: Large river, physical habitat, connectivity, boundaries, ecotones. 1. Introduction While almost all large fluvial systems in the Northern Hemisphere have been regulated, the The Paraguay River forms an hydroecologi- Paraguay River is still in a nearly pristine stage cal corridor that crosses the central part of South and may be useful as a natural model for restora- America from the tropical headwater spring tion planning in similar systems of the tropical brooks at 15° S (closely linked with Amazonian and subtropical zones. Moreover, insights from headwater streams) to the subtropical zone of the largely preserved river systems may be used to confluence with the Paraná River (27° 17’ S). develop models for restauration planning in those Large parts of the upper and middle catchments rivers where pristine reference sites have been of the Paraguay River contain one of the more lost (Wantzen et al. 2005). Several studies in important ecological regions of the world, the temperate rivers (Drago 1980, 1981, 1989, 2007; huge Pantanal wetland which in turn, regulates Amoros, Roux 1988; Baker et al. 1991; Ward, the hydrosedimentological regime of the Lower Stanford 1995; Arscott et al. 2000; Tockner et al. Paraguay segment (Drago et al. 2008, this issue 2000; Pringle 2001; Amoros, Bornette 2002; Figs. 1 and 2). Marchese, Ezcurra de Drago 1992; Marchese et 1 2 6 al. 2002; Drago et al. 2003) as well as in tropical ranging from 5 to 100 km in length. In spite of that rivers (Hamilton, Lewis, Jr. 1987; Hamilton et al. length ranges, they represent a uniform set of 1996; Ezcurra de Drago et al. 2004; Marchese et physical, chemical, and biological conditions with- al. 2005; Wantzen et al. 2005), identify fluvial in a large river segment. Because of that homoge- landscape dynamics and connectivity betweeen neity in their environmental characteristics, they main channel and floodplain as key factors in are usually chosen as the principal sampling units. controlling habitat heterogeneity and biotic diver- Furthermore, the reach is also a common unit of sity. Few studies of this kind exist in neotropical field description among fluvial geomorphologists rivers (e.g. Ezcurra de Drago et al. 2004; (Frissell et al. 1986; Amoros et al. 1987; Petts, Marchese et al. 2005), however they are urgently Amoros 1996; Fitzpatrick et al. 1998). For exam- needed as habitat destruction proceeds very fast ple, the Lower Paraguay functional sector includes (Wantzen et al. 2005). a 80 km long reach (from Bermejo mouth to the The primary goal of this paper is to contrib- confluence with the Paraná), which is strongly ute to the physical characterization, hierarchical affected by the high inputs of solid suspended load ordination and evolution trends of the aquatic from Bermejo River (Drago et al. 2008 this issue, habitats of the Lower Paraguay River (Drago et Fig. 1A,B). A functional set on the Lower Paraguay al. 2008, this issue Fig. 1), as well as the descrip- may develop an area as large as 4500 ha, as in the tion of the major types of water-water boundaries case of the Lake Herradura functional set, an aban- and ecotones. doned meander of the main channel (Drago et al. 2008 this issue, Figs. 1, 3, 4). The character and evolution of each functional set within the fluvial 2. Materials and methods segment is determined by the river-floodplain dynamics and the connection degree with the main Functional classification channel or the active floodplain channels. For instance, the former floodplain areas remote from As in previous papers (Drago et al. 2003; the Paraguay main channel are dominated by the Wantzen et al. 2005) we adopt the hierarchical rainfall, tributary input and groundwater seepage, ordination of the riverine landscape structure and receive river water only during higher floods developed in the Fluvial Hydrosystem Approach (Drago et al. 2008, this issue Figs.3, 5). The divi- (FHA; Amoros et al. 1987; Petts, Amoros 1996). sion of the functional units were based on a com- The target of this statement is to compare the flu- bination of the fluvial conditions of the site, as vial segments or their different reach assemblages topographic slopes, water depth, and frequency and the corresponding types of aquatic habitats and duration of the inundation and drought phases. that conform the Paraguay River hydrosystem In the cited case of the Lake Herradura set, func- (Wantzen et al. 2005). The remainder on this sec- tional units of the lake has a surface of 560 ha, tion as well as the site description has been devel- whereas the marsh unit develops an area of ca. oped in Drago et al. (2008 this issue). 2000 ha (Drago et al. 2008, this issue Figs. 1, 3: Measurements of the water temperature and salin- HL, 4: 7, 8, 9). The flow pulse (sensu Tockner et ity were made with a hand-held WTW series 300 al. 2000) is also a very important factor to condi- probes, the turbidity with a Hach turbidimeter, and tioning the functional units in short temporal the water velocity with an AOTT current meter. scales. The hydro-functional classification of the A functional sector describes river segments lentic units was mainly based on their geomorphic differentiated by changes in channel pattern- flood- connection and the degree of connectivity with the plain type, valley slope, width, and the confluence river water (Drago et al. 2008, this issuee Table I, effect from tributaries with different water and Figs. 4, 5, 11,12, 13). sediment discharge and hydrochemistry. In the large South American rivers, the scale as well as the channel pattern of each functional sector or 3. Results segment may vary greatly, sometimes largely over- coming 100 km in length. That is the case of the The meandering-floodplain functional sector Paraguay River, where its upper segment shows an of the Lower Paraguay includes five functional assemblage of reaches with different lengths and sets which embraced twenty one functional units. patterns (Wantzen et al. 2005). River sectors may Within these functional units, sixty one mesohabi- be divided into functional sets (Petts, Amoros tats were delineated encompassing those located in 1996) which are closely linked with specific fluvi- the main channel as well as in the floodplain com- al geoforms as braided or meander reaches, aban- plex (Fig. 1.). However, we must stress that partic- doned meanders, ridge and swale topography, etc. ular riverine areas often provide an ecologically The terms “reaches” or “stretches” are used as syn- distinct habitat at different water levels, as some onymous of functional sets for the shorter sections minor temporary floodplain channels which may forming a segment or functional sector, which show lentic conditions during drought phases 1 2 7 FUNCTIONAL SET FUNCTIONAL UNIT MESOHABITAT - substrate-defined patches - mobile sand bedforms CENTRAL STRIP - rocky outcrop - logjam - substrate-defined patches - meander scour hole BANK STRIP - rocky outcrop - slackwater area - logjam MAIN CHANNEL - aquatic vegetation belt CHANNEL BAR - substrate-defined patches - associated slackwater area - substrate-defined patches CHANNEL ISLAND - chute channel - scroll lake - levee lake FLOATING SUBSTRATA - floating macrophyte patches - driftwood - substrate-defined patches CENTRAL STRIP - confluence scour hole - logjam - substrate-defined patches - meander scour hole - chute channel FLOODPLAIN CHANNEL BANK STRIP - scroll lake - logjam - aquatic vegetation belt CHANNEL BAR - substrate-defined patches - associated slackwater area FLOATING SUBSTRATA - floating macrophyte patches - driftwood DIRECT CONNECTED LAKE - open water INDIRECT CONNECTED LAKE - vegetated shoreline FLOODPLAIN WATER BODY ISOLATED LAKE - vegetation-free shoreline MARSH - substrate-defined patches SWAMP - macrophyte-defined patches - substrate-defined patches CENTRAL STRIP - rocky outcrop - logjam - substrate-defined patches - meander scour hole BANK STRIP - rocky outcrop - slackwater area - aquatic vegetation belt - logjam TRIBUTARY - substrate-defined patches CHANNEL BAR - associated slackwater area - logjam - substrate-defined patches CHANNEL ISLAND - chute channel