Macrophyte Species Composition in Streams of Latvia under Different Flow and Substrate conditions/Makrofuutide Liigiline Koosseis Lati Vooluvetes Erineva Voolukiiruse Ja Substraadi Tingimustes

INTRODUCTION Streams are heterogeneous environments with differences in physical and chemical parameters. Different studies worldwide obviously indicate that the knowledge of the macrophyte species composition and abundance provides important information on the aquatic ecosystem (e.g., Best, 1995; Dawson & Szoszkiewicz, 1999; Baatrup-Pedersen et al., 2003). Macrophytes are a key component in the functioning of streams where they grow in relatively high abundance (Clarke, 2002). The physical stream environment has a major impact upon the development and growth of submersed macrophytes (Baatrup-Pedersen & Riis, 1999). In shallow, low-energy streams where macrophytes are able to grow abundantly, plants greatly influence the functioning of the ecosystem (Sand-Jensen et al., 1989). Substrate and flow velocity are the two most important factors influencing the distribution of macrophytes in streams. And vice versa, the growth of macrophytes has important impacts upon flow resistance, flow velocities, and sediment dynamics. Macrophytes grow abundatly under such current velocity and in sediments where they can be best rooted and withstand the erosive force of the water during periodic scour events (Chambers et al., 1991). Velocity is an important controlling factor of substrate stability and composition of macrophytes. Several researchers have argued that flow velocity is the main factor in controlling macrophyte composition and biomass in streams (Westlake, 1967; Nilsson, 1987; Chambers et al., 1991; Riis & Biggs, 2003; Haslam, 2006). Flow velocity has often been found to affect the distribution of macrophytes in streams (e.g., Riis et al., 2000; Demars & Harper, 2005). River surveys have indicated that many macrophyte species are associated with sediment of particular particle size (Holmes, 1983; Haslam, 2006). Substrate stability is a significant controlling factor because a stable substrate allows rooting and establishment of macrophyte communities. Mobile substrates prevent this, resulting in a limited potential for plant community development (Riis & Biggs, 2003; Haslam, 2006). Despite the apparent importance of water movement in regulating macrophyte communities in lowland rivers, the nature of the processes and the factors controlling their dynamics are not well understood (Franklin et al., 2008). For an assessment of the ecological status of a stream using macrophytes a certain minimum plant quantity is required. The variability of macrophyte richness is linked to physical factors in the environment, which make an important contribution to the pattern of macrophyte distribution (Abou-Hamdan et al., 2005). Since there are still numerous unaffected streams in Latvia, knowledge of the riverine vegetation in Latvia could be of regional importance on a European scale; for example, Baatrup-Pedersen et al. (2008) found a high similarity of species-based predictions in Denmark before intensive land use started there around 1900 with vegetation in Latvian and Lithuanian streams. The aim of the study was to examine the role of different environmental factors in the formation of macrophyte vegetation in middle-sized lowland streams in Latvia, focusing on the importance of flow velocity and substrate type of streams. The objectives were to investigate the macrophyte vegetation at 131 stream sites throughout the country, and to supplement each survey with a description of environmental factors (substrate type, flow velocity, shading, stream width, and water depth). MATERIAL AND METHODS Study area The study area covers the whole territory of Latvia. There is a dense network of streams in Latvia. The total number of streams is up to 12 500, of which only 17 exceed 100 km (Klavins et al., 1999). The streams are classified as lowland streams with averagely low flow velocities and modest hydrological variability. However, spring flow velocities may be high and summer velocities low. …