Short communication Effects of phosphorus and nitrogen amendments on the growth of Egeria najas Sidinei Magela Thomaz a, * , Patricia A. Chambers b , Sandra Andre ´a Pierini c , Gabrielli Pereira d a Universidade Estadual de Maringa ´, Nupelia, Av. Colombo 5790, 87020-900 Maringa ´, PR, Brazil b National Water Research Institute, P.O. Box 5050, 867 Lakeshore Blvd., Burlington, Ont. L7R 4A6, Canada c Faculdade Integrado de Campo Moura ˜o, Rodovia BR – 158, km 207, Jardim Batel, CEP 87 300-970, Campo Moura ˜o, PR, Brazil d Programa de Po ´s-Graduac ¸a ˜o em Ecologia de Ambientes Aqua ´ticos Continentais, Universidade Estadual de Maringa ´, Departamento de Biologia, Nupelia, Av. Colombo 5790, 87020-900 Maringa ´, PR, Brazil Received 16 February 2006; received in revised form 16 October 2006; accepted 23 October 2006 Abstract The effect of nutrient addition on the growth of E. najas was evaluated in a dose response experiment using sand amended with phosphorus (P) and nitrogen (N), and in enrichment trials with N and P amendments to natural sediments. Plants, water and sediment came from lagoons of the Upper Parana ´ River Floodplain and from Itaipu Reservoir (Brazil). Relative growth rates (RGRs) of E. najas shoots, based on dry mass (DM), varied from 0.03 to 0.060 d 1 for both nutrients. Root:shoot biomass ratios were related to sediment exchangeable P (r = 0.419; P = 0.03) and N (r = 0.54; P = 0.006), however root RGR was not related to sediment nutrient concentrations. When natural sediments were amended with N and P, neither shoot nor root RGRs differed among treatments for substrata from either the reservoir or the floodplain lagoons (P > 0.05). Comparison of nutrient concentrations measured in natural sediments collected from several sites in both the Upper Parana ´ River Floodplain (range 49– 213 mgPg 1 DM; 36–373 mgNg 1 DM) and Itaipu Reservoir (range 43–402 mgPg 1 DM; 7.9–238 mgNg 1 DM) showed that sediment N and P from these systems usually exceeded minimum requirements necessary for E. najas growth, as measured in the dose response experiment. Together, these results indicate that E. najas, at least in early stages of development, responds to sediment nutrient amendments and relies upon bottom sediments to meet its N and P requirements and that for at least two Brazilian ecosystems, growth of this species is not limited by insufficient sediment N or P. Thus, reducing N and P in water is not enough to control E. najas growth in short time periods in these ecosystems. # 2006 Elsevier B.V. All rights reserved. Keywords: Egeria najas; Phosphorus; Nitrogen; Sediment; Brazil; Macrophyte 1. Introduction Availability of nutrients is considered one of the main factors affecting the abundance and composition of aquatic plant assemblages (Van et al., 1999). Of the many nutrients required for growth, nitrogen (N) and phosphorus (P) are the elements typically of shortest supply in aquatic ecosystems and therefore most likely to affect the growth of submerged species (Barko et al., 1991; Carr and Chambers, 1998). Aquatic macrophyte distribution has been found to be strongly determined by N and P, and different species may occur along nutritional gradients (Robach et al., 1995). Positive relation- ships between nutrient availability and aquatic macrophyte biomass have also been found both in laboratory experiments and in the field (Grane ´li and Solander, 1988; Carr and Chambers, 1998). However, controversy remains as to whether sediment or water is the major source of nutrients for rooted aquatic macrophytes. Submerged plants exhibit great pheno- typic plasticity and are adapted to absorb and translocate nutrients from sediment to leaves, especially in limiting conditions (Twilley et al., 1977; Barko et al., 1991; Sutton and Portier, 1995). Although several studies have emphasized the importance of root uptake (Carignan and Kallf, 1980; Barko and Smart, 1983, 1986; Smart and Barko, 1985; Chambers et al., 1989; Barko et al., 1991; Barko and James, 1998), others (such as Robach et al., 1995; Eugelink, 1998; Madsen and Cedergreen, 2002) argue the importance of shoot uptake. www.elsevier.com/locate/aquabot Aquatic Botany 86 (2007) 191–196 * Corresponding author. Tel.: +44 261 4617. E-mail addresses: smthomaz@nupelia.uem.br (S.M. Thomaz), sandrapierini@grupointegrado.br (S.A. Pierini). 0304-3770/$ – see front matter # 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.aquabot.2006.10.004