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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 Paraná 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 0.060d−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 Paraná River Floodplain (range 49–213μgPg−1 DM; 36–373μgNg−1 DM) and Itaipu Reservoir (range 43–402μgPg−1 DM; 7.9–238μgNg−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. [Copyright &y& Elsevier]

Abstract: Incidence data of two native submerged aquatic macrophytes (Egeria najas Planch. and Egeria densa Planch.) were obtained in eight arms of a large (1350km2) subtropical reservoir (Itaipu Binacional Reservoir, Brazil-Paraguay). Environmental variables were measured simultaneously. Two large-scale surveys in the same localities identified by a global positioning system were carried out in April 1999 (n =235) and January 2001 (n =230). Logistic regressions were used to test the effect of environmental variables on the likelihood of E. najas and E. densa presence or absence. The two species were found under different environmental conditions: conductivity, light attenuation coefficient (k) and fetch were, in this order, the most important environmental variables in predicting the probability of occurrence of E. najas, whereas light attenuation coefficient was the main predictor of the probability of occurrence of E. densa. Thus, both species were negatively affected by the light attenuation coefficient. However, this effect was stronger in E. densa. The small area occupied by these species may be accounted for by the permanent high turbidity of Itaipu Reservoir. Additionally, the dominance of E. najas over of E. densa can be explained by the probably higher light requirements of E. densa. In other reservoirs worldwide, with higher water transparency, the opposite is frequently true. Between 1999 and 2001, an episodic water-level drawdown (5m) caused the disappearance of submerged vegetation. After water-level normalization, previous vegetation presence (in 1999) was an important predictor of the probability of occurrence of E. najas in 2001. [Copyright &y& Elsevier]

The ability of two submerged freshwater macrophyte species, Egeria najas Planchon and Egeria densa Planchon to use HCO3− as a source of inorganic carbon for photosynthesis was assessed by a pH drift experiment. The pH values and different HCO3− concentrations were set up at the beginning of the experiments, in low and high alkalinities (ca. 100 and 800 μmol). Results of pH drift experiment and [CT]/Alk quotient indicated that both species used HCO3− and were adapted to low inorganic carbon availability. Nevertheless, different efficiencies of HCO3− use were evident based on the photosynthesis rates and pH values attained at the end of the experiments. Higher photosynthesis rates and pH values were observed in the presence of E. najas both at low and high alkalinities. Both species used more efficiently HCO3− in high alkalinity. The results suggest that the efficiency in using HCO3− depends on the concentration of this ion in water uptake ability of these species. Both E. najas and E. densa preferred CO2 in both alkalinities. By adjusting the data with a Michaelis–Menten equation, it was shown that E. najas is slightly more efficient in using CO2 than E. densa, when the concentrations of this dissolved gas in water are low. Thus, the low CO2 concentrations characteristic of several Brazilian freshwater ecosystems may be advantageous to the colonization of E. najas, compared to E. densa. [Copyright &y& Elsevier]