Nutrient Absorption by the Submerged Macrophyte Egeria densa Planch.: Effect of Ammonium and Phosphorus Availability in the Water Column on Growth and Nutrient Uptake

The relative importance of phosphorus and nitrogen absorption fkom water and sediments by Egeria densa Planch. (Hydrocharitaceae), a subinerged macropliyte very common in Pampean streams in Argentina, was experiiiientally assesced. Two experiments were performed to determine the importance of water as opposed to sediment cource of phosphorus, and to investigate the effect oftype of nitrogenouc ion dissolved in water on nitrogen uptake by this species. In the first experiment, the concentration ofsoluble reactive phosphorus i i i water had an effect on E. densa biomacc and on tissular phocphoruc concentrations. Absorption rates of phocphorus from water were higher than from the sediment. In a separate experiment, we found that E. densa absorbed more nitrogen from water when it was precent as ainmonium than when it was present as nitrate. Thic differential absorption increased total nitrogen concentration in the macrophytic tissues, but biomacc yield was not affected. Nitrogen uptake from the cediment was negligible. In our experiments, absorption rates from water and sediments were higher for phosphorus than for nitrogen. This suggests that mechanicms of uptake and retention are more efficient for phosphorus than for nitrogen in E. densa.