A comparison of nutrient uptake efficiency and growth rate between different macrophyte growth forms

Aquatic macrophytes grow abundantly in many lowland streams and play a key role in ecosystem functioning, such as nutrient retention. In this study, we performed a microcosm experiment to quantify and compare the contribution of two freshwater macrophyte growth forms to nutrient cycling. We measured and compared inorganic nitrogen (NH4-N and NO3-N) and phosphorus (PO4-P) uptake kinetic parameters (V-max and C-min) in 12 submerged and seven amphibious plant species. We tested whether relative growth rate (RGR) was related to high V-max and low C-min, and quantified changes in nutrient uptake kinetic in a subset of six out of 19 plants species during the growth season. Uptake rates of NH4-N were higher in submerged compared to amphibious plants, whereas uptake rates of NO3-N were significantly higher in amphibious species; PO4-P uptake kinetics were not significantly different between the two growth forms. There were also significant seasonal differences in V max NH4-N rate among both submerged and amphibious species and in V-max NO3-N among amphibious species. Highest uptake rates were observed in summer for both submerged and amphibious species. Overall, we found that nutrient uptake kinetics differed between the two growth forms within and between seasons. Consequently, the presence of both growth forms should extend the period of nutrient uptake across the year and enhance nutrient uptake within seasons. We conclude that higher functional diversity enhances annual nutrient uptake in streams and that stream restoration efforts should consider increasing the niche space available for both submerged and amphibious species.