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.

Submerged freshwater plant communities do not show species complementarity effect in wetland mesocosms

It is a generally accepted theory that ecological functions are enhanced with increased diversity in plant communities due to species complementarity effects. We tested this theory in a mesocosm study using freshwater submerged plant beds to determine if increasing species number caused overyielding and species complementarity. We applied a maximum of four species in the plant beds corresponding to the typical species number in natural freshwater plant beds. We found no clear effects of species number (1-4) on biomass production and thus no conclusive overyielding and complementarity effect. This may be explained by low species differentiation among the four species in plant traits relevant for resource acquisition in freshwater, or that other species interactions, e.g. allelopathy, were inhibiting overyielding. The existing knowledge on species complementarity in aquatic plant communities is sparse and inconclusive and calls for more research.