Does Water Brownification Affect Duckweeds in Freshwaters? Results from a Laboratory Experiment

Abstract

The influence of water brownification (an increase in water color) on floating-leaved macrophytes appears to be an important issue in predicting the consequences of climate change in freshwater ecosystems. To fill the gap in the knowledge in this area, we performed a laboratory experiment aiming to test the impact of humic acid (HA) on Lemna minor sampled from an oxbow lake. Plants were exposed over seven days to four different additions of pure HA (resulting in DOC concentrations between 5.0 ± 0.1 mg dm−3 and 36.5 ± 0.4 mg dm−3). Changes in duckweed total fresh biomass, leaf area and the root length were measured, as well as the concentrations of total phosphorus, phosphates and nitrates in the water. The study did not find any significant decrease in either the biomass or leaf area of L. minor under conditions of enhanced HA content. However, we found that a moderate increase in HA in water promoted the growth of duckweed roots, thus, increasing the biomass but not the leaf area. In contrast, a large increase in HA in the water suppressed the growth of roots in L. minor, which was also in coincidence with less nutrient depletion in the water.