The roles of nitrogen and phosphorus in regulating the dominance of floating and submerged aquatic plants in a field mesocosm experiment

• I studied how nitrogen, phosphorus, and their ratio drove floating plant dominance.
• A survey of 38 Michigan ponds showed nonlinear responses to all three variables.
• In a field mesocosm experiment, floating plant cover responded only to nitrogen.
• Submerged plant communities explained floating taxa composition at the end.

Free-floating plant outbreaks have attracted the interest of scientists and managers alike due to their strong effects on ecosystem structure and function in shallow lakes and ponds. I evaluated the roles of nitrogen (N), phosphorus (P), and N:P ratios in driving the dominance of duckweeds (Lemnoideae and Wolffioideae) in survey data from 38 ponds in Michigan, USA. I also conducted a mesocosm experiment manipulating the variables in enclosures in two experimental ponds with floating and submerged plants. Floating plants exhibited nonlinear responses to total N, total P, and N:P in the survey data. Experimentally, N drove the initial growth of floating plants. Later in the experiment, floating taxa composition often shifted from Lemna minor L. to filamentous green algae and Wolffia sp., and the shift was associated with increased pH and submerged plant composition. These results support N controlling floating plants, but complex interactions between nutrients, pH, and primary producer community composition need further study.