Discontinuous variation in community structure over space and time is present in many natural systems, including temporary and semipermanent ponds. My observational work shows that several alternative community states exist in the aquatic plant communities of Michigan wetlands, including domination by floating and submerged plants. I conducted an enclosure experiment in two experimental ponds (originally dominated by submerged plants) to test the role of nutrients in driving competitive dominance of floating plants (duckweeds Lemnaceae). I focus this presentation on the responses of floating plants to nutrient additions manipulating nitrogen and nitrogen:phosphorus ratios.
Results/Conclusions
I found duckweeds to respond to nutrients in a threshold-shaped response. The lower two nutrient treatments exhibited lower floating plant cover than the higher three treatments. In addition, while these experiments were performed in two identically built experimental ponds, there were strong differences in floating and submerged plant cover by pond. While these differences were particularly apparent at the end of the experiment (with one pond having all enclosures codominated by floating and submerged plants), it is informative to examine the initial trajectories of floating plants in the two ponds as well. At intermediate and high nitrogen levels, floating plants exhibited fast increases in one pond but more gradual increases in the other pond. In one intermediate nitrogen level (also proportionally low in phosphorus added), these differences in trajectories by pond continued throughout the experiment. This sensitivity to initial conditions at intermediate levels of a controlling variable may indicate evidence for alternative stable states. Combined with other studies, this work will improve our understanding of the existence, controls, and consequences of alternative community states in wetlands.
