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, characterized by the dominance of different producer functional groups. In 2008, an enclosure experiment was performed in two experimental ponds (originally dominated by submerged plants) to test the role of nutrients in driving producer communities. I added equal amounts of floating plants to all of the enclosures and then manipulated both the nitrogen levels and nitrogen:phosphorus ratios in five treatments.
Results/Conclusions
Phytoplankton abundance was highest in the highest nitrogen:phosphorus ratio treatment, but analysis so far suggests that submerged and floating plants did not respond much to differences in nutrient ratios. Plant composition did respond to total nitrogen levels in the replicates of one of the ponds, with more floating and less submerged plants in some of the higher nitrogen treatments. However, the replicates in the second pond did not show this pattern (they were all codominated by both plant types), and my preliminary data analysis suggests the shapes of the responses do not conform to predictions based on alternative stable states theory. Combined with other studies, this work will improve our understanding of the existence, controls, and consequences of alternative community states in temporary wetlands and inform management of floating plant outbreaks.
