Effects of clonal fragmentation and nutrient availability on the competitive ability of the floating plant Salvinia natans

Floating plants are widespread in aquatic ecosystems. Most floating plants are clonal and their clones can be easily fragmented as a result of natural events or human activities. Such clonal fragmentation may decrease the growth and thus the competitive ability of floating plants. We tested these hypotheses with the widespread floating, clonal plant Salvinia natans. We simulated disturbance-mediated fragmentation of S. natans clones at two levels (low or high) by cutting clones initially consisting of six connected ramets into two fragments with three connected ramets each or six fragments with one ramet each), with no fragmentation as the control. These fragmented or intact clones were grown under two nutrient levels (low or high) in the presence or absence of the floating plant S. polyrhiza. High nutrient availability significantly increased the biomass and ramet number of S. natans, and the presence of S. polyrhiza reduced them. Furthermore, the negative effect of competition of S. polyrhiza on S. natans was greater under high than under low nutrient availability. Clonal fragmentation of S. natans significantly decreased its growth, but did not affect its competitive response or effect. Thus, nutrient availability can affect competition between S. natans and S. polyrhiza, but clonal fragmentation cannot. Our results suggest that clonal fragmentation and decreasing nutrient availability can decrease the spread of some clonal floating plants in aquatic ecosystems and that increasing disturbance to fragment clones and reducing nutrient load in water could be efficient at controlling the spread of some floating plants.