Interspecific relationships between submerged and emergent aquatic plants along a nitrogen gradient in a mesocosm experiment

The responses of plant populations and communities to environmental factors play a key role in the ecological protection and restoration of lakes. However, little is known about how interspecific relationships respond to environmental gradients, especially for relationships between different growth forms such as submerged and emergent aquatic plants in shallow lakes, which can further affect species composition as community dynamics. We carried out a mesocosm experiment to investigate the responses of two focal aquatic plants (Ceratophyllum demersum and Phragmites australis) at two density levels and six levels of total nitrogen (TN) levels. Our results suggested that interspecific competition exist between C. demersum and P. australis, especially at high nitrogen levels. The competition relationship was mainly influenced by variations of relative growth rates of C. demersum and population density of P. australis along the nitrogen gradient. The interspecific neighbor effect on C. demersum shifted from facilitation to competition as TN increased, whereas on P. australis showed no significant facilitation effect along the TN gradient. Ceratophyllum demersum and P. australis adopted stress-tolerance and competition strategies for nitrogen, respectively, in co-culture. As a result, the dominant species in the plant community is likely to shift from the submerged macrophyte (C. demersum) to the emergent macrophyte (P. australis) as TN increases. Moderate TN (2.5 mg/L) can potentially support high biodiversity in macrophyte-dominated shallow lakes in our study. This finding has implications for different community composition at different nitrogen levels. Furthermore, it has important implications for protecting biodiversity and guiding vegetation restoration in shallow lakes.