Abstract
Pistia stratiotes is one of the worst weeds in the world, and it adversely affects the environment and biodiversity. A large gap remains in the quantitative knowledge about the growth, biomass and life cycle of this invasive macrophyte under natural conditions in Egypt. Therefore, this study aims to determine the monthly variation in P. stratiotes population dynamics in the South Nile Delta (Egypt) to identify the potential weak stages in the species' life cycle and thereby suggest the best time to begin control of this invasive species. Sampling was performed monthly from May 2013 to February 2014 at three sites along the Al-Sero Drain; three randomly distributed quadrats (each of 0.5x0.5m) were used at each site. Generally, the Al-Sero Drain is characterized by having alkaline, brackish and moderately eutrophic waters. The shoot systems started to grow in May, reached their maximum biomass of 320.7g DM m(-2) in September and then decreased and reached their lowest value in February. The root biomass increased from 2.3g DM m(-2) in May to 26.6g DM m(-2) in August and decreased to a minimum of 4.9g DM m(-2) in February. The relative growth rate of the plant was highest during the summer, whereas the lowest relative growth rate occurred in the winter. In conclusion, to suppress or eradicate this macrophyte from a drain, the harvest should be performed in May, when the biomass is low, and the offshoot production is just beginning (before the formation of large mature individuals).