The floating menace : Evaluation of methods to eradicate Nymphoides peltata in Lake Mälaren

Invasive alien species are the second biggest threat to global biodiversity. Biodiversity loss results in loss of ecosystem functions and resilience. Freshwater ecosystems are more sensitive to invasions; therefore, their biodiversity is decreasing at a much faster rate in comparison to terrestrial ones. This thesis focuses on one of three invasive alien aquatic plants present in Sweden: Nymphoides peltata, a sediment-rooted, floating-leaved plant originating from middle Europe and Asia. Since the early 1980’s N. peltata has been present in Lake Galten, the most western part of Mälaren. The current eradication work is carried out by light depletion through floating frames that shade the N. peltata colony. The aim of this work was to evaluate the efficiency of this method by assessing eventual recolonization of N. peltata in eradicated sites and suggesting possible improvements. Information and data were collected by reviewing existing literature on invasive species eradication as well as conducting interviews with experts followed by field observations in Munkhammar bay of Lake Galten. The choice of floating frames as the main eradication method has been proven valid since it counters N. peltata clonal growth and fragmentation capacity. Despite a reduction by 1.7% of total N. peltata coverage from 2019 to 2020, correlated with the increase in frame coverage, field observations showed that recolonization is happening. Suggested improvements are the use of frames as spreading barrier, to reduce clonal growth and fragmentation of large colonies, together with introducing a free-floating fiber sheet to properly seal the gap between shoreline and frames. A proactive covering of vulnerable shores with fiber sheet is also recommended. However, the lack of a coordinated national action plan and laws’ inadequacy weakens the possibility of a successful eradication. Tempestive adjustments of current legislations are needed since climate change will allow N. peltata to spread further north and produce viable seeds.