Artificial wave breakers promote the establishment of alien aquatic plants in a shallow lake.
We surveyed the occurrence of three emergent alien aquatic plants, Gymnocoronis spilanthoides, Alternanthera sessilis, and Alternanthera philoxeroides, in Lake Kasumigaura from 2013 to 2017. To determine the effects of shoreline modification on the expansion of alien aquatic plants, we compared the relative occurrence of alien aquatic plants in varying shoreline conditions modified by shoreline hardening and wave breakers. The distribution of alien aquatic species increased continually in Lake Kasumigaura over the study period, although the rate of expansion differed among years. Furthermore >64.6% of the vegetative fragments were found within 2 km of the nearest patch. The construction of artificial wave breakers, which were installed parallel to the shoreline, had a positive influence on alien aquatic plant patch establishment. Especially, parallel wave breakers increased the patch establishment of G. spilanthoides by about 2.27-fold compared to the natural environment. Perpendicular wave breakers increased alien patch establishment by about 1.63-fold. For lakeshore management, it is important to consider both alien plant suppression and levee protection. We further suggest prompt action to prevent the expansion of alien aquatic species around pumping stations, which can also result in the relocation and expansion of invasive species.