Water hyacinth ( Eichhornia crassipes ) invasion and establishment in a temperate river system

Water hyacinth ( Eichhornia crassipes ) is an invasive aquatic macrophyte that is prolific throughout many regions worldwide, but it has recently been detected in several areas of the Midwest United States, outside of its historical range. We assessed the occurrence of water hyacinth in the upper Illinois River to understand the current distribution and potential establishment pathways within a large river system in a temperate climate. We estimated plant biomass and occurrence of seeds in substrate and the water column at locations where water hyacinth occurred and other areas throughout the study area. Biomass of seeds varied temporally and spatially, but no potential water hyacinth seeds germinated during germination trials and germination rates for unscarified water hyacinth seeds from an established population were extremely low. Interestingly, a high abundance and diversity of seeds were detected below water hyacinth mats indicating that water hyacinth root systems may also act as a barrier to hydrochoric seeds in large river systems. Generally, the occurrence of water hyacinth mats and propagules during our study did not pose any apparent issues for commercial navigation due to low overall occurrence and biomass, late occurrence in the growing season, and nongermination of seeds. However, presence of potential water hyacinth in the seed bank and rapid proliferation late in the growing season could pose issues for recreational navigation in low‐flow areas and suggests increased risk if environmental conditions become more suitable for establishment. Occurrence of water hyacinth in the upper Illinois River likely relies on biomass reintroduction annually by humans, and behavioural modification or legal means may be necessary to reduce future occurrences.