The Water Fern is native to northern, southern, and central America. It was introduced to botanical gardens but was dispersed into the wild – likely by machinery and on the clothes of garden workers. It is a hardy plant that can overwinter as plant fragments and inhabit areas with low nutrient conditions. It can grow to cover the entirety of a water surface, blocking light and influencing oxygen levels in the water which can be detrimental to other species. It can also block movement of animals, including the movement of amphibians to the surface. There are 2 recorded sightings of water ferns in the River Thame – at Gizzel Pond and Newell’s Pond in 1988.