Spread of an Aquatic Invasive Plant, Iris pseudacorus, Following Replacement of a Hydraulic Structure

Freshwater ecosystems are heavily modified by hydraulic structures (embankments, dams, dikes, causeways, etc.). International efforts to restore freshwater ecosystems, along with increased stress on hydraulic structures due to climate change-induced flooding, extreme rain events, and sea-level rise, have resulted in increased incidences of breaching and removal of these structures. However, modification of hydraulic structures can facilitate the dispersal of aquatic invasive species. Such an unintended outcome became apparent following the replacement of an ageing earthen dam at Greenburn Lake (South Pender Island, British Columbia) that appears to have facilitated the spread of Iris pseudacorus (Yellow flag iris) into previously uninvaded wetland habitat. While best practices to prevent this plant’s downstream dispersal were implemented, the observed spread offers several practical recommendations to mitigate this risk in similar projects. We recommend that aquatic invasive species be explicitly considered prior to construction as part of an environmental impact assessment. Specifically, the spread of I. pseudacorus could have been mitigated through 1) enhanced efforts to clean machinery and equipment, 2) timely off-site disposal of organic material and sediment containing reproductive propagules, 3) cutting and smothering of I. pseudacorus within the work area at minimum three months before construction, 4) shifting timing of construction to fit within ecologically defined work windows that minimize the spread of I. pseudacorus seeds, and 5) use of physical barriers to capture suspended and buoyant propagules. The utilization of similar methods may reduce the spread of aquatic invasive plants during the maintenance or removal of other hydraulic structures.