CRISEP: Tackling Invasive plants on waterways

Logo: Canal & River Trust

By Charles Hughes, Environmental Scientist & INNS Project Manager

Person in high-vis hosing down a trail path
Himalayan Balsam Trial - Dredging and Revegetation (Canal & River Trust)

Invasive non-native species (INNS) are one of the top global threats to biodiversity. In 2023, IPBES published an extensive assessment (1) on invasive non-native species and their control, with key findings including; “60% of global species extinctions have been caused, solely or alongside other drivers, by invasive alien species” and it is estimated that the “global annual costs of biological invasions in 2019 was over $423 billion”. This puts the impacts into a global perspective highlighting the significant threat that this issue poses to biodiversity, people and the economy.

Closer to home in the UK, recent assessments undertaken by CABI (2023) estimate that INNS cost the UK economy approximately £4 billion per annum (2). At the Canal & River Trust alone, invasive non-native plants can cost over £700k per annum to manage and control. So yes, the impacts of INNS is something that everyone, not just in the environmental/conservation sector, needs to be aware of. So… what are we at the Trust doing to help alleviate this issue. For the Trust, INNS can have far-reaching impacts from operational difficulties including navigation and water control disruption to environmental issues such as deterioration of protected sites (SSSIs and SACs) as well as direct threats to protected species such as Luronium natans.

Closeup of a Weevil sitting on a leaf surrounded by raindrops
Floating Pennywort Weevil - Bittell Reservoirs SSSI 2023 (Corin Pratt at CABI UK)

In 2021, the Trust entered into a strategic partnership with Severn Trent to develop and deliver a project (CRISEP) to tackle four priority invasive plants across our network. This was to be delivered on our waterways and funded by Severn Trent worth £600,000 spread out over a 4-year duration. The key species were Japanese Knotweed, Giant Hogweed, Floating Pennywort and Water Fern alongside secondary trials for Himalayan Balsam. To date, we have delivered 313km of invasive vegetation management across our waterways as well as scientific research.

Often the difficulty of managing INNS is being able to work collaboratively with landowners and stakeholders at a catchment-scale. We are both fortunate and ‘unfortunate’ that we own and manage such extensive waterways with respect to INNS. It is typically much easier for us to deliver works on our assets (because we own them) but at the same time comes with a huge liability, a greater risk of spread, and a significant price tag!

Heavy machinery dredging and pulling up Floating Pennywort
Floating Pennywort Removal on Cannock Extension Canal SSSI_SAC (Canal & River Trust)

In 2023 alone as part of CRISEP, we have mechanically removed >1,700 tonnes of Floating Pennywort. On the River Soar, just one waterway in the East Midlands, this made up around 74% of this value last season. However, we don’t want to just rely on singular management techniques or those which may be less sustainable or just because they’ve always been done that way. For example, in the last two years we have worked in partnership with CABI UK (the experts on biological control!) to release a weevil from South America Listronotus elongatus on our network. This is to investigate its viability in controlling this very invasive plant. This has in itself brought challenges, particularly in the case of climatic variations between the south and north of England. Temperature is a key factor in determining the viability and success of the weevil for example (e.g. prolonged colder periods will impact overwintering and breeding) but is still showing some real promising results for the future, something we need to consider with future UK climate change projections.

Talking of climate change, another subsequent aspect we have to consider is the potential impact of nutrient enrichment on aquatic invasive plants. With predicted warmer, wetter winters and hotter drier summers, this is likely to impact on discharges (both point and diffuse sources) during heavy rainfall events as well as risks associated with low oxygen conditions during high temperatures in the summer (as seen very evidently in 2022). These for example all have an impact on the nutrient availability to invasive plants and algae, some of which like Floating Pennywort can grow up to 23cm a day in optimal conditions.

Three men posing for a photo as one holds up an award
CIEEM Awards NGO Impact Award Winner 2022 (Chartered Institute of Ecology & Environmental Management (CIEEM))

During the CRISEP project we have worked with consultants from the UK and EU to develop nutrient budgets for canals and lakes to understand how nutrient loading could be increasing growth of a number of aquatic invasive plants on our network. This work was to determine the external load, through stormwater discharges and agricultural runoff, and the internal load of phosphorus release from sediments. In one case study, we determined that one of our small lakes (0.8ha) contained up to 1,400kg of releasable P within sediment! Furthermore, the extensive growth of aquatic invasive plants on the surface were exacerbating anoxic conditions (very low oxygen levels) indicating suitable conditions for release of sediment-bound phosphorus. This low oxygen environment changes the chemical structure of metal compounds such as those of iron, which in their oxidised form bind with nutrient ions such as phosphate but, when in their low-oxygen (reduced) forms, will release phosphate into solution potentially becoming available for these undesirable species.

From mechanical removal, biological control to developing our own techniques for other species such as dredging and revegetation to suppress Himalayan Balsam, it is important as environmental professionals that we work in a multi-disciplinary fashion. We must bring in broad expertise across the sciences to help solve some of these big environmental issues. Hopefully, some day we wont just be reporting on the sad state of affairs as seen in the above assessments, but more on the incredible solutions that we have developed in the face of adversity!


Canal & River Trust Webpage: Canal & River Trust 

Project Webpage: Canal & River Invasive Species Eradication Project 2021-2025 | Canal & River Trust 


(1) Thematic Assessment Report on Invasive Alien Species and their Control | IPBES secretariat

(2) Invasive non-native species cost UK economy an estimated £4bn a year, new CABI-led study reveals -

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Posted On: 02/05/2024

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