Sensitivity of macrophytes to the herbicide diquat dibromide grown singly and in mesocosms

Native macrophyte communities are essential components of healthy aquatic ecosystems. They play an important role in maintaining aquatic biodiversity, ecosystem functionality and water and sediment quality. On the contrary, invasive macrophyte species are a significant threat to the ecosystem’s health, since they can oust native vegetation and reach nuisance levels that reduce oxygen and accelerate eutrophication. Aquatic herbicides, such as diquat dibromide, are widely used as chemical control options to manage invasive macrophytes. However, application of diquat dibromide endangers native macrophyte populations, as the chemical is non-selective which means that non-target plant species will also be affected by its toxic mode of action. The high effectiveness of diquat dibromide in controlling aquatic weeds can dramatically change the ecosystem through removal of plant growth, resulting in a high potential for indirect effects on aquatic biota such as invertebrates, amphibians and fish.

Despite those potential threats to aquatic environments, data on the effects of chronic, ecologically relevant concentrations of diquat dibromide on non-target macrophytes are limited. Past research with macrophytes has mainly concentrated on determining effective eradication techniques for nuisance growths, and only in a few cases have macrophytes been used as test species in ecotoxicity tests for pesticide registration. Current risk assessment procedures for herbicides in the United States, Canada and the European Union mostly rely on toxicity tests with alga species, even though the necessity of testing of macrophytes is acknowledged by authorities (Maltby et al., 2010; US EPA, 2001) and an official testing guideline for the rooted macrophyte Myriophyllum spicatum (OECD, 2014) is available for use. The importance of including native macrophyte species as test species for risk assessment cannot be overlooked, as they are growing directly in the target area where diquat dibromide is applied for aquatic weed control.


Addressing this lack of knowledge, my master thesis focuses on the sensitivity of native and invasive macrophytes to diquat dibromide exposure. My aim is to provide phytotoxicity data for herbicide risk assessment and the upcoming re-evaluation of diquat dibromide’s registration in Canada. I am using test systems of different complexity, i. e. single species tests and a mesocosm experiment, to represent two essential tiers of pesticide risk assessment. By performing single species dose-response tests, I am aiming at investigating the effects of diquat dibromide on native and invasive macrophytes grown in small systems without interactions between biota. In addition, I included the same macrophyte species in a mesocosm experiment that was designed to assess potential effects and the risks diquat dibromide pose to aquatic biota at a larger scale by simulating natural environmental conditions.



Maltby, L.; Arnold, D.; Arts, G.; Davies, J.; Heimbach, F.; Pickl, C.; Poulsen, V. (2010): Aquatic macrophyte risk assessment for pesticides. SETAC Europe Workshop AMRAP. Wageningen, Netherlands. Boca Raton, London, New York: CRC Press.
Organisation for Economic Co-operation and Development (OECD) (2014): Test No. 239: Water-sediment Myriophyllum spicatum toxicity test. OECD Guidelines for the testing of chemicals: OECD Publishing.
United States Environmental Protection Agency (US EPA) (2001): Proposal to update non-target plant toxicity testing under NAFTA. Scientific Advisory Panel Briefing. Joint Presentation by Health Canada Pest Management Regulatory Agency and US EPA Office of Prevention, Pesticides, and Toxic Substances. With assistance of M. Davy, R. Petrie, J. Smrchek, T. Kuchnicki, D. Francois.