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.