No time to dye: dye-induced light differences mediate growth rates among invasive macrophytes

Invasive, submerged macrophytes negatively alter aquatic ecosystems and biodiversity through disruption of ecological structure and functioning. These plants are especially challenging and costly to control, with relatively few successful eradications. We examine the efficacy of dye treatments to control three invasive, submerged macrophyte species: Elodea canadensis Michx., Elodea nuttallii (Planchon) H. St. John and Lagarosiphon major (Ridley). Using an experimental mesocosm approach, growth rates of each species were monitored in relation to five light treatment groups: light, 1×, 2×, 3× dye dosage, and complete darkness (range: 2 70 to 0 μmol·m -2 ·s -1 ). Dye presence did not negate growth in any of the tested species, but the effects of treatments on invasive macrophyte growth rates differed across species. In dyed conditions, E. canadensis exhibited significantly greater increases in length compared to E. nuttallii and L. major , whilst E. nuttallii and L. major were lower and statistically similar. However, L. major significantly increased length relative to Elodea spp. in dark conditions. Similarly, for biomass changes, Elodea spp. gained significantly more biomass than L. major under light and dyed conditions, but not in the dark. Our findings suggest that the tested dye concentrations are not sufficient to halt the growth of these plants. However, under certain conditions, they could potentially help to reduce densities of invasive macrophytes by slowing growth rates and reducing biomass in select species. Differential responses to light could also help explain species replacement dynamics under varying environmental contexts. Overall, while further empirical research is required, management actions that reduce light could help control aquatic macrophytes in combination with other actions, but could also simultaneously mediate shifts in community assembly.