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

Invasive, submerged macrophytes negatively alter aquatic ecosystems and biodiversitythrough disruption of ecological structure and functioning. These plants are especiallychallenging and costly to control, with relatively few successful eradications. We examinethe efficacy of dye treatments to control three invasive, submerged macrophytespecies: Elodea canadensis Michx., Elodea nuttallii (Planchon) H. St. John andLagarosiphon major (Ridley). Using an experimental mesocosm approach, growthrates of each species were monitored in relation to five light treatment groups: light,1×, 2×, 3× dye dosage, and complete darkness (range: 270 to 0 μmol·m-2·s-1). Dyepresence did not negate growth in any of the tested species, but the effects oftreatments on invasive macrophyte growth rates differed across species. In dyedconditions, E. canadensis exhibited significantly greater increases in lengthcompared to E. nuttallii and L. major, whilst E. nuttallii and L. major were lowerand statistically similar. However, L. major significantly increased length relativeto 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 notsufficient to halt the growth of these plants. However, under certain conditions,they could potentially help to reduce densities of invasive macrophytes by slowinggrowth rates and reducing biomass in select species. Differential responses to lightcould also help explain species replacement dynamics under varying environmentalcontexts. Overall, while further empirical research is required, management actionsthat reduce light could help control aquatic macrophytes in combination with otheractions, but could also simultaneously mediate shifts in community assembly.