Effects of climate change on late-season growth and survival of native and non-native species of watermilfoil (Myriophyllum spp.): Implications for invasive potential and ecosystem change

The combined effects of global climate change and invasive species can lead to shifts in ecosystem structure and function. We assessed how a rise in water temperature influenced the late-season growth, survival, and invasive potential of an invasive aquatic plant, Eurasian watermilfoil, (Myriophyllum spicatum Linnaeus) in comparison to a co-occurring native species northern watermilfoil (Myriophyllum sibericum Komarov). We examined the interrelationship between water temperature, watermilfoil species, and the aquatic ecosystem including periphyton growth and zooplankton abundance. The study was conducted using replicated mesocosms (3785 l), with water heaters used to provide a range of temperatures. Late-season shoot survival of M. sibiricum was lower than that of M. spicatum across all temperature treatments with a mean survival rate of 24% and 96% respectively. M. sibiricum also showed significantly slower rates of plant growth (mean growth of 3.3 cm over the duration of the field season compared to 7.6 cm for M. spicatum) and reduced vigor compared to M. spicatum, with an average of less than half the number of growing meristems. Zooplankton densities averaged over 20 times higher in mesocosms with M. sibiricum compared to those with M. spicatum. Periphyton biomass was best explained by water temperature with an increase in growth in warmer water. Our study suggests that warming water as a result of climate change will extend the growing season for M. spicatum, but not for M. sibiricum. This could provide the former species with a competitive advantage over the latter.

► The role of water temperature on survival and growth of watermilfoil was assessed.
► We also assessed water temperature, milfoil, and effects on the aquatic ecosystem.
► The non-native species of watermilfoil continued to grow and survive with warmer water.
► Warmer water did not halt late-season senescence for the native milfoil species.
► Zooplankton densities were over 20 times higher with the native species.