Effects on local oxygen conditions by the invasive macrophyte Myriophyllum spicatum

Myriophyllum spicatum, Eurasian watermilfoil, is a submerged aquatic plant invasive to North America. Several characteristics found in M. spicatum provide reasoning behind its invasion success such as its ability to spread and grow rapidly as well as displace other surrounding native species. However, Eurasian watermilfoil’s effects on ecosystem functioning (such as dissolved oxygen) and how such functioning differ from effects of native vegetation have seldom been studied. Using data collected in field, we used statistical models including Gaussian multivariate linear effect models and structural equation modelling (SEM), to investigate the effect of vegetation type and cover on dissolved oxygen (DO) and temperature gradients. Here, we show that invasive Eurasian watermilfoil colonies, relative to native submerged vegetation, can have a direct effect on DO gradients. These changes in DO conditions were driven by both an increase in surface oxygen concentrations and a decrease in bottom layer oxygen concentration in dense M. spicatum vegetation. Furthermore, we find that the differences in DO gradients could be predicted from M. spicatum’s direct impact on oxygen concentration and not indirectly via its effects on water temperature. Our results demonstrate that dense colonies of M. spicatum can directly affect DO concentrations and may do so more than native macrophytes which could explain its rapid spread and potential impacts on ecosystem functioning. •Linear models test variation in DO and temperatures among vegetation.•SEM is used to assess direct and indirect effect sizes of M. spicatum on DO.•M. spicatum exhibited sharper declines in DO with depth than native vegetation.•M. spicatum increase surface-level DO, and decreased bottom-layer DO.•Vegetation type (native and M. spicatum) did not impact water temperatures.