Effects of water depth on the growth of an invasive species, Myriophyllum aquaticum

The aquatic plant Myriophyllum aquaticum , native to South America, has been introduced to China as an aquarium ornamental plant species over the past 20 years and has now established itself as an invasive species in multiple regions of southern China. In the present study, we conducted a controlled pot experiment with Myriophyllum aquaticum planted at seven different water depths (0, 25, 50, 75, 100, 125, and 150 cm) to investigate its growth patterns and adaptive mechanisms in various aquatic environments. As expected, underwater light decreased exponentially with increasing water depth. Spectral analysis indicated significant attenuation across all wavelength bands, with the blue light band being reduced to a greater extent than the red light band, consequently leading to a gradual elevation in the red-to-blue ratio (Red/Blue) with depth, which has a significant effect on the survival rate of M. aquaticum . With an increase in water depth, the survival rate of M. aquaticum showed a decreasing trend; the plants did not survive at a depth of 150 cm. The effect of water depth on the growth and reproduction of M. aquaticum is evident. The growth indices, namely plant height, the number of stem nodes, internodes, the number of branches, the number of tillers, root length, wet weight, and the RGR were all shown to decrease with increasing water depth. Growth conditions gradually diminished with the increase in depth: the Chl-a content of the M. aquaticum leaves gradually decreased, and when the water depth was ≥ 50 cm, the chlorophyll synthesizing ability of the leaves gradually decreased. Increased water depth – and the corresponding stress of low light – resulted in an increase in the malondialdehyde content of the leaves. The results of this experiment demonstrate that M. aquaticum is more likely to become established in shallow-water areas (depth up to 25 cm).