Comparative effects of water level variations on growth characteristics of Myriophyllum aquaticum

Myriophyllum aquaticum is a non-native aquatic plant that has invaded a range of habitats in the United States and Europe. Myriophyllum aquaticum reduces native species richness at local scales, water quality, habitat quality for fish and wildlife and impacts on human uses. Despite the submersed leaf form, M. aquaticum is not typically a weed problem as water level increases, although little data exist describing basic biological and ecological mechanisms affecting growth. The objective of this study was to quantify changes in M. aquaticum growth as water levels increase from 0 (representing shoreline habitat) to 137 cm under controlled mesocosm conditions. Total biomass at 0 cm was 96% greater than plants grown at a depth of 137 cm. Biomass of emergent shoots, stolons and roots were also greater when M. aquaticum was grown at the 0 cm water level. Submersed shoot biomass was on average 99% greater between 37 and 77 cm. Submersed shoots comprised a small fraction, 0.1–12% of total biomass, depending on water level. Total plant length was 25% greater when plants were grown at water levels from 0 to 77 cm, compared with plants grown at 97, 117 or 137 cm. Nuisance growth is dependent upon plants emerging from the water column; as water levels increase, this becomes more difficult for plants, as a result of the reduced photosynthetic ability of submersed leaves. These results can be used to identify optimal areas for M. aquaticum growth and possibly predict areas of new infestations.