Growth of a native versus an invasive submerged aquatic macrophyte differs in relation to mud and organic matter concentrations in sediment

• Organic matter (OM) and mud content in sediment can determine macrophyte growth.
• OM had a stronger influence than mud on the growth of the two submerged plant species.
• Egeria najas (native) growth increased continuously over the OM gradient.
• Hydrilla verticillata (invasive) growth had a quadratic response to the OM gradient.
• Sediment OM partially explains the distribution of these species in the River Paraná.

Eutrophication has reduced the colonization of submerged plants in some freshwater ecosystems, and organic matter (OM) together with grain size in sediment may mediate such decreases. We tested the isolated and combined effects of sediment mud and OM concentrations on the early growth of two species of submerged macrophytes, Egeria najas (native) and Hydrilla verticillata (invasive). We hypothesized that the sediment OM concentration has more important effects than the mud concentration on plant growth and that E. najas is more successful than H. verticillata in highly organic sediment. We tested these hypotheses using mesocosms with several combinations of mud and OM concentrations in the sediment. We used plant length, the number of lateral branches, root dry mass and total plant dry mass as response variables. Both OM and mud were found to be important determinants of the growth of both species, but the former had a stronger influence than the latter. However, the responses of all plant attributes differed between the species. For example, the growth of E. najas increased linearly with increasing OM concentration, but H. verticillata responded to this variable with a quadratic tendency. The decrease in the growth of the invasive species at high OM concentrations may be associated with its lower tolerance to phytotoxic compounds released by the decomposition that occurs in more organic sediment. In conclusion, our experimental data support the importance of the sediment OM in the growth of the native E. najas and invasive H. verticillata. However, future studies should investigate the adaptation mechanisms that allow both plants to colonise such distinct sediment types.