Do invasive exotic and native freshwater plant species respond similarly to low additional nitrate doses?

Nutrient status of freshwater ecosystems has a significant influence on biological invasions, species richness and community structure. The role of phosphorus in driving these effects has been widely reported while its co-limitation with nitrogen and other elements has received more recent attention. In a greenhouse experiment, we investigated the growth responses of two invasive exotic (Egeria densa and Salvinia molesta) and one native (Vallisneria spiralis) freshwater plant species to additional low concentrations of nitrate nitrogen (N-NO3). The species were grown at five nitrate concentrations (0.02, 0.05, 0.1, 0.5, and 0.9 mg N-NO3 L−1). We found that the growth of E. densa and V. spiralis increased with increasing nitrate concentration. Surprisingly, S. molesta had the fastest growth rate at the midrange nitrate concentration of 0.1 mg N-NO3 L−1 and its leaf production was not affected by nitrate treatment. Irrespective of nitrate concentration, the invasive exotic species, particularly S. molesta, showed much greater growth responses than the native V. spiralis. We conclude that freshwater plant growth responses to low nitrate concentrations will be species specific but the faster growth rates of S. molesta provide an example of how differences between co-occurring invasive exotic species and native species could have profound effects on the structure and function of freshwater ecosystems under changed environmental conditions.