Growth and morphology of four freshwater macrophytes under the impact of the raised salinity level of the Lower Rhine

Abstract

The growth and morphology of Potamogeton lucens L., Potamogeton perfoliatus L., Potamogeton nodosus Poir. and Ranunculus circinatus Sibth. were studied in relation to salinity. Plants were grown for 8 weeks in microcosms under ambient climatic conditions at (sodium) chloride concentrations which presently occur in the main channel of the Lower Rhine and its connected lakes (7.0 mmol l−1 Cl−), which have a poorly developed vegetation, and at concentrations which presently occur in infrequently flooded lakes (1.4 mmol l−1 Cl−1) within the floodplain of the Lower Rhine, which are rich in aquatic macrophytes.The sodium chloride level of the main channel negatively affected biomass production and growth rates for all three Potamogeton species, but not for R. circinatus. The reductions in biomass and growth rates of the Potamogeton species concurred with a decline in their leaf ratios, suggesting that the growth reduction might be related to a reduced leaf cell expansion. Root:shoot ratios increased in all selected species, whereas flowering decreased in response to higher salinity. The relevance of salinity as a stress factor is discussed in relation to the occurrence of these macrophytes in the Lower Rhine and its floodplain lakes.