Growth and photosynthetic performance of Phragmites australis in estuarine waters: a field and experimental evaluation

Abstract

Surveys of South African estuaries showed that in estuaries that had a gradient of decreasing salinity up the length of the estuary salt marshes occurred near the saline middle-lower reaches and reeds and sedges in the brackish upper reaches. However, in certain estuaries common reed Phragmites australis occurred in the lower reaches of estuaries. The plants often extend into the intertidal zone where they are inundated with saline water (35 ppt). This study tested whether P. australis occurred at these sites because its roots were located in fresh or brackish water. In P. australis stands in the Goukou and Keurbooms estuaries interstitial water salinity was lower than surface water salinity. There was an increase in interstitial salinity towards the water’s edge and this was associated with a decrease in the height of P. australis. Laboratory studies tested whether P. australis could survive tidal inundation with saline water (35 ppt) if its roots and rhizomes were located in freshwater (0 ppt). Plants which were supplied with freshwater to the roots but were tidally inundated with saline water had greater stem elongation and less dead leaves than plants that were supplied with 20 ppt salinity to the roots. P. australis will probably only survive intertidal flushing with saltwater if its roots and rhizomes are located in brackish water (<20 ppt). This accounts for the presence of P. australis in saline estuaries at the sites of freshwater seepage.