Effects of salinity and sulfide on the distribution of Phragmites australis and Spartina alterniflora in a tidal saltmarsh

Abstract

In laboratory studies, the short-term response of Phragmites australis and Spartina alterniflora to root immersion in solutions of different salinity and sulfide concentration was measured as the rate of ammonium depletion (a proxy for root uptake) from an initial concentration of 20 μM. From 0 to 20 ppt in the absence of sulfide, ammonium uptake as a function of dry root weight decreased for both Phragmites (from 29.7 ± 3.5 s.e. to 8.2 ± 1.5 μmol N g −1 h −1) and Spartina (from 25.5 ± 3.5 to 9.0 ± 1.5 μmol N g −1 h −1). With an average sulfide concentration of 582 μM at 20 ppt, the rate of ammonium uptake for Spartina (10.9 ± 2.2 μmol N g −1 h −1) was not significantly different from the rate in the absence of sulfide. In contrast, the rate of ammonium uptake was significantly lower for Phragmites when the average sulfide concentration was increased from 0 to 375 μM (1.7 ± 1.6 μmol N g −1 h −1). In a tidal saltmarsh, the average porewater sulfide concentration in Phragmites australis stands (124 ± 206 μM) was almost an order of magnitude lower than that measured in stands of Spartina alterniflora (989 ± 166 μM). These field and laboratory results support the notion that increased sulfide in the rhizosphere reduces the ability of Phragmites to take up nutrients relative to species such as Spartina that are better-adapted to sulfidic soil conditions, thus restricting the distribution of Phragmites in tidal saltmarshes.