Sediment chemistry associated with native and non-native emergent macrophytes of a Hudson River marsh ecosystem

Abstract

In tidal freshwater marshes of the Hudson River, coverage byPhragmites australis andLythrum salicaria has increased greatly over the past twenty years, althoughTypha angustifolia is still the predominant vegetation. Prior to any attempts at marsh restoration via removal of exotic/invasive plant species, we wanted to describe the current relationship between these plants and sediment nutrient pools. Extant stands (n=3 of each) ofT. angustifolia, L. salicaria, andP. australis were sampled with porewater equilibrators in the spring and summer of 1995 and summer 1996 to measure porewater ammonium, nitrate, and phosphate. Porewater pools of phosphate were significantly lower (p<0.05) in stands ofL. salicaria in summer, with concentrations only half those measured in stands ofP. australis andT. angustifolia. Porewater ammonium did not differ among plant communities, and nitrate was undetectable in sediments associated with all three communities. Sequestration of nutrients in above-ground biomass differed significantly among plant species, indicating differential demand on sediment nutrient pools. There were significant decreases in porewater ammonium from spring to summer. Growing season estimates of nitrogen incorporation into above-ground plant tissue are more than adequate to explain the removal of ammonium from porewater for all plant communities. Similarly, plant uptake of porewater phosphate was several times greater than springtime standing stocks of dissolved inorganic P. Concentrations of porewater phosphate remained high in the summer, indicating rapid replenishment from other sediment phosphorus pools. Depletion of porewater ammonium in the summer and low N:P in plant tissues suggest N limitation of these marsh plants. Our data suggest that marsh management practices intended to shift the relative vegetation coverage towards native and non-invasive species should consider the subtle but ecologically significant effects on nutrient cycling.