Sediment physico-chemistry associated with natural marsh development on a storm-deposited sand flat

Abstract

Intertidal sediment physico-chemistry was compared at 2 naturally developing barrier island marshes in Virginia, USA. One site was located on the intertidal portion of a sand flat, deposited during a coastal storm in October 1991. The other site was an undisturbed, fringing marsh aged at 14 to 16 yr. Three samphng stations were established at equivalent elevations at both sites. At the marsh site, these sampling stations represented tall and short forms of Spartina alterniflora (low and intermediate elevation stations) and a h ~g h marsh zone of mostly Spa]-tlna patens (high elevation station). At both sites, sediment across the intertidal zone was characterized by <1.5'% organic matter and > 9 0 % sand. Relative to the marsh site, the sand flat site had significantly higher pore water salinity and redox potential at the upper elevation stations, higher pH at all stations, and lower hydrogen sulfide a t the 2 lower elevation stations. Organically bound nutrients generally were higher in the root zone of the marsh site, but inorganic nutrients were a n order of magnitude higher at the low elevation station of the sand flat compared to all stations at the marsh site. The higher concentration of nutrients a t the sand flat site may arise from a nutrient-rich mud flat buried during the storm event. This high concentration of nutrients could serve a s a n external nutrient source to the root zone of the developing marsh and may also lead to a net export of inorganic nutrients to the flooding tidal water. A predictable zonation of intertidal vegetation on the sand flat should exist given the present patterns in sediment physico-chemistry. Based on nutrient standing stocks, the most favorable growing conditions for S. alterniflora exist at the low elevation station of the sand flat, where the tall form of this species is expected to dominate. The short form of S. alternjflol-a is expected to colonize the intermediate elevation station. A mixed community of species (S patens, Distichlis spicata) probably will develop at the high elevation station, where the sediment 1s most oxidized. Vegetation zonation at the marsh site was not readily explained by patterns in sediment physico-chemistry measured in this study. Current research by the authors indicates that nutrient dynamics, controlled by subsurface hydrology, dlctates nutrient supply at the marsh site and leads to a higher exposure to nutrients in the root zone of the tall form of S. alterniflora. The present study represents one of few involving measurements of s e d ~m e n t physico-chemistry in naturally developing marshes during the initial stages of development.