Nitrogen has been determined to be the limiting nutrient in most salt marsh systems. Nitrogen becomes available to a salt marsh from several sources. These sources include inputs from tidal flood waters (both organic and inorganic forms), fixation by blue green algae, freshwater discharge into the high marsh, and direct precipitation (both wet and dry) onto the marsh surface. The atmospheric source is one of the least understood components of the salt marsh nitrogen cycle.
Wet deposition of atmospheric nitrate (NO3-) and ammonium (NH4+) has been measured on a weekly basis at the VCR-LTER site from March 1990 to the present. The samples are collected from northern Hog Island. In order to establish a link between atmospheric inputs and groundwater concentrations of inorganic nitrogen in a back-barrier high marsh system, three transects were established on three separate `Parramore Pimples'. Each transect extended from the pimple surface (upland/fresh groundwater), through the brackish transition zone and out into the high marsh region (saline groundwater). Porewater samples were collected on a monthly basis from sippers which were installed along each transect.
The `Parramore Pimples' are isolated, elevated land forms surrounded by salt marsh. These features tend to be round in shape, and range in size from approximately 10-250 meters in diameter. The pimple surfaces are elevated above the surrounding marsh by 1-3 meters. Several hundred of these land forms are scattered throughout the back barrier marshes of Parramore Island. The features which are at least 30 meters in diameter are able to support an assemblage of terrestrial vegetation because a shallow fresh water lens has developed beneath the surface. This lens constitutes a body of fresh water, recharged by precipitation, that rests on top of the denser, saline groundwater below. The presence of the fresh water coupled with the raised relief of the features allow each pimple to be characterized as a small upland ecosystem. The vegetation zonation around the features is typical of upland systems observed on the barrier islands of Virginia. The pimples therefore present an ideal study site to investigate the upland/high marsh boundary conditions because they are independent hydrologic systems void of any direct anthropogenic nutrient inputs.
The concentrations of NO3- and NH4+ in the wet deposition varied considerably from individual events, but generally exceeded the inorganic concentrations of porewater nitrogen measured along each of the three transects. During the summer months the average concentrations of NO3- and NH4+ were respectively 35.0 micromoles/l and 23 micromoles/l in the wet deposition. Through the summer of 1994, porewater NO3- concentrations were generally less than 3 micromoles/l, while NH4+ concentrations were usually less than 15 micromoles/liter. Following a large precipitation event (54.6mm) on 5-August, 1994, significant increases in the NO3- concentrations were measured in the transition (brackish, shrub zone) zone of each pimple (figure 1). The brackish shrub zone is the transition region between high marsh and upland. Beneath the shrubs in this region a spongy organic layer has developed from the partial decomposition of large quantities of wrack which has been deposited in this zone. Precipitation water may be held in this spongy layer for several days following an event.
The results from this study indicate that porewater inorganic nitrogen concentrations along the high marsh/upland transition in this region are relatively low. Higher concentrations in the wet deposition than in the porewater indicates rapid assimilation of atmospheric nitrogen into the system. The increases in porewater NO3- concentrations following extreme precipitation events are quickly depleted, as the concentrations return to background levels within 2-3 weeks. The NO3- is either utilized immediately by the vegetation, or reduced to NH4+. However, corresponding increases in NH4+ concentrations were not observed. Therefore the NO3- was most likely consumed by plants.