Sediment variables known to affect the growth of Spartina alterniflora were measured during the growing seasons of 1993 and 1994 in a developing marsh (~15 years of development) as part of a larger study of natural marsh development on the Virginia barrier islands. All variables were measured within and just below the root zone on two transects at two intertidal elevations representing the tall- and short-growth forms of S. alterniflora. Measurements included pore water salinity, Eh, and pH and the standing stock of ammonium, phosphate, nitrate/nitrite, and hydrogen sulfide. Sediment-bound inorganic and organic fraction of nutrients were also measured. Above-ground production of tall-form S. alterniflora (2735 g m-2 growing season-1) was significantly greater than the short form (328 g m-2 growing season-1). Gradients in the above-listed secondary factors known to adversely affect nutrient uptake fail to explain the pattern of S. alterniflora production. A significant increase in height of S. alterniflora upon application of nitrogen, but not phosphorus fertilizer, confirmed nitrogen limitation in both growth forms of the plant. However, there was no significant difference in existing nitrogen standing stocks along the production gradient that would explain the patterns in production. A clearer explanation of the production gradient of S. alterniflora, as it relates to nitrogen limitation, can be obtained from a combination of nutrient standing stock with tidal cycle pore water flux. Hydraulic head was measured hourly over seven tidal events (three flood events and four ebb events) along two piezometer transects extending from the upland through the marsh. The horizontal component of subsurface discharge (liters m-2 h-1) was calculated as Darcian discharge using sediment hydraulic conductivity and head differences between the different vegetation zones. The vertical component of discharge was calculated in the same way only using the head difference between 0.7 and 0.1 m in depth. Infilitration was calculated as a volume flux by combining the average depth to the water table during the time period when the respective zones were flooded with the measured specific yield from that zone. All calculations were normalized to a 1 m2 by 0.2 m depth of marsh. Only those components representing an input to the root zone were summed to give the total volume input of water to the zone for each hour. The area was then calculated under a curve that represented the total input rate as a function of time (Figure One). These time-integrated results revealed a 2.1 times higher discharge rate over a complete tidal cycle through the root zone of the tall-form S. alterniflora compared to the root zone of the short form of S. alterniflora. When hourly measurements of nutrient concentrations were combined with the appropriate discharge input component, the resultant nutrient input revealed significantly more ammonium passing through the tall-form S. alterniflora root zone (294 umoles ebb event-1) compared to the short-form root zone (66.7 umoles ebb event-1) during the ebb tide. There was no significant difference in ammonium input during the flood tide event or nitrate input over either flood or ebb tide events between the zones. Both growth forms of the plants were nitrogen limited, but this limitation in the tall-form may be alleviated to a greater extent because the lower marsh root zone receives 4.4 times the nitrogen than the short-form root zone over a single ebb tide event. There has been speculation in the literature that an increased root zone nutrient flux may lead to a larger supply of nutrients to S. alterniflora and aid in alleviating nutrient limitation. The results of this study demonstrate that this mechanism can be important, especially in developing marsh ecosystems where sediment nutrient standing stocks are poorly-developed.
Figure 1. Instantaneous hourly total dicharge input and nutrient input rates to the tall and short-form Spartina alterniflora root zones averaged over three flood tide events and four ebb tide events.