DISTANCES FROM TIDAL CREEK AND THEIR EFFECT ON NUTRIENT CHEMISTRY AND SALINITY IN SPARTINA ALTERNIFLORA AND JUNCUS ROEMERIANUS.


Jamey T. Watt: Dr. Robert Christian (E.C.U.) and Dr. Linda Blum (U.Va.).


The initial construction phase of a small scale (in comparison to a proposed LTER experiment) experimental plot on Phillip's Creek Marsh in Brownsville, VA is complete. This "pilot" plot (3m x 4m) was designed as a feasibility study to test pumping technology from Phillip's Creek and to establish baseline conditions in salinity, ammonium, and phosphate for future work. This plot was enclosed and partitioned parallel to the marsh slope to contain half the area as Spartina alterniflora and half as Juncus roemerianus. Six other open, undisturbed plots of the same dimensions were chosen for the next phase but have been designated as controls here (Figure 1). The plots have approximately the same relative elevation and were installed in late June 1993 to begin collecting data for the establishment of seasonal trends in sediment chemistry. In the coming months, these undisturbed control plots will become a major part in the full scale manipulation study.

Preliminary results from both the pilot plot and the six undisturbed control plots indicate decreasing pore water salinity with increasing distance from the creek in marsh areas vegetated by J. roemerianus. In contrast, the S. alterniflora vegetation in the pilot plot shows increasing salinity along the same path of increasing distance from the tidal creek. However, the S. alterniflora salinities collected from the six undisturbed control plots did not continue the trend.

The six undisturbed control sections indicate a higher concentration in average pore water salinity within the J. roemerianus vegetation compared to that of S. alterniflora (Figure 2). This also seems to be the case regarding salinities in the pilot plot, however, the other manipulations underway obscure this result. Support for the difference between vegetation types results from how the two different plants deal with their saline environment. J. roemerianus is a salt "excluder"; the salt stays in the pore water. On the other hand, S. alterniflora is a salt "excreter"; the salt is taken up from the pore water and then excreted from the leaves.

This study examined pore water concentrations of phosphate (PO4=) in the six undisturbed control plots and found higher levels in the S. alterniflora than the J. roemerianus (Figure 3) . Whether the difference results from uptake, mineralization, or exchange is unknown.

From the data collected in the six undisturbed control plots, it appears that distance from the tidal creek plays a major role in determining pore water ammonium (NH4+) concentrations. The two near locations of both the S. alterniflora and J. roemerianus had higher levels of ammonium than the two far locations (Figure 4).

The samples from this analysis were collected by suction lysimeters on six dates ranging from July 20, 1993 to August 11, 1993. Tidal creek water was not pumped into the pilot plot at regular intervals until late August 1993.