Both sites had similar grain size (> 90% sand), but the sand flat had significantly lower organic content compared to the marsh site. The sand flat was characterized by significantly lower hydrogen sulfide and higher pH, salinity, and redox potential (Table 1).
Nutrient pools were generally similar between the sites. One exception was at the tall-form S. alterniflora portion of the intertidal zone where soluble and extractable ammonium and phosphate were approximately ten times higher at the sand flat site (Table 2). Only organic phosphorus was significantly higher at the marsh site in the S. alterniflora zones . Results of a depth profile of ammonium and phosphate at both sites revealed a nutrient-rich source at depth at the sand flat, possibly a mud flat buried in the storm event. In contrast, the marsh site has evolved to the point of having generally lower nutrient pools which are mostly concentrated in the root zone.
Measurements of subsurface hydrology at the marsh site revealed a 1.6 times higher discharge rate through the root zone of the tall form of S. alterniflora compared to the root zone of the short form of S. alterniflora. Calculated nutrient flux over the tidal cycle revealed significantly more ammonium passing through the tall-form S. alterniflora root zone (226 µmoles) compared to the short form root zone (125 µmoles). This corresponds with an eight times greater 1993 growing season production (2735 g m-2) for the tall form of S. alterniflora compared to the short form (328 g m-2).
A manipulation experiment at the marsh site, involving the interruption of subsurface pore water flux, effectively reduced horizontal flux through the root zone by 20-40% in the treatment plots. By October there was a small decrease in S. alterniflora shoot height and tissue percent nitrogen in the treatment plots (67.2 cm; 1.54%) compared to the control plots (70.2 cm; 1.91%), however this difference was not statistically significant. Another growing season may be necessary to observe any significant effect.
The greater calculated rate of pore water flux through the tall-form S. alterniflora root zone could account for the larger values of production compared to the short form. Nutrient limitation in the tall form may be alleviated to a greater extent as the lower marsh root zone receives 1.8 times the nitrogen than the short form root zone over a single tidal cycle.
Gradients in nutrient standing stock at the sand flat site appear to be the factor that will dictate the predicted zonation at that site, where the most favorable growing conditions exist at the most frequently-flooded zone. Gradients in nutrient standing stock or other edaphic factors that affect nutrient uptake fail to explain the existing vegetation gradient at the young marsh site. Nutrient availability based on subsurface hydrologic flux could be an important factor in S. alterniflora production gradients in this latter site and may have important implications for similar types of developing marsh ecosystems.
TABLE 1. SUBSTRATE CHEMISTRY FROM A STORM-DEPOSITED SAND FLAT
AND UNDISTURBED MARSH**
SAND FLAT MARSH SITE
TS SS HM TS SS HM
SALINITY (ppt) 33.1 *34.6 *33.8 33.1 29.9 30.5
[0.3] [0.4] [0.5] [0.4] [0.5] [0.4]
pH *7.1 *7.2 *7.4 6.8 6.9 6.8
[0.02] [0.02] [0.01] [0.03] [0.02] [0.04]
REDOX POTENTIAL (mV) *66.4 83.3 *223 25.4 75.5 178.6
[3.2] [5.4] [10.2] [13.9] [4.6] [11.4]
Fe (II) (uM) 28.7 *26.1 *2.3 25.9 19.4 13.2
[2.7] [2.4] [0.5] [2.0] [1.2] [2.0]
Fe (III) (uM) 1.3 0.2 2.0 0.1 0.5 0.9
[1.2] [0.1] [1.0] [0.1] [0.2] [0.3]
HYDROGEN SULFIDE (uM) *6.8 *2.5 0.2 15.9 8.1 2.1
[1.3] [0.6] [0.1] [2.9] [2.1] [0.5]
TABLE 2. SUBSTRATE NUTRIENT CHEMISTRY FROM A STORM-DEPOSITED
SAND FLAT AND UNDISTURBED MARSH**
SAND FLAT MARSH SITE
TS SS HM TS SS HM
NITROGEN
SOLUBLE (uM) *246 *50.4 11.9 9.0 5.1 4.0
[17.4] [4.3] [2.7] [1.5] [0.7] [0.6]
ORGANIC (umol/g) 8.0 7.7 5.7 9.1 11.0 6.4
[0.9] [1.7] [0.8] [1.7] [2.6] [1.1]
EXTRACTABLE (umol/g) *0.84 0.26 0.03 0.09 0.26 0.09
[0.17] [0.05] [0.01] [0.01] [0.08] [0.02]
PHOSPHORUS
SOLUBLE (uM) *99.8 14.3 3.8 19.8 11.1 1.1
[4.7] [1.5] [0.5] [2.6] [2.2] [0.2]
ORGANIC (umol/g) *0.43 *0.13 0.98 1.40 1.43 1.73
[0.12] [0.07] [0.43] [0.40] [0.36] [0.36
EXTRACTABLE (umol/g) *4.7 *4.3 3.6 2.6 2.3 3.1
[0.4] [0.4] [0.4] [0.3] [0.3] [1.1]
* significantly different than corresponding station in marsh (p < 0.05)
** values are average of 6 months from 10 and 40 cm in depth
standard error is given in brackets
TS = TALL SPARTINA
SS = SHORT SPARTINA
HM = HIGH MARSH