THE SALT FLATS OF THE VCR-LTER: A SYNTHESIS.
Salt flats are hypersaline upper mid-littoral sites partially or
totally devoid of vascular vegetation. They are usually found on arid or
semi-arid tropical and subtropical shorelines. The salt flats at the
VCR-LTER are probably near their latitudinal limit on the U.S. Atlantic
coast. Salt flat research at the VCR-LTER has focused on the: (1)
characterization and comparison of the VCR-LTER salt flats with their
tropical counterparts, (2) causes of upper mid-littoral vegetation
die-back, (3) environmental conditions associated with salt flat spatial
patterns and (4) quantification of the upper mid-littoral hydrological
cycle in order to evaluate salinity buildup. Only the results about the
first and third topics will be summarized below. There are several
similarities between VCR-LTER and tropical salt flats. First, they both
usually develop between the Mean High Water Spring (MHWS) line and the
upland boundary. Second, they both have hypersaline profiles at depth.
(which separate salt flats from seasonally hypersaline sites, that are
hypersaline only at the marsh surface). Third, they both may develop
porewater salinities above 100 ppt, which is lethal to all vascular
marsh plants. The salt flats at the LTER-VCR also show considerable
diversity at the micro-scale spatial level. In the mainland (at Quinby)
salt flats develop on clay and the hypersaline profile is only 0.5 m
deep. On the barrier islands, salt flats develop on pure sand (Crescent
Island) or in a mixture of silt and sand, with hypersaline profiles
deeper than 5 m (at Hog Island). Their physico-chemical characteristics
are also variable, with porewater concentrations of nutrients and
sulfides varying by orders of magnitude (see Table 1)
. Salt flat micro-spatial distribution patterns were evaluated with a
Principal Component Analysis (PCA) of selected environmental variables. The
resulting eigenvalues were used in a multiple regression against
salinity. The analysis revealed that porewater salinity buildup is
mainly controlled by changes in drainage as a function of topographical
slope. This variable accounts for 71 % of the observed variation in
porewater salinity in the upper mid-littoral zone. The same procedure
was used to examine the difference in porewater salinity regimes,
attained at different salt flats at different localities. The results
showed that the combined variation of sediment hydraulic properties and
micro-climate as the main variable, followed by variation in total tidal
flooding, both variables accounting for 43 % and 15 % of the regional
variation in porewater salinity, respectively.
Table 1. Selected environmental settings of VCR-LTER salt flats.
QUINBY CRESCENT ISLAND HOG ISLAND
Porewater 53 100 120
salinity (ppt)
Depth of 0.5 >3.0 >5.0
hypersaline
profile
(meters)
Topographical 1 / 553 1 / 372 1 / 333
slope
(ratio)
Tidal flooding 354 1,049 401
(hours / year)
Porewater 31.5 4.9 1,602.8
ammonium
(micromoles /
liter)
Porewater 0.1 21.7 252
phosphate
(micromoles /
liter)
Porewater 4.1 0.3 2,624.2
sulfide
(micromoles /
liter)
Grain size CLAY - 45 CLAY - 0 CLAY - 3
SILT - 38 SILT - 0 SILT - 23
SAND - 17 SAND - 100 SAND - 64