ecological groups, "upland" and "aquatic remnant"
On rare occasions the crust types intergrade. Crusts
crusts (Brostoff 1998). The upland crusts (correspond-
with characteristics of both upland and aquatic remnant
ing to the rough, uneven crusts and pedicled crust types
types have been reported on soft pans between dunes
of Johansen) occur predominantly on dunes, alluvial
situated on a larger playa at Edwards Air Force Base
slopes, and other areas that do not pond water. The
(Brostoff 1998). The crusts in question were smooth and
dominant constituent species (Microcoleus, lichens,
thin and had upturned edges and conspicuous black
mosses) of these crusts are often destroyed or at a com-
areas on a light gray background. Normally, these pans
petitive disadvantage when submerged even for short
do not become inundated, but may remain moist for pro-
periods. However, these crusts are frequently most
tracted periods of time because of discharge of water
abundant on nonponded areas adjacent to areas that
from surrounding dunes (possibly analogous to
do pond based on a study of a playadune system in
groundwater seepage). During periods of heavy rain-
California (Brostoff 1998). Thus, based on both docu-
fall they do pond. Similar crusts have also been seen in
mented biology and empirical evidence, the upland
Utah and New Mexico, and have been produced under
crusts may be used as negative evidence for protracted
laboratory conditions (Brostoff, personal observations).
periods of standing water.
Along playa/dune isoclines, the crust types may be
The aquatic remnant crusts form in areas of previ-
either adjacent or disjunct in distribution (Brostoff 1998)
ously standing water (Fig. 5). Although relatively little
(as in the transition shown in Fig. 3b). Locations where
attention has been paid to them, they may be domin-
upland and aquatic remnant crust types were found
ated either by bacteria or by one or more species of
adjacent were those where water levels were adjacent
algae--usually the same ones that are found in the
to dunes. Those locations with disjunct distributions
upland crusts (Brostoff et al. 1996, Brostoff 1998). These
represent instances where maximal water levels were well
crusts range from beige or brown to red in color, some-
below the level of dunes. Further, the most conspicu-
times with a distinct green cast on their underside. The
ous upland crust formation was adjacent to areas that
darker colored crusts are more often dominated by algae;
ponded for the longest periods of time.
based on very preliminary laboratory results, it is
The use of crusts for retrospectively determining the
thought this darkening is an extracellular algal product
OHW mark is promising. Current work at ERDC and EL
(an extracellular polysaccharide or possibly a pigment).
on both restoration and modeling crust-dominated eco-
The crusts range from 1 to 2 cm (0.39 to 0.78 in.) up to
systems will yield results applicable to delineation in
15 cm (6 in.) across and from <1 mm (0.039 in.) to a few
two areas: (1) understanding the relationship between
mm thick. The thinner crusts characteristically show
moisture and crust formation and (2) differentiating
upturned edges. The surface may be smooth, or show
causal factors in algal-dominated vs. bacteria-dominated
rosette or mesh-like patterns with units roughly 100 mm
aquatic remnant crusts. At least at present, because the
(0.0039 in.) wide by 1 mm (0.039 in.) long. These pat-
relation between crust characteristics and inundation
terns may appear as impressions in the surface, old algal
is probably site specific, conservative predictions of
sheath material, or algal filaments. Based on laboratory
OHW from crust presence should be based on local
observations (Brostoff and Rundel 1998), the patterns,
knowledge. Ideally, some local crust formation should
when present, are evidence of ponding; however,
be observed such that it can be unambiguously linked
smooth-surfaced crusts are not necessarily negative evi-
with the presence of water. Another caveat is that crusts
dence. Crust formation has been followed for two wet
may blow away, so the absence of the aquatic remnant
dry cycles at Edwards Air Force Base, California (EAFB),
crusts should not be used as negative evidence for
and there is good correspondence between areas of
OHW.
flooding during the wet season and presence of aquatic
Diatoms, in addition to being a crust component, may
remnant crusts during the dry season (Brostoff 1998,
be found among playa sediments (Busch and Kubly
and unpublished observations). During the season, the
1980). Because of the high turbidity, diatoms are not
crusts may decrease in width (i.e., fracture into smaller
able to live in playa waters. Two other potential habi-
pieces) and, during dry periods, blow away (Fig. 5b, c).
tats for diatoms exist around playas: (1) the aerial habi-
This wind erosion is exacerbated by anthropogenic dis-
tat, or "dry soil," which is populated by ubiquitous soil
diatoms such as Navicula and Hantzschia, and (2) moist
turbance (Brostoff 1998). Some crusts (predominantly
the algal rather than bacterial-dominated ones) appeared
to wet areas around the margins of standing water pop-
to remain stable for several years. Although formal lab-
ulated by a group of diatoms often found around lake
edges, including Achnathes, Denticula, Epithemia,
oratory comparisons were not performed, there were no
Gomphonema, and Meridion (Busch and Kubly 1980).
apparent macroscopic differences between recently
formed crusts and those which remained dry over one
However, using species assemblages to demarcate the
inundation period.
border between inundated and non-inundated areas
17
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