Geoscience Reference
In-Depth Information
channel edges. In addition to accretion by
marine processes, alluvium deposited near the
mouths of rivers builds layers of marsh substrate
of sediment and organic matter (Mann 2000).
Over long periods this accumulation allows
marsh elevation to maintain itself in areas expe-
riencing relative sea-level rise (Dionne, Dalton
and Wilhelm 2006). Sea-level anomalies allow
estuarine marshes to expand landward in the
case of rising sea levels or seaward in the case
of the opposite (Dame, Childers and Koepl er
1992). However, questions remain regarding
marsh integrity in the face of rapidly rising sea
levels during this century. Studies have sug-
gested several possible scenarios, including
the contraction of estuarine marshes that are
bounded by higher elevation developed land on
one side and rising sea level on the other. Other
instances may see high marsh areas becoming
low marsh or being completely drowned out.
Intricate patterns of channels and creeks
allow sea water to reach far inland during high
tides and l ood vast sections of New England
coastal marshes (see Fig. 4-4). Daily tidal action
not only provides an important l ushing func-
tion, transporting materials in and out of the
marsh, but also introduces and deposits vital
nutrients to the low and high marsh zones.
Away from the daily inl uence of tides and with
greater inl uxes of fresh water, both brackish
and fresh-water marsh systems dominate. These
inland zones may support a greater diversity of
plant species. Species competition may inl u-
ence the location of halophytes across a marsh.
In addition, studies have indicated that salinity
tolerance, distance from water, and tidal inl u-
ence are also important variables in determining
species zonation (Bertness and Ellison 1987;
Sanderson, Foin and Usten 2001). However, con-
siderable variability may be observed from site
to site based on local conditions and latitudinal
inl uences (Mann 2000).
Special morphological, root, and cellular
adaptations are common in marsh plants. Across
northern New England, salt marshes are domi-
nated by tall forms of
Spartina alternil ora
(smooth cordgrass), an obligate halophyte, bor-
dering creek and drainage channel banks within
what is considered the low marsh. The short
Figure 16-36.
An almost dry panne feature with
blue-green algae and surrounded by short form
S.
alternil ora
. Photo by Firooza Pavri.
form of
S. alternil ora
(generally
0.4 m tall)
frequently occurs adjacent to this zone.
Spartina
patens
(saltmeadow cordgrass), by contrast, may
be found in extensive stands across higher sec-
tions of the marsh and may at times be inter-
spersed with
Distichlis spicata
(saltgrass).
Juncus
gerardii
(saltmeadow rush) is generally found
toward the upper edges of the high marsh.
Pools and panne (or pan) systems are found
dispersed across the high marsh landscape and
provide unique microhabitats. These areas retain
water from tidal l ooding for varying periods of
time. Pools are deeper depressions and vary in
species composition depending on local factors.
They provide habitat for i sh stranded during
l ooding, and forage sites for bird populations.
Pannes generally dry out between periods of
inundation and are varied in their composition.
Due to evaporation, salinity levels may be quite
high. Some pannes are bordered by short form
S. alternil ora
, others are dominated by mud,
blue-green algae, or forbs along their edges (Fig.
16-36).
The upland edges of a marsh are important
transition zones. These are areas only occasion-
ally l ooded by exceptionally high tides. In New
England,
Typha angustifolia
(narrow-leaf cattail)
and
Phragmites australis
(common reed) in
monotypic stands are often found across ter-
restrial upland edges. Some have attributed
these invasive communities of
Phragmites
to
increased nitrogen loading (Bertness, Ewanchuk
and Silliman 2002).
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