S
SALT MARSH ACCRETION
Peddrick Weis
Department of Radiology, Rutgers – New Jersey Medical
School, Newark, NJ, USA
Definition
Salt marsh accretion is defined as growth by deposition of
suspended particles during flooding (allochthonous
growth) and by accumulation of plant material, both roots
and decomposed material from plants growing in the
marsh (autochthonous growth) (Schuerch et al., 2012).
Process
Salt marsh accretion consists of the addition of both sediment and detritus from above and adding root tissue
below the salt marsh surface. The sediment components,
both organic and inorganic suspended solids, are waterborne particles that have been trapped in the salt marsh
vegetation. The detritus is decaying plant material from
dieback following the growing season. Added together,
the peaty soil of a salt marsh was reported to be >80 %
organic matter in a Chesapeake Bay salt marsh (Langley
et al., 2009) and, in a Rhode Island salt marsh, 91 %
and 96 % in low and high marsh, respectively (BrickerUrso et al., 1989). There are many factors influencing
the rate and quality of accretion, as shown in Figure 1.
The result of the many interacting factors is that salt
marsh accretion is keeping ahead of sea-level rise in many
but not all regions.
Suspended solids
Materials washing downstream or resuspended from
under open water tend to settle out in salt marshes, where
the water movement is slow. These materials include both
organic and inorganic components. Since suspended
sediment particles settle slowly to the bottom, settling
out is aided by natural meandering streams; conversely,
if straight channels are dug into the marsh to aid drainage
(as occurs with ditching for mosquito “control”), then the
tide ebbs faster with less time for settling out. If the
upstream materials are prevented from reaching an estuary, as occurs with dams on and dredging of rivers, and
by construction (urban development), a marsh has less
material to accumulate for the accretion process.
Vegetation
Since high and low marsh have different species of vegetation and since different species are characteristic of these
marsh segments on the several continents, it follows that if
the several species cause different rates of accretion to
occur, then different marsh areas and geographic regions
will have different accretion rates. The phenomena relating to accretion include stem density, plant biomass
(both above and below the marsh surface), and rate of
detritus formation following autumnal senescence. In fact,
it has been suggested, on the basis of both observations
and modeling, that vegetation actively engineers its landscape, building its environment to soil heights that optimize its stability (Marani et al., 2013).
Stem density
The flow of water during tidal flooding is somewhat
impeded by the plant stems. The physical transport of
suspended solids is thus slowed down – reduced 16 %
according to Wolanski et al. (2000) – allowing more time
for (1) adhering to plant stems and (2) settling out of the
water column to the marsh surface.
When observing a salt marsh at low tide, the high tide
line on plant stems can be seen because sediments have
adhered to the intertidal portion of the vegetation. However, Li and Yang (2009) quantified the amount of sediment adhering to vs. trapped by three species of plants
M.J. Kennish (ed.), Encyclopedia of Estuaries, DOI 10.1007/978-94-017-8801-4,
© Springer Science+Business Media Dordrecht 2016
SALT MARSH ACCRETION
Peddrick Weis
Department of Radiology, Rutgers – New Jersey Medical
School, Newark, NJ, USA
Definition
Salt marsh accretion is defined as growth by deposition of
suspended particles during flooding (allochthonous
growth) and by accumulation of plant material, both roots
and decomposed material from plants growing in the
marsh (autochthonous growth) (Schuerch et al., 2012).
Process
Salt marsh accretion consists of the addition of both sediment and detritus from above and adding root tissue
below the salt marsh surface. The sediment components,
both organic and inorganic suspended solids, are waterborne particles that have been trapped in the salt marsh
vegetation. The detritus is decaying plant material from
dieback following the growing season. Added together,
the peaty soil of a salt marsh was reported to be >80 %
organic matter in a Chesapeake Bay salt marsh (Langley
et al., 2009) and, in a Rhode Island salt marsh, 91 %
and 96 % in low and high marsh, respectively (BrickerUrso et al., 1989). There are many factors influencing
the rate and quality of accretion, as shown in Figure 1.
The result of the many interacting factors is that salt
marsh accretion is keeping ahead of sea-level rise in many
but not all regions.
Suspended solids
Materials washing downstream or resuspended from
under open water tend to settle out in salt marshes, where
the water movement is slow. These materials include both
organic and inorganic components. Since suspended
sediment particles settle slowly to the bottom, settling
out is aided by natural meandering streams; conversely,
if straight channels are dug into the marsh to aid drainage
(as occurs with ditching for mosquito “control”), then the
tide ebbs faster with less time for settling out. If the
upstream materials are prevented from reaching an estuary, as occurs with dams on and dredging of rivers, and
by construction (urban development), a marsh has less
material to accumulate for the accretion process.
Vegetation
Since high and low marsh have different species of vegetation and since different species are characteristic of these
marsh segments on the several continents, it follows that if
the several species cause different rates of accretion to
occur, then different marsh areas and geographic regions
will have different accretion rates. The phenomena relating to accretion include stem density, plant biomass
(both above and below the marsh surface), and rate of
detritus formation following autumnal senescence. In fact,
it has been suggested, on the basis of both observations
and modeling, that vegetation actively engineers its landscape, building its environment to soil heights that optimize its stability (Marani et al., 2013).
Stem density
The flow of water during tidal flooding is somewhat
impeded by the plant stems. The physical transport of
suspended solids is thus slowed down – reduced 16 %
according to Wolanski et al. (2000) – allowing more time
for (1) adhering to plant stems and (2) settling out of the
water column to the marsh surface.
When observing a salt marsh at low tide, the high tide
line on plant stems can be seen because sediments have
adhered to the intertidal portion of the vegetation. However, Li and Yang (2009) quantified the amount of sediment adhering to vs. trapped by three species of plants
M.J. Kennish (ed.), Encyclopedia of Estuaries, DOI 10.1007/978-94-017-8801-4,
© Springer Science+Business Media Dordrecht 2016