SALT MARSHES (tidal marshes)
I. Definitions and Characterization
"Salt marshes are communities of emergent herbs, grasses, or low shrubs rooted in soils that are alternately inundated and drained by tidal action"II. Environmental Characteristics
high
temperatures and evaporation lead to the salt pans (can be 100
psu)
–
areas without plants, surrounded by salt meadows
3. West coast
Spartina alterniflora -> Salicornia
D. Causes of Zonation
1. unlike the rocky
intertidal,
in the salt marsh competition often determines the
upper boundary and physical tolerances set the lower
boundary
a. Competitive dominance sets upper limits
b. Salinity
tolerances set the lower limits
c.
Experimental evidence
2. mutualisms –
a. Spartina alterniflora and fiddler crabs
b.
Spartina and the mussel, Geukensia demissa
3. disturbance
a. Ice scours
b. High tides and wrack
c. Salt build-up in mid-marsh
d. Salt-tolerant spikegrass (Distichlis) invasion
e. When the sediment is covered with spikegrass, evaporation decreases,
salinity decreases, and other more competitively dominant plants invade
4. grazers generally have
relatively little influence –
a. periwinkles graze, but they are controlled by crab predators
b. geese, wild boar, feral horses and nutria
V. Productivity
A. Productivity is high, mostly from vascular plants
1. Vascular plants --
50-3,000
g C/m2/yr depending on latitude and nutrients
2. Algal production lower
-- from 20-60% of vascular plant production
B. Evidence for nitrogen limitation (addition of
N changes outcome of competition)
VI. Interactions and Food Webs
A. Few herbivores – salt marsh plants high in salt,
contain lots of silica and cellulose, and are low in nutrients; some
have
chemical defense compounds
B. At times grazing is important; snow geese,
periwinkles
C.~90% of the production becomes detritus
D. Some of that carbon and energy may be exported
beyond the marsh
E. Many organisms migrate in and out of the marsh
with the tides
F. Oysters and mussels historically formed dense
beds at the edges of coastal salt marshes;
filtered incoming water, transferring lots of nutrients in fecal
material
to the marshes;
over-harvesting, diseases and eutrophication have caused massive
declines
in these shellfish and degradation of marshes
G. Role of predators; blue crabs, fish
Human Interactions with Estuaries and Salt Marshes
I. ‘Ecosystem services’
A. Shelter coasts from erosion
B. Salt marshes filter sediments and nutrients from
the water column, reducing turbidity and eutrophication in the
estuaries
C. Habitat for waterfowl and migrating birds
D. Support fisheries – shrimp, shellfish
E. Nurseries for many juvenile fish
II. Human Impacts
A. Traditionally thought of as wastelands that
should
be reclaimed for human use -- housing developments,
marinas, seaports, industrial parks, cities, garbage dumps…
1. Dikes and drainages;
fill
2. 1/3 of all estuaries
in the US are completely gone
3. dredging of navigation
channels increases exposure of estuaries and marshes to wave action
B. Rivers input modifications
1. carry pollutants
2. carry extra nutrients
leading to eutrophication
a. phytoplankton blooms
(some toxic)
b. anoxia – mouth of Mississippi and the Gulf of Mexico (‘dead zone’);
Chesapeake
3. damming and diversion
often reduces freshwater flow from rivers
C. Exotic species – ports and ballast water
1. Asian clam, 10,000 clams/m2
in SF Bay
2. green crab in NE U.S.
– voracious predator, including on economically important shellfish
3. nutria
4. >200 exotics in San
Francisco
Bay
5. cordgrass (Spartina)
has also been transplanted to other parts of the world (Europe, China),
covering mud flats and oysterbeds \
6. phragmites
D. Salt marsh restoration
1. restore tidal flow by
unblocking connections to the sea
2. dredge some accumulated
settlements
3. seed/transplant Spartina
4. attract birds with decoys
and solar-powered tape recorders with bird calls