Sept. 30, 1997
Continued... Tidal Salt Marshes, Mangroves, and Seagrasses
Tidal Salt Marshes
Chemistry- chemically what determines what happens to the vegetation.
Salinity- - -
Nitrogen- - - - - -salt
marshes influenced by these factors because they are continually flushed
with seawater
Phosphorus- - - -and
have anaerobic soils
Iron- - - - -
Sulfur- - -
Why do plants grow really well or not at all? ( Puerto Penasco
example in class)
-The Water Table-
Underneath
the marsh is an aquifer. Water risies to the surface via capillary action.
When water gets evaporated,
salts saty behind. The highest salinity is around the pams. The lowest
salinity levels are around the
low intertidal zone where it is continually flushed by seawater. Rain collects
under the dunes and becomes
a perched brackish aquifer. However, it is hard for plant roots to get
down to this water level ( 40 ft.
or more).
-Right above the high intertidal zone is an area called the supralittoral
zone. This zone contains lush growth, all salt-tolerant plants, but not
necessarily halophytes.
Influences and Adaptations to Salinity:
There is not a precise measurement for salinity because seawater has a mixture of salts. 85% of seawater is NaCl. Seawater is also composed of MgSO4 (Epsom salt), CaSO4 (plaster of Paris), bicarbonate, and KCl.
The units for salinity measurement are: 1. ppt - g/L total salts. There
are 32-35ppt in the ocean, and 40ppt in desert seas. 2. moles/L - The molecular
weight of NaCl is 58, seawater has 500-600mM. 3. pressure units - the osmotic
pressure effect. Distilled water has 0MPa, seawater has - 2-3Mpa. (1 Pa
= 1 newton/m2 .) Pressure can also be expressed in bars, seawater is measured
to be 20-30 bars. 4. Electrical Conductivity (EC) - a measurement of how
well electricity conducts through water. Seawater has 60 dsm.
Mechanism of Salt Tolerance:
The problem with seawater is that Na is toxic to cells, and salty water has low water potential. In normal cells the water and solutes inside give the cell turgor and water continues to move into the cell because there is a lower water potential inside. In seawater, the lower water potential is on the outside and water leaves the cell. This creates small, shriveled up cells.
Halophillic bacteria can take seawater into their cells because they have developed enzymes that can tolerate salt.
Algae cells make organic compounds to lower their water potential, which keeps the salts out. Algal enzymes are not at all tolerant of salts.
Higher halophytic plants have developed a vacuole inside of their cells which will keep NaCl contained, while the cytoplasm has only organic solutes and a low NaCl concentration.
Emergent plants face a special problem due to the large amount of water flowing through them and high transpiration rates. The amount of water that a plant needs to process for growth is 200-500g per g of dry matter. (200 g for C4 plants, 500 g for C3.) It seems that any amount of salt in the water would load the plant with salts. It does not happen this way, plants have developed a very efficient system for filtering out salts at the root system. 99% of the salt is excluded from the plant, and what does get in is put in vacuoles.
Characteristics of Halophytes: 1. Exclude most NaCl efficiently from
roots. 2. Many are C4 (requiring less water). 3. Can store NaCl in cell
vacuoles, especially in the leaves. 4. Can excrete salts into the leaves
through salt glands or bladders. 5. Can become succulent to dilute or concentrate
NaCl as needed.
Can you discuss the different factors that effect salinity?
Influences of Nitrogen:
Salt marsh vegetation almost always limited by nitrogen. The plants need
alot of it to make proteins, amino acids, etc. Salt marsh vegetation, however,
does not get much nitrogen. 99% N -organic ( not available to plants because
they can not take complex compounds from the soil, slow decomposition without
O2), 1% N - ammonia ( can only use this type of nitrogen), and a little
nitrate.
Influences of Phosphorus:
Phosphorus is usually available in high levels. It is not a limiting factor.
Influences of Iron and Other Metals:
These metals tend to be very abundant and can even reach toxic levels ( seawater has them, so the metals build up in the soil from continual flushing.
Influences of Sulfur:
Sulfur in an abundant ingredient in seawater (MgSO4-Epson Salts). When it reaches marsh soils, the sulfur gets reduced by sulfur reducing bacteria. These bacteria, in turn, produce hydrogen sulfide (H2S-toxic at even low levels, makes that rotten egg smell). It also makes iron pyrites and iron sulfide. When you plow this land, and then try to reclaim it, it gets oxidized, which in turn, makes sulfuric acid ( H2SO4) - acidic soils = no growth.
Natural Sulfur Cycle:
Sulfate reducing bacteria=organic matter=H2S=Sulfur oxidizing bacteria=back
to beginning
-decomposition of organic matter takes place in this cycle ( 70 % of decomposition)
-consequences of this includes: 1) lack of O2
2)
seawater has so much sulfur
Ecosystem Structures and Energy Diagrams: Vegetation and Production in
Ecosystem Structure
-how cycles interact in self contained systems ( even though these are
not self contained because it interacts with seawater)
Vegetation and Distribution:
Tidal Salt Marshes: Spartina (cordgrass) dominates the east and
west coasts of North America, making up the main marsh system of the coastal
US. S. foliosa is found on the west coast, while S.alternifloria
(smooth cordgrass) and S. patens (short cordgrass) are found on
the east coast. The reason these species are of so much interest is their
high productivity. Spartina supports a detrital food chain.
Eugene Odum has done a lot of work with Spartina and has developed the concept of tidal energy subsidy. Odum measured the productivity of Spartina and found 750 g/L2 in the high intertidal zone, 2300 g/L2 in the intermediate zone, and 4000 g/L2 in the low intertidal zone. This productivity is as high as any agricultural crop, even though Spartina faces low levels of nutrients, high salinity, and anaerobic soil. The tidal energy subsidy theory explains that tidal action brings oxygen and nutrients to the plant as well as flushes salts deposited on its leaves.
2 interesting cases of S. altternifloria:
1) In the 1970's, this species was brought to the south San Francisco
Bay area. It quickly spread all of the way up to Puget Sound, replacing
all of the low growing halophytes. Now the coast is dominated by Spartina
marshes.
2) In the late 1890's, Spartina was brought to a marsh in Poole,
England. It spread and then crossed with a local species S.maritina
to make a male sterile hybrid S.townsendii. The new species spread
vegetatively and doubled its chromosomes to produce another species S.anglica
which is fertile. All of these species have taken over England's marshes.
Associated species in salt marshes are species that grow in fronts or pockets of the dominant species (usually Spartina).
Positive estuaries have a river at the back and get less saline going away from the sea. The true halophytes are found at the front, less salt tolerant wetland plants are found at the back.
In a positive estuary the back zone is colonized by Juncus, Scirpus, and Phragmites. The middle zone is dominated by Spartina or mangroves, with patches of halophytes like Batis and Salicornia. The front is colonized by annual Salicornia before Spartina can get going, this is because Salicornia can easily colonize mud flats.
Negative estuaries are like the Colorado River delta with no current river flow. These areas are saltiest near their backs.
In a negative estuary, the back zone is covered in Allenrolfia, Atriplex, and Distichlis. The front zone has algae and seaweed as well as Salicornia.
A salt marsh will typically only have about a dozen species living in it.
In the Gulf of Mexico, the species that are found are Distichlis palmeria (Nypa grass), and Salicornia bigelouii (a succulent annual) in the low zone. The middle zone is occupied by the bushes Salicornia subterminalis and Salicornia virginica. There are also the ground covers Patis maritima, Cressa truxillensis, Suaeda esteroa, Atriplex barclayonia, Frankenia grandifolia, and Monathochloe littoralis. In the high zone larger bushes are found such as Atriplex canescens, Allenrolfia occidentalis and Suaeda torreyana.
Sept. 30, 1997
Mangroves.
Mangrove is used to refer both to the tree and to the ecosystem. ( The old word for mangrove was mangal.)
Mangroves are halophytic trees found in salty, saturated soils. Mangroves are found in tropical regions between the Cancer and Capricorn tropics. There are two big groups of mangroves with only one similar species. The western group ( or New World) has only 8 species, and the eastern group ( or Old World) has 40 species. In the west, the species are found in the genera Auicennia germinaus, Rhizophora mangal, Rhizophora racemosa, and Lagoncularia racemoas. In the east, the genera are Auicennia marina, Rhizophora mucronata, Lumitzera, Nypa, and several others.
The speciation of mangroves is the same as that for seagrasses and corals. Distribution happened 70 - 135 million years ago in the Cretaceous period, also the age of dinosaurs. Laurasia was the center of origin for the mangroves, and when the continents divided few made it to the west. Today nearly all of the species are found in the east , which also has the richest flora. There is no overlapping of species because of this splitting up of Pangea.
This vegetation grows so thickly that nothing else can grow underneath them. They can not tolerate frost. If there is 3 to 4 days of frost, the whole mangrove line will die ( except maybe Rhizophora because it is more cold tolerant).
Mangrove swamps were cleared before because: 1)they were attributed to contained and spreading diseases like malaria, and 2) people were scared of them and attributed to where all the scary monsters lived, 3) not as productive as salt marshes.
Four Types of Mangrove Ecosystems:
1) Fringes: usually, narrow, only flushed with seawater, 39o/oo. Salinity
increases the further you move from the coast (60o/oo).Trees
can grow 13 m in height. Mostly found in Mexico, Texas, and Florida.
2) Rivering Type: freshwater influence. Trees can grow more than 20 m tall.
Most productive of all mangroves. Tidal river. Salinity
only reaches 10 to 20 o/oo because of freshwater flushing.
3) Basin Mangroves: Trees can grow 10 m in height. Dominated by spp. of
Rhizophora (red), Avicennia(black), and Laguncularia(white).
Dwarf stands:grow like shrubs, with bare spaces in between them. Rare tidal
flooding.
4) Overwash: In Ten Thousand Islands, in Florida. Rhizophora sit
a hill that is completely uncovered during low tide, but only the
tops of the trees stick out during high tide.
Chemistry:
Salinity: Has a major effect on the distribution of vegetation. Do not
require salt, but they will tolerate it ( they like freshwater better).
Slow growing, use salt to keep out invading species.
Dissolved O2: The only O2 comes from what the plants make themselves.
Acidity (sulfides high): This is not a problem for mangroves because they
have an aerobic zone around their roots where the acidity
builds up. ( Can not use this soil if mangrove swamp is cleared. Soil is
called "Cat Clays"- not very
useful soil, very acidic. Wnat to restore mangroves on this land. Howard
Teal used to write books about
dereclamation in the 1970's, but now he goes around the world helping people
build mangrove swamps).
Ecosystem Structure: Canopy- 3 to 4 spp. at most, often just 1 or 2 spp.
Understory-almost
no plants because the canopy grows so thickly. One species call the mangrove
fern, Acrostichum,
can grow under here, but one in Puerto Rico. Only a few spp. of this genus.
Zonation: It is thought that mangroves are actually building land seaward.
There is no real evidence that planting these will build up
the land ( old theory). Most likely has a cyclic process. Rhizophora
seedling will germinate and grow right on the tree.
It will then drop off into the seawater. Where ever they land, they drop
roots and grow). In Molokai, Hawaii, sugar
planters planted Rhizophora mangle becasue they thought it would
build up the coast, and they would have more
coast for planting sugar. They ended up bringing in a real pest spp. It
overpopulated and grew extremely dense. The
hawaiians hate the tree and want to cut the down to restore fish ponds.
The U.S. Corps Engineers say they can not
do this. They want to protect all coastal vegetation.
Productivity: 10 % of grass salt marshes, 1 to 5 tons/ hectare/ year of
dry matter ( compare this to the dry matter produce in the
salt marshes).
Values: The mangroves stabilize the shoreline. It provides habitats and
nursery areas. They also create sinks for nutrients.
Adaptations: They must be salt tolerant. Plants take up salt, just enough
NaCl so that they can balance outside salinity. Interesting
Study: Scholander points out that Rhizophora does not have any salt
glands and therefore must exclude
salt. However, Avicennia does have salt glands on the back of its
leaves so it must therefore excrete salt.
His theory would only hold for his own experiments, but in reality: 99.9%
of salt is withheld from plant, let a little
bit in to make inside move saltier than outside to let them osmotically
adjust. Some do have salt glands or drop
their leaves in order to rid of excess salts.
Viviparous seeds: seeds germinate/grows on parent because it is hard to
grow in seawater. Rhizophora propagule can float in seawater for
months ( air tight). As soon as it touches gound, it grows roots.
After you clear a mangrove swamp, you usually can not grow them again there because you changed the contour of the land when you cut them down.