On our trip to Costa Rica, a class of Marietta College
students disembarks to study a mangrove swamp. The mangrove swamp is a
wetland - submerged only at high tide - and its placement between the shore and
the coral reef in tropical areas makes it a crucial part of the ecology of the
coral reef itself - hence its placement in this section otherwise dealing with
oceanic habitats. Come with us to explore this vitally important habitat.
As you can see from the map above, mangroves are found along
the shore in many of the same areas that coral reefs are found (see map below)
Mangroves
are very important to the adjacent coral reefs in that they filter out silt and
nutrients that would otherwise go out to the reef and smother corals and
encourage algal growth. In addition, they serve as refuge and nursery for the
young of many reef fish. Ecologically, mangroves are exciting systems in their
own right.
Mangroves require warm tropical conditions and a shallow slope -
they don't do well along cliffs, for instance. They are particularly
prominent at river mouths or estuaries. Unfortunately, these are some of
the same places people like to build, and in order to do so they often cut down
the mangroves. In the long term this is foolish, as many have found out
when hurricanes or tsunami have come ashore without the mangroves to buffer the
coast. In the tsunami of 2004, many areas of Indonesia that were still
protected by their mangroves had relatively little damage to human structures.
In the Caribbean, there are 3 common types of mangroves. The photo
above shows all three of them in their typical habitat (this was a beach in
northern Jamaica where Columbus had been shipwrecked 500 years earlier).
Each of them has its own unique way of dealing with high salt concentrations.
Closest to the water - in fact in the water at high tide - are the Red
Mangroves (Rhizophora mangle). The roots of the red mangrove
are distinctive, with long arching aerial prop roots that help anchor the plant
in the sediment. These roots are firmly mired in the organic muck;
decomposition in this muck releases nutrients that the tree can use, but there
is no oxygen. The roots of the red mangrove are able to obtain water from
the ocean by pumping magnesium ions into the root. These positive ions
force other positive ions, such as sodium, out of the root. The high
concentration of magnesium in the root creates a high osmotic potential, and
this in turn attracts water in from the surrounding seawater. The net effect is
to set up "reverse osmosis" or to exclude salt from the root.
Oxygen to support the cells moving all those magnesium ions is provided through
air channels in the roots.
Next inland, usually above the high tides, are the Black Mangroves (Avicennia
germinans). These trees deal with salt by excreting it onto the
leaves; also, like the red mangroves, the roots of the black mangrove are
metabolically very active. To supply the roots with oxygen in the
oxygen-poor sediments the black mangrove has an extensive development of
pneumatophores. These structures grow up out of the soil and their spongy
construction helps convey oxygen down to the roots. Unlike the red
mangrove which actively excludes ocean salts from entering at the root, the
black mangrove allows the salt to enter but excretes it on the surface of the
roots and the leaves. You can often find salt crystals on the leaves of
the black mangrove. It can survive the higher salinities then the
red mangrove; such higher salinities might be expected further up on the beach
where the black mangroves are found.
Furthest inland are the White Mangroves (Laguncularia racemosa).
Neither aerial prop roots or pneumatophores are usually visible (but either may
be present if conditions warrant; the pneumatophores take the form of peg
roots)). Like the black mangrove, the white mangrove excretes salts
on the leaf surface.
You may have noticed that the scientific names for the mangroves differ
greatly (Rhizophora mangle, Avicennia germinans, Laguncularia
racemosa). This is because the word "mangrove" indicates an ecological
rather than a taxonomical grouping. For instance, when we
speak of oaks or maples we are referring to trees of the genera Quercus
and Acer, respectively. However, when we speak of mangroves we are
speaking of tropical, salt-tolerant trees that grow along the shore.
Hence, the 3 species of mangrove mentioned all hail from different genera and
are not closely related to each other.
Another interesting tidbit is the way the trees segregate themselves in the
habitat. Red mangrove seeds are much larger than either of the other two
and simply aren't carried very far inland. Black mangrove seeds are
smaller and will be washed further up the beach by the tides. White
mangrove seeds are the smallest and are carried the furthest inland. Thus,
each tree species has a seed that is adapted to be carried to the appropriate
habitat for the tree to grow.
Here are 3 views of mangrove communities, all from
Florida. Above left is at the marina at Flamingo in the Everglades
National Park; you see the mangroves growing down to the water at high
tide. Above you see the shoreline at Flamingo with a black mangrove
to the left and mangrove-covered islands in the distance. To the
lower left is an estuary near Naples, Florida which is dominated by red
mangroves with their distinctive prop (aerial) roots.
Down in the roots.
Above, red mangrove prop roots arch
down to the water. Note that there is very little vegetation on the
ground itself. This is due to flooding by high tides and the high
salt concentration, as well as the effect of shading by the dense mangrove
canopy above. In the image to the upper right, a black mangrove at
Tamarindo in Costa Rica sends out its roots through the muck;
pneumatophores stick up to obtain oxygen for the roots. Curiously,
this black mangrove is to seaward of the red mangroves behind it; a
reverse of the normal situation. Right, a look through the red
mangrove roots at low tide reveals various color bands resulting from the
effects of high and low tides. At high tide this area will be
submerged and fish will be free to forage through the roots for prey
feeding in turn on the organic sediments. One of the key detrital
components in those sediments are the leaves of the mangroves
themselves. The roots also trap and hold soil.
Near Tampa, Florida
Tamarindo, Costa Rica
Tamarindo, Costa Rica
Black Mangroves: If you look carefully in the shade
under the black mangrove at the upper left you will see the dense stand of
pneumatophores poking up from the sand. As the roots stretch away
from the tree, they send up pneumatophores every few inches (above).
At the water's edge in Tamarindo, Costa Rica (left) the pneumatophores
form a thick mat. Together with the roots under them, they help to
hold the soil in place where the kayaks are drawn up. Below, the
flower and leaves of the black mangrove from Bahia Junquilla, Costa
Rica. Note the salt crystals on the leaves.
.
Above: Pneumatophores of the Black Mangrove.
Life Cycle of the Red Mangrove: Flowers of the red mangrove
(above, right) are fertilized and begin to develop. The propagule or
seedling, does not drop from the tree immediately, but continues to grow
in place, reaching about 6" in length (15cm). When it does drop
off, the propagule (also known as a pencil) can float. It is heavier
at the root end, and eventually the lower end makes contact with soil and
begins to grow (below). If there are no storms or other
disturbances, the red mangrove seedling and its companions can advance the
shoreline as they stabilize the soils beneath them. In nature
however, storms tend to keep the system in balance. Human cutting of
mangroves can cause severe erosion problems during major storms or tsumani,
as we learned in 2004 and 2005 with the Indonesian Tsumani and Huricane
Katrina. See also:
Mangrove Ecosystem, Ding Darling National Wildlife Refuge,
Sanibel Island, Florida
Crabs are important parts of the mangrove ecosystem
playing a key role in the cycling of nutrients. A fiddler crab is
pictured below; these crabs have one large claw and one small claw.
They live in burrows in the sand; the burrows bring oxygen into the soil
and the excavated soils are nutrient rich.
The other crabs pictured here are "mangrove crabs" we haven't
bee able to identify yet. These shy crabs were quite abundant on the
roots of the red mangroves at Tamarindo, Costa Rica.
Burger's Fiddler Crab -- This fiddler crab (Uca
burgersi)was one of
many scurrying among the mangrove roots in a mangrove swamp in Florida.
The fiddler crabs live in burrows in the mud; the openings to the burrows
are above the water level. Below: The Halloween Crab, Gecarcinus
quadratus.
Touring the estuary at Tamarindo, Costa Rica
The endangered American Crocodile (Crocodylus
acutus) is an inhabitant of the mangrove swamp. As the swamps
are cut, the crocodiles lose their habitat. Above: juvenile
crocodile, Playa Nancite, Costa Rica. Above right: Crocodiles in
river, Costa Rica. Right: Crocodile in Everglades National Park,
Florida. Note that the individual on the bottom is pushing a
mangrove leaf with its nose.
Below: Endangered Manatees also use mangrove swamps to some extent.
The estuary at Tamarindo, Costa Rica is lined with mangroves.
Birds of the mangroves:
Many, many species of birds live in, on or among the
mangroves. 3 species are pictured here. Above left the Reddish
Egret (Egretta rufescens), photographed as it dances through
the shallows at Ding Darling National Wildlife Refuge, Florida.
Above right, the Roseate Spoonbill (Ajaia ajaja)
uses its unique bill to strain small invertebrates from the mud.
Photographed on Estero Island, Florida. Below are two views of the Yellow
Crowned Night Heron (Nyctanassa violacea) as it feeds among
the red mangrove roots at Ding Darling National Wildlife Refuge,
Florida. The individual to the right has caught a crab; the heavy
beak of these birds makes short work of the tough crab exoskeleton.
The Cassiopeia jellyfish - also known as the
upside-down jellyfish - is found in the waters of a mangrove swamp.
These unique jellyfish lie on their backs in shallow, clear water.
They harbor endosymbiotic algae (much like corals); the algae
photosynthesize and pass extra sugars on to the jellyfish. If it
weren't for the filtration supplied by the mangrove roots, silt and other
agents would cloud the water and ruin the habitat for these invertebrates.
Right - even in the waterlogged mangrove swamp, termites
can be found. Of course, in the swamp they build their nests (out of
a material known as carton composed of chewed up wood pulp
and termite saliva) high up in the branches.
Oysters and Sea Stars (left): Oysters are
commonly found growing attached to the roots of the red mangroves.
The Sea Star, Echinaster spinulosus, preys on the oysters.
This picture was taken in Florida; when I picked up the sea star I noticed
that a drop of water falling from the sea star into the water near the
oysters would cause them all to close ("Clam up"). Water
falling from my hand did not elicit the same response.
The 3 images above and to the right show southern Florida at three
points in time. Overall, mangroves have done pretty well, and that's
a good thing because of the extensive development elsewhere. The
coral reefs of the Florida Keys are quite sensitive to nutrient enrichment
and sediment, and both of these are made worse by the increasing
urbanization and intensive agriculture that have taken over much of south
Florida. Much of the everglades has been converted to agriculture,
and in some places only a thin band of mangroves exists to absorb the
released nutrients. The two photos below show what happens when the
system is no longer able to keep the nutrients under control. Note
the development of a "black zone" of dead algae and other
phytoplankton of the southwestern coast of Florida over the period of a
few weeks in the winter of 2002. Obviously, the problem in south
Florida go beyond what the remaining mangroves can deal with - a restructuring
of human modifications will be needed to protect the reefs in the keys.
What's at stake - extensive eutrophication (algal growth
fueled by nutrients running off agricultural fields and from urban areas)
spreading into the Gulf of Mexico off the coast of Florida. Intact
ecosystems, including mangrove swamps, help prevent eutrophication and the
problems it caused.
