Tour of Mangrove Swamps

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.

More on Crocodiles: http://www.flmnh.ufl.edu/natsci/herpetology/brittoncrocs/csp_cacu.htm

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.

More on these birds at: http://www.mbr-pwrc.usgs.gov/id/framlst/infocenter.html




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.
Ecological Role of Mangroves:

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.


For more on mangroves see:



From the Marietta College Biology Department Field Trip to Costa Rica:

Mangroves at Bahia Junquillal, Costa Rica     Mangroves at Tamarindo, Costa Rica


Proceed to Coral Reefs Proceed to Mangrove Swamps
Proceed to Coral Reef Fish Proceed to Sandy Shores
Proceed to Coral Reef Invertebrates Proceed to Rocky Shores