Mangroves
‘Walking Trees’
Jonathan Lowrie
A mangrove is a
type of tree that grows in tropical regions at river mouths, bays, coastal
lagoons and islands. They occur in
many regions through out the world; some of the best-studied mangrove forests are
in the Florida ecosystem. In the Florida Keys, they create a fringing network
around most islands and grow at hundreds of shallow locations offshore. They are one of Florida’s true native
species. They thrive in their
salty environment because they are able to obtain freshwater from
saltwater. Some of the species do
this by excreting salt through their leaves; others block absorption of salt at
their roots.
Mangrove roots
act as a physical traps to filter the water systems. They trap debris and silt,
stabilizing the near shore environment, and clarifying adjacent waters, and
facilitate photosynthesis in other marine plants. They also provide an attachment substrate for various marine
organisms. Many of these attached
organisms filter water through their bodies and, in turn trap and cycle
nutrients. Sponges, barnacles, oysters, mussels, shrimps and oysters are all
efficient filter feeders that attach to mangrove root systems. The Florida ecosystem has an estimated
470,000 acres of mangrove forests, and they all contribute to the purification
of the state’s water quality. This
ecosystem traps and cycles various organic nutrients, chemical elements, and
acts as a nutrient sink for important nutrients. Mangroves shed and drop about
7.5 tons of leaf litter per acre per year. The constantly shed leaves are quickly broken down by
bacteria and fungi and released into the water, providing food for sealife.
I cannot
overemphasize the relationship between mangroves and their associated
wildlife. Mangroves provide a
secure and safe haven for young fishes, crustaceans, and molluscs. They also provide food for many marine
species like snapper, damselfish, tarpon, and shrimp. Without a healthy mangrove system, the vitality and health
of the sport and commercial fisheries would decline. 74% of the game fish and 90% of the commercially valuable
sealife in Florida depends on the mangrove.
Most animals
find shelter in the roots, or the complex branch structures of mangroves. The upper branches serve, as rookeries
for coastal birds, like the brown pelican. The roots also offer habitat for mammals, amphibians,
reptiles, countless unique plants, and other invertebrate life. This root
structure that penetrates into the water provides substrate for a variety of
bivalves, and macro-algaes to attach.
This dense coverage provides shelter for juvenile invertebrates and
fish.
Worldwide, more
than 50 species of mangroves exist.
Of these, only three are found in Florida waters. The best known is the Florida Mangrove,
Rhizophora mangle. It’s characterized by aerial roots and
concealed prop roots, which provide support for soft muds and stabilize
sediments. It typically grows along the water’s edge, where tidal flushing is
sporadic and the water is nutrient poor.
The red mangrove is easily identified by the tangled mass of reddish
roots called prop roots. These
projections from the trunk have earned this mangrove the name ‘walking
mangrove’. This tree can easily
reach 30 feet in nature.
The Black
Mangrove, Avicennia germanans,
usually occupies higher elevations than the Red Mangrove. They are characterized by the presence
of small pencil-like vertical root shots called pneumatephores. These root shoots stand in dense arrays
near the high tide line, enabling the tree to get oxygen from the
atmosphere. The underside of the
leaf surface has a whitish residue, which is excreted salt. This will remain unless rinsed by a
passing thundershower.
The White
Mangrove, Laguncularia racemosa
usually occupies the highest elevations farther upland than the red or black
mangroves. It grows on elevated
grounds above the high tide mark.
Unlike its counterparts, it has no visible aerial root system, as the
root system resembles that of most terrestrial trees. The leaves are thick and succulent, rounded at both ends,
and appears uniform in color on both sides.
Many threats abound to mangrove habitats. Hurricanes can damage 100,00 acres in a
few hours, as did Hurricane Donna in 1960. Even the recent Hurricane Georges
that swept across Puerto Rico and the keys was responsible for mangrove habitat
damage. However, all of the storm damage cannot equal the impact humans have
had on these forests. Shoreline
development has replaced Mangroves with marinas, dredged channels, airports,
seawalls and commercial and residential construction. Over 55% of shallow water mangroves were lost in the upper keys
in past 15 years. Forty percent of
the loss was from filling of the habitat to make way for new construction. This staggering loss occurs not just in
the keys, but all over the Florida coastline. Other threats include illegal dumping, oil spills, agricultural
run-off that contains herbicides and pesticides. Freshwater and street water runoff has also altered the
salinity in some habitats causing mangrove die-backs.
Many
organizations are studying habitat loss of the mangroves. Looking at aerial photos of the same
habitats from the 1940’s and 1950’s, scientists can see how pronounced the
habitat destruction is. In
Florida, state and local laws were enacted to protect the mangroves. Local laws vary, but most in
municipalities it’s illegal to take any rooted plant, or disturb the trees or
associated wildlife in any way.
The penalty includes heavy fines and possible jail time.
Mangroves are
even now being kept in home aquariums.
They certainly can make a lovely habitat tank, allowing a touch of realisms
for the shoreline tidal flat, or mangrove forest. With the uptake of nutrients, they will certainly contribute
to the overall vitality of the captive ecosystem, but because of slow growth,
an uptake, they are not ‘miracle natural filters’. The most common species for home aquariums is the Red
Mangrove. It’s available as a propagule, and ships well, in moist bags.
This species
does well in most seawater tanks, given a deep substrate to plant it in, and
plenty of light. I have had
success with normal fluorescent bulbs, but have found compact fluorescent bulbs
or metal halides to work best.
Salinity is less critical, as long as rapid fluctuations do not occur. One way to jump-start these ‘seeds’ is
to soak them in a separate container of water containing a small quantity of miracle
Grow or similar fertilizer. Its important
to note that you should never put the fertilizer in your main aquarium
display. These propagules should
be firmly embedded in the sediment, so that the bottom 2 to 3 inches is
covered. If you have a healthy propagule,
then you should see the beginning of growth in just a few weeks. I have also had some that take as long
as six months before noticeable growth shows- so patience is vital. Its best not to disturb the root system
once it has begun to grow.
Before any
mangroves are added to the tank, a few considerations must be made. They are coastal vegetation, and are
not found on the reef, but as an associated habitat. One could never find mangroves, and small polyped corals
grouped together. Planting one in
a typical reef, would only cause problems for the tree or the corals. Mangroves are relatively slow growers,
yet seem to expand by leaps and bounds when confined in a small aquarium. Considering the overall height of 45
feet, they do grow slowly, but they can get over three feet tall in an aquarium
in less than 6 months with adequate lighting. The size of aquarium, and how much room you have to offer
the tree plays a critical role in keeping mangroves.
I have some
mangroves in a lagoonal system, complete with sea grasses, and some coastal
corals, like Sidastrea, and Condylactus anemones. I also have a system utilizing
mangroves from Indo-Pacific and have the roots structured to create a land
mass, where mudskippers, archer fish, can call home. The possibilities for little captive microcosms are nearly
endless. Its best to get the mangroves
as propagules with just a few roots starting. If the plant is more established, it commonly fails from
system shock.
Mangroves are a
vital habitat we should all strive to preserve. They most certainly add beauty and habitat to our home
aquariums, and open up many new avenues of aquascaping. If you ever have a chance to visit the
coastal regions of Florida, I highly recommend a visit to a mangrove forest, as
the abundance of wildlife makes this habitat one of the most diverse anywhere
on earth.
