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spring 2014
Aren’t Mangroves Just Mosquito-
Infested Swamps?
OK, so seagrasses aren’t grasses, and they have achieved
coastal water domination. But what about mangroves?
Although mosquitoes do live among mangroves, these
trees are much more than a home for insects. Mangroves,
which are woody plants, are much pickier about where
they take root than seagrasses are. Mangroves grow in
tropical and subtropical latitudes along the land-sea
interface in bays, estuaries, lagoons, backwaters and tidal
rivers. About two-thirds of the coastline in the tropical
areas of the world is lined by mangroves.
Mangroves are most recognizable by their dense tangle
of prop roots that make the trees appear to be standing
on stilts above the water. The aerial roots allow the trees
to handle the daily rise and fall of tides; most mangroves
get flooded at least twice a day. Mangroves do not require
saltwater to live, but they are unique in their tolerance of
saline environments. Like seagrasses, mangroves make
use of a unique set of adaptations to do so.
The recognizable aerial roots not only give the trees
stability but also allow the mangroves to use atmospheric
oxygen, which is important because the majority of sedi-
ment in which mangroves live is anaerobic. If you look
closely at the roots you’ll notice little bumps, which are
called lenticels. It is through these that gas exchange occurs.
Other adaptations to the saline environment include
physiological mechanisms that allow the tree to excrete
salt. Some of the leaves become covered in visible
salt crystals as the tree expels the salt; if you can’t see
the salt, you may be able to taste it (try it!). There are
also reproductive adaptations. Most mangroves are
viviparous, which means they give birth to live young.
The young trees germinate on the parent tree — in
the form of propagules — rather than germinating
externally from a seed. Tidal fluctuations distribute
these buoyant propagules once they fall into the water.
Connected to Coral Reefs
The brilliant blue water you see on the reef is in part due
to mangroves and seagrasses. These inshore habitats
improve water quality at the reef. Corals depend on clear
water that’s low in nutrients. The source of most nutrients
and sediment is land; when rivers and streams empty into
the ocean, the water tends to be high in nutrients such as
nitrogen and phosphorous and is generally very turbid,
which means it contains suspended sediment.
Sediment in the water column reduces light penetration
and can settle, smothering coral colonies and impairing
coral feeding. Seagrass leaves and mangrove roots slow
the flow of water, allowing suspended material to settle on
the bottom. Consequently, the water that flows through
these habitats out to the reef has low turbidity.
Generally, the water over a coral reef is very low in dis-
solved nitrogen and phosphorous. When nutrient levels
are too high on a reef, faster-growing phytoplankton and
seaweeds can outcompete and/or smother corals and
eliminate settling habitat for coral larvae. Seagrasses and
mangroves are nutrient sinks, buffering nutrient and chem-
ical inputs to the marine environment. Microorganisms
that live among the seagrasses and mangroves recycle these
nutrients back into the marine ecosystem.
Many of the animals you see while diving on coral reefs
spent the initial stages of their lives hiding among blades
of seagrass and mangrove roots. These coastal habitats
act as nurseries, offering food and safe haven from the
predators of the reef. Juvenile mullets, perch, whiting,
tailors, bream, snappers and emperors all live among
seagrass beds and mangroves. Once these fish grow larger
and stronger, they make their way farther offshore.
Vital Ecosystems
Coral reefs are not the only beneficiaries of these coastal
habitats. Seagrasses and mangroves provide extensive
advantages to the overall tropical marine ecosystem.
Everything needs to eat, and there are plenty of
herbivores that use seagrass blades and mangrove leaves
as food. These include vertebrates and invertebrates
alike; even residents of the coral reef use these inshore
habitats as feeding grounds. While mangrove leaves
are on the tree they are food for insects, but when the
leaves fall into the water many marine herbivores such
as mollusks and crabs feed on them.
These herbivores are the primary-level consumers,
which in turn support an array of secondary consumers,
including small fish and juveniles of predators such as
barracudas. When those juveniles mature, they become
third-level consumers. Larger predatory animals such
as herons, cormorants, sharks, barramundi, salmon and
crocodiles are also attracted to the seagrass meadows by
the schools of fish that seek shelter there.
Even decaying mangrove leaves and seagrasses are
important to the entire ecosystem as they provide
a base for the detrital food web. Bacteria, fungi and
other microorganisms colonize and decompose the
fallen plant material, creating detritus, a protein-rich
food source used by worms, microscopic organisms,
nematodes and crustaceans. Small fish eat these
detritivores (as well as detritus), and these fish become
prey for larger fish, birds and other animals.
Nearshore reefs protect coastline from wind and waves
and help prevent erosion. Likewise, mangrove forests stabi-
lize coastlines, limiting erosion from storm surge, currents,
waves and tides. Seagrasses are also important in stabilizing
