Of Florida’s estimated 469,000 acres of mangrove forests, the Indian River Lagoon contains roughly 8,000 acres of mangroves, which play critical ecological roles as fish nurseries, shoreline stabilizers, and pollution mitigators.
However, an estimated 76 to 85 percent of this acreage has become inaccessible as nursery habitat for local fisheries through construction of mosquito ditches and impoundments. Though mangroves are protected in Florida by state law, mangrove habitat loss is a problem globally as these edge ecosystems are removed to make way for commercial and agricultural development.
Worldwide, more than 50 species of mangroves exist. In Florida, the mangrove community consists of three main species of true mangroves: the red mangrove, Rhizophora mangle, the black mangrove, Avicennia germinans, and the white mangrove, Laguncularia racemosa.
Buttonwood (Conocarpus erectus), while often considered a fourth mangrove species, is technically a mangrove associate, as it lacks the specialization of true mangroves and grows in uplands beyond the reach of tides.
In the IRL, the most recognizable and common species is the red mangrove. Dominating the shoreline from the upper subtidal to the lower intertidal zones, red mangroves are easily identified by their tangled, reddish “prop roots” that grow outward and down from the trunk into the water and underlying sediments. The tree often appears to be standing or walking on the surface of the water.
In the tropics, trees may grow to more than 80 feet (24 meters) in height; however, in Florida, trees typically average around 20 feet (6 meters) in height. Dwarf mangroves are prevalent in Merritt Island and the northern reaches of the lagoon, but also occur as far south as the Keys.
Leaves have glossy, bright green upper surfaces and pale undersides. Trees flower throughout the year, peaking in spring and summer. The seed-like propagules of the red mangrove are pencil-shaped, and may reach nearly a foot in length (30 cm) as they mature on the parent tree.
Black mangroves typically grow immediately inland of red mangroves. Though they may reach 65 feet (20 meters) in some locations, Florida populations typically grow to 50 feet (15 meters.) Instead of prop roots, black mangroves feature thick stands of pneumatophores, stick-like branches which aid in aeration that grow upward from the ground. Leaves are narrower than those of red mangroves, and are often encrusted with salt. Black mangroves flower throughout spring and early summer, producing lima bean-shaped propagules.
White mangroves are more prominent in high marsh areas, typically growing upland of both red and black mangroves. White mangroves are significantly shorter than red or black mangroves, as well as the least cold tolerant, generally reaching a maximum 50 feet (15 meters) in height under optimal conditions. Leaves are oval and flattened, with two glands at the base, known as nectaries, that produce sugar. Trees flower in spring and early summer. Propagules are small, measuring just over a third of an inch (roughly 1 cm).
Mangroves occur in dense, brackish swamps along coastal and tidally influenced, low energy shorelines. In Florida, mangrove forests extend from the Florida Keys to St. Augustine on the Atlantic coast, and Cedar Key on the Gulf coast. Factors such as climate, salt tolerance, water level fluctuation, nutrient runoff, and wave energy influence the composition, distribution, and extent of mangrove communities.
Temperature also plays a major role in mangrove distribution. Typically, mangroves occur in areas where mean annual temperatures do not drop below 66°F (19°C). Mangroves are damaged under conditions where temperatures fluctuate more than 50°F (10°C) within short periods of time, or when they are subject to freezing conditions for even a few hours. Stress induced by low temperatures leads to decreasing structural complexity in black mangroves, with tree height, leaf area, leaf size and tree density within a forest all negatively impacted.
Mangroves have several important traits that allow them to thrive in a harsh saline environment that often excludes other species.
Soils in mangrove areas tend to be fairly oxygen-poor, preventing many types of plants from taking root. Mangroves have adapted to these anoxic conditions by evolving broad, shallow root systems instead of deep-reaching taproots. However, specialized aerial roots allow mangroves to absorb oxygen despite their waterlogged environments.
Red mangroves’ prop roots act to both stabilize the tree and aerate the roots. Above-ground portions of prop roots have many small pores, called lenticels, that allow oxygen to diffuse first into spongy air spaces called aerenchyma near the outer edges of the root, then into underground roots. Lenticels are also hydrophobic, or water-repelling, and block water from entering the trees’ roots even during high tide.
Instead of buttressing prop roots, black mangroves’ aerial pneumatophores form dense thickets of finger-like protrusions around the base of trees. Sometimes called “dead man’s fingers” in reference to their gnarled, spooky appearance, these pneumatophores function similarly to red mangroves’ prop roots, and can reach 8 inches (20 cm) in height.
As an upland species, white mangroves lack pneumatophores and prop roots. However, when submerged for prolonged periods of time, the tree may develop “peg” roots to aerate the subsurface cable roots.
Mangroves can grow in either fresh or salt water, depending on which is available. However, they are largely confined to estuaries and upland fringe areas that are at least periodically flooded by brackish or salt water. One reason mangroves rarely develop in strictly freshwater zones may be due to more intense competition from freshwater plants.
That makes salt management critical. Mangroves accomplish this either by preventing salts from entering their tissues, or by excreting excess salts that are absorbed. Red mangroves block salts at their root surfaces via a reverse osmosis process that allows water to diffuse freely into plant tissues. Black and white mangroves, as well as buttonwoods, excrete salt to balance concentrations, which may be up to ten times higher than in species which exclude salts. Salt-excreting species can absorb high-salinity water, then excrete excess salts using specialized salt glands located in the leaves.
Mangroves employ two unique methods to increase their chances of reproductive success: vivipary, and water-borne dispersal of young plants.
In plants, vivipary is akin to gestation in animals: seeds or embryos begin to develop before they separate from the parent. In red and black mangroves, seeds partially germinate and develop into seedlings (propagules) while still attached to the parent tree, rather than dropping off to await germination in favorable conditions. White mangroves are not considered to be viviparous; however, germination in this species often occurs during the dispersal period.
Propagules eventually detach from the parent and float in water for a certain period of time, during which germination completes. In red mangroves, this is approximately 40 days; in black mangroves, 14 days; and in white mangroves, 8 days.
There are five distinct types of mangrove forests based on water level, wave energy, and pore water salinity.
Tall mangrove fringe forests occur along protected coastlines and the exposed open waters of bays and lagoons. Red mangroves are typically the dominant species. Daily tidal cycles inundate fringe, as well as export buoyant materials such as leaves, twigs and propagules from mangrove areas to adjacent shallow-water areas. This export of organic material provides nutrition to a wide variety of organisms and provides for continued growth of the fringing forest.
Mangrove overwash islands are also subject to tidal inundation, and are dominated by red mangroves. The major difference between fringe and overwash islands is that the entire island is typically flooded during each tidal cycle. Because overwash islands are unsuitable for human habitation, and because water acts as a barrier to predatory animals such as raccoons, rats and feral cats, overwash islands are often the site of bird rookeries.
Riverine mangrove forests occur on seasonal floodplains in areas where natural patterns of freshwater discharge remain intact. Salinity drops during the wet season, when rains cause extensive freshwater runoff; during the dry season, salinity increases when estuarine waters are able to intrude more deeply into the river system. Nutrient availability is also highest during low-salinity periods, supporting optimal mangrove growth.
Basin mangrove forests are a very common community type, and the most commonly altered. Basin mangrove forests occur in inland depressions which are irregularly flushed by tides. Periodic hypersaline conditions can stunt or kill trees. Black mangroves tend to dominate in basin communities, but certain exotic species including Brazilian pepper and Australian pine are also successful invaders. Basin mangrove forests contribute large amounts of organic debris to adjacent waters, including whole leaves and dissolved organics.
Dwarf mangrove forests occur in areas where lack of nutrients, freshwater, and inundation by tides limit tree growth. Any mangrove species can be dwarfed; trees are generally no taller than three feet (1 meter). Dwarf forests are most common in South Florida, in the vicinity of the Everglades, but can occur throughout mangroves’ range.
Long viewed as inhospitable, marginal environments, mangroves provide numerous environmental benefits.
Often referred to as the bridge between land and sea, mangroves provide their coastlines a critical first line of defense against erosion and storms. Aerial roots trap sediments in and assist in preventing coastal erosion. As storms approach and pass by, branches in the canopies and roots in the water reduce the force of winds and waves, blunting the edge of strong storm impacts.
Mangroves are extremely absorbent carbon sinks—but managed improperly, they stand to be supercharged sources of carbon that could exacerbate global warming processes. The 34 million acres (14 million hectares) of global mangrove forest occupies the equivalent of only 2.5 percent of the Amazon rain forest. As of 2000, that land area was found to hold an estimated 6.4 billion metric tons of carbon, or 8 percent of the Amazon basin’s approximately 76 billion tons of sequestered soil carbon.
But over the next 15 years, deforestation of mangroves for agriculture, aquaculture, timber and human development resulted in the loss of up to 122 million metric tons of soil carbon—equivalent to Brazil’s total carbon dioxide output in 2015 of 447 million tons of CO2.
Mangroves also reduce environmental pollutants. Like many other tidal wetlands, mangrove communities accumulate nutrients – including harmful toxic metals and trace elements. Mangrove roots, invertebrates, epiphytic algae, bacteria and other microorganisms can sequester nutrients in their tissues, often for long periods of time.
Mangroves were once believed to create soils. It’s not hard to see why: their complex aerial root structures trap the abundant leaf litter that falls from the trees each year, as well as any other passing organic detritus carried past by water and wave action. Mangrove forests are among the world’s most highly productive ecosystems.
As living material dies and is decomposed, tidal flushing helps distribute this material to areas where other organisms can use it—creating the conditions for highly biodiverse communities.
Leaf litter, including twigs, propagules, flowers, small branches and insect refuse, is a major nutrient source to consumers in mangrove systems. Generally, leaf litter is composed of approximately 68 to 86 percent leaves, 3 to 15 percent twigs, and 8 to 21 percent miscellaneous material.
Leaf fall in Florida mangroves has been estimated to be 2.4 dry grams per square meter per day on average, with significant variation depending on the site. Once fallen, leaves and twigs decompose fairly rapidly, with black mangrove leaves decomposing faster than red mangrove leaves. Tidal and frequently flooded areas have faster rates of decomposition and export than other areas. Decomposition of red mangrove litter proceeds faster under saline conditions than under freshwater conditions. As the decay process proceeds, nitrogen, protein, and caloric content within the leaf all increase.
Mangroves provide habitat for a wide variety of species. An estimated 220 fish species, 24 reptile species, 18 mammal species, and 181 bird species that all utilize Florida mangroves as habitat during their life cycles. A wide diversity of invertebrates and crustaceans also rely on mangrove prop roots as habitat for at least part of their life cycles. Mangrove roots are particularly valuable juvenile fish habitat—even more so than any seagrass beds that may be nearby, which are notoriously rich habitats for fish. Fish densities in mangroves can be up to 35 times higher than in adjacent seagrass beds.
Many other species make use of mangrove areas for foraging, roosting, breeding, and other activities. Prop roots host sessile (fixed) organisms such as bryozoans, tunicates, barnacles, and mussels. More mobile organisms including crabs, shrimp and snails also use prop roots for feeding and refuge.
The canopy provides shelter to organisms that can migrate from the water’s surface, including lagoonal snails, mangrove crabs, isopods and insects, and of course, birds. The tiny-to-microscopic organisms that dwell in the upper sediments of mangrove forests, or benthic infauna, are also present in surprising abundance given the harsh conditions.
In upland mangrove communities, plants including bromeliads, orchids, ferns and other epiphytes appear in the canopy. Common upland arboreal animals include jays, wrens, woodpeckers, warblers, skinks, anoles, snakes, and tree snails.
In the understory of upland hardwood communities, animals including snakes, rodents, insects and mammals like gray foxes, deer and bobcats reside. Many of these animals enter mangrove forests daily for feeding, but return to the upland community at other times.