Along the barrier island system in east central Florida, sand dunes along the shoreline abound, and can be further subdivided into foredunes, dune crests, swales, and secondary dunes. Foredunes and dune crests absorb most of the punishing action of winds and tides, while swales and secondary dunes that lie protected behind the primary dune, are relatively stable in comparison. 

Inland of the dune system lie the scrub zones and maritime hammocks that have been built upon stable backdunes. Beyond hammocks, the land begins to fall toward the Indian River Lagoon where the mangrove fringe is located. Mangrove areas border both the east and west margins of the lagoon along much of its length. Within the lagoon itself are various submerged aquatic habitats such as seagrass beds and oyster reefs, as well as the many spoil islands which arose as the result of dredging in the lagoon. Beyond the mangrove fringe are the fresh water swamp, hardwood hammock, and upland forest habitats that characterize interior Florida.
 

Topographic profile of representative Indian River Lagoon habitats:  1) mangrove/salt marsh;  2) submerged habitats within the Indian River Lagoon;  3) mangrove fringe;  4) oak forest/maritime hammock;  5) oak scrub;  6)  saw palmetto scrub;  7) sea oats foredune;  8) beach. 

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BEACH:
Beaches lie at the interface between the land and the ocean. East coast beaches in Florida, especially those in the Indian River Lagoon area, are generally dynamic, high-energy, areas.  Key physical processes in beach and dune formation are wave action, erosion, sand accretion by winds, overwash, and the deposition of salt spray. 

The slope of a beach and the shape of its dunes are heavily influenced by tides, wind patterns, storm events and the movement of sand that often accompanies these events. Sand is typically deposited on beaches as waves break on the shoreline and their energy dissipates. Whatever particulates that had been suspended in the wave are deposited on the beach and then dragged down the face of the beach again in the wave’s backwash. Since the energy of backwash tends to be far less than the initial energy of the wave, there is typically a net onshore transport of sand. However, hurricanes and their accompanying storm surges often have the effect of either eroding sands offshore, or overwashing and destabilizing dune systems, redepositing sands inland.

Wave action tends to shape the beach slope as well, with high-energy waves tending to increase the steepness of the slope, and lower-energy waves resulting in flatter beach profiles. On high-energy beaches in the IRL region, beach profiles change seasonally. In summer, waves tend to occur as swells that move sediments up the beach, building berms and providing sands for dunes. However, during fall and winter, the steep waves that accompany storms erode beaches and flatten out the beach profile, depositing eroded sands seaward on longshore bars.

Important biological factors that influence beaches center around the ability of plants to colonize and grow while withstanding the adverse effects of being buried in sand, inundated by sea water, and dry conditions.  Plants occurring on beaches and dunes tend to occupy specific regions according to their individual growth patterns and environmental tolerances. Most beach plants occupy the area closest to the shoreline in the pioneering zone, which extends landward from the wrack line on the upper beach through the dune area. Pioneering species must be highly specialized to tolerate the severe environmental challenges they face. The most successful pioneering species in coastal zones are halophytic, meaning that they are able to thrive under highly saline conditions. Many of these same plants also have high growth rates, with some plants actually stimulated to grow faster as they become buried in sand.

At first glance, beaches may appear to support comparatively few animal species; however, beaches are complex habitats that support many species of animals unique to shorelines, many of them too small to notice. Successful animal inhabitants of beaches include the often overlooked but highly abundant meiofauna that live between sand grains, and the more familiar species of annelid worms that burrow into the substratum. Various bivalve and snail species, as well as many species of small crustaceans such as isopods and amphipods inhabit the wrack line along the shore.

Beyond the meiofauna, many species utilize beaches for nesting and feeding grounds.  Several species of birds, including the black skimmer (Rynchops niger), the least tern (Sterna antillarum), the royal tern (Sterna maxima), and the sandwich tern (Sterna sandvicensis) are known to nest on beaches;  while many other bird species frequent beach areas for feeding.  Six of the 7 species of sea turtles depend on Florida beaches for nesting during the summer. In fact, the Florida coastline is the most important nesting area for sea turtles in the western Atlantic.  Other species that use beaches for feeding include some fishes such as the Florida pompano and several drums that employ the surf zone to prey on animals that either wash out of the sand due to wave action, or come close enough to the shore to be captured. Some mammals are also known to utilize beaches as feeding grounds. Among these are raccoons, feral cats and foxes, which are known to patrol the wrack line at the high water mark and scavenge eggs from sea turtle nests.

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DUNES:
On virtually any barrier island, wind and sand combine to create sand dunes. Dunes play a vital role in protecting coastlines and property. They act as buffers against severe storms, protecting the lands beyond the dune from salt water intrusion, high wind and storm surges. Dunes also act as sand reservoirs, which are important for replenishing coastlines after tropical storms, hurricanes, intense wave action, or other erosional events.

Vegetation colonizing the upper beach and foredune must be well adapted to periodic disturbance, and is generally characterized by the presence of salt-adapted, grassy species. Growth of these colonizing species must keep pace with the rate of sand build-up along the foredune if the plants are to survive. On the foredune, beach pioneers such as railroad vine (Ipomoea pes-caprae) and shoreline sea purslane (Sesuvium portulacastrum) meet the primary species of dune colonizers. Sea oats (Uniola paniculata), a coarse grass that grows as tall as 6 feet and spreads laterally via rhizomes is the principal dune colonizer. Sea oats and 2 other dune-building species, bitter panic grass (Panicum amarum) and beach cordgrass (Spartina patens), have growth patterns in which upward growth is actually stimulated by burial in sand. Subsequent lateral growth in these plants allows for the construction and stabilization of a continuous dune ridge. Other plant species that colonize foredunes must be able to grow at a relatively fast rate to prevent their burial in sand

The dune crest is the area where herbaceous vines and grasses begin to be replaced by shrubby or woody species. Common herbaceous plants of the dune crest include sea ox-eye daisy (Borrichia frutescens), beach sunflower (Helianthus debilis), firewheel (Gaillardia pulchella), and annual phlox (Phlox drummondii). Also common on dune crests are several woody species including sea grape (Coccoloba uvifera), saw palmetto (Serenoa repens), and the invasive Brazilian pepper (Schinus terebinthifolius). Many of the woody species growing on dune crests are often observed to be low-growing and shrubby, despite their growing as robust shrubs or trees in areas inland of the dunes. Much of the reason for this growth habit is due to the well-drained, low nutrient soils of dunes, as well as to the effects of high winds and salt spray. Though most grasses and vines found on dune crests are well adapted to saline conditions, the tender terminal buds of many trees and shrubs growing on dune crests and in swales are killed upon contact with salt spray, resulting in the salt-pruned, windswept canopies commonly seen in the low, stunted trees of Florida’s dune communities.

Swales, located between dunes, gain an increased measure of protection from winds and salt spray as the dune system builds over time. Because swales can be scoured down nearly to the water table, they are able to support freshwater plants, though most plants that grow in swales have some degrees of salinity tolerance as well. Stands of sea grape (Coccoloba uvifera), saw palmetto (Serenoa repens), and the invasive Brazilian pepper (Schinus terebinthifolius) are common woody species on dune crests and in swales.

Backdunes and secondary dunes generally support a wider variety of vegetation than do foredunes. Additionally, the same species that grow as low shrubs or stunted trees on dune crests, grow in backdune areas as well; though in these more protected locales, they are often able to attain full height. Saw palmetto (Serenoa repens), cabbage palm (Sabal palmetto), live oak (Quercus virginiana), and prickly pear cactus (Opuntia stricta), are all common inhabitants of backdunes and secondary dunes.

A number rodents, some of which are becoming increasingly rare, utilize dune habitats. The threatened southeastern beach mouse (Peromyscus polionotus niveiventris) can be found in disjunct populations from Cape Canaveral to Sebastian Inlet. Other rodents that inhabit dunes include the cotton mouse (Peromyscus gossypinus palmarius), cotton rat (Sigmodon hispidus littoralis), and rice rat (Oryzomys palustris).   Rabbits, including the eastern cottontail rabbit (Sylvilagus floridanus), and the marsh rabbit (Sylvilagus palustris paludicola), are also observed on dunes. Several other mammals such as gray foxes (Urocyon cinereoargenteus), raccoons (Procyon lotor), feral pigs (Sus scrofa), and feral cats (Felis catus) also use dunes for feeding.

Many species of shorebirds utilize dunes for feeding; and several species also nest in dune habitats. Among the nesting species are the willet (Catoptrophorus semipalmatus), American oystercatcher (Haematopus palliatus), and Wilson’s plover (Charadrius wilsonia), which prefer nest sites in dune areas with sparse grass or herbaceous cover. The laughing gull (Larus atricilla), Caspian tern (Sterna caspia), and the gull-billed tern (Sterna nilotica) also nest in dunes, but prefer areas with somewhat more dense coverage.

Reptiles are also common inhabitants of dunes. Several species of anoles, among them the green anole (Anolis carolinensis), and the brown anole (Anolis sagrei), are quite common. Gopher tortoises (Gopherus polyphemus), while not plentiful, can often be observed in stable backdune areas. Many different types of snakes also live and feed in dune systems. Eastern diamondback rattlesnakes (Crotalus adamanteus), yellow rat snakes (Elaphe obsoleta quadrivittata), eastern coachwhip snakes (Masticophis flagellum), Florida rough green snakes (Opheodrys aestivus carinatus), and coastal dunes crowned snakes (Tantilla relicta pamlica) all utilize grassy dunes or more woody areas of backdunes as habitat.

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MARITIME HAMMOCKS:
Maritime hammocks, also known as coastal strand, are characterized as narrow bands of forest that develop almost exclusively on the stabilized backdunes of barrier islands, inland of primary dunes and scrub. Generally dominated by species of broad-leaved evergreen trees and shrubs, maritime hammocks are climax communities influenced heavily by salt spray. Soils are predominantly composed of either sand or peat. Many factors influence whether particular species will be successful colonizers of the maritime forest. Strong winds, low nutrients, unpredictable supplies of freshwater, erosion, sand-blasting, storm exposure, sand migration, and overwash from the ocean during storm events, are all major influences; however, tolerance to salt spray is generally cited as the key factor controlling vegetative cover in maritime forests.

Trees closest to the ocean are subject to onshore winds carrying sand and salt spray, which acts not only to prune terminal buds in the canopy top, but also encourages growth of lateral buds, producing over time, the familiar windswept shape of maritime forest canopies. Streamlining of the canopy profile assists growth by both deflecting damaging winds up and over the forest and sheltering the understory from large temperature fluctuations, reducing warming of the soil during the day, and preventing heat loss at night. Additionally, because trees on the windward edges of the forest show increased growth in their lateral buds, they are somewhat denser overall than more interior trees. As winds carrying salt spray blow across the dense canopy, salts are deposited at the outer fringes of the forest, while the interior trees are protected.  This feature allows trees in the interior forest to assume characteristic heights and growth patterns resembling those of mainland forests.

The vegetative composition of maritime forests is diverse, and depends heavily on prevailing physical conditions.  There are 3 major types of upland broad-leaved forests of barrier islands:  temperate broad-leaved forest, also known as evergreen forest; southern mixed hardwood forest; and tropical forest. Temperate broad-leaved forests are dominated by Quercus virginiana (live oak), and Sabal palmetto (sabal palm) communities. Southern mixed hardwood forests are dominated by Magnolia grandiflora (Southern magnolia), Ilex opaca (American holly), Cornus florida (flowering dogwood), Carya glabra (pignut hickory), and Fagus grandiflora (American beech). Tropical forests are dominated by both evergreen and deciduous species such as Mastichodendron foetidissimum (mastic), Eugenia spp. (stoppers), Lysiloma latisiliqua (wild tamarind), and Bersera simaruba (gumbo limbo).

Many different animal species inhabit Florida’s barrier island communities. In maritime hammocks, insects, small mammals, reptiles and birds dominate the fauna. Common inhabitants include wading birds such as great blue herons (Ardea herodias), great egrets (Casmerodius albus), snowy egrets (Egretta thula), little blue herons (Egretta caerulea), tricolored herons (Egretta tricolor), night herons (Nycticorax spp.), brown pelicans (Pelicanus occidentalis), various ducks, warblers, and others. Birds of prey such as red-shouldered hawks (Buteo lineatus), Cooper’s hawks (Accipiter cooperii), sharp-shinned hawks (Accipiter striatus), and bald eagles (Haliaetus leucocephalus), also utilize hammocks for feeding, roosting and nesting. Small mammals such as eastern cottontails (Sylvilagus palustris), mice (Mus spp.), Norway rats (Rattus norvegicus); and larger mammals such as river otters (Lontra canadensis), and wild boar (Sus scrofa), may thrive in hammock habitats. Reptiles include softshelled turtles (Frionyx ferox), gopher tortoises (Gopherus polyphemus), cottonmouth snakes (Agkistodon piscivorus), southern black racers (Coluber constrictor priapus), Atlantic saltmarsh snakes (Nerodia spp.), eastern diamondback rattlesnakes (Crotalus adamantus), indigo snakes (Drymarchon corais couperi), as well as a variety of skinks and lizards which prey on the abundant insect, frog, and small mammal population.

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MANGROVES:
The term mangrove is loosely used to describe a wide variety of often unrelated tropical and subtropical trees and shrubs which share common characteristics.  Globally, more than 50 species in 16 different families are considered mangroves. In Florida, there are 3 main species of true mangroves:  the red mangrove (Rhizophora mangle), the black mangrove (Avicennia germinans) and the white mangrove (Laguncularia racemosa).  The buttonwood (Conocarpus erectus) is often considered a fourth mangrove species, however, it is classified as a mangrove associate because it lacks any morphological specialization common in true mangrove species, and because it generally inhabits the upland fringe of many mangrove communities.   

Red mangroves, Rhizophora mangle, dominante the shoreline from the upper subtidal to the lower intertidal zones and are distinguished from other mangroves by networks of prop roots that originate in the trunk of the tree and grow downward towards the substratum.  Red mangroves may attain heights of 80 feet or more, with leaves a glossy, bright green at the upper surface, with somewhat more pale undersides. Trees flower throughout the year, peaking in spring and summer. Seedlings, called propagules, remain attached to the parent tree and grow as long, pencil-like structures that  may reach 12 inches in length before they drop from the parent tree.  

Black mangroves typically are found growing immediately inland of red mangroves and may reach 70 feet in height.  They are characterized by their conspicuous pneumatophores, vertical branches that may extend upward in excess of 8 inches from cable roots lying below the soil.  Pneumatophores develop into extensive networks of fingerlike projections that surround the bases of black mangroves to provide them with proper aeration.  The leaves of black mangroves tend to be somewhat narrower than those of red mangroves and are often found encrusted with salt.  Black mangroves flower throughout spring and early summer, producing 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, generally reaching 50 feet in height.  Their leaves are oval in shape, and somewhat flattened.  Trees flower in spring and early summer, and produce small propagules which measure only 2.5 inches in diameter.

Mangrove forests occur brackish swamps along 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.   Mangroves are damaged under conditions where temperatures fluctuate more than 50°F within short periods of time, or when they are subject to freezing conditions for even a few hours. 

Mangroves perform a vital ecological role providing habitat for a wide variety of species.  The prop root zone provides sessile filter feeding organisms such as bryozoans, tunicates, barnacles, and mussels with an ideal environment.  Mobile organisms such as crabs, shrimp, snails, boring crustaceans, polychaete worms, many species of juvenile fishes, and other transient species also utilize the prop root zone of mangroves as both a refuge and feeding area.  The arboreal canopy is used by species able to migrate from the water’s surface to the mangrove canopy.  Snails such as the coffee bean snail (Melamphus coffeus), angulate periwinkle (Littorina anguilifera), and ladderhorn snail (Cerithidea scalariformis) are among the most common of the invertebrate species in mangrove canopies.  Also common are many species of crustaceans such as the common mangrove crabs Aratus pisoni, Goniopsis cruentata, Pachygrapsus transverses, and Sesarma spp., the isopod Ligea exotica, and many species of insects and birds. 

In addition to providing vital nursery and feeding habitat to the animal community, mangroves also assist in shoreline protection and stabilization.  Prop roots of red mangroves trap sediments in low-energy estuarine waters, and thus assist in preventing coastal erosion.  Mangroves also assist in buffering the coastal zone when tropical storms and hurricanes strike.  Because mangroves encounter damaging winds and waves before inland areas do, the branches in their canopies, and their many prop roots create friction that opposes and reduces the force of winds and waves.  Thus, coastlines are protected from severe wave damage, shoreline erosion and high winds (Gillet1996 In: Feller 1996).

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SEAGRASSES:
Seagrasses are a type of submerged aquatic vegetation (SAV) have evolved from terrestrial plants and have become specialized to live in the marine environment. Like terrestrial plants, seagrasses have leaves, roots, conducting tissues, flowers and seeds, and manufacture their own food via photosynthesis. Unlike terrestrial plants, however, seagrasses do not possess the strong, supportive stems and trunks required to overcome the force of gravity on land. Rather, seagrass blades are supported by the natural buoyancy of water, remaining flexible when exposed to waves and currents.

Though often confused with marine algae; seagrasses are more closely related to terrestrial plants.  Like plants that grow on land, seagrassses are structurally complex and specialized, while algae are relatively simple and unspecialized.  While algae possess only a tough holdfast that assists in anchoring the plant to a hard surface, seagrasses possess true roots that not only hold plants in place, but also are specialized for extracting minerals and other nutrients from the sediment. In marine algae, all cells possess photosynthetic structures capable of utilizing sunlight to produce chemical energy. In seagrasses, however, specialized structures called chloroplasts, which occur only in the leaves, perform photosynthesis and supply energy to the plant.  Further, while algae are able to take up minerals and other nutrients directly from the water column via diffusion,  seagrasses transport minerals and nutrients in vein-like tissues called xylem and phloem. Finally, while most algae lack specialized reproductive structures, most seagrasses have separate sexes and produce flowers and seeds, with embryos developing inside ovaries.

Within seagrass communities, a single acre of seagrass can produce over 10 tons of leaves per year. This vast biomass provides food, habitat, and nursery areas for a myriad of adult and juvenile vertebrates and invertebrates. Further, a single acre of seagrass may support as many as 40,00 fish, and 50 million small invertebrates. Because seagrasses support such high biodiversity, and because of their sensitivity to changes in water quality, they have become recognized as important indicator species that reflect the overall health of coastal ecosystems.

Seagrasses perform a variety of functions within ecosystems, and have both economic and ecological value. The high level of productivity, structural complexity, and biodiversity in seagrass beds has led some researchers to describe seagrass communities as the marine equivalent of tropical rainforests. While nutrient cycling and primary production in seagrasses tends to be seasonal, annual production in seagrass communities rivals or exceeds that of terrestrially cultivated areas.

As habitat, seagrasses offer food, shelter, and essential nursery areas to commercial and recreational fishery species, and to the countless invertebrates that are produced within, or migrate to seagrasses. The complexity of seagrass habitat is increased when several species of seagrasses grow together, their leaves concealing juvenile fish, smaller finfish, and benthic invertebrates such as crustaceans, bivalves, echinoderms, and other groups. Juvenile stages of many fish species spend their early days in the relative safety and protection of seagrasses. Additionally, seagrasses provide both habitat and protection to the infaunal organisms living within the substratum as seagrass rhizomes intermingle to form dense networks of underground runners that deter predators from digging infaunal prey from the substratum. Seagrass meadows also help dampen the effects of strong currents, providing protection to fish and invertebrates, while also preventing the scouring of bottom areas. Finally, seagrasses provide attachment sites to small macroalgae and epiphytic organisms such as sponges, bryozoans, forams, and other taxa that use seagrasses as habitat.

Economically, Florida’s 2.7 million acres of seagrass supports both commercial and recreational fisheries that provide a wealth of benefits to the state’s economy. Florida’s Department of Environmental Protection (FDEP) reported that in 2000, Florida’s seagrass communities supported commercial harvests of fish and shellfish valued at over 124 billion dollars. Adding the economic value of the nutrient cycling function of seagrasses, and the value of recreational fisheries to this number, FDEP has estimated that each acre of seagrass in Florida has an economic value of approximately $20,500 per year, which translates into a statewide economic benefit of 55.4 billion dollars annually. In Fort Pierce, Florida alone, the 40 acres of seagrass in the vicinity of Fort Pierce Inlet are valued at over $800,000 annually. When projected across St. Lucie County’s estimated 80,000 acres of seagrass, this figure increases to 1.6 billion dollars per year.

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OYSTER REEFS:
Oyster reefs, often referred to as oyster bars, are common submerged habitats in the southern United States. Oyster reefs in Florida are found in nearshore areas and estuaries of both coasts, but grow especially vigorously near estuarine river mouths where waters are brackish and less than 10 meters deep. Within the Indian River Lagoon, oyster reefs may be found in the vicinity of spoil islands and impounded areas. In addition to being commercially valuable, oyster reefs serve a number of important ecological roles in coastal systems: providing important habitat for a large number of species; improving water quality; stabilizing bottom areas, and influencing water circulation patterns within estuaries.

Oyster reefs are built primarily by the eastern oyster, Crassostrea virginica, through successive reproduction and settlement of larvae onto existing reef structure. Oysters in Florida spawn from late spring through the fall. The planktonic larvae that develop require a hard substratum to settle upon in order to complete development to the juvenile stage, and prefer to settle on the shells of other oysters. Thus, over time, continued settlement and subsequent growth of generations of oysters may form massive reef structures consisting of staggering numbers of individuals. Luntz (1960), estimated that 5,895 oysters, the equivalent of 45 bushels, occurred within a single square yard of oyster reef.

As successive generations of oysters settle and grow, reefs become highly complex, with many structural irregularities and infoldings that provide a wealth of microhabitats for many different species of animals. Wells (1961) listed 303 different species utilizing oyster reef as habitat in North Carolina. Common Indian River Lagoon species associated with oyster reefs include bivalves such as the hard clam (Mercenaria mercenaria) and bay scallop (Argopecten irradians concentricus); space competitors such as the scorched mussel (Brachidontes exustus), ribbed mussel (Geukensia demissa), the jingle shell (Anomia simplex), and barnacles of the Balanus genus; gastropod mollusks such as the conchs (Melongena spp. and Strombas spp.) and rocksnails (Thais spp.); numerous sponge species; flatworms; polychaete worms; amphipods; isopods; shrimp; and fishes such as blennies, gobies, spadefish, snappers, drum, and seatrout, among others.

Beyond providing smaller organisms with habitat, oyster reefs also provide food to a wide variety of secondary consumers. Many species of fish prey upon oyster reef associates; while others such as the black drum (Pogonias cromis) and cow-nosed ray (Rhinoptera bonasus) prey upon oysters themselves. Other species that utilize oyster reefs for foraging and feeding include the xanthid crabs, also known as mud crabs; swimming crabs of the genus Callinectes; mollusks such as the thick lipped oyster drill (Eupleura caudata), the sharp-rib drill (E. sulcidentata), the Atlantic oyster drill (Urosalpinx cinerea), the Tampa drill (U. tampaensis), the knobbed whelk (Busycon carica), the lighthire whelk (B. contrarium), and the pear whelk (B. spiratum pyruloides); flatworms such as oyster leeches (Stylochus spp.); boring sponges (Cliona spp.); and annelid worms (Polydora spp.).

Oyster reefs also contribute to improved water quality in areas where they occur. Oysters are filter feeders that strain microalgae, suspended particles of organic matter, and possibly dissolved organic matter from the water column over their gills in order to feed. Under optimal temperature and salinity conditions, a single oyster may filter as much as 15 liters of water per hour, up to 1500 times its body volume. Spread over an entire reef, for an entire day, the potential for oysters to improve water clarity is immense. Additionally, since oysters are sessile, and bioaccumulate some potential toxins and pollutants found in the water column, they have been used to assess the environmental health of some areas.

Over-harvesting, as well as persistent diseases such as MSX and Dermo have taken a devastating toll on many oyster populations along the east and Gulf coasts. In recent years, oyster reef restoration has been a concern for resource managers all along the East Coast of the United States, but especially in areas where oyster harvesting has historically been commercially important. In the late 1800s, for example, annual oyster harvests in the southeastern United States routinely topped 10 million pounds per year, and peaked in 1908 when the harvest was nearly 20 million pounds. However, annual harvests since that time have declined steadily. Today, annual harvests for oysters in the southeast averages approximately 3 million pounds per year.

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MOSQUITO IMPOUNDMENTS:
Florida’s mangroves and salt marshes have historically been problem areas in one important respect:  they are preferred breeding habitat for salt marsh mosquitoes (Ochlerotatus taeniorrhynchus and O. sollicitans).  These mosquitoes are an important nuisance species that affect the health of humans and domestic animals.  Salt marsh mosquitoes do not reproduce by laying their eggs in standing water.  Rather, they deposit eggs in the moist soils of high marsh above the water line in tidal wetlands (Provost 1976).  Eggs will remain dormant, often for long periods of time, until water levels rise in response to rains or tides.  Eggs hatch in the water and undergo several larval stages before developing into adult mosquitoes within 5 days. 

Mosquito control impoundments are areas of salt marsh or mangrove forest that have been diked to allow control of water levels for purposes of mosquito control without the use of pesticides.  Within the dikes, perimeter ditches are flooded artificially in order to raise water levels and reduce the amount of breeding habitat available to salt marsh mosquitoes.  Simply keeping water levels artificially high reduces the available area in which mosquitoes may lay eggs. 

The first impoundments in Florida were built in Brevard County in 1954, with other counties soon following.  By the 1970’s, in excess of 40,000 acres of Florida’s coastal wetlands had been impounded (Rey and Kain 1990).  The majority of impoundments were constructed at the mean high water level and then flooded year round, closed off from adjacent estuarine waters.  Some, however, were allowed to drain during the winter months, but were flooded again as mosquito breeding season approached. 

Although impoundment for mosquito control is an effective method of controlling mosquito populations, there are often severe environmental impacts on impounded wetlands isolated from adjacent estuaries.  Particularly important are issues of water quality degradation, isolation of important fishery species from critical nursery habitats, interruption of nutrient flow between wetlands and estuarine waters, creation of unnaturally high water levels, and excessively saline (hypersaline) conditions that may develop in closed impoundments when evaporation of water occurs. 

Fish species were greatly affected by closed impoundments, with numbers of some species being significantly reduced in species that utilized salt marsh or mangrove areas as nursery grounds (Harrington and Harrington 1961, Snelson 1976, Gilmore et al. 1982, Rey et al 1990).  Tarpon (Megalops atlanticus), ladyfish (Elops saurus), common snook (Centropomus undecimalis), mullet (Mugil cephalus), and other species important to commercial and recreation fisheries were adversely impacted by closed impoundments.  Marine invertebrates were also impacted by isolation of impounded wetlands, with biodiversity and species abundance changing dramatically in some areas.  In some areas, the invertebrate community became more characteristic of freshwater wetlands than marine or estuarine wetlands (Brockmeyer et al. 1997).

Beginning in 1974, seasonal impoundment was combined with active water management.  This strategy of allowing for impoundments to be adequately flushed by tides not only controlled salt marsh mosquitoes, but also helped to retain black mangroves and other vegetation, and allowed the return of juvenile fishes to nursery areas unavailable to them in closed impoundments.  This management strategy is currently referred to as Rotational Impoundment Management (RIM). 

Under RIM, estuaries retain many of their natural functions, and their primary productivity can rival that of unaltered wetlands (Lahmann 1998, Rey et al. 1990b).  Culverts remain open between the impoundment and the estuary from October to May to allow water exchange and use of impoundments by transient fish species and invertebrates.  Then, during the summer months, culverts are closed and impoundments flooded to the minimum levels needed to prevent oviposition in salt marsh mosquitoes.  Low areas of the surrounding dike, called spillways, insure that water levels do not exceed prescribed levels, thus preventing injury to vegetation.  RIM has proven to be an effective strategy for controlling mosquitoes while minimizing serious environmental impacts to estuaries.

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