Lying behind the barrier islands is a 4- to 6-mile band of marshland comprising about 393,000 acres (Spinner 1969). Nearly 286,000 acres of this is covered by a single species of marsh grass, known as saltmarsh or smooth cordgrass. (See Table 12 for scientific names of marsh plants not given in the text.) The remaining 107,000 acres consist of several other types of salt, brackish, and freshwater marshes.
Formation and sediment characteristics
Tidal marshes are formed in conjunction with barrier island development. When sea levels rise, dune and beach ridges become partially submerged. Water filling the trough between these structures and the mainland or other barrier islands is subject to relatively less energy perturbation than open waters, and clay and silt sediments suspended in the water are deposited, leaving a mud-type substrate. In time, salt-tolerant marsh plants such as cordgrass stabilize the area.
Deposition continually occurs on the tidal marsh, but at a very slow rate. The added weight of the sediment contributes to submergence and the accumulation of additional sediments (Hoyt 1968d). As flood tides rise above creek banks and inundate the marsh floor with a shallow layer of water, the energy maintaining the sediments in a suspended state is reduced, and the sediments drop out of suspension. As the tides recede, some sediments are resuspended, but this amount averages less than that of flood tides. The receding tide waters form an extensive drainage system of tidal creeks and rivers (Fig. 26). Some tidal channels erode deeply enough to expose Pleistocene sands and frequently penetrate barrier island deposits (Hoyt et al. 1966).
Four sources contribute to the suspended material that is deposited in the marsh: (1) continental shelf, (2) mainland rivers, (3) the marsh itself, and (4) organic deposits (Levy 1968). Continental rivers are the principal source of this material which is distributed and sorted by longshore currents, waves, and tidal currents (Neiheisel and Weaver 1967).
Superficial deposits are up to 6 ft deep. Cross-bedding related to channeling is common (Henry and Hoyt 1968). Total depths of recent marshes range from 30 to 50 ft (Hoyt et al. 1964).
Grain size of sediments in the marsh range from clay to fine sand. Large amounts of sand from reworked sandy sediments are locally common. Teal and Kanwisher (1961) have analyzed the salt marsh substratum for percentage of sand, silt, clay, organic matter, and roots. Their results show that higher marsh sites have more sand and less organic matter than lower marsh sites. When saturated on high tides, the upper layers of marsh mud contain 50-70% (wet weight) water (Teal and Kanwisher 1961).
Two distinct layers of salt marsh sediments are revealed in cross-section. The aerated and leached sediments of the upper few centimeters are brown. The sediments below are black and rich in reduced organic end products including hydrogen sulfide, methane, and ferrous compounds.
Although marsh soils are normally neutral or slightly alkaline, some of them have the potential to become extremely acid under certain conditions. When marsh soils containing large amounts of organic matter are regularly flooded and anaerobic conditions exist, the sulfates in sea water are reduced and precipitated as sulfides. These conditions occur most commonly at the mouths of larger rivers, in brackish water marshes thickly vegetated with species such as big cordgrass and reed cane. So long as anaerobic conditions prevail, soil pH remains high. But when the soils are exposed to prolonged aeration (as in drained areas, dikes, or spoil areas), the sulfides are oxidized, and one of the products is sulfuric acid. The soil becomes extremely acid (pH may drop to as low as 2.0) and no plant growth can occur. Such soils are then called "cat clays." The acidity is so severe that it is not feasible to correct the condition by liming and the area may remain barren for many years (Edelman and Staveren 1958; Fleming and Alexander 1961).
The U.S. Fish and Wildlife Service has classified the wetlands of the United States into 20 types (Shaw and Fredine 1956). Six of these are coastal marshes occurring in Georgia. These are characterized in Table 10. The extent of these types of marsh on the Georgia coast is as follows (Spinner 1969):
More specific marsh types may be recognized according to plant associations. Many factors contribute to the determination of plant composition of coastal marshes. These include water levels and fluctuations, salinity, type of substratum, acidity, available nutrients, and fire, among others. Salinity and inundation are most important, and gradients or zonations of vegetation related to these factors are commonly evident. Intolerance for salinity and inundation prevent most species from occupying tidal salt marshes (Table 11), and species diversity is greatest in shallow, freshwater marshes. The harsh combination of critical limiting factors in the marsh produces conditions allowing a few tolerant species such a competitive advantage that they develop pure stands. On the Georgia coast, the most extensive of these monospecific marshes are smooth cordgrass, needlerush, and giant cutgrass.
TABLE 10. Coastal wetland types occurring on the Georgia coast.a
TABLE 11. Relative salt tolerance of some plants important in the coastal marshes of Georgia.a
Freshwater and brackish marshes
Freshwater marshes occur primarily near the mouths of larger mainland streams and are most extensive at the mouth of the Altamaha River. They may extend for some distance up the rivers before being replaced by cypress-gum or hardwood swamps. Much of the area now covered by freshwater marsh was cypress swamp before it was cleared and diked for rice culture. Shallow fresh water marshes contain a variety of species including cattails, several bulrushes, smartweeds, aneilema, arrowhead, arrow arum, and others. The deeper fresh water marshes are more extensive, occupying about 25,000 acres along the Georgia coast. In many areas this marsh type is comprised almost exclusively of giant cutgrass. Stands of sawgrass occur intermittently. Around the deeper margins of the marsh, stands of cattail are common and wild rice occurs in sporadic stands. In the deeper creeks and potholes, submersed and floating-leaved plants are dominant.
As salinities increase to brackish conditions (about 0.5-2%), giant cutgrass is replaced primarily by big cordgrass and, to a lesser extent, by salt marsh bulrush.
Most indigenous plants cannot survive salinities approaching sea strength; they are replaced in the salt marsh by a few species with high salinity tolerances. Included in this group are smooth cordgrass, needle rush, saltgrass, glasswort, salt meadow cordgrass, and sea oxeye (Borrichia frutescens).
The salt marshes of the southeastern states have been the subject of much study. Studies of plant associations in the salt marshes of North Carolina have been reported by Wells (1928), Reed (1947), Bourdeau and Adams (1956), and Adams (1963). Kurz and Wagner (1951) reported on salt marsh vegetation in Florida and at Charleston, S.C. General treatments of salt marshes include those of Townsend (1925), Ragotzkie et al. (1959), Chapman (1960), and Teal and Teal (1969).
Of the local marsh plants only smooth cordgrass is adapted to both the salinities and the tidal fluctuations of the low tidal marsh. Teal and Teal (1969) describe the adaptive mechanisms in smooth cordgrass that enable it to exist in the marsh. Root membranes prevent the entry of much salt into the plant, and the cells selectively absorb sodium chloride to maintain osmotic pressure and prevent plasmolysis. Special glands on the leaves excrete excess salt.
Ducts in the stems carry oxygen to the roots of the plant where it is used in the oxidation of iron sulfides to soluble iron compounds that are used by the plant. The high iron requirement of smooth cordgrass is one of the factors that restrict it to the salt marsh (Adams 1963).
Smooth cordgrass and most other salt marsh plants grow best in fresh water, at least under laboratory conditions (Taylor 1939). But they do not commonly occur in freshwater marshes, partly because they are unable to compete with more vigorous species. Their tolerance to salt stress enables them to persist in the salt marsh free of competition. Thus, zonation in the salt marsh is primarily related to elevation as it determines frequency, depth and duration of inundation, and soil salinity.
The zones of the salt marshes of the southeastern United States have been classified in various ways (Wells 1928; Penfound 1952; Adams 1963; Teal 1958; Stalter and Batson 1969, and others). Following is a description of the usual gradient in vegetation in Georgia salt marshes, proceeding generally from the tidal creek landward (Fig. 27). Unless otherwise indicated, Spartina used hereafter refers to smooth cordgrass.
A portion of the creek banks exposed at every low tide is devoid of vegetation. These banks are composed of sand, mud, or oyster shells. Slumping occurs along some banks and results in relatively large deposits being exposed directly to tidal currents. The upper slopes of the banks are vegetated with smooth cordgrass. The cordgrass grows tallest here (3-10 ft), and Teal (1958) designated this zone the "tall Spartina edge marsh." The cordgrass grows to about 3 ft on top of the levees. Teal (1958) called this zone the "medium Spartina levee marsh."
Behind the levees the marsh is thickly vegetated with smooth cordgrass and is covered by every tide for several hours each day. Sand content is 0-10% (Teal 1958). Teal (1958) called this area the "short Spartina low marsh." Stalter and Batson (1969) designated this zone together with the creek banks and levees as the "low low marsh."
With increasing elevation toward the edge of the marsh, inundation is to a lesser depth and is for a shorter period of time. Sand content is from 10 to 70% (Teal 1958), and salinity of the soil beneath the cordgrass progressively increases (Penfound and Hathaway 1938; Bourdeau and Adams 1956; Stalter and Batson 1969). A dwarf form of smooth cordgrass occurs in this zone that is probably a genetic variant, environmental conditions reinforcing the differences between the two forms (Broughton and Webb 1963; Stalter and Batson 1969). Teal (1958) termed this zone the "short Spartina high marsh," and Stalter and Batson (1969) called it the "high low marsh."
At elevations where the marsh is flooded for only about one hour each day, the dwarf Spartina gives way to other species: notably glasswort, saltgrass, sea oxeye, and sea lavender (Linomium carolinianum). Sand content is 85-95% in this zone (Teal 1958). This zone is called the "Salicornia-Distichlis marsh" (Teal 1958) or the "low high marsh" (Stalter and Batson 1969).
Sandy, unvegetated areas are locally common in the higher portions of the marsh. These areas are commonly called salt barrens or salt pans. Subject to infrequent flooding and rapid evaporation, the barrens are too saline to support vegetation. Glasswort occurs around the margins and grades into saltgrass.
Salt meadow cordgrass (which, farther north on the Atlantic coast, forms extensive meadows that are harvested for hay, straw, and upholstery stuffing) occurs on the Georgia coast only at the rim of the marsh in a zone that is flooded only a few times each week. Salinity decreases abruptly in this zone (Bourdeau and Adams 1956). Salt meadow cordgrass occurs only where relatively low salinities prevail. Other plants that characterize this zone are high tide bush (Iva frutescens), groundsel tree (Baccharis halimifolia), and salt myrtle (Baccharis glomeruliflora).
Needlerush marsh occurs at slightly higher elevations that are infrequently flooded, especially where salinity is lower. It occurs as a narrow zone around the salt marsh on the islands, and forms extensive pure stands in many areas adjacent to the mainland.
Animals that complete their life cycles in the salt marsh have morphological, physiological, and behavioral adaptations for coping with extremes of salinity, inundation, and exposure in the marsh, and the activity cycles of most species are keyed to the tides.
Following is a discussion of some of the animal forms occurring in the salt marsh: their distribution, abundance, and ecological relationships. This information was obtained from published material and from our own field observations. The discussion is extended to include rails and waterfowl in the freshwater marshes; otherwise, it is restricted to animals of the salt marsh.
1. Mammals. The harshness of the salt marsh restricts the numbers of resident mammals to a few species. Raccoons are one of the most abundant mammals, and marsh rabbits are common along the edges of the marsh adjacent to high ground. Mink are more common than otter, but both of these carnivores are seen infrequently. The rice rat is common along the levees of tidal creeks.
Mammals that have adapted to living or feeding in the marsh are highly mobile. Racoons feed in the marsh at low tide and are inactive at high tide regardless of whether it is night or day. They retreat to the higher ground on the mainland or islands at high tide or construct a bed of cordgrass above the high-tide level. In freshwater marshes nearby, their activity is mostly nocturnal (Ivey 1948). Mink and otter are well adapted to feeding in the aquatic environment of the marsh, but they often retire to higher ground for nesting and denning purposes. Kale (1965) stated that mink may spend much of their life in the marsh. He observed that they constructed beds of dead cordgrass on the high ground of the marshes or used hollow tree trunks washed into the marsh. The rice rat is well adapted to spend a 24-hour day in the salt marsh. The rat constructs its own nest in the tall Spartina or occupies an abandoned nest of the long-billed marsh wren (Sharp 1967).
Food for marsh mammals is abundant but is of limited diversity. Raccoons are known to feed heavily upon fiddler and squareback crabs (Uca spp. and Sesarma spp.), two of the most abundant animals in the marshes. They also prey on the eggs of diamondback terrapins (Coker 1906), clapper rails (Oney 1954), and marsh wrens (Kale 1965). They are not considered a limiting factor on wren populations (Kale 1965). The food habits of marsh mink are not known for the coastal region of Georgia. Teal and Teal (1969) state that clams and crabs are the principal foods of marsh mink in autumn, and Wilson (1954) reported that fish occurred in 61% of the digestive tracts of mink from marshes of northeastern North Carolina. Oney (1954) listed the mink as a predator on clapper rail eggs. Otters feed mostly on fish. Wilson (1964) found fish in over 90% of otter digestive tracts and fecal samples from northeastern North Carolina.
The rice rat is preferentially carnivorous. Studies on Sapelo Island (Sharp 1962, 1967) reveal that during the summer the rats feed primarily on fiddler and squareback crabs and the larvae of the rice borer, with other insects occurring as incidental food items. In the fall, crabs are the major food items, but small amounts of seed and fibrous portions of smooth cordgrass also are used. Kale (1965) attributed considerable egg loss and nestling mortality of the long-billed marsh wren to predation by rice rats. Sharp (1967), however, found no remains of wrens in 22 rats examined, and he suggested that the wren is only an incidental food item during the summer.
Marsh rabbits are characteristic of high marsh and normally do not occur in tidal salt marsh (Tomkins 1955; Teal 1962). Marsh rabbits are strictly herbivorous, but their exact food habits are unknown.
Harvest of marsh furbearers such as raccoon, mink, and otter generally is low in the coastal area because of the low market value of pelts, especially from this area. There are an estimated 300 fur trappers (Spinner 1969). Raccoons are taken for food by a small number of hunters; there is no closed season. The only management for mink and otter is the regulation of harvest by an established trapping season.
2. Birds. Kale (1965) stated that three species of birds were intimately associated with the salt marsh community in coastal Georgia. These are the long-billed marsh wren, the clapper rail or marsh hen, and the seaside sparrow. Kale (1965) studied the ecology and bioenergetics of the marsh wren, and Oney (1954) studied the biology, distribution, and limiting factors of the clapper rail in Georgia. The seaside sparrow has not been intensively studied in Georgia.
The following discussion of the long-billed marsh wren is based primarily on Kale's (1965) study in the vicinity of Sapelo Island. During the breeding season the marsh wren establishes territories averaging 100 m2 in the tall Spartina edge marsh. Singing males establish breeding territories, and nesting and brood rearing take place within the territory. The peak of the breeding season is May to July. Predation is the primary cause of mortality of eggs and young. Major predators are rice rats, raccoons, and mink. However, mortality factors are minor limiting factors on marsh wren populations; Kale concluded that the highly developed territorial and colonial behavior of the marsh wren was the principal factor limiting population size. A discussion of food habits and bioenergetics of the wren is included in the section on food webs and energy flow.
The clapper rail is a common game bird in coastal salt marshes. Clapper rails begin nesting in the medium Spartina marsh early in April. Oney (1954) found that some nests were inundated by as much as 19 inches of water during high tides and hatched successfully. Teal and Teal (1969) also reported that the eggs of the marsh hen can withstand tidal inundation. The major nest predators probably are raccoons, mink, and crows. The major food is the squareback crab, which occurs in the soft mud areas of the tidal streambanks. Other foods include fiddler crabs and periwinkle snails (Littorina irrorata). During the winter months when the crabs become inactive, clapper rails shift to other foods. Richard Heard (pers. comm.) observed that rails feed on snails (Melampus sp.) during cold weather.
The clapper rail is hunted by a small number of hunters5000 in 1968 (Spinner 1969)during the high tides of September-November. Prior to 1948 the rail harvest approached 86,000 birds (Oney 1954). However the use of outboard motors in rail hunting was prohibited by Federal law beginning in 1948 and rail hunting declined sharply. Oney (1954) recommended that outboard motors be allowed in rail hunting.
The king rail, a close relative of the clapper rail, occurs in the freshwater and brackish marshes. Meanley (1969) reported that king rails nested in giant cut grass and softstem bulrush along tidal canals on the Savannah National Wildlife Refuge. The fiddler crab (Uca minax) of the latter habitat is a preferred food of the rail. King rails feed on freshwater insects, fish, crustaceans, and amphibians that are abundant in mats of alligator-weed in the canals and ditches (Meanley 1969).
The willet, although common on beaches, is more common in the salt marsh. It feeds on fiddler and squareback crabs and periwinkle snails in the short Spartina low marsh (Tomkins 1965b).
The seaside sparrow has not been studied in coastal Georgia. Kale (1965) observed that in his study area the seaside sparrow fed primarily on the ground. Howell (1932) listed small crabs, pelecypods, gastropods, dragonflies, grasshoppers, spiders, and beetles as foods.
Wading birds use the marsh as a feeding and nesting area. Five species frequent the marsh in large numbers throughout the year. Other species occur seasonally in the marsh, especially during the summer. Several large rookeries are located on some of the marsh islands (Fig. 34, Appendix 4).
Great blue herons, little blue herons, common egrets, snowy egrets, and Louisiana herons are common residents of the salt marshes. During high tides, they feed in shallowly flooded marsh grass. Snowy and common egrets are most frequently encountered in the marsh at low tide. Cattle egrets have been observed feeding on fiddler crabs near tidal creeks; they feed more commonly near agricultural areas inland.
During the summer of 1970, a large wading bird rookery was discovered on a marsh island in the Altamaha River (Fig. 34). This rookery contained, in addition to large numbers of white ibis, 75-100 nesting pairs of glossy ibis. Hebard (1950) reported glossy ibis nesting in Georgia, but Burleigh (1958) did not acknowledge his record, and Sciple (1963) considered it too "tenuous" to accept. The observation of glossy ibis nesting in Georgia in 1970 fills the hiatus in the breeding distribution for this species as noted by Teal (1959).
Georgia is within the Atlantic flyway of migratory waterfowl, and the coastal region is on the southern portion of the flyway. The major species of ducks occurring in this flyway include mallard, black, pintail, gadwall, baldpate, shoveler, green-winged teal, scaup, ring-necked, and canvasback. All of these species occur in varying numbers along the Georgia coast. The number of waterfowl wintering on the Georgia coast has been affected by refuges farther north where many waterfowl spend the winter instead of migrating farther south to their natural wintering areas. For example, very few Canada geese now winter on the Georgia coast, and populations of wintering mallards are significantly reduced by northern refuges.
Most waterfowl management on the Georgia coast is on lands owned by state and Federal wildlife agencies. Despite the fact that several areas in private ownership offer excellent opportunities for attracting waterfowl, little private management is practiced as compared with South Carolina, for example. There are a few marsh areas near the Satilla River that are under the management of private hunting clubs, but the management programs of these clubs are minimal.
Freshwater marshes have greater variety of aquatic plants (Table 12) and are most heavily used by waterfowl. Except on Blackbeard Island, most coastal waterfowl management is in old rice field impoundments and newly diked marsh in brackish water areas. Water control devices are strategically placed in the dikes, and water levels are manipulated to favor plants used by waterfowl. These include in brackish impoundments submersed plants such as widgeon-grass, musk-grass, and pondweed, and emergent plants such as dwarf spikerush and salt marsh bulrush, and in freshwater impoundments smartweeds, aneilema, and panic grasses.
TABLE 12. Some naturally occurring plants of significance in waterfowl management.
The salt marshes (not including potholes and tidal streams) have limited appeal as feeding areas for most species of waterfowl. Lynch (1968) stated that the "true worth of the southern tidal marsh lies not so much in its direct appeal to waterfowl, but rather in its subtle contributions to waterfowl food chains of adjacent environments." Foods available to waterfowl in salt marshes are mainly animal forms. In tidal creeks and potholes widgeon-grass, a submersed aquatic, is the most important duck food. Most dabbling and diving ducks utilize this plant. The black duck, the most common duck using the salt marsh, feeds primarily on snails. Many species of waterfowl use areas within the salt marsh as resting and loafing areas (Teal and Teal 1969).
The open waters of the tidal creeks are especially attractive to diving ducks and other birds preferring animal foods. Pied-billed grebes, red-breasted and hooded mergansers, and large numbers of scaup commonly feed in the tidal creeks.
At low tide the exposed mud flats of the tidal creeks and marshes are preferred feeding areas for many species of shorebirds such as willets and greater yellowlegs.
Species and preferred habitats of birds occurring on the coast of Georgia are tabulated in Appendix 3.
3. Reptiles. The diamondback terrapin is the only reptile inhabiting the salt marsh throughout the year; it is very common there. Formerly the diamondback was a "gourmet's delight" and commanded high prices in northern markets; about 1900, marketable females commonly sold for $30-36 per dozen (Coker 1906). Only the females reach a marketable size of about 6 inches; males seldom exceed 4.5 inches in length. Experimental rearing studies were conducted by state and Federal agencies (Coker 1906; Hildebrand 1929, 1932; Barney 1922), and although artificial production was successful, it did not prove to be commercially profitable. Small numbers of terrapins are currently taken from Georgia marshes for personal use, and a few turtles are still sold commercially.
The terrapin feeds mainly on periwinkles in the short Spartina marsh during periods of high tides. Martof (1963) reported that the peak in egg-laying in the marshes near Sapelo Island occurred late in May and early in June. Female terrapins dig nests and lay their eggs slightly above the high tide line. Raccoons are their main predators.
Alligators sometimes are observed in the tidal creeks and sounds when they move to and from freshwater and brackish marshes. A few alligators feed in the salt marsh.
Approximately 50 species of insects occur in the salt marshes (Teal and Teal 1969). Some of these complete their life cycle in the marsh, others only breed or spend a portion of their life cycle there. The salt marsh grasshopper (Orchelimum fldieinium) is restricted to the salt marsh and is one of the few marsh insects that have been studied intensively. Smalley (1960) found this species to be the primary grazer of smooth cordgrass in Georgia. It is more fully discussed in the section on food webs and energy flow. The plant hopper, Prokelisia marginata, very common in the marshes, sucks the juices of smooth cordgrass (Teal and Teal 1969).
An ant, Crematogaster clara, feeds on the surface of the cordgrass but lives within the stem of the plant. During high tides, a specially adapted individual blocks the entrance to the nest with its head and thus prevents the colony from being flooded (Teal and Teal 1964).
Two species of salt marsh mosquitoes occur on the coast, Aedes taeniorhynchus and A. sollicitans. Aedes taeniorhynchus is the more common species, but A. Sollicitans is more aggressive in attacking humans (Bidlingmayer and Schoof 1957). Both species breed above the intertidal zone and lay their eggs in potholes and depressions that are covered only by excessive amounts of rainfall or above-normal high tides (King et al. 1960). Oviposition occurs on moist soil or grassy areas within the zone (King et al. 1960). Saltgrass is a good indicator of these zones. However, it is not essential to the breeding of either species as both commonly breed in the cracks that occur in silt dredged from coastal rivers (H. F. Schoof, pers. comm.). The eggs can withstand droughts of 4-6 months, and eggs may hatch within hours following flooding. The detritus-feeding larvae live in the tidal pools before transforming into adults.
The salt marsh mosquito is a vector of the dog heartworm (Dirofilaria immitis). Although certain arboviruses have been isolated from these mosquitoes, diseases of public health significance, such as encephalitis, have not been traced to A. taeniorhynchus or A. sollicitans (H. F. Schoof, pers. comm.).
Three blood-sucking midges, Culicoides furens, C. hollensis, and C. melleus, occur in the coastal area of Georgia. These noxious insects breed in the marsh and are very abundant during the summer.
Deer flies (Chrysops spp.) are extremely common in the coastal region. During the peak emergence in mid-May (Snoddy 1970), they are a severe annoyance to inhabitants of the area. Breeding takes place in habitat similar to the breeding sites of salt marsh mosquitoes. However, the larvae of deerflies are semiaquatic; they are seldom found in areas which are covered with tides.
Several species of crabs are common in the marsh. The brown squareback crab (Sesarma cinereum) occurs on the landward side of the marsh, and the purple squareback crab (S. reticulatum) and mud crab (Eurytium limosum) occupy the soft mud area on the levees of tidal creeks in the dense cordgrass (Oney 1954; Teal 1958).
Teal (1958) studied the factors responsible for the distribution of the three species of fiddler crabs in the Georgia marsh. He determined that Uca pugnax and U. minax select sites with a muddy substratum suitable for digging burrows that do not collapse when flooded by tides. Uca minax, the largest species, prefers marsh habitat in brackish water and has the highest tolerance to fresh water. Uca pugilator, commonly called the sand fiddler, selects sandy marsh areas for its burrows. Teal noted that this crab is excluded from some sandy marsh habitat by competition from the other two crabs. Uca pugilator plugs its burrow before tidal inundation and, although the burrow is in sandy marsh, the air trapped within the burrow prevents collapse (Teal and Teal 1969). All of the fiddlers feed on organic detritus, and food is not a limiting factor in their distribution (Teal 1958). All of the fiddler crabs reach the adult stage in one year.
Two species of snails, the periwinkle and Melampus lineatus, are common in the salt marshes. The shell of the periwinkle is sealed by a horny, protective shield (operculum) attached to the foot, an adaptive protection against desiccation. Melampus is susceptible to desiccation because it has no operculum and therefore must remain near damp sites. It moves up and down the cordgrass stalks with the tides, and feeds on detritus deposits on the plant. Experiments with Melampus indicate that it has a biological clock timed to tidal movements and begins to climb the Spartina stalks before the tidal water arrives (Teal and Teal 1969).
Primary productivity is the rate at which energy is fixed or carbon is stored by the photosynthetic and chemosynthetic activities of producer (autotrophic) organisms. The fixed energy becomes available for consumption by consumer (heterotrophic) organisms within the system. Therefore, any analysis of total productivity is really an analysis of the rate of plant growth (Schelske and Odum 1961).
Research at the University of Georgia's Marine Institute reveals that the marsh-estuaries of Georgia are highly productive ecosystems. Schelske and Odum (1961) list five factors that are responsible for this high productivity. These are: (1) three types of primary production units; (2) tidal ebb and flow; (3) abundant supplies of nutrients; (4) rapid regeneration and conservation of nutrients; and (5) year-round production with successive crops.
Primary production units are smooth cordgrass (Smalley 1959), benthic algae (Pomeroy 1959), and phytoplankton (Ragotzkie 1959). Each producer unit occupies a different "production niche." Spartina thrives in the intertidal zone and produces two-thirds to three-fourths of the total primary production. Net Spartina production is about 6580 kcal/m2/year (Teal 1962). Maximum production occurs during the summer months. Mud algae contributes one-third to one-fourth of the total primary productivity (Schelske and Odum 1961). Pomeroy (1959) determined gross productivity to be approximately 1800 kcal/m2/year and net production to be 1620 kcal/m2/year. Thus the mud algae are the most efficient producer systems in that only 180 kcal/m2/year is lost to respiration. Production rates were approximately the same throughout the year. During the summer, production was high when marshes were flooded and light intensity was reduced. However, in winter, when insolation was less, maximum productivity occurred when the marsh was exposed. Phytoplankton production is greater than previously thought (Schelske and Odum 1961), but exact production figures are not available. Schelske (1962) has determined that nutrients are not a limiting factor on phytoplankton production. However, high turbidity in estuarine waters limits light penetration and phytoplankton production below a depth of about 6 ft. Phytoplankton production increases when the marsh is flooded because the surface area is four to five times greater at high tide than at low tide (Ragotzkie and Bryson 1955).
Tidal action is perhaps the most important factor influencing primary production. Twice daily, tides of approximately 7 ft carry essential nutrients into the marshes, export detritus and nutrients back into estuaries, and provide a large surface area for phytoplankton production. Tidal flushing maintains a desirable vertical distribution of nutrients and detritus.
Nutrients are abundant and probably are not a limiting factor in estuarine production (Schelske and Odum 1961). The nutrients of the fertile estuarine waters are contributed by the marshes and not directly by inflowing rivers (Schelske and Odum 1961; Thomas 1966).
A significant factor relating to primary production is the rapid turnover of nutrients within the system. Such is the case in Georgia estuaries where nutrients are not "tied up" but move rapidly through the system (Pomeroy 1960). Fewer nutrients are needed for production if they are continually available to organisms (Schelske and Odum 1961).
Nutrients are conserved in the system by recycling; they are repeatedly reused with little loss to the system. The horse mussel, for example, is an important biological agent in the recycling of phosphorus (Kuenzler 1961), an essential element in estuarine production.
The three primary producers sustain production on a year-round basis (Schelske and Odum 1961). Spartina produces two crops per year, mud algae produce at a fairly constant rate, and phytoplankton contribute to production throughout the year (Schelske and Odum 1961).
The combined effect of these factors produces one of the most naturally fertile areas of the world. The net production of the marshes and estuaries amounts to approximately 2000 gm/m2/year or 10 tons (dry weight) per acre of organic material (Odum 1961).
Energy flow and food webs
The most comprehensive studies of energy flow in the salt marsh have been conducted at the University of Georgia's Marine Institute. These studies have revealed that energy stored by the primary producers follows two pathways through the ecosystem (Fig. 28): the grazer food chain and the detritus food chain (Odum 1961, 1963). The grazer food chain has four primary consumers: Orchelimum fidieinium (Smalley 1960), Prokelisia marginata, Trigonotylus sp., and Ischnodemus badius (Marples 1966). Smalley (1960) found that during a period of about 100 days, salt marsh grasshoppers consumed approximately 2% of the total Spartina crop. The utilization efficiency of salt marsh grasshoppers (0.57%) was low because Spartina grows throughout the year at varying rates and because the grasshopper has a short life span (4 months) and ingests only leaves (Smalley 1960). Total energy flow for the grasshoppers and planthoppers is summarized in table 13. Odum (1961) reported only 5% of the total Spartina production is utilized by the herbivores Orchelimum and Prokelisia; therefore, most of the marsh production is available to detritus-algae consumers.
The base of the detritus food chain is dead Spartina which is attacked by microorganisms. Odum and de la Cruz (1967) stated that "organic detritus is the chief link between primary and secondary productivity" (autotrophic and heterotrophic levels). It has been suggested by several workers that bacteria may be an important link in the food chain, making some of the hard-to-digest materials available as well as serving as food themselves. They colonize the detritus particle and, upon ingestion, may be stripped off and utilized as food (Burkholder and Bornside 1957; Teal 1958, 1962; Pomeroy et al. 1969). Burkholder and Bornside (1957) estimated that the standing crop of bacteria in the marsh is 107 bacteria per gm wet mud.
TABLE 13. Summary of energy flow in the salt marsh.a
Teal (1962) considered the important detritus-algae feeders to be fiddler crabs, oligochaetes, periwinkle snails, and nematodes among the deposit feeders, and Modiolus demissus and Mamayunkia aestuarina among the suspension feeders. Marples (1966) reported that the periwinkle snail and fiddler and square-back crabs were the most important detritus feeders. Several investigators have determined rates of energy flow for some of these forms. Teal (1962) presented some of his own data and summarized the data of others relating to these detritus feeders. This information is summarized in Table 13.
Predaceous insects (secondary consumers) feed on the herbivorous insects and in turn are fed upon by marsh wrens and rice rats (tertiary consumers). The marsh wren was shown by Kale (1965) to be also a secondary consumer on the grazer food chain since it also fed on herbivorous insects. Predators of the detritus feeders include mud crabs, raccoons, clapper rails (Teal 1962), and the diamondback terrapin. Additional predators which have been observed to prey upon fiddler crabs (detritus feeders) are red drum, willet, white ibis, herring gull, gull-billed tern, boat-tailed grackle, whimbrel, snowy egret, cattle egret, and little blue heron. Table 13 summarizes the energy flow data presented by Teal (1962) for the mud crab, raccoon, and clapper rail.
The utilization of organic matter in the salt marsh (Fig. 28) accounts for approximately 55% of the total primary productivity. Therefore, roughly 45% (4.5 tons per acre) of the production is available for utilization by and support of an abundance of estuarine animals (Teal 1962), including numerous finfish and shrimp which sustain an important industry.
In summary, 6.1% of incident light energy is utilized by the salt marsh in gross production and 1.4% of light energy becomes net production (2000 gm/m2/yr). Fifty-five percent of the net production is consumed by the marsh inhabitants and 45% is available for export into the estuarine system (Odum 1961; Teal 1962).
Last Updated: 1-Apr-2005