Ecological Setting

What is a salt marsh?

The salt marsh-tidal creek ecosystem is a highly productive coastal wetland that occurs between upland areas, such as forests and urban environments, and estuaries, where fresh and salt water mix. As an intertidal habitat, the surface of the salt marsh is under water at high tide and dry at low tide. A dendritic, or finger-like, network of tidal creeks winds through the marsh and facilitates the movement of tidal water onto the marsh surface and back into the estuary.

Along the Atlantic coast of the United States, the salt marsh-tidal creek ecosystem occurs from Maine to Florida. It is, however, most abundant in the Southeast (North Carolina, South Carolina, Georgia, Florida). South Carolina and Georgia each have approximately 350,000 acres of salt marshes and tidal creeks; North Carolina has about 225,000 acres. These three states combined support nearly two-thirds of salt marsh habitat found along the east coast. The northeast Florida Atlantic coast also has approximately 83,000 acres of salt marsh and tidal creek habitats.

Map of Southeastern salt marsh abundance

In the Southeast, salt marsh-tidal creek ecosystems are generally found in estuaries along the flanks and behind barrier islands such as Kiawah Island, SC, and Tybee Island, GA, as well as along the flanks of larger estuarine systems like the Savannah River, the Charleston Harbor, or the Cape Fear River.  They are dynamic systems with varying levels of salinity, or salt in the water.  Freshwater is 0 parts per thousand (ppt) and the Atlantic Ocean is 36 ppt.  As you travel up the river, salt marshes and tidal creeks have a low salinity, less than 5-10 ppt.  Closer to the ocean, salt marshes and tidal creeks are saltier with salinities ranging between 20-35 ppt.  Between these two extremes, the salinity of the salt marsh-tidal creek ecosystem varies greatly depending on tidal stage, amount of rainfall, and size of river system.  This guide focuses on salt marsh-tidal creek systems which have salinities between 10-35 ppt, and are dominated by a plant called smooth cordgrass, scientifically known as Spartina alterniflora.

Aerial view of Kiawah Island, South Carolina

Kiawah Island, South Carolina

Today’s salt marsh-tidal creek systems began to form about 12,000 years ago, at the end of the Pleistocene epoch when the glaciers that covered much of the earth started to melt and sea level began to rise.  As sea level increased and salt water moved inland, bays and lagoons formed behind barrier islands along the Southeast coast.  Freshwater input from major river systems deposited sand and fine sediments in the shallow regions creating sand bars and mud flats.  The elevation of the sand bars and mud flats rose above low tide, giving salt tolerant plants (halophytes), particularly Spartina alterniflora the opportunity to grow.  The roots and stems of Spartina trapped incoming sediment and stabilized it, thus creating a marsh platform. As vast expanses of salt marsh grew, runoff from adjacent uplands in combination with the rising and falling tides resulted in the creation of numerous tidal creeks which transect the marshes.

The abiotic or physical properties of the salt marsh-tidal creek ecosystem are extremely variable and strongly influence the system’s biotic properties, which include the productivity, diversity, and abundance of plants and animals that occur there.  The flora and fauna of this ecosystem are adapted to changes in environmental conditions that occur hourly, daily, seasonally, annually and on even longer time scales.  The major physical variables influencing the salt marsh-tidal creek ecosystem include the tides, salinity, sediments, air and water temperature, precipitation, and sunlight.

The Southeastern salt marsh-tidal creek ecosystem has semi-diurnal tides, meaning they experience two high tides and two low tides each day, each lasting about six hours.  The tidal range, or difference between low and high tide, ranges from about ten feet (3m) in Georgia to about three feet (1m) in North Carolina and Florida.  Tides result from the moon and sun’s gravitational pull on the earth’s oceans.  Approximately twice a month, around the new moon and full moon, tides reach their maximum height (spring tides).  When the moon is at the first quarter or third quarter, the tide’s range is at its minimum height (neap tides).  The concave shape of the Southeast coast in combination with the broad and shallow continental shelf from Cape Hatteras, NC to West Palm Beach, FL, termed the South Atlantic Bight, funnels large volumes of water into this area causing the largest tides to occur in the middle of the Georgia coast.  Maximum tidal length decreases to the North and South of this point.

Tides continually move salt water into and out of salt marsh-tidal creek systems. On flooding tides, the marshes and creeks are flooded with higher salinity water as well as fine sediments and nutrients. During periods of heavy rainfall and ebbing tides, the creek and marsh can be inundated with freshwater, decreasing salinity. Salinity of the water is a major factor determining which plant and animal species will successfully inhabit the salt marsh-tidal creek ecosystem. Organisms living in the salt marsh-tidal  creek ecosystem must be adapted to survive in variable salinities.

Salt Marsh at low tide

Low tide

Salt Marsh at mid tide

Mid tide

Pond view during high tide

High tide

As the water moves into the marsh on the flooding tide, the suspended particles of fine sediment settle out on the marsh surface and edges of tidal creeks.  The stems of Spartina are especially effective at reducing tidal currents and facilitating fine sediment deposition. The greatest amount of fine sediment accumulates where Spartina stems are the most dense. The sediment in salt marsh-tidal creek systems can range from coarse sand to very fine mud. Traversing a sandy area in a marsh is quite easy; however, walking in muddy sediments called pluff mud can be difficult and frustrating.

The Southeast coast has a subtropical climate.  Average monthly winter air temperatures range between 38-59° F (3-15°C).  Average monthly summer air temperatures range between 70-90°F (21-32°C).  Rainfall varies along the Southeast coast; however, it averages around 50 inches (1,270mm) per year.

Researchers in pluff mud

Researchers in pluff mud

Temperature and sunlight greatly influence the color and condition of salt marsh plants, particularly Spartina alternifloraSpartina alterniflora is an annual plant reflecting the seasons in its growth and color.  In spring, Spartina begins to grow and the marsh takes on a brownish green color caused by the combination of new growth and dead stems remaining from the previous year.  In the summer, Spartina reaches its maximum height and takes on a distinct bright green color.  By fall, small white flowers develop along the upper stalks, forming seed heads full of hundreds of seeds no bigger than a grain of rice.  The leaves then turn a golden brown color and the seeds are dispersed.  In winter, Spartina stems turn brown and die.  The stems break off at the base and accumulate on the marsh surface as mats called “wrack.”  Winds and tides, decomposition by bacteria and fungi, and grazing of marsh animals, such as the marsh periwinkle snail, break the wrack into smaller and smaller pieces, called detritus.

Salt Marsh in the Spring

Spring

Salt Marsh in the Summer

Summer

Fall

Salt Marsh in the Winter

Winter

Spartina reproduces in three ways including: (1) seed formation and dispersal by wind and tides, (2) fragments of living plants that break off and form new ones, and (3) roots, called rhizomes, which spread underground and sprout new plants.

The salt marsh-tidal creek ecosystem may appear to be a homogenous environment, or monoculture, dominated by Spartina, but within the marsh system a variety of zones and habitats occur including the upland border, marsh platform, marsh hammocks, high marsh, low marsh, tidal creeks, mud flats, sand flats, and oyster reefs.  Between the estuary and the uplands, gradual changes occur in the type and abundance of the marsh plants and animals.  Elevation (height above mean sea level), tidal height, and salinity are the dominant environmental factors controlling the zonation or distribution of habitats.  Elevation change as small as a couple of inches is all that is needed to produce changes in the types of plants and animals that occur.  When you visit the salt marsh, you can see these changes as you walk from the drier and higher upland down towards the creek.

When you first approach the salt marsh by land you will find yourself in the marsh upland border.  This is the zone between the high marsh and the uplands.  The elevation in this transition zone is above the highest tides.  Plants occurring here are tolerant of salt spray and occasional storm surges.  They are also adapted to living in coarse sandy soils that do not retain freshwater.  Plants living in the marsh upland border generally have thick, waxy leaves able to store freshwater during periods of low rainfall.  Sea ox-eye daisy and marsh elder are characteristic plants of the upland border.

Upland Border

Upland Border

As you look out at the marsh from the upland border, you will see the marsh platform.  The marsh platform is the primary surface of the salt marsh and refers to the flat, broad area extending landward from the water.  The platform accumulates sediment brought in with the tide and provides a stable surface for vegetation growth.  Tidal creeks and rivulets (very small tidal creek like structures) dissect the platform creating the low marsh and the high marsh.

Marsh Platform and tidal creek in Florida

Marsh platform and tidal creek in Florida

The marsh platform supports glasswort and black needlerush in higher elevations to Spartina in lower elevations. Many terrestrial organisms, such as raccoons and great blue herons, and aquatic organisms, such as blue crabs, red drum, and spot, feed throughout the marsh platform.

Marsh hammocks are essentially islands in the middle of the salt marsh. Marsh hammocks vary greatly in size from less than an acre to several hundred acres.  Most are small (less than a few acres) and undeveloped; however, some larger ones are developed for residential homes. The secluded nature of undeveloped marsh hammocks provides an important refuge for wildlife from nearby development.  They play a particularly important role as resting, nesting, and feeding areas for small migrating birds. The colorful painted bunting, for example, nests on most hammock islands.  Marsh hammocks are also one of the last remaining habitats for some amphibians and reptiles, including the diamondback terrapin. Deer, bobcat and raccoon use hammocks as refuges during high tide, while wading birds and alligators (in fresher areas) find hammocks suitable for nesting grounds.  Plant life on a marsh hammock can also be very diverse.  A half-acre hammock may support only a few small live oaks, while larger hammocks support maritime forest communities with live oak, wax myrtle, saw palmetto, yaupon holly and many other plant species.

Small Marsh Hammock

Small marsh hammock

Within the marsh platform, the high marsh zone is covered with salt water for only about one to two hours each day, with the upper extent of the high marsh flooding only a few times a month during spring tides.  The transition from the relatively muddy low marsh to the sandier high marsh requires only a few inches change in elevation.  Spartina begins to experience competition for resources from plants, such as black needlerush, saltgrass, and glasswort in the high marsh, and may only reach heights of three to twelve inches (eight to thirty cm).  While any plant found in the high marsh needs to be salt-tolerant, freshwater running off from the mainland is also essential for their growth and success.

Salt panne with common glasswort

Salt panne with common glasswort

The thin layer of water over the high marsh evaporates quickly resulting in high levels of salt in the sediment.  In certain areas of the high marsh, called salt pannes, sediment salinities approach 100 ppt during the summer.  The succulent plant, common glasswort, is able to store salt in its branch-like leaves and is the dominant vegetation of salt pannes.

Salt excretion on leaf of Spartina

Salt excretion on leaf of Spartina

The low marsh zone spans from the tidal creek bank to the high marsh and is covered with saltwater for half of the day.  In the Northeast, salt marshes tend to have narrow bands of low marsh; however, in the Southeast the low marsh can often be found covering miles of habitat, making it a predominant habitat of many salt marsh-tidal creek systems. The low marsh zone provides an abundance of food, including ribbed mussels, fiddler crabs, and small invertebrates for larger animals.  Only one type of grass, Spartina alterniflora, has the adaptations needed to withstand the amount of tidal flooding and salt content experienced by the low marsh.  These adaptations include: (1) glands along the grass blades that excrete salt, making it possible for Spartina to consistently be exposed to saltwater; and (2) an extensive root-rhizome system that acts as an anchor, stabilizing it during the tides and holding it steady against storms and high wave energy.  Spartina growth and production in the low marsh varies. Immediately adjacent to the creek bank, where the greatest amount of pore water flow and nutrients exist, Spartina can reach heights between five to eight feet (1.5-2.4m).  In the interior region of the low marsh, where less nutrients are deposited and salinity is higher, Spartina height ranges from two to four feet (0.6-1.2m).

Tidal creeks are branching structures that meander through and shape salt marshes.  They are the major water link between salt marshes and the open estuaries.  Tidal creeks are also a conduit for stormwater runoff from the upland to the open estuaries. These creeks, some almost dry at low tide, are particularly important as nursery areas for many species of fish and invertebrates. Wave after wave of recreationally and commercially valued species, including spotted seatrout, red drum, spadefish, spot, black drum, blue crab, white shrimp, and brown shrimp, enter tidal creeks as juveniles to continue their life cycle before moving to deeper water.  Even some fish we normally associate with the open ocean, such as grouper and barracuda develop in tidal creeks during their earliest years.  The quiet, protected creeks provide an abundant food supply and give juveniles respite from predation as well as access to the marsh platform at high tide.  Large fish predators, such as flounder, will lurk at the mouths of tidal creeks feeding on the smaller organisms that are flushed out on ebbing tides.  Wading birds and other avian predators also feed on organisms in the shallow creeks at low tide and on the marsh platform at high tide.

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Aerial view of winding tidal creek

Winding tidal creek

Fiddler crab near its burrow

Fiddler crab near its burrow

The low marsh and tidal creek zones are the predominant areas pluff mud occurs, particularly along the edges of creeks.  Pluff mud gives off the characteristic “rotten egg” sulfuric smell many quickly associate with salt marsh-tidal creek systems.  While the odor seems foul to some it is actually a healthy indicator of a process called anaerobic respiration.  Anaerobic respiration uses sulfates from the water and releases hydrogen sulfide into the mud, creating that sulfuric odor. Mud flats lack oxygen, however certain bacteria and fungi in the mud thrive in low oxygen environments. This anoxic habitat may not seem well suited for animal life at first glance. However, burrowed into the mud are numerous organisms such as clams, fiddler crabs, and small worms which have adapted to live in low oxygen environments.  These organisms in turn attract larger predators that feed on them.

Intertidal mud flat


Mud flat

Mud flats are intertidal, non-vegetated, soft sediment habitats occurring in the areas of tidal creeks with weaker currents.  Fine sediment particles tend to accumulate in these areas and form muddy intertidal habitat.  Sand flats are intertidal, non-vegetated environments that occur in areas with stronger currents.  Sand particles tend to accumulate in these areas and form sand bars.  Mud and sand flats are both depositional features, meaning tidal currents continually modify their size and shape.  Sand and mud flats are inhabited by abundant populations of burrowing clams, crustaceans and worms, and are important feeding grounds for shore and wading birds, fish, crabs, and rays.

One of the most recognized habitats in the salt marsh-tidal creek ecosystem is the oyster reef. Spawning peaks from April to October and is dependent upon temperature and food availability. Early life stages are free floating in the water as plankton, but the final larval stage must attach to a hard surface and transform into a small oyster, called spat, before it can continue to mature. Most often, the shells of other oysters, dead or alive, turn out to be the best solid surface for attachment.

Spat growing on oyster shell

Spat growing on oyster shell

As oysters continually grow and build upon each other, expansive reefs are formed in tidal creeks.  Oysters are filter feeders, and adult oysters are capable of filtering up to four gallons of water an hour.  This makes oysters extremely important in improving water quality by filtering out particles from the water, including bacteria, algae, detritus and suspended sediments.  One of the best indicators of impaired water quality is when oyster reefs are restricted from being harvested because levels of fecal bacterial indicators are too high.  Oyster reefs are also vital in protecting shorelines from erosion and provide habitat for crustaceans, worms, and fish.  The reef is a particularly important refuge for juvenile fish as it provides a bountiful food supply as well as shelter from predators.  For all of the above reasons, oysters are often referred to as a keystone species, or a species that shapes an ecosystem and on which a number of other species in the ecosystem rely.

Unfilitered Oysters

Before filtering

Filtered Oysters

After timed oyster filtering

Section of an oyster reef

Section of an oyster reef

As discussed, the twice-daily ebb and flow of the tides greatly influences life in the salt marsh-tidal creek ecosystem.  The rapid and regular transition from wet to dry, salty to fresh, and cold to hot depending upon season, greatly limits the types of plants and animals that can survive in this ecosystem.  However, the rich soil and abundant sunlight make this ecosystem very productive, allowing the animals and plants adapted to these changing conditions to develop abundant populations which contributes to the complex and intricate food web of this ecosystem.

The salt marsh-tidal creek ecosystem serves as primary habitat for a rich variety of animal life.  Some animals live in the ecosystem permanently, while others are transient.  From the worms living in the mud to the birds flying over it, a number of organisms use the marsh.  Raccoons can be seen scouring the mud for a meal of mussels and fiddler crabs, while a diamondback terrapin lays eggs on a nearby hammock.  Small invertebrates, like the mud snail and periwinkle snail, graze directly on sediments and Spartina stems for algae and microorganisms.  When the tide comes in, blue crabs feed on the periwinkle snails that climb up the stalks of Spartina, and wading birds and red drum in turn prey upon the blue crabs.  Large fish predators hunt in deeper sections of tidal creeks preying on shrimp, crabs, and small fish that may wash into the nearby estuary on the ebbing tide.

Animal tracks through the salt marsh

Animal tracks through the salt marsh

Spartina leaves and stems and the algae growing on the marsh surface form the base of the salt marsh-tidal creek food web. Most of the nutrient production of the salt marsh-tidal creek ecosystem is consumed as part of what is referred to as a detritus-based food web.  In the fall and early winter, tidal currents, waves, wind and storms dislodge and break up decaying leaves and stems of Spartina, and deposit this material throughout the salt marsh-tidal creek ecosystem as wrack.  Microscopic organisms such as bacteria, fungi and small algae attach to this dead plant matter and break it into smaller and smaller particles.  The detritus, including the microorganisms attached to it, are consumed by a wide variety of organisms including mussels, snails, worms, oysters, and crustaceans, as well as juvenile and adult fish.

Spartina wrack

Spartina wrack

The salt marsh-tidal creek ecosystem is crucial as a nursery ground.  Some estuarine animals, such as mud minnows and snails, complete their life cycle within this ecosystem.  Others, such as white and brown shrimp, red drum, mullet, and blue crab as well as many other recreationally and commercially valuable fish species spawn in the open estuary and ocean where initial developmental occurs.  After a few weeks, the early life stages of these ocean and estuary spawning organisms are transported up tidal creeks via tides and into the marsh where food is abundant and they are relatively safe from predators.  These juveniles are adapted to, or can tolerate, the naturally low dissolved oxygen levels found in these systems for long periods of time whereas the larger animals cannot.  As they grow, the juvenile animals continually move between the flooded marsh platform at high tide and tidal creeks at low tide, in search of food and protection from predators too large to swim into the shallow water.  This process allows rapidly growing juveniles to mature in relative safety.  Some species will stay in the marsh and continue to live and reproduce, while others will move out into the open water of the estuary to complete their life cycle.

Map of Blue Crab Life Cycle

Blue Crab life cycle

Shrimp LIfe Cycle

Shrimp life cycle

Many species of birds feed on the small fishes and invertebrates that live in the salt marsh.  Terns and osprey dive for fish visible in the shallow water, while sandpipers and their relatives probe exposed flats for small snails and marine worms. Only two birds, the clapper rail and marsh wren, nest in Southeastern salt marshes.  Clapper rails, often heard but seldom seen, hunt for fiddler crabs among the stalks of Spartina and black needlerush, where they build their nests above the reach of the tides.  Marsh wren, prolific singers during the spring breeding season, weave tall Spartina stems into messy nests along the creek banks.

One of the few permanent reptiles of the salt marsh-tidal creek ecosystem is the diamondback terrapin which moves into the grass during high tide to feed on periwinkle snails and will move to high ground to lay its eggs above the high tide mark.  This small turtle comes in a variety of colors, from spotted and light green to dark brown and black.  Alligators often pass through brackish marshes but usually avoid high salinities.

A few mammals enter the marsh platform to forage while the mink, an excellent swimmer, searches the marsh and small creeks for fish, shrimp, and other aquatic prey.  Bottlenose dolphins will leave the deeper water of the estuary to find prey in shallow creeks.  If you are lucky, you may see dolphins feeding along creek banks, a practice called strand feeding or mudding!

As a result of these biological interactions, the salt marsh-tidal creek ecosystem is one of the most productive and complex ecosystems on the planet.  In fact, the amount of biomass the salt marsh-tidal creek ecosystem produces, recycles, and transports is rivaled only by the rainforest!

Birds Probing for prey in the marsh

Birds probing for prey in the marsh

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