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West Meadow Creek in Winter, North Shore of Long Island NY

Spartina salt marshes are broad nearly level meadows dominated by several species of grasses that are tolerant of salt and anoxic sediment. Broad meadows are interspersed with creeks that are biologically rich, owing to the rich supply of organic matter and the provision of protected habitats for many species.

A salt marsh is “born” by the arrival of a seed or the rafting of a plant of the cord grass Spartina alterniflora. The grass spreads asexually by means of a subterranean rhizome system. The grass becomes dense and forms a baffle, which encourages the deposition of fine particulate sediment, including organic matter (salt marsh peat). This, in effect, causes a rise of the sediment surface and makes the habitat more terrestrial. As this happens, other somewhat less salt-tolerant grasses are able to invade. Eventually, this series of invasions and takeovers leads to a vertical zonation of grasses and a spread of the entire marsh system.

SAS = Spartina alterniflora – short form

SP = Spartina patens, the next higher grass species

In the foreground we see the Spartina patens zone, which is higher (though that is hard to tell from the photo) than the S. alterniflora zone (taller grass in the background). S. patens is less salt tolerant than S. alterniflora.

This is a schematic cross section of a marsh with creeks. Note the tall form of Spartina patens, which tends to live adjacent to creek edges, which are bathed in nutrient-rich water. The short form lives in higher sites away from creek edges. The ribbed mussel Geukenzia demissa often lives among Spartina spp. and its production of nitrogen-rich feces and pseudofeces enhances grass growth.

The high intertidal marsh mussel Geukensia demissa is ubiquitous in Spartina salt marshes. The late Charles Lent showed that this mussel was capable of breathing air, and often got more of its oxygen from air than from water.

The diagram at left shows that Geukensia demissa is more elongate than the mussel Mytilus edulis, and lives semi- infaunally in the sediment, with byssal threads attached to sedimentary grains or rhizomes of Spartina. Steven Stanley demonstrated that the musculature apparently helps in dragging the mussel down into the sediment, as opposed to that of M. edulis, where the foot retractor muscles act to appress the mussel to the rocks.

Ribbed mussels can be quite densely packed. Mark Bertness and colleagues demonstrated that this mussel enhances the growth of Spartina. The mechanism may relate to biodeposition of nitrogen-rich feces and pseudofeces. The byssal threads of this mussel also help bind the sediment and may retard erosion. Another common salt marsh resident, the fiddler crab Minuca pugnax, also aids in Spartina growth by burrowing and aerating the sediment. Note the barnacles attached to mussel shells, which is possible owing to the mussel’s semi-infaunal habit.

 

 

A new section of marsh must start by the colonization of a seed or by rafting of an adult. In this case, a clump was initiated probably by arrival of a seed, but all of the plants in this picture are connected by a rhizome, suggesting that this small founding population arose from a single successful colonizing seed.

Courtesy of Mark Bertness, Photo by Irv Mendelssohn

The aerenchymal tissue allows Spartina to exchange gases, even when surrounded by anoxic soil. The tissue in this photograph is visible as a series of circular passageways around the periphery. Plants cannot use nutrients efficiently without oxygen, so this tissue allows a connection between the aerobic leaves to the stems, which are surrounded usually by anoxic water.

In the fall, Spartina leaves senesce and eventually calve off at the base. This material then is deposited on the sediment surface. By the time the particulate organic material is deposited on the surface, nearly all nitrogen has been removed by microbes, so it is very carbon-rich and nitrogen-poor. The carbon is mainly in the form of cellulose, which is largely indigestible by invertebrates.

The senesced leaves accumulate on the sediment surface as shown at left. This material will smother any grass beneath, but a high spring tide or a storm may remove it all quite rapidly and transport it out of the marsh system to the shelf.

This material has been the source of much controversy over its value as food to marine invertebrates. On the one hand it consists mainly of virtually indigestible cellulose. But, when broken into small particles, it can be a substratum for bacteria and microalgae, and may be consumed by animals such as oysters.

This meadow is rather high in the intertidal and is dominated by Spartina patens, which occurs higher in the intertidal zone than S. alterniflora. S. patens is less salt-tolerant than S. alterniflora. In the foreground is a small band of S. alterniflora.

The picture was taken at Flax Pond, a salt marsh on the north shore of Long Island, near Stony Brook, New York.

Here again we see a lovely Spartina alterniflora salt marsh in the fall. In the foreground is a mud flat that is covered with the sea lettuce Ulva rotundata, which decomposes and is a source of nutritious and digestible particulate organic matter for the benthic invertebrates in the flat.

Here we see a large patch of Spartina alterniflora that is overgrowing a mud flat. “Cage” arrows point to cages that were used in an experiment to study the competitive interactions between the mud snails Ilyanassa obsoleta and Hydrobia totteni. The larger Ilyanassa obsoleta displaces Hydrobia.

A large area of salt marsh may be covered by wrack for some time, which probably smother the underlying plants. In the high intertidal and supratidal, such open bare areas are prone to strong evaporation and salt deposits begin to accumulate. Research by Mark Bertness shows that the salt depresses seed germination and invasion by surrounding plants, making the salt pan a locally stable state.

Salt pans are stable states, unless a heavy rain occurs that can dissolve the salt and allow seeds to germinate. Sometimes a plant or group of plants may be able to spread into a favorable microsite in the salt pan. D. spicata can spread into the marsh by forming runners, which allows a group of plants to shoot up. This creates a local spot where shade permits water to dissolve salt faster than it precipitates, which may allow yet more plants to spread via a rhizome system and may also allow more seeds within the sediment to germinate.

Spartina alterniflora consists of tough leaves composed of cellulose. Very few grazers attack the plant. The flowers, however, are consumed by a large number of insect grazers, which often cause a failure of seed set. Courtesy of Mark Bertness

Melampus bidendatus in human hand

If you turn over a piece of wood or look at high intertidal grass blades at the time of low tide, you might find this snail, which is only a few mm in length. It is a pulmonate snail and therefore must be in air for some part of the day to get oxygen. Unlike other pulmonates, however, it has a planktonic larvae, which swims for two weeks. Larvae are released at the time of spring high tide and settle high in the marsh at the next spring high tide. This maximizes the chance that they will release larvae when they are covered by water and that the larvae can settle in a part of the tide zone that is exposed to air for most of the day.

Several species of Salicornia can be found in eastern American Spartina salt marshes in the highest part of the intertidal zone. They are all very salt tolerant. This plant is only about 5 cm high.

This individual is standing on the periphery of a salt creek, stalking prey, which may include smaller fish and crabs.

SALT MARSHES AT RISK

Salt marshes are among the most productive of shore habitats, but they are endangered and on the decline in many regions. The simplest loss is the dredging and digging of salt marshes, which are being replaced by shoreline development such as docks, hardened shorelines, and housing developments. The obvious losses can be found in our coastal cities. For example the island of Manhattan, New York, was once fringed entirely by Spartina salt marshes. With a few small stretches as exceptions, most of the marshes have been replaced by hardened sea walls. Unfortunately, such removal has extended to outer suburban and even rural parts of urban areas. On Cape Cod, Massachusetts, USA about 30 percent of marshes have been removed. In New England USA at least half of the salt marshes are gone. In Jamaica Bay, New York, over 50 percent of marshes are gone. This pattern extends world wide. In the United Kingdom, salt marshes are expected to be gone by the end of the century.

Along with direct removal, pollution and nutrient enrichment is a major factor in salt marsh degradation. Nitrogen pollution tends to favor the above ground growth of the lower salt marsh species Spartinal alterniflora, and below ground root growth is reduced. Invasive species from the high marsh are also displacing native salt marsh species. Worse of all, the delicate balance of marsh growth is being affected by sea level rise throughout the world. Sea level rise has been followed for the last several decades in the northeast North American region, and the news is bad. New York and New England marshes are especially at risk from sea level rise. Forested regions of New England and the small rivers tend to produce very small amounts of sediment, which starves coastal marshes of sediment that is needed to grow as sea level rises. In addition, New England salt marshes respond to sea level rise by poor investment of resources into marsh roots, which further weakens and reduces marsh vertical and lateral growth. Recent studies show that Long Island marshes are doing poorly from sea level rise owing to these general factors. The future of salt marshes is unfortunately grim.

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