Sitting between the tides and the uplands allows a salt marsh to be fed by several nutrient-rich water sources. Seawater contains many of the elements used by plants. Rivers also carry inorganic compounds released from decaying plants and animals. Storm runoff collects still more nutrients on its trip over the landscapes draining into the estuary.
These nutrients are absorbed by the estuary’s 2,230 acres of marsh grasses, rushes, sedges, succulents, algae and flowering plants. From May to October they convert the sun’s energy into food energy through photosynthesis. Marsh fertility is great, actually comparable to highly productive farmland.
Floral Arrangements
The flow of the tide from the water's edge and the seep of fresh water from the uplands and groundwater, creates varying environmental conditions, or zones, across a marsh. These zones favor different plant species. Even from a distance, you can see from the variegated greenery how the plants transition from one group to the next. They arrange themselves in response to sediment conditions, such as oxygen, nutrient, and salinity levels. Other factors include the competitive nature of neighboring plants, and the occasional disturbances of smothering debris rafted in by the tide.
Low Marsh
At the water's edge the conditions are too stressful for most plants. The tides saturate low marsh peat, drastically reducing the amount of oxygen that diffuses below the surface. Though an ebbing tide gives the peat a chance to drain, evaporation can leave high concentrations of salt on the sediments. Salt draws moisture from the cells of any plant not equipped to prevent dehydration in a marine or estuarine environment.
One plant able to thrive in spite of the inhospitable conditions is salt marsh cordgrass, also called smooth cordgrass. It would fare well in freshwater, too, but for its inability to compete with freshwater plants. So it dominates the lower zones of salty and brackish marshes, growing in four- to five-foot stands.
Cordgrass excretes excess salt, which solidifies
into crystals on its leaves. This is one way it copes with the conditions
of the lower marsh. To learn about its other adaptions for intertidal life,
visit the plant kingdom link on the Flora & Fauna page.
Most cordgrass growth is not eaten by herbivores. When it dies at the end of the growing season, the leaves release organic compounds and nutrients into the tides before the fungi and bacteria set in for the decomposition process. Leaf litter not carried away by the tides mixes into the sediments. Over time these layers of marsh peat have built up to where only the high tides reach
High Marsh
At this elevation, salt marsh hay starts to dominate, growing in cowlicked meadows with other kinds of grasses, rushes, and sedges. Unable to oxygenate the sediment around its roots like cordgrass, marsh hay tolerates only extreme high tides.
Salt marsh hay (Spartina patens)
But the dense growth of its roots and rhizomes (underground stems) gives it a competitive advantage over cordgrass and other plants. Though cordgrass moves quickly into bare patches, its roots and rhizomes are too widely disbursed to beat out salt hay in the competition for space and resources.
Scattered
among the salt hay, you can find black grass and spike grass. In irregularly-flooded,
brackish marshes you see species like salt marsh bulrush and Olney three-square.
Fresh water flows encourage purple loosestrife, narrow-leaf cattails,
and common reed.
Dragonflies, grasshoppers, praying mantis, spiders, and green head flies find food or shelter among these plants.
Pools & Pannes
Marshes typically have pools and pannes in the upper zone. Some were created when the marsh was forming, as sediments were deposited unevenly by the tides. These depressions remain filled even at low tide. Other depressions formed after the marsh was established. Many times vegetation is smothered by materials rafted onto the landscape by the tides, including wrack (dead marine vegetation), or even a chunk of marsh ripped off by ice movement. If no plants colonize the area, the tide sometimes fills the resulting bare patch. Pannes are shallower and dry up during long dry spells. Pools are the permanent water holes.
Pools
and filled pannes are shelter for numerous animal species like small or
juvenile finfish, swamp hydrobia (tiny snails), shrimp, and baby green
crabs. Sticklebacks, eels, and the incredibly tolerant mummichogs, are
some of the fish found in those waters. These creatures have to be able
to endure environmental extremes. Shallow water levels are prone to swinging
temperatures. Plus, depending on their location, the pools and pannes
may not be refreshed every tidal cycle, resulting in hypersalinity from
evaporation, or low salinity from rainfall.
If pannes evaporate, they leave ideal open space for solitary plants like the marsh glasswort, marsh orach, and seaside lavender. They are able to capitalize on the light and nutrients of the bare patch since they tolerate the hyper-salinity caused by evaporated tidal water. In some cases those plants can establish themselves in a panne right away, instead of the standing tide water. These may have sprouted from seeds left there by the wrack causing the disturbance.
Common glasswort is the first plant to colonize
this bare patch of a marsh in Greenland. Much of the marsh was actually
disturbed by design. It was bulldozed as part of a restoration project
to restore saltwater flows. The old practice of draining marshes has been
found to hurt rather than help mosquito eradication efforts. The tides
carry in forage fish species, like the mummichogs, which feed on the mosquito
larvae.
Grasses and sedges, with their dense root systems, eventually overcome these solitary plants. Another way they dominate is through vegetative reproduction. They send out underground stems called rhizomes, which sprout new plantlets. These clones draw on the water and nutrients of the parent plant as they stake out new territory. Since solitary plants like sea lavender reproduce only by seed, they are ultimately a transitional plant, preceding the community of dense grasses. This is why it's common to find sea lavender scattered throughout grass only as single plants or in small groups.
Food Sources
As with cordgrass, very little of this abundant plant life is grazed by herbivores. Chewing insects like grasshoppers consume their seeds, and leaf beetles consume their leaves. Sap sucking aphids are at work on plants in the upland borders of the marsh.
Ladybug at Sandy Point marsh prepares to lunch on
aphids. This beetle can consume dozens of aphids in a day.
Microscopic algae growing on sediment and plant surfaces are a big food source for invertebrates, since they produce food through photosynthesis yet lack the tough structural fibers most animals are unable to digest. By far, most plant growth is eaten only after it dies, providing the basis for a nutritious blend called detritus: decomposing plants and animals, along with the agents of this decomposition—colonies of bacteria and fungi. Algae and detritus are important food sources for the worms, mussels, crabs, periwinkles and amphipods living on or in the regularly flooded sediments of the low marsh.
Of the fish swimming through the estuary's creeks, ditches, and pannes, mummichogs are the most common. They, too, feed on the detritus. Schools of American silversides swim in with the high tide to feed on detritus, as well as small animals, like amphipods, mysid shrimp, copepods, and horseshoe crab eggs. Other fish feeding in marsh creeks during certain seasons include white perch, tomcod, river herring, and smelt.
Marshes serve as important nurseries for some fish to spawn or rear juveniles. When these fish swim away they take the food energy they have consumed in the marsh and pass it up the food chain to predators in the estuary (e.g. crabs, fish, and birds), and the ocean (e.g. herring and striped bass).
Birds also benefit from marsh productivity. Canada geese consume cordgrass. Red-winged black birds and swallows catch insects. Herons and egrets eat small fish and crustaceans. Hawks, like the northern harrier, catch small mammals. Various upland mammals, including deer, raccoon, mink, and otter, also find food or shelter in the marshes.
Marsh Formation
Most marshes in the Northeast are estimated to be between 3,000 and 4,000 years old. Their formation started where currents slowed and sediments dropped from the water, typically in sheltered embayments and at the mouths of rivers.
The sediments eventually layered high enough in intertidal areas to allow cordgrass seedlings to take root, since they withstand submersion for up to half of each tidal cycle. As the grass spread, its roots bound sediments together and its blades provided added drag on water flow, allowing more sediments to drop from the water column.
Eventually the layers of sediment and grass combined to form peat, which accumulated high enough above the tides to cultivate other plant species. Thousands of years later this gradual building continues, allowing the marshes to stay above sea level, which rises less than one-tenth of an inch each year in New Hampshire.