Tidal Wetland Ecology of
Long Island Sound (Part 2)
(Part 1)
(Part 3)
Tidal Wetland Ecology of
Long Island Sound (Part 2)
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Tidal Wetlands as Estuarine Ecosystems (continued)
Most of the tidal wetlands along the Sound are salt marshes, where summer salinity averages about 20 to 30 ppt. Such an
environment is termed polyhaline. This high and variable salinity is an important factor contributing to the relatively low species
diversity. In contrast, brackish marshes occur where salinities range from 0.5 to 18 ppt, and freshwater tidal wetlands are
located where there is no detectable salt in the water (less than 0.5 ppt). An important concept is that all coastal marshes
are tidal, but not all tidal marshes are salty.
Moving up an estuary, away from the Sound, salinity starts to drop while species diversity begins to increase. Tidal action is still operative, and typical salt marsh plants and animals still dominate, but some characteristics of salt marshes are lost, while less salt tolerant plants and animals appear. This transition is gradual, but by the time salinity averages 15 ppt there is a distinctive brackish marsh community. The technical term for such relatively salty brackish environments is mesohaline. Farther up the estuary, where salinities average just 5 ppt - 8 ppt, typical salt marsh plants and animals have largely disappeared; these low salinity brackish wetlands are termed oligohaline. Continuing upstream on the Sound's two major tidal rivers, the Connecticut and Housatonic, tides remain important but salinity essentially disappears. Distinctive fresh tidal marsh communities border the rivers in these areas. Freshwater tidal marshes are limited in Connecticut, but they are highly productive and support an extremely diverse assemblage of plants and animals.
THE SALT MARSH COMMUNITY
As previously explained, there are many factors, such as marsh elevation, duration of tidal flooding, salinity and soil aeration that can affect the distribution of both the plant and animal populations as one moves from the tidal creeks that dissect these wetlands to the low and high marsh and on to the upper border. Often four rather distinctive belts from the creek or bay front to the upland can be recognized, as shown in Figure 1. The following discussion of the ecology of salt marshes is also organized by elevation, beginning with tidal creeks and ending at theupper edge of the marsh.
Creeks and Ditches
We'll start our look at the plants and animals that make up a typical Connecticut salt marsh community in a tidal creek.
Although some creeks may drain completely, the center of others will contain a few centimeters to a third of a meter or more
of water, even at low tide. The almost constantly flowing water results in a moderately firm, even a bit sandy, channel bottom.
Ditch-Ggrass (Ruppia maritima), not really a grass at all, may grow here. This highly branched little plant is covered with a
thin layer of mud, at low tide, and often extends onto the small mud flat that separates the channel from the grasses of the
marsh itself. It also occurs in tidal pools and mosquito ditches on the high marsh.
The tidal creeks and mosquito ditches are nursery areas which provide food and refuge for fish, crabs and other animals (Fig. 2). The most abundant fish of the creeks and ditches are the Common Mummichog (Fundulus heteroclitus), the Striped Killifish (Fundulus majalis), the Sheepshead Minnow (Cyprinodon variegatus) and the Atlantic Silverside (Menidia menidia). Young-of-the-year Winter Flounder (Pleuronectes americanus) may also be found in these creeks.
Fig.2 Click on figure to see typical salt marsh food web.(109K)
Mummichogs often enter creeks and ditches on the flooding tide with relatively little food in their guts and leave them on the following ebbing tide full of food. This is true regardless of marsh inundation, and therefore it appears that the creeks and ditches are important foraging areas. Major components of the diet are detritus (partially decomposed organic matter), algae, amphipods, isopods, copepods and insects. Annelids and young fiddler crabs are also consumed in significant amounts. Mummichogs also feed extensively on the marsh surface when it is covered by high tides. Saltmarsh Snails (Melampus bidentatus), Saltmarsh Isopods (Philoscia vittata) and Saltmarsh Amphipods (Orchestia grillus) are among the high marsh animals in the Mummichog diet. On a Delaware marsh, it was found that when Mummichogs were denied access to the marsh surface by enclosures, their growth was retarded compared to that of fish able to forage on the marsh surface at high tide. It is becoming clear that in some systems, the emergent marsh provides a significant nutritional resource for this fish (Fig. 3).
The diet of the Striped Killifish is similar to that of the Common Mummichog, another small fish. Atlantic Silversides feed on copepods, shrimp, annelids (including Nereis succinea), plant material and small fish, among other things. Young Winter Flounder consume annelids, amphipods, flat worms, detritus, algae and shrimp. Sheepshead Minnows are primarily herbivores, ingesting living plant material and detritus.
The fish of the tidal creeks and ditches are eaten in turn by larger predatory fish such as Bluefish (Pomatomus saltatrix), Fluke (Paralichthys dentatus) and Striped Bass (Morone saxatilis) that forage in shallow water. In addition, at least some Atlantic Silversides, and perhaps large numbers of them, appear to migrate during the winter to inner continental shelf waters where they become prey to offshore predators. Consequently, these fish may be important in transferring energy from shallow saltmarsh - estuarine systems to open water as secondary production. Other predators of marsh fish include crabs and birds such as Terns, Snowy Egrets and Great Blue Herons.
Common Mummichogs spawn at night during high water of spring tides. In New England, the eggs are deposited in mats of algae on the marsh, on the mud-detritus substrate around the bases of Smooth Cord-grass (Spartina alterniflora) or in sand near the high water mark. Spawning in these sites keeps the embryos well aerated and tends to protect them from adult Mummichogs and other aquatic predators. The eggs are resistant to drying and hatch when they are submerged, usually by the next series of spring tides. After hatching, the larvae are transported by the rising tide to pools on the high marsh. The fry (baby fish) may spend 6 to 8 weeks in shallow pools on the marsh surface; and by the time they reach 3 cm in length, most of them have entered the ditches and tidal creeks.
Blue Crab (Callinectes sapidus) and Green Crabs (Carcinus maenas) are often present in the tidal creeks and ditches. Green Crabs, which may be especially abundant, eat shrimp, algae, detritus, small clams, annelids, fish and living marsh Cord-grass. Blue crabs are active scavengers but also prey upon fish, clams and small crabs, among other things. Shore Shrimp (Palaemonetes spp.) and Sand Shrimp (Crangon septemspinosa) may also be numerous in the creeks and ditches. The first is a detritus-algae feeder, whereas the Sand Shrimp is a predator that feeds on amphipods, other small invertebrates, and small fish, including young winter flounder.
Just a couple of meters of smooth, dark mud separate the low tide channel of the creek and the first stems of grass marking the lower edge of the marsh proper. It may be covered with slowly moving snails, up to more than 1000 per square meter, appropriately called Mud Snails (Ilyanassa obsoleta). These forage on the mud surface, eating microscopic algae, dead plants and animals, and algae such as Sea Lettuce (Ulva lactuca).
The mud separating the creek bottom from the marsh grass may not be wide, but it is quite soft and can easily be more than knee deep. It is principally fine grained silts and clays deposited by the slower moving water away from the main channel. Walking through it may be difficult until you reach the grass, where roots and rhizomes give these sediments at least a bit of structure.
Low Marsh - The Smooth Cord-grass Belt
The low marsh itself is essentially a pure belt of Smooth Cord-grass that extends from a bit below mean sea level to
approximately mean high tide, with the actual width of this belt varying with the steepness of the intertidal slope (Fig. 4).
Smooth Cord-grass stems, which can reach more than one centimeter in diameter, are scattered, with densities of tens to a
couple of hundred per square meter. This grass can be extremely productive, converting sunlight, carbon dioxide, and water
into plant material at a rate that matches high yielding corn crops. The height and productivity of this grass are directly related
to the mean tide range. By flowering in mid August, plants at the western end of the Sound, where mean tide range exceeds
two meters, can reach over two meters tall; at the eastern end of Connecticut, where Long Island Sound becomes Fishers
Island Sound and mean tide range is only about 0.8 meters, low marsh Cord-grass is only about one meter in height.
The most conspicuous animals of the low marsh are the Ribbed Mussel (Geukensia demissa) and Fiddler Crabs (Uca sp.). These animals may be found along the seaward edge of the marsh, as well as along the banks of the tidal creeks and mosquito ditches. The Ribbed Mussel is usually partly buried in the marsh mud and anchors itself by means of byssal threads, which are produced by its foot, to the Cord-grass rhizomes and other firm objects such as shells (Fig. 5).
Fig.5 Ribbed Mussels clustered around Smooth Cord-grass stems in a low marsh community. (W.A. Niering)
(95K)
It may occur at densities of more than 1000/square meter; small mussels may also occur at low densities on the high marsh. Where they occur in large numbers, mussels stabilize the sediments and help prevent erosion by binding the soil-root complex with byssal threads. This animal is a filter feeder, removing phytoplankton, bacteria and detritus from the water that it pumps through its gills. The smaller particles are ingested, whereas the larger particles, together with wastes, are deposited on the marsh surface, thus enriching the sediment. It has been shown that the productivity of the Cord-grass is positively correlated with the abundance of the Ribbed Mussel. For example, removal of mussels from experimental plots results in a reduction in grass productivity or biomass at the end of the growing season and conversely, addition of mussels increases Cord-grass growth.
Two species of fiddler crabs are common on Connecticut marshes; the Marsh Fiddler or Black Fiddler (Uca pugnax) and the larger Red-jointed Fiddler (Uca minax). In the very sandy marshes on Cape Cod or the south bays of Long Island, a third species, the Sand Fiddler (Uca pugilator), is found. The male and female fiddler crabs look remarkably different, the males being distinguished by the possession of one very large claw that is used for sound production, mating displays and aggression. These crabs inhabit burrows which they dig in the marsh mud and from which they emerge to feed. The burrows allow tidal water carrying oxygen and nutrients to more easily reach the Cord-grass roots; and as the burrows collapse and are re-dug by the crabs, the upper layer of sediments is extensively tilled. These facts probably help explain why the growth and productivity of Cord-grass is greater in places with high fiddler crab densities. Roots and rhizomes of the Cord-grass, in turn, stabilize the soft muds in which they grow and reduce the tendency of the crab burrows to collapse at high tide. In addition, the tall stems and dense leaves of the Cord-grass provide the crabs cover from predators.
The Fiddler Crabs feed during the day, primarily at low tide, but they are inactive during cold weather and the hottest part of warm days. They eat detritus, bacteria and algae from the marsh surface. Using their small claw (s), they scoop up mud and transfer it to their mouth, where their mouthparts separate edible organics from sand, silt and clay. The organics are ingested while the inorganics are returned to the marsh surface. Alert to potential predators, the Fiddlers rapidly dart into their burrows when they are disturbed.
Marsh crabs (Sesarma reticulatum), which also dig burrows in the mud, are relatively rare on southern New England
marshes but are common farther south. They are omnivores, with the outer leaves of Smooth Cord-grass as a major portion
of their diet, supplemented by the occasional Fiddler Crab. Other invertebrates of the low marsh include the Striped Sea
Anemone (Haliplanella luciae), the Common Clamworm (Nereis succinea), the Rough Periwinkle (Littorina saxatilis),
and the Mud Snail (Ilyanassa obsoleta).
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