Tidal Wetland Ecology of Long Island Sound (Part 1) (Part 2) (Part 3)

R. Scott Warren, Professor of Botany,
Paul E. Fell, Katherine Blunt Professor of Zoology,
Connecticut College

Wetlands are transitional zones between terrestrial and aquatic systems where the water table is usually at or near the surface, or the land is actually covered by water either permanently or periodically. Wetlands typically support hydrophytes (plants adapted to wetland conditions) and the substrate is hydric (wet) mineral and/or organic soil that is usually poor to very poorly drained. Tidal wetlands occur at the land/ocean interface where daily tidal action moves water in and out of the systems. Along the Atlantic and Gulf coasts of the United States, tidal wetlands are found from northern Maine to southern Texas. From all but the southern tip of Florida, where mangrove swamps occur, these coastal wetlands are tidal marshes, where grasses are the predominant vegetation. Long Island Sound marshes are technically classified as Estuarine Emergent Wetlands, because the Sound is an estuary, and the vegetation emerges above the water level.

Tidal Wetlands as Estuarine Ecosystems
Although these wetlands have a north-south range of over 2,000 kilometers (1,200 miles), Atlantic and Gulf tidal wetlands are remarkably homogeneous in their plant and animal communities. The basic physical and biological structure of these communities comes from a few species of Cord-grass in the genus Spartina, particularly Smooth Cord-grass (Spartina alterniflora). Tidal marsh animal communities are dominated by various snails in the genera Melampus and Littorina, the Ribbed Mussel (Guekensia demissa), three different fiddler crabs (Uca spp.), and several different minnows (Fundulus spp.).

Between Maine and Texas, however, there are regional differences in the vegetation and animal populations reflecting both climate and coastal geomorphology. Long Island Sound's tidal marshes are within the New England type and are representative of the tidal marshes found from southern New Jersey to central Maine. In this region the marshes have often formed in drowned river valleys and contain considerable deposits of peat. Below New Jersey there is a transition to Mid-Atlantic tidal marshes, while moving north of Penobscot Bay in Maine, with extreme tide ranges and cool, short growing seasons, the shift is to a Fundy type tidal marsh. The last takes its name from the Bay of Fundy, located between Maine, New Brunswick and Nova Scotia.

The sun, tides and salts in water all play important roles in the functioning of tidal wetlands. They are the principal non-living influences which "organize" these complex communities, and help to differentiate the regions within a marsh system, as illustrated in Figure 1.

Fig. 1 Click on figure to see idealized zonation of communities on a typical Connecticut salt marsh. (125K)

Marsh areas at the lowest elevations are submerged by all or most high tides and are termed low marsh. In all but low salinity and freshwater tidal wetlands, low marsh is vegetated by essentially pure stands of Smooth Cord-grass. The upper border, inundated only by occasional spring high tides, lies along the upland edge, at the highest elevations. This border between wetland and upland is dominated by a few characteristic grasses and shrubs. Between these two extremes of very wet to very dry habitat lies the high marsh, which is flooded by most spring high tides, but may also be continuously free of tidal water for a week or more at a time. Relatively short, fine-stemmed grasses dominate high marsh vegetation. Transitions between low marsh, high marsh and upper border may be gradual or quite abrupt and, although linked by tidal flooding, these three regions are characterized by distinctive assemblages of plants and animals and they are, in many ways, very different environments.

Virtually all earth's ecosystems are "solar powered " - photosynthesis by green plants converts light energy into sugars, the biological energy unit, which sustains all other forms of life. In tidal marshes this solar power is supplemented with "tidal power" - the movement of tidal water. Tidal circulation does many different kinds of work for these wetlands (analogous to the work farmers put into their fields) and this tidal powered work allows the extremely high biological productivity of these communities. For example, tides deliver the suspended sediments necessary for the continued vertical growth of the marsh surface in the face of continuing sea level rise (see the Evolution and Development of Tidal Marshes chapter). Flooding tides also enhance plant growth since they deliver oxygen enriched water to marsh soils and remove potentially toxic materials as they exit. Tidal waters are also the vehicle carrying plant nutrients onto the marsh in periods of abundance, and transporting excess nutrients back to the estuary at other times of the year. These exported nutrients support the growth of phytoplankton (single celled plants suspended in the water) which are the foundation of marine food chains. Flooding frequency and duration, and marsh surface relief also strongly influence patterns of soil saturation and resulting oxygen availability, which in turn are major factors controlling the distribution of plant species within these wetlands. Tides also link the marshes to estuarine waters and the offshore coastal zone environment. They deliver saltwater from the marine environment, which mixes with and is diluted by freshwater from uplands, to produce salinity gradients which also act to characterize and help organize these systems.

Finally, tides are critical to animals that live on the marsh as well as those in estuarine waters, both for nutrition and reproduction. As described below, flooding tides allow fish and crabs access to prey on the marsh surface and deliver nutrients to sessile (stationary) organisms like Ribbed Mussels (Guekensia demissa) and Striped Anemones (Haliplanella luciae). Stems of low marsh grass, accessible only at high tide, provide protected places for Mummichog (Fundulus heteroclitus) to lay their eggs. Reproduction of a number of invertebrates is also dependent upon the spring tides which regularly flood the high marsh in two week cycles.

The common bond between all types of tidal wetlands is tidal action; the amount of salt present in the flood waters and soil is what distinguishes one type of tidal wetland community from another. The concentration of salts (principally sodium chloride, common table salt) dissolved in the open ocean is approximately 3.5 % or 35 ppt (parts per thousand). Salinity in Long Island Sound varies seasonally and with proximity to major sources of fresh water such as the Connecticut, Housatonic and Thames Rivers, but is generally between 27 and 32 ppt. Sea water is carried into estuaries and embayments by tidal action, where it mixes with and is diluted by freshwater from rivers and streams. The resulting ocean to fresh salinity gradient is constantly moving and changing over hours, weeks and seasons.

Tidal Wetlands Productivity

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