Tidal freshwater marsh

Figure 8. Sawgrass dominates this tidal freshwater marsh. (Photograph by the author. More about the photograph)

Figure 8 shows a tidal freshwater marsh. The dominant plant here is sawgrass, the same species that occurs abundantly in the Everglades. Here it is growing along a blackwater river in front of a swamp forest. The area illustrated here is close enough to the sea so that the influence of ocean tides causes the water level to rise and fall twice each day, but the area is too far from the seas to receive salty water. Hence the term “tidal freshwater marsh”.

Freshwater marshes often develop along the interface between forests and the river itself. The marshes are extraordinarily productive and produce tall stands of grasses and sedges in a single growing season. Figure 8 shows such a marsh in late March. If you look closely you can make out the beginnings of new growth at the base of last years plants. These have died back in the fall after withdrawing much of their energy rich material into their roots for use in the next growing season. By the end of that season the plants will be five to seven feet tall and will have converted over 3,000 grams of carbon from carbon dioxide in the air to organic carbon in the plants. The process by which this conversion was made is, of course, photosynthesis.

These riverside habitats provide a particularly favorable place for photosynthesis. The river brings fertilizing nutrients such as phosphorus and nitrogen to the plants, and the opening above the river allows full sunlight to reach the riverbank. Together these resources sustain the large photosynthesis rates that are required to produce the tall plants that characterize these habitats. The most common of these plants are cat tails, saw grass and a mix of tall sedges such as common three square and whitetop.

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