Tidal Wetlands Primer: An Introduction to Their Ecology, Natural History, Status, and Conservation

1. Definitions and Classification of Tidal Wetlands and Estuaries

1 1 Definitions and Classification of Tidal Wetlands and Estuaries Tidal wetlands are saline and freshwater marshes, swamps, banks, and shores subjected to flooding by tides. They are mainly low-lying, relatively flat plains formed by deposition of river-carried (fluvial) and/ or marine sediments. They also include gently sloping beaches and other sloping landforms in areas with large tide ranges. Some are intertidal shorelines at the base of cliffs formed on material eroded from the cliffs. Tidal wetlands may cover vast areas that are miles wide or be limited to narrow, fringing bands only a few feet or meters wide. Regardless of their origin, their location beside oceans, estuaries, and tidal rivers promotes an exchange of water, nutrients, sediments, and biota between the land and the sea that has made tidal wetlands some of the world’s most productive natural environments . Throughout this book, the terms “tidal wetlands” and “coastal wetlands” are used interchangeably. Readers should recognize, however, that wetlands along the Great Lakes have also been referred to as “coastal wetlands” because of the lengthy coastlines associated with these huge waters. Incidentally, these freshwater lakes are also tidally influenced due to their huge size, yet the tidal effect is minimal—only an inch or more. Water levels vary more due to seiches in which strong winds in one direction cause water levels to rise on one side of the lake and fall on the opposite side. Also note that a few researchers have applied the term “coastal wetlands” more broadly to include all wetlands in coastal watersheds and not strictly to tidal wetlands (Field et al. 1991; Stedman and Dahl 2008a; Stedman et al. 2010). Vegetated tidal wetlands are typically found in deltas of major rivers, sheltered embayments behind barrier islands and spits and along the shores of bays, and on the floodplains of tidal rivers (Figure 1.1). They also occur in open water along lowenergy , unprotected shores. Intertidal sites that are protected from the full force of ocean waves and strong tidal currents, and that are alternately flooded and exposed by tides, favor colonization by vascular hydrophytes (water-tolerant plants). More exposed ­ locations are typically devoid of such plants (e.g., mudflats and beaches). Seaweeds (macroalgae) may colonize high-energy, surf-pounded rocky shores in northern climes where they find suitable substrates for attachment by their holdfast organs. Hydrology, salinity, sedimentation and erosion rates, and other factors profoundly influence plant and animal life and make coastal wetlands interesting places for scientific research as well as for nature observation . Most tidal wetlands along the Atlantic Coast are marshes colonized by hydrophytic plants: salt-tolerant ( ­ halophytic) plants in salt and brackish regions and freshwater hydrophytes upstream along fresh tidal waters (Figure 1.2). Where tidal ranges are high or in exposed locations in which Figure 1.1. Aerial view of tidal wetlands: (a) along the New Jersey coast showing tidal wetlands formed behind barrier islands and along the Mullica River and (b) tidal wetlands in a Connecticut embayment off Long Island Sound. (a: Copyright Geospatial Division, MDA Information Systems Inc., Gaithersburg, MD) a b Figure 1.2. Marshes dominated by herbaceous species are the most common type in North America, while tidal flats predominate in regions with extremely high tidal ranges: (a) Nova Scotia salt marsh and (b) Maine tidal flat. a b 4  Chapter 1­ currents and wave action are strong, intertidal flats devoid of macrophytic plant life are particularly extensive due largely to longer periods of submergence. As we shall later see, human influences not withstanding , the fate of tidal marshes and swamps­ ultimately rests on their response to changes in sea level (i.e., their ability to maintain a certain elevation above mean sea level), the presence of low-lying lands to allow for their continued landward movement, and the ability of hydrophytes to colonize­ nonvegetated sites (e.g., tidal flats, deltas, and overwash sands on the leeside of barrier islands). Since tides affect bodies of water ranging from the ocean (marine waters) to estuaries to freshwater tidal rivers (tidal fluvial), a diverse group of wetlands have formed along their shores. They include vegetated types such as salt marshes, brackish marshes, estuarine forests, oligohaline marshes, and tidal freshwater marshes and swamps as well as nonvegetated types such as coastal beaches, tidal flats, and rocky shores. Marshes are dominated by herbaceous (nonwoody) plants, whereas swamps are characterized by trees and shrubs. Other coastal wetland...