Charles T. Roman is a coastal ecologist with the US National Park Service and professor-in-residence at the University of Rhode Island Graduate School of Oceanography. David M. Burdick is Research Associate Professor of Coastal Ecology and Restoration in the Department of Natural Resources and the Environment at the University of New Hampshire.

A Synthesis of Research and Practice on Restoring Tides to Salt Marshes

CHARLES T. ROMAN AND DAVID M. BURDICK

The structure and ecological function of salt marshes are defined by many interacting factors, including salinity, substrate, nutrient and oxygen availability, sediment supply, and climate, but hydrology (the frequency and duration of tidal flooding) is a dominating factor (e.g., Chapman 1960; Ranwell 1972; Daiber 1986). When tidal flow is restricted there can be dramatic changes to physical and biological processes that affect vegetation patterns, fish and avian communities, and biogeochemical cycling, among others. Throughout the developed coastal zone, roads and railroads that cross salt marshes often have inadequately sized bridges and culverts that restrict tides (fig. 1.1). Tide gates are also a common feature, eliminating or dramatically restricting flood tides from entering salt marshes but allowing for some drainage on the ebb tide. Other tide-restricting practices that have been ongoing for centuries include impoundments for wildlife management purposes (Montague et al. 1987) and diking and draining to facilitate grazing and agriculture (Daiber 1986; Doody 2008). Diking is particularly extensive in Atlantic Canada (Ganong 1903), Europe (Davy et al. 2009), and the United States (e.g., Delaware Bay, Sebold 1992; San Francisco Bay, Nichols et al. 1986).

With tidal restriction there are often dramatic changes in vegetation as salt and flood-tolerant species of the salt marsh are displaced by plants typically found in fresher and drier conditions. Under regimes of tidal restriction, Spartina-dominated (cordgrass) marshes in the northeastern United States have been invaded by the aggressive Phragmites australis (common reed), often in dense monocultures, and other less salt-tolerant herbaceous and woody species (e.g., Roman et al. 1984; Burdick et al. 1997; Crain et al. 2009). Phragmites marshes, when compared to short-grass Spartina meadows, reportedly do not provide suitable habitat for birds, especially those that typically nest in salt marshes (Benoit and Askins 1999; DiQuinzio et al. 2002). Fish abundance, species composition, and food web support functions are altered by tidal restriction when compared to tide-unrestricted systems (e.g., Dionne et al. 1999; Able et al. 2003; Raposa and Roman 2003; Wozniak et al. 2006). Feeding, reproduction, and nursery function can be much reduced or eliminated based on studies documenting the response of the dominant East Coast marsh fish, Fundulus heteroclitus (mummichog), to Phragmites invasions (Able and Hagan 2000; Able et al. 2003; Hunter et al. 2006). Tidal restriction can result in significant subsidence of the sediment surface and acidification of salt marsh soils, with subsequent declines in marsh primary production and export (e.g., Anisfeld and Benoit 1997; Portnoy 1999). Water quality concerns, especially low levels of dissolved oxygen in tide-restricted marshes, have been reported with detrimental effects on estuarine fauna (Portnoy 1991).

The practice of restoring tidal flow to degraded tide-restricted salt marshes has been actively pursued for decades. In Delaware Bay (New Jersey) over 1700 hectares of salt marsh that had been diked and cultivated for salt hay are now undergoing tidal restoration (Weinstein et al. 1997; Philipp 2005). Similarly, restoration efforts through the natural or deliberate breaching of dikes are under way in the United Kingdom and other parts of Europe (Pethick 2002; Wolters et al. 2005; Davy et al. 2009), Bay of Fundy (Byers and Chmura 2007), San Francisco Bay (Williams and Faber 2001; Williams and Orr 2002), the Pacific Northwest (Thom et al. 2002), and elsewhere. Along populated coasts, managers are also engaged in programs to restore tidal flow to degraded salt marshes by removing tide gates and enlarging culverts, bridge openings, and other flow restrictions, with numerous examples...