1494  2003 Estuarine Research Federation Estuaries Vol. 26, No. 6, p. 1494–1504 December 2003 Relationships of Nitrogen Loadings, Residential Development, and Physical Characteristics with Plant Structure in New England Salt Marshes CATHLEEN WIGAND 1, *, RICHARD A. MCKINNEY 1 ,MICHAEL A. CHARPENTIER 2 ,MARNITA M. CHINTALA 1 , and GLEN B. THURSBY 1 1 U.S. Environmental Protection Agency, Office of Research and Development, National Health and Environmental Effects Research Laboratory, 27 Tarzwell Drive, Narragansett, Rhode Island 02882 2 OAO Corporation, 27 Tarzwell Drive, Narragansett, Rhode Island 02882 ABSTRACT: We examined the vascular plant species richness and the extent, density, and height of Spartina species of ten Narragansett Bay, Rhode Island (United States) fringe salt marshes which had a wide range of residential land development and N-loadings associated with their watersheds. Significant inverse relationships of tall S. alterniflora with species richness and with the extent and density of S. patens and short S. alterniflora were observed. Extent and density of S. patens and extent of short S. alterniflora were positively and significantly related with plant species richness. Marsh elevation and area did not significantly correlate with plant structure. Flood tide height significantly and inversely cor- related with S. patens, but did not significantly relate to S. alterniflora or plant species richness. Marsh width significantly and positively correlated with plant species richness and S. patens and inversely correlated with tall S. alterniflora. Signif- icant inverse relationships were observed for N-load, % residential development, and slope with S. patens, short S. alterniflora, and species richness, and significant positive relationships with tall S. alterniflora. The marsh slope and width were significantly correlated with N-load and residential development that made it difficult to determine to what extent anthropogenic stressors were contributing to the variation in the plant structure among the marshes. At five marshes with similar slopes, there were significant inverse relationships of N-load with S. patens (density and extent) and a positive relationship with tall S. alterniflora (extent). Although there were no significant relationships of slope with the plant metrics among the five sites, other physical factors, such as the flood tide height and marsh width, significantly correlated with the extent and density of Spartina species. Significant relationships of N-load with plant structure (albeit confounded by the effect of the physical characteristics) support the hypothesis of competitive displacement of dominant marsh plants under elevated nitrogen. It is likely that the varying plant structure in New England marshes is a response to a combination of natural factors and multiple anthropogenic stressors (e.g., eutrophication and sea level rise). Background Although salt marshes are often considered ni- trogen (N) poor (Valiela and Teal 1974), non- point source pollution to coastal waters has dra- matically risen in recent decades and the N status of critical habitats such as wetlands may also be affected by these increased nutrient loads (Carpen- ter et al. 1998; Deegan 2002). Recent research in New England salt marshes has shown increasing stable N isotopic ratios in salt marsh biota that has been attributed to increasing wastewater contribu- tions from adjacent watersheds (McClelland and Valiela 1998; Valiela et al. 2000a; McKinney et al. 2001; Wigand et al. 2001). It has been demonstrat- ed in fertilization studies at a few New England sites that Spartina patens, which is the superior com- petitor when N is limiting, is out-competed and * Corresponding author: tele: 401/782-3090; fax: 401/782- 3030; e-mail: wigand.cathleen@epa.gov displaced by Spartina alterniflora under N-enriched conditions (Levine et al. 1998; Emery et al. 2001). These fertilization studies suggest that under nu- trient-enriched conditions where S. alterniflora is both the best competitor for N and most tolerant of physical stresses an entire salt marsh could be- come dominated by this one species (Levine et al. 1998; Emery et al. 2001). It has generally been demonstrated, although not specifically in salt marsh plant communities, that there is decreasing plant species diversity in macrophyte-based ecosys- tems with increasing nutrient availability (Tilman 1984, 1987; Goldberg and Miller 1990). The New England salt marsh plant structure is described as a patchy mosaic of S. alterniflora (short and tall forms), S. patens, Distichlis spicata, Juncus gerardii, Salicornia europea, and bare patches that are influenced by elevation, tidal inundation, salin- ity, sediment oxygen levels, interspecific competi- tion, and availability of N (Redfield 1972; Nixon