RESEARCH ARTICLE Restoring Ecological Function to a Submerged Salt Marsh Camille L. Stagg 1,2,3 and Irving A. Mendelssohn 1 Abstract Impacts of global climate change, such as sea level rise and severe drought, have altered the hydrology of coastal salt marshes resulting in submergence and subsequent degradation of ecosystem function. A potential method of rehabilitating these systems is the addition of sediment- slurries to increase marsh surface elevation, thus ame- liorating effects of excessive inundation. Although this technique is growing in popularity, the restoration of ecological function after sediment addition has received little attention. To determine if sediment subsidized salt marshes are functionally equivalent to natural marshes, we examined above- and belowground primary production in replicated restored marshes receiving four levels of sedi- ment addition (29–42 cm North American Vertical Datum of 1988 [NAVD 88]) and in degraded and natural ambi- ent marshes (4–22 cm NAVD 88). Moderate intensities of sediment-slurry addition, resulting in elevations at the mid to high intertidal zone (29–36 cm NAVD 88), restored ecological function to degraded salt marshes. Sediment additions significantly decreased flood duration and fre- quency and increased bulk density, resulting in greater soil drainage and redox potential and significantly lower phyto- toxic sulfide concentrations. However, ecological function in the restored salt marsh showed a sediment addition threshold that was characterized by a decline in primary productivity in areas of excessive sediment addition and high elevation (>36 cm NAVD 88). Hence, the addition of intermediate levels of sediment to submerging salt marshes increased marsh surface elevation, ameliorated impacts of prolonged inundation, and increased primary productiv- ity. However, too much sediment resulted in diminished ecological function that was equivalent to the submerged or degraded system. Key words: net annual primary production, sea level rise, sediment-slurry addition, severe drought, sudden marsh die-back. Introduction The deterioration of coastal wetlands is a significant concern in Louisiana’s Mississippi River Delta complex, where wet- land loss occurs at a rate of 77 km 2 /year (Barras et al. 2003). A multitude of factors have contributed to the decline of these vegetated systems, including canal dredging, levee construc- tion, geological subsidence, and eustatic sea level rise (Boesch et al. 1994; Jelgersma 1996; Turner 1997; Day et al. 2000). Additionally, severe weather events, such as hurricanes and drought, have been linked to the acute loss of coastal wet- lands (McKee et al. 2004; Barras 2009). For example, in 2000 a severe drought caused the sudden die-back of over 100,000 ha of smooth cordgrass (Spartina alterniflora )-dominated salt marsh in southern Louisiana (McKee et al. 2004; Alber et al. 2008). Although some areas recovered after this expansive disturbance, in many cases, salt marshes were converted to 1 Department of Oceanography and Coastal Sciences, Louisiana State University, Baton Rouge, LA 70803, U.S.A. 2 Address correspondence to C. L. Stagg, email staggc@usgs.gov 3 Present address: I.A.P. World Services, Inc., U.S. Geological Survey, National Wetlands Research Center, Lafayette, LA 70506, U.S.A. © 2010 Society for Ecological Restoration International doi: 10.1111/j.1526-100X.2010.00718.x unvegetated mudflats that eventually subsided. Materne et al. (2006) documented up to a 15 cm decrease in elevation at die- back-affected areas relative to unaffected salt marshes. Thus, drought-induced subsidence altered the natural hydrology of the affected areas, resulting in longer periods of inundation accompanied by low redox potentials, high sulfide concentra- tions, and minimal vegetative recovery (Schrift et al. 2008). To ameliorate the effects of excessive inundation, hydrauli- cally dredged sediments were added to a die-back-affected marsh, with the notion that an increase in elevation would improve soil drainage and ultimately vegetative growth (Mendelssohn & Seneca 1980). The use of hydraulically dredged, fine-grain sediments can decrease flood duration (Schrift et al. 2008) and increase bulk density and soil nutri- ent concentrations (Mendelssohn & Kuhn 2003), resulting in greater aboveground biomass, plant density, and cover (Mendelssohn & Kuhn 2003; Slocum et al. 2005; Schrift et al. 2008). However, the effect of sediment-slurry addition in coastal marshes on ecological function has received little atten- tion (but see Edwards & Mills 2005). The goals of restoration are to return a degraded system to pre-degradation conditions based on ecological services, which depend not only on ecological structure, but also on ecosystem 10 Restoration Ecology Vol. 18, No. S1, pp. 10–17 SEPTEMBER 2010