Location: Kings Bay, Georgia
Oceanic Region: Southeast Atlantic
Salt Marsh Habitat Restoration in the Southeast United States
The destruction and degradation of intertidal salt marsh habitats is a major concern in southeastern Atlantic coastal environments. Recent increases in the numbers of people living within 100 km of the coastline have resulted in the exploitation and inevitable destruction of marshes. Predictions of accelerated growth in U.S. coastal populations---an additional 60 to 200% over the next 10 years (Culliton et al., 1990)---only increase the likelihood of detrimental impacts on native salt marshes. The loss of marsh habitat can limit the ability of coastal environments to handle storms, run-off, sedimentation, and pollution effectively. Marsh loss also negatively affects nutrient cycling, groundwater quality, ecosystem biodiversity, and trophic stability. Additional impacts on mammal, bird, fish, and invertebrate populations reduce the recreational and commercial value of both nearshore and offshore coastal ecosystems. Restoration ecology, including creation of new or rehabilitation of existing marshes, offers the best and perhaps only hope to alleviate most or all of the negative consequences attributed to marsh loss (Dobson et al., 1997). However, the ability of past or future restoration efforts to reestablish natural functioning marshes within a reasonable time frame remains in question.
Studies that assess the success of marsh restoration efforts have identified numerous structural (biomass, density, and species composition) differences between natural and restored marshes. The reestablishment of natural marsh characteristics typically is dependent on the variable being measured and the age of the restored marsh (Craft et al., 1999). Visually comparable stands of vegetation can be created quickly when one or a few marsh plant species such as the smooth cordgrass, Spartina alterniflora, are transplanted into appropriately prepared, previously barren sites along the U.S. eastern and Gulf coasts. Restored marsh plant growth and aboveground primary productivity can equal that of natural marshes within 5 years after planting. However, faunal characteristics often require decades or longer before the reestablishment of natural levels within restored marshes. In some restored marshes, macrobenthic species composition and abundance have remained dissimilar up to 17 years after completion of the restoration effort (Sacco et al., 1994). Decade-long differences in the presence of microorganisms, organic matter accumulation, and nutrient cycling also can exist between natural and restored marshes.
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Figure 1
Kamehameha marshes (clockwise from upper left): NE view of the restored marsh after initial planting in 1996; same NE view of the restored marsh 2 years later in 1998; adjacent natural marsh along the North River in 1998.
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Figure 2
The structural inequalities between restored and natural marshes studied to date suggest either directly or indirectly that differences in ecological processes may exist and affect the natural functioning of restored marshes. Salt marshes serve a number of critical, ecological roles in coastal ecosystems that include providing food resources and refuges from predation (Nixon, 1980; Dame, 1989). Identification of how ecological functions vary between natural and restored marshes is necessary to assess accurately the ability of restoration efforts to minimize or reverse impacts of coastal habitat loss. The objective of this case study is to explore whether southeastern U.S. restored and natural salt marshes are similar ecologically by determining differences in resource availability and experimentally examining the possible functional effects on a marsh-dependent faunal species.
Studies were conducted within adjacent natural and restored Kamehameha salt marshes along the North River on the Kings Bay Naval Subase at Kings Bay, GA, USA (81°32’W, 30°47’N). The extensive North River natural marshes exhibit differences in the types of plants with increasing elevation and composition typical of southeastern intertidal habitats. Tall ( › 1 m), low-marsh cordgrass (S. alterniflora) plants along the tidal channels (Fig. 1) grade into shorter ( ‹ 0.5 m) plants in the less frequently flooded high-marsh areas. Stands of black needle rush (Juncus roemerianus) commonly separate the high-marsh from pine-oak uplands. In spring 1995, approximately 6 hectares of pine upland were logged. Roots and debris were removed to a depth of 45 cm, and the site was graded to typical intertidal elevations. A network of tidal channels was dug, intersecting and connecting the site to the North River. Intertidal elevations in the site were planted in November 1996 with 10.2 cm diameter plugs of S. alterniflora placed 0.9 m on center (Fig. 1).
Plant and sediment characteristics were determined within natural and restored Kamehameha marshes during the summers of 1997 and 1998. Spartina alterniflora stem densities (live, standing-dead, and shoot) were collected from 0.25 m2 quadrants randomly placed within typical low-, mid-, and high-marsh elevations. Sediment was sampled within each marsh elevation using a 2.1 cm diameter corer to a depth of 2 cm. Individual cores immediately were frozen at
-20°C and processed later. The ash-free dry mass (AFDM) of sediment samples was calculated as the difference between sediments dried to constant mass at 60°C and then ashed at 450°C for › 4 h. A one-way analysis of variance, consolidating the variation among elevations, was used to evaluate the differences between marshes (natural, restored) on each date.
The growth of a ubiquitous marsh resident species, the salt marsh periwinkle (Littoraria irrorata) (Fig. 2), was followed within natural and restored marshes in a series of mark-recapture experiments. Littoraria irrorata depend directly on the marsh for food, foraging on S. alterniflora and sediments, and are the natural prey of commercially important fish and crabs. Potential differences in snail growth would indicate a lack of functional similarity between natural and restored marshes. Experimental snails were collected randomly in May of 1997 and 1998 from the same natural marsh along the St. Marys River, approximately 40 km from the Kamehameha marshes. The collection of snails from a distant, common site was intended to eliminate possible confounding effects that might result from using snails collected directly within the Kamehameha experimental marshes. Individual St. Marys snails were measured (length, width, height, wet weight), marked with numbered tags, and 200 each released within natural and restored Kamehameha marshes (Fig. 2). Snails released in May of each year were recaptured from the field in August and re-measured. Sample tests were used to compare the differences in snail shell length distributions between the dates (May, August) and the marshes (natural, restored).
Consistent with other studies of newly restored marshes, plant density and sediment organic content varied between natural and restored Kamehameha marshes. In 1997, approximately 1.5 years after initial planting, S. alterniflora total stem densities were significantly greater (a probability of occurrence higher than .001 following an X test) in natural compared to restored marshes (Fig. 3). Results were different the next year when there was no significant difference in total stem density between natural and restored marshes (Fig. 3). Sediment organic content (Fig. 3) consistently was significantly greater in natural marshes both in 1997 and 1998.
Shell growth of recaptured L. irrorata varied between marsh and year. The number of snails recaptured in August ranged between 34 to 55% with consistently fewer snails recovered from within the natural marsh. Initial May shell length distributions for recovered snails (Fig. 4) were statistically similar between marshes in both 1997 and 1998. Shell length increased on average between 6.5 and 11.6% during the May to August period. By August, snail shell length distributions were significantly different between natural and restored marshes in 1997 but not in 1998. Snails greater than 19 mm in length were more common within the restored compared to natural marshes in 1997 (Fig. 4). Changes in shell length were reflected by increases in snail wet weight. Restored marsh snail wet weights in 1997 and 1998 increased by 32 and 29% while natural marsh snail wet weights only increased by 22 and 21%, respectively.
Figure 3
Natural and restored Kamehameha marsh surface sediment ash-free dry mass and Spartina alterniflora total stem density in 1997 and 1998. Values are means + standard errors for 10 to 30 samples collected throughout the marshes.
Figure 4
Size frequency distributions for marked snails released within natural and restored marshes in May and recaptured in August of 1997 and 1998.
Experimental results suggest that the 1- to 2-year-old restored Kamehameha salt marsh was similar to natural marshes in the way it functions. Snail growth, a measure of trophic stability and food availability, within the restored marsh was greater than or equal to growth in the natural marsh. Increases in snail shell length should have been reduced significantly if the restored marsh was not equivalent functionally to the natural marsh. Results from marsh comparisons of snail growth were unexpected given the age of the Kamehameha restored marsh. Past restoration studies indicate that greater structural differences typically exist between natural and young or newly restored marshes. Structurally, the 1-to 2-year-old Kamehameha restored marsh paralleled results from previous studies with plant stem density and sediment organic content generally greater within adjacent natural marshes. However, structural differences were not translated into expected functional differences between the marshes. Snail growth within the restored marsh, indicated by the greater than or equal increases in length and wet mass, did not reflect the relative structural differences between restored and natural marshes.
A number of factors may have contributed to the observed greater-than-expected growth of restored marsh snails. The quantity of food resources, duration of foraging times, or quality of available food all can affect faunal growth rates. Measurements of food resource amounts varied between the Kamehameha marshes, but the relative difference would suggest greater growth of natural, not restored, marsh snails. Spartina alterniflora stem densities, a measure of macrophyte production, and sediment AFDM, an estimate of detrital and microalgal availability, typically were greater within the natural marsh. The greater availability of food did not translate into greater rates of snail growth within the natural marsh. Food amounts may not limit snail growth once a certain threshold is reached (for example, X stems per m2). Within the Kamehameha restored marsh, the threshold appears to have been reached by the end of the first full growing season.
Evidence that the growth of restored marsh snails may be explained by differences in the time available for foraging is either lacking or speculative. If snails feed exclusively during high tides, flooding frequency and depth along with predator avoidance may affect the time that intertidal snails are able to forage for food. There was no indication that the restored marsh flooded less frequently or to shallower depths than the natural marsh (Walters, pers. obs.). A comparison of selected locations confirmed that low-, mid- and high-marsh sites were at comparable elevations within the adjacent natural and restored marshes. However, the reduced recovery of marked snails in the natural marsh suggests that predation pressure may have been greater within the natural marsh. Increased predation pressure (for example, greater numbers of blue crabs entering the natural marsh) may have affected snail behavior (for example, stem climbing to avoid predation) and reduced the time available for foraging. The ability of snails to feed during periods of low tide also could decrease the effects of decreased foraging during high tides. Unfortunately, direct evidence to indicate whether predation pressure was greater within the Kamehameha restored marsh, or whether L. irrorata were able to adjust their foraging behavior based on the presumed increased predation pressure, is unavailable.
Although food amounts generally were greater within the natural marsh, the quality of available food may have been greater within the restored marsh. Studies indicate that food quality (nutritive value and palatability) can affect feeding behavior and the overall growth and survival of marine organisms. Spartina alterniflora within the restored marsh may have benefited from the initial release of sediment-bound nutrients that often occurs in disturbed habitats. The increased availability of nutrients could have produced plant material of greater nutritive value, resulting in the equivalent or greater growth of restored marsh snails.
The relatively young Kamehameha restored marsh was able to support snail growth rates that were greater than or equal to the rates observed within adjacent natural marshes. Results do not suggest that either long-term or additional functional differences do not exist between the Kamehameha marshes. Numerous studies of marsh restoration efforts have documented decade or longer differences in structural elements that could easily be reflected in functional differences between marshes (Craft et al., 1999). Clearly, sediment organic matter is reduced significantly within the Kamehameha restored marsh and will likely continue to affect biogeochemical cycling for years to come. However, for a typical marsh resident, snails, the 2-year-old restored Kamehameha marsh is functionally "as good as" the well-established natural marshes along the North River.
Acknowledgements
This study would not have been possible without support from J. Garner, P. Schoenfeld, and R. Wilkenson of the Facilities and Environmental Division, Kings Bay Naval Base, Kings Bay, Ga.
References
Craft, C., J. Reader, J. N. Sacco, and S. W. Broome. 1999. Twenty-five years of ecosystem development of constructed Spartina alterniflora (Loisel) marshes. Ecological Applications 9: 1405-1419.
Culliton, T. J., M. A. Warren, T. R. Godspeed, D.G. Remer, C.M. Blackwell, and J. J. McDonough, III., 1990. Fifty years of population change along the nation's coasts. The 2nd report of a coastal trends series. NOAA Publication.
Dame, R .F. 1989. The importance of Spartina alterniflora to Atlantic Coast estuaries. Reviews in Aquatic Sciences. 1: 639-660.
Dobson, A.P., A. D. Bradshaw, and A.J.M. Baker. 1997. Hopes for the Future: Restoration ecology and conservation biology. Science 277: 515-522.
Nixon, S. W. 1980. Between coastal marshes and coastal waters---A review of twenty years of speculation and research on the role of salt marshes in estuarine productivity and water chemistry. In P. Hamilton & K. B. MacDonald, eds., Estuarine and wetlands processes with emphasis on modeling. New York: Plenum Press.
Sacco, J. N., E. D. Seneca, and T. Wentworth. 1994. Infaunal community development of artificially established salt marshes in North Carolina. Estuaries 17: 489-500.
Key Principles
- Habitat restoration
- Functional similarity
- Trophic stability
Ethical Considerations
- Is marsh restoration a viable answer to the destruction of coastal habitats?
- Can naturally functioning salt marshes be restored effectively?
Author
Keith Walters
Department of Biology
Middle TN State University
Murfreesboro, TN 37132
kwalt@mtsu.edu
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