CHAPTER FOUR Conclusions and Implications
The overall goal of this study was to describe the pattern and process associated with variations in geomorphology in a developing barrier island salt marsh, on a spatial and a temporal scale. In a young salt marsh, the variations in pattern are clearly related to the spatial variability of the marsh surface, both locally, and on the larger scale of the ecosystem. In addition, the function of the marsh, in terms of the rates of production, the physiological development of the plants, and the relative importance of the biotic and abiotic interactions that define marsh function, also varies spatially in relation to the landscape controls. As a part of the spatial variability in function, this study has illustrated that the rate of change of pattern, as well as a change in function, as described by the succession of the ecosystem, is also determined by the landscape.

4.1 Landscape controls on patterns 

The patterns present in the system were described by: the chemical and physical status of the substrate, the biomass of the primary producer, and the physiology of the primary producer. There are clearly trends in the physico-chemistry of the substrate in relationship to the creekbank in a young marsh. The substrate closest to the creekbank is richer in organic material, carbon and nitrogen, and has a lower salinity than the substrate farther away. The biomass of the primary producer is also different, depending on the proximity to the creekbank, and the nitrogen content of the plants differs as well. At the level of the ecosystem, the variation in the type of catchment also has an effect on both the substrate chemistry. The marsh which drains an upland catchment has lower salinities than the marsh which drains a salt flat. Salt is detrimental to S. alterniflora growth, and therefore, there is lower S. alterniflora production in the marsh with higher salinities. In addition to having lower overall production, the plants in the marsh with higher salinities exhibit a distinctly different physiological make-up.

4.2 Landscape controls on process 

Because the structure of the marsh is different in different places depending on the morphological controls, the function of the marsh in that region is also different. As discussed in the previous paragraph, the high salinities of creek Y2, the creek that drains a salt flat, have a detrimental effect on the plant. The plant is osmotically stressed by the salt concentration, and reacts by changing its physiological makeup. Nitrogen is accumulated to counteract the osmotic imbalance between the plant and its environment. Thus, these plants are physiologically, and thereby functionally different than plants in a low salinity environment. Further from the creekbank, the sediment is less well flushed, salts and toxins accumulate, and the production is lower. The lower production at a distance from the creekbank, as well as the lower production as determined by high salinities, changes the relative importance of the abiotic-biotic feedbacks as illustrated in Figure 1.2. Because plants are less productive, a smaller amount of organic material is contributed to the sediment at the end of the growing season, and therefore fewer nutrients will be made available to the plants the following year. These changes lead to variability in the temporal patterns of change in these marshes.

4.3 Temporal changes in pattern: succession 

The use of this chronosequence allowed an examination of the changes in pattern that take place in a salt marsh on a successional time scale. It was illustrated that as the marsh ages, there are predictable patterns that develop. The sediment becomes more highly organic, and the relative fractions of sand, silt and clay in the sediment change. There is an increase in the porewater nutrients, an increase in porewater sulfide, and a decrease in the redox potential. The biomass of the plants also changes, as does the reproductive strategy, and the elemental composition of the plant tissue.

4.4 Landscape controls on the rate of succession 

As discussed previously, the variability in the landscape, both the local slope and elevation, as well as the relative location of the marsh within the upland/marsh complex is important in determining both the pattern and process exhibited by the system. The biotic-abiotic feedbacks which define the function of the marsh are also responsible for the changes that we see over a successional time scale. Thus, the current function of the marsh controls the rate at which the marsh changes over time. For example, (1) greater production leads to more organic soils, which increases the nutrient content of the sediment, and (2) greater production leads to greater entrainment of particulates, which over time changes the character of the substrate. In addition, as was illustrated in Chapter 2, the processes that occur within the creeks themselves are also important in changing the patterns observed, and thereby the function of the marsh adjacent to the creek. Thus, the landscape or morphology has the power to accelerate the rate of succession in a salt marsh.

4.5 Implications for community structure 

While the overall community structure in these marshes was not addressed experimentally, it is possible to address the implications that the variable structure and function caused by the variable geomorphology may have on the community. Salt marsh communities are primarily detritus-based systems. The dominant resident invertebrates are fiddler crabs (Uca pugnax and U. pugilator), snails (Littoraria irrorata and Ilyanassa obsoleta) and the ribbed mussels (Geukensia demissa). For the most part, these organisms are detritivores. Thus, in a young marsh where there is very little organic material, it is unlikely that there will be a large, diverse community of detritivores. In the young marshes, the only invertebrates observed were the sand fiddler crabs (U. pugilator). However, in the older marshes there were mussels, mud fiddler crabs, and both species of snails (pers. obs.). Within the older marshes there appeared to be some spatial variability in the distribution of the invertebrates based on elevation. At the creekbank, which has a lower elevation, fewer invertebrates were observed as compared to the interior of the marsh. This may be related to inundation time and their susceptibility to predation by blue crabs (Callinectes sapidus) at low tide (B. Silliman, pers. comm.). Thus, the landscape may be controlling the distribution of the invertebrates as well, which has obvious implications for the cycling and recycling of energy within the system. 

Another potential implication for the control that the landscape has, indirectly, on the community structure is related to the elemental composition of the plants. Plants with a higher N content are likely to be more palatable to grazers, and are likely to produce more nutritious detritus. Thus, the S. alterniflora in older marshes, which has a higher N content may be preferentially grazed upon. This may also be the case within the younger marshes, where the plants subject to higher salinity have a higher N content. If so, this would add an additional stressor to an already stressed plant, and would alter the functional status of that marsh to a greater extent. These are ideas which deserve further attention in the future.

4.6 The "strategy of ecosystem development" 

In Odum's classic (1969) paper he developed a strategy of ecosystem development. He discussed that succession is an orderly and predictable process that is controlled by the interactions between organisms and their environment. From this study, which has been an evaluation of the processes controlling the structure, the function and the rate of change in both structure and function in a salt marsh, it is evident that the interactions between the organisms and their environment is a very important part of the strategy of development. Another idea that has become clear is that it is important to carefully define the boundaries of the ecosystem when pattern and process are being evaluated. Marsh ecosystems exist within the larger marsh/upland complex, and the upland can have important effects on the structure and function within the marsh. Thus, the strategy of ecosystem development depends on the function of the ecosystem, and this function is controlled by the landscape, the hydrology, the chemistry, and the biology of the ecosystem.

4.7 The implications of this study for salt marsh restoration 

In recent years, there has been an increased effort made to mitigate salt marsh destruction by restoring damaged marshes, and by creating new ones (Race & Christie 1982, Mitsch & Wilson 1996). Narrow, fringing marshes are particularly important in stabilizing shorelines (Lugo & Brinson 1979). Studies such as this one, which examine the structure and function of the salt marsh ecosystem during the early stages of marsh development, are important in facilitating an understanding of how a 'natural' young marsh works. 

Salt marshes are an important ecological and economic resource (Mitsch & Gosselink 1993, Lugo & Brinson 1979). In recent years, there has been an increase in efforts to restore and create marshes for mitigation of destroyed marshes. Understanding the patterns and processes associated with marsh development is crucial to planning successful restoration efforts, and to evaluating the success of past restoration efforts. As discussed by Mitsch and Wilson (1996), one of the greatest needs in wetland mitigation projects is a better understanding of wetland function. Many mitigation projects fail because of a lack of communication between the engineers who design and restore wetlands and the ecologists who study them.

4.7.1 Getting the hydrology right

It is commonly accepted that the hydrologic regime is the primary controlling factor in a wetland (Mitsch & Gosselink 1993). Therefore, getting the hydrology "right" in a created marsh is of utmost importance. Indeed, many restoration efforts fail because of improper hydrological regimes (Mitsch & Wilson 1996). The key to this is two-fold, because there are two major factors that control the hydrology of a salt marsh: the height of the seawater table, and the height of the groundwater table. First, it is important to establish an ideal relationship with the groundwater table by placing the marsh in an ideal landscape. If a wetland is placed improperly in a watershed, the success and function of the marsh will be altered (Mitsch & Wilson 1996, Bedford 1996, Zedler 1996). The results of this study indicate that the relationship of the marsh to the upland is quite important in determining the input of groundwater to the marsh; this is especially true in the narrow, fringing marshes characteristic of the back-side of barrier islands. If there is adequate inputs of freshwater, the salinities will be maintained at a lower level, and the production of S. alterniflora will be enhanced.

The impact of the seawater table is determined by the elevation and slope of the marsh surface. The higher the elevation relative to sea level, the more ET is important in driving the salinity (de Leeuw et al. 1991). At lower elevations, and especially at creekbanks, the action of tidal flushing is more important in controlling the salinity (Agosta 1985, Yelverton & Hackney 1986). Thus, by establishing a marsh surface that is properly placed relative to sea level, the salinity, and therefore the productivity of the marsh can be optimally controlled.

4.7.2 Controlling the rate of development 

This study suggests that there are several factors that can influence the rate at which succession proceeds in a marsh: the elevation, the slope, the salinity, and the productivity of the marsh. All of these factors have been related to landscape level controls, whether through the existence of spatially varying morphology at the creekbank, or through the placement of the marsh in the larger scale marsh-upland complex. Where Spartina alterniflora is growing successfully, the marsh matures at a faster rate. If morphological conditions are established to maximize productivity in a created marsh, then successional processes will proceed more quickly, and the marsh will achieve functional maturity at an earlier point in time.

4.7.3 Evaluation of restoration activities 

Often the success or failure of a restoration project is evaluated based on the species present, or the biomass achieved. This evaluation is generally made by comparison with a companion or reference marsh, a nearby marsh that is mature (Race & Christie 1982) and that meets the standards of function and sustainability (Brinson & Rheinhardt 1996). From this study, we see that the biomass in a very young marsh, and the biomass in a mature marsh are not all that different. However, it is also evident from this study that a 15 year old natural marsh and a mature marsh are significantly different in their function. There are great differences in the physico-chemistry, the factors controlling production are different, and the strategies of production are also different. It follows that a 15 year old restored marsh and a mature marsh will also be significantly different in function, even if they do not differ in biomass. Creating a functional understanding of marsh pattern and process during natural development will aid in the critical evaluation of restoration activities.