CHAPTER ONE Introduction: Geomorphological and Hydrological Controls on Pattern and Process in a Developing Barrier Island Salt Marsh
1.1 Environmental controls on barrier island pattern and process 

The physical environment exerts controls on organisms which determine the distribution and abundance of species. These physical controls on ecological processes range in scale from macro-scale factors, such as geographic position and climate, which are on the order of tens to thousands of kilometers, to micro-scale chemical factors which can act over millimeters or microns. The scale of the physical forces which are the subject of this study are meso-scale forces which are on the order of meters to kilometers. This study examines how these meso-scale physical factors, such as the local and regional landscape, control patterns and processes within a barrier island salt marsh on a temporal, or successional time scale, and on a spatial scale within the salt marsh.

Hayden et al. (1995) put forth the idea that the vegetation patterns on barrier islands are controlled by the interaction of three free surfaces: the land surface, the seawater surface, and the fresh or groundwater surface. Where these surfaces meet determines the hydrological regime, the substrate physico-chemistry, and thereby the vegetation. In general, the relationship between the landscape, the hydrology, the chemistry, and the biology dictates the pattern and process, or function of these ecosystems. Figure 1.1 shows, schematically, the relationship between these factors.





Figure 1.1. The interrelationship of environmental controlling factors and the biology in a barrier island ecosystem.



The vegetated portion of the intertidal zone closest to the flooding source is dominated by Spartina alterniflora Loisel. The production of this macrophyte is controlled by the physico-chemical make-up of the substrate. These plants are limited by the availability of nutrients (especially nitrogen), by the presence of sulfide or a lack of oxygen, and by high porewater salinities. These physico-chemical factors are controlled by the local hydrologic conditions (Mitsch & Gosselink 1993). The hydrological conditions considered in this study are both above and below the sediment surface. The action of the tides causes subsurface hydraulic gradients, as well as movement of flooding water over the surface of the marsh. Groundwater fluxes occur within the subsurface of the marsh. The elevation and slope of the local landscape are important in determining the magnitude of the tidally induced hydrological processes, and the regional landscape determines the extent of the groundwater fluxes into the marsh. For example, the slope of the surface associated with a creekbank will lead to increased lateral flushing of the sediments by flooding water, and proximity to an upland with a freshwater reservoir will lead to an influx of low salinity water. These processes both act to lower the local salinity, and create a more optimal growing environment for S. alterniflora. In this fashion, the physico-chemical and biotic patterns are controlled by the landscape.

As illustrated in Figure 1.1, there are feedbacks between the biotic factors and the chemistry, and the biotic factors and the hydrology of the marsh. For instance, the production of organic matter by S. alterniflora leads to an increase in the porewater nutrient pool through remineralization. It is the nature of these biotic-abiotic relationships that defines the function of a salt marsh ecosystem. Figure 1.2 is a more detailed schematic outlining the various biotic and abiotic interrelationships that are important in creating the observed pattern and process in a salt marsh. This figure is organized hierarchically (geomorphology hydrology chemistry biology) to show the "chain of command" within the marsh. However, there are feedbacks between all levels. This schematic framework will be used throughout this study to illustrate the various relationships that define the function, or "process", of a salt marsh.

1.2 Ecological succession on a barrier island

Barrier islands are highly dynamic environments (Hayden et al. 1991) which are subjected to frequent disturbances by storms and tides. These disturbances rework the substrate, and as a result the platform upon which the vegetation exists is frequently altered. This causes alteration of the free land surface. When the sediment surface is completely reworked, such that it is devoid of vegetation, the successional clock is reset. Because this happens in different regions at different times, barrier islands consist of a mosaic of different aged marshes. This environment affords the opportunity to examine the process of succession in salt marshes by employing a "space-for-time" substitution where the different aged marshes can be examined contemporaneously. This is a luxury that happens infrequently in more stable environments.

Once vegetation becomes established on the platform, the successional process begins, and there is then interactive control on marsh function between the biotic and the abiotic as illustrated in Figure 2.2. There is control on the organisms by the structuring forces of the environment, as well as control of the environment by the organisms. Because, ultimately, the local and regional landscape exert control over the organisms, the process associated with the succession may occur at different rates in different locations. To revisit a previous example, where the productivity, and thereby organic matter deposition and remineralization are greatest, the interaction between the biotic (S. alterniflora production) and the chemical (nutrient status) will be exaggerated, and therefore the rate at which the marsh achieves a mature functional state will be accelerated in this region.

1.3 Scope of this study and objectives 

This study poses questions of both a structural and functional nature. First, it asks the question "what are the defining characteristics of these young marshes?" Second it asks the question "why?", i.e. "why do we see the patterns that we see?", and "what are the factors that are controlling these patterns?". Finally, it asks "how?". "How do these controlling factors influence the marsh so that we observe these patterns?"

The first objective of Chapter 2 is to describe the process of ecological succession in a salt marsh by examining a series of different aged marshes. The focus of this chapter is the dynamics in the region of the creekbank. As discussed previously, this study relies on a "space-for-time" substitution, where marshes of different ages are used to simulate the process of aging. Traditional definitions of succession are based on the sequential replacement of communities until a climax community is achieved. In the case of a salt marsh, there is a single dominant primary producer from the primary sere to the climax sere. Because the aging process cannot be defined by the primary producer community, another approach must be taken. The approach used here is an overall description of the biotic and physico-chemical nature of the system at each age. Given the interrelatedness of the various factors described in Figure 2.2, I expected to see changes in the biomass and physiological status of the primary producers, a change in the concentration of porewater nutrients and other solutes, and a change in the organic content and the grain size of the sediments. These changes were observed, although the rate at which the variables achieve a "mature" level is not consistent between variables.

In addition to looking at temporal changes, I also examine the spatial patterns in each of these marshes. There is a well known variation in marsh structure and function as distance from the edge of a creek increases. This area has greater primary production due to the increased flushing of the sediments. The surface morphology causes this increased flushing because of the slope and elevation of the surface relative to sea level. Because the primary production is greater closer to the creekbank, I expected that perhaps there is a difference in the abiotic-biotic interactions that are responsible for creating the changes associated with marsh maturation. Indeed, it appears that the marsh matures more quickly closer to the creekbank.

Overall, by examining the spatial variability in marshes of different ages, I can examine the successional process both within and between marshes. This can be used first, to define a rate of succession and second, to examine if this rate is different depending on location within the marsh. The final question addressed in this chapter is: "why is there is a difference in successional rates on a spatial scale?" or "how does the variation in surface morphology affect the rate of succession?". This chapter attempts to illustrate the functional reasons why a physical parameter, surface morphology, can affect an ecological process, succession. 

The third chapter focuses on the very young marshes. Two young tidal creek ecosystems of approximately equal age and regional morphology were examined in order to determine if the successional trajectory is the same in all locations. It was observed that the general pattern of succession is the same between these two systems, but that the rate of change was different. The overall hypothesis for this chapter is that the regional morphology is important in controlling the successional processes within the marsh. In addition, this study is an attempt to place these marshes in the context of the entire back-barrier upland/marsh complex, and to show that the marshes do not exist in isolation from the rest of the immediate environment.

Again, at first, structure questions like "what do these young marshes look like?", and "what are the apparent differences between them?" are posed. Second, I ask the function question "what is responsible for these differences?". In this case, the difference between sites cannot be found in the immediate vicinity of the creekbank, and one must look further, in a spatial sense, to find the answer. There are differences between marshes that are the result of differences in the entire 'catchment', and do not relate to the morphology of the intertidal zone. In particular, this study examines how a difference in the geomorphology of the upland/marsh complex can result in variation in the subsurface hydrological processes in the region of the creekbank, how a difference in subsurface hydrology can change the physico-chemistry of the sediment, and ultimately, how the physico-chemistry relates to the production of the dominant plant species. In general, it attempts to establish a link between geomorphology, hydrology, chemistry and biology. Ultimately, the goal of this chapter is to illustrate that the successional process can vary between marshes as a result of the regional landscape.

1.4 Significance of this study 

"Wetlands are among the most important ecosystems on Earth" (Mitsch & Gosselink 1993). Marshes are important for shoreline stabilization, have been touted for their potential capacity as natural wastewater treatment facilities, and overall play a key role in the productivity of the larger estuarine/coastal ecosystem. Understanding the pattern and process of these important ecosystems is imperative in attaining an overall understanding of the function of coastal ecosystems. Although the function of mature marshes has been addressed in scientific research for the past few decades, the function of salt marshes during the early stages of succession has not been fully addressed. In gaining a knowledge of the factors controlling the development of marshes, a further understanding of how these systems can be preserved, restored and created will be achieved. 

In recent decades there have been increasing efforts to restore and create wetlands for the mitigation of wetland destruction elsewhere (Mitsch & Wilson 1996). However, one of the primary reasons for the failure of such projects is that there isn't a clear understanding of the function of wetlands (Mitsch & Wilson 1996). This seems to be especially true of the function of young, naturally developing wetlands. For example, many studies make use of a natural mature companion wetland in order to compare the function of the newly created wetland and to evaluate the overall success of the project in creating a sustainable system (Brinson & Rheinhardt 1996). As this study suggests, young and old marshes do not differ greatly in terms of the primary producers, but they do differ in many other physico-chemical factors. Thus, unless an understanding of the function of a natural immature marsh is achieved, these comparisons are not appropriate. Studies such as this one, which examine the pattern and process, or function of immature marshes are imperative in the design, implementation, and evaluation of restoration activities.