parameters and related processes, most significantly mean sea level, is likely to impact wetland distribution, including that of mangrove and saltmarsh. It has long been hypothesised that changes in the relationship between wetland elevation and water level can have significant impacts upon mangrove and saltmarsh, threatening their survival (Woodroffe 1990). However, the relationship between water level and wetland elevation is not simple, with many biophysical feedbacks between water level, inundation depth, duration and frequency of inundation, groundwater levels,
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sedimentation, and plant productivity; all of which have the potential to influence wetland surface elevation relative to sea level (Cahoon et al. 2006; McKee et al.
2012; Rogers & Saintilan 2008; Whelan et al. 2005). This concept, many factors affecting wetland elevation relative to sea level, was best described by (Pethick 1981) and is referred to as the negative feedback loop (Figure 3.10). The negative feedback loop is foundational for projecting the response of wetlands to sea level rise and is a central component to the SET technique. Tides bring sediments into wetlands which are positioned between mean sea level and highest astronomical tide, over time building wetland elevation, with the subsequent response being a decrease in elevation (Figure 3.10).
Figure 3.10: The negative feedback loop: relationships between sedimentation and elevation in coastal wetlands (Saintilan et al. 2009).
Several studies have investigated the relationship between wetland elevation, water levels, and various biophysical feedbacks in mangrove and saltmarsh environments in Southeastern Australia, including Homebush Bay. Vertical sediment accretion, one of the key drivers of surface elevation change (Figure 3.10), has been found at to contribute to 67% of surface elevation change in saltmarsh settings, and 51% in
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mangrove settings in a study of multiple sites in Southeastern Australia in a variety of geomorphological settings using the SET-MH technique (Rogers 2004). No
consistent relationship was observed between surface elevation change and sediment accretion at all sites, as below ground processes influence the surface volume, meaning that surface elevation gain can be greater than, or less than the degree of accretion that is occurring(Rogers 2004). At Homebush Bay no significant difference observed between rates of sediment accretion and surface elevation change (Rogers 2004). The study also found that at Homebush Bay surface
elevation change rates exceeded sediment accretion rates overall, suggesting that below ground processes are significant at Homebush Bay (Rogers 2004).
In studies investigating surface elevation at multiple sites surface elevation change has also been strongly correlated to Southern Oscillation Index (SOI) and rainfall, which may affect both above and below ground processes (Rogers 2004). At Homebush Bay surface elevation variability was found to be directly influenced by changes to groundwater and tidal inundation, surface elevation variability explained between 70-85% by SOI (Rogers & Saintilan 2008). However, the drivers of surface elevation change are regionalised, for example increasing mangrove and saltmarsh surface elevation strongly correlated to position within the tidal prism, defined by inundation depth and distance to the tidal channel at the Tweed River in
Northeastern NSW (Rogers et al. 2014), in contrast to the findings at Homebush Bay.
Wetland distribution dynamics have also been found to be scale dependant in the temporal context. The spatial extent of mangrove encroachment into saltmarsh varies regionally, and therefore requires a regionalised driver, such as rainfall and water level variations (Rogers et al. 2014). Both rainfall and water level variations were found by this study to relate to short term variability in surface elevation,
however cross correlation confounded the identification of a single driver of the short term variability observed. At a larger temporal scale, the long term trend of mangrove invasion into saltmarsh was been found to correlate to global sea level trends, rather than any particular localised factor (Rogers et al. 2014). While these specific findings only directly apply to the study site, the Tweed River, they recognize the impact of
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temporal extent on wetland distribution dynamics, and the implications of cross correlation.
Comparison of studies investigating processes operating at individual sites emphasises the spatial variability of the biological, geological and hydrological processes that contribute to surface elevation change within individual wetlands in Southeastern Australia. Geological contributions, specifically sedimentation and subsidence, are the dominant contributors to surface elevation change in
Southeastern Australia (Rogers 2004). However, accretion and subsidence rarely completely explain changes in surface elevation, therefore other processes must be at work in wetlands (Rogers 2004). Changes in below ground biomass, a key
biological contributor to surface elevation change, have been attributed to surface elevation gains greater than sediment accretion at Homebush Bay (Rogers 2004) and surface elevation decreases at Kooragang Island, Hunter Rover, NSW, where saltmarsh biomass has been lost due to encroachment by mangroves (Rogers et al.
2013). Hydrology, including groundwater, tidal cycles and rainfall, has also been found to play a key role in contributing to surface elevation change. At Kooragang Island storms, which result in higher sediment deposition, have been found to have a pronounced short term influence on accretion trends, and therefore surface elevation over the same period (Rogers et al. 2013). Tidal cycles also appeared to have a short-term effect on surface elevation at the site, causing shrink-swell of sediments.
Longer term, shrink-swell of sediments at Kooragang Island was found to be affected by large fluvial flows, drought and elevated estuarine water levels (Rogers et al.
2013). On the other hand, surface elevation change at Homebush Bay was also found to be affected by hydrological processes, but at this site a high correlation found between groundwater levels and surface elevation change (Rogers & Saintilan 2008). Overall, geological, biological and hydrological processes may all influence surface elevation change in Southeastern Australian wetlands, the magnitude and nature of these effects both spatially and temporally variable.