## 4 Discussion  
### Carbon Dynamics in Mangrove Wetlands  
Our hypothesis, that mangroves along a hydrologically disturbed Creek sequestered less carbon than mangroves along an undisturbed reference Creek proved correct. There were also differences when carbon sequestration was compared across the different hydrogeomorphic settings. Rates of carbon sequestration in the reference riverine mangroves were the highest recorded, followed by mangroves in the undisturbed fringe mangroves. The disturbed riverine mangroves sequestered statistically less than did the undisturbed riverine mangroves, while the disturbed fringe mangroves sequestered a little less than half of what was sequestered in the undisturbed fringe mangroves. The basin mangroves sequestered similar amounts of carbon in the undisturbed and disturbed site. These results may be explained several ways. The primary force dictating long-term carbon accretion in sediments is the hydrologic regime. Both freshwater inflow and coastal tidal activity have the capacity to lessen sediment and, hence, carbon accumulation. However, it is more likely  
Figure 7: Carbon sequestration rates for seven different mangrove sites in four different hydrogeological settings for the hydrologically disturbed Hamilton Avenue Creek (HA) and reference Susan’s Creek (SC). Bars depict mean (\(\pm\) standard error) carbon sequestration values (\(n\) = 3). Similar letters indicate no statistical differences, alpha equals 0.05.  
that a decrease in freshwater inputs may be causing lower carbon accumulation in the disturbed tidal Creek. [PERSON] [26; 46] formulated and defended the outwelling hypothesis, which described mangrove ecosystems as net exporters of organic material and crucial links to subsidizing the relatively unproductive oceanic shelf systems. Some have suggested that this idea was an over-simplification and that coastal nutrient dynamics are much more complex [9]. [PERSON] _et al._[47] showed that there was a net influx of suspended sediments during high tides that were not exported at ebb tide. Further expanding upon this idea, [PERSON] and [PERSON] [48] found that rainfall and temperature are the main factors dictating the fate of carbon in coastal sediments; study sites with high annual temperature (>29 \({}^{\circ}\)C) and low precipitation (<200 mm\(\cdot\)year\({}^{-1}\)) exported more carbon than did sites with low annual temperature (<15 \({}^{\circ}\)C) and high precipitation (>4000 mm\(\cdot\)year\({}^{-1}\)). They suggest that the export of senesced plant material is approximately 50% of annual litterfall; in their study the export averaged 202 g-C m\({}^{-2}\cdot\)year\({}^{-1}\)[48]. Tidal activity in mangrove forests can either increase or decrease long-term carbon sequestration. Incoming tidal fluxes provide a subsidy to the mangrove forest, bringing in nutrient-rich allochthonous material and carrying away high-salinity waters. If there is inadequate freshwater flushing, salinity levels can increase, resulting in increased physiological stress on mangrove trees, causing them to displace more energy on osmotic regulation within the plant and diminishing energy available to growth [9]. This added stress ultimately leads to less leaf and stem production available to be incorporated into sediments. The majority of this oceanic material does not get re-exported at ebb tide and is incorporated into the sediment as a coarse mineral fraction or fine organic detritus, either helping to stabilize the coastline or increasing nutrient availability in the soil. If mangroves are not flushed adequately with freshwater from upland or precipitation, then high concentrations of salt can accumulate in the sediments and increase the respiratory energetic costs to mangroves. However, the high variability among different mangrove communities makes it difficult to generalize and expect similar outcomes when these systems are exposed to differing levels of tidal inundation, freshwater inputs, precipitation, and, subsequently, productivity.