Relationship between water elevation and organic soil depth in restored subtropical wetlands

Background/Question/Methods

Wetlands are important sinks in the global carbon cycle as flooded, anaerobic conditions retard organic matter decomposition, thus increasing soil carbon storage. To increase understanding of wetland ecosystem function and evaluate the impacts of hydrological restoration on carbon storage, this study assessed the relationship between water levels and organic soil depth at the Disney Wilderness Preserve (DWP).  This 4,654-hectare former ranch, now managed by the Nature Conservancy, underwent hydrological and ecological restoration from 1994 to 2012 to mitigate Disney and airport development. The restored site contains 1,416 hectares of wetlands situated at the headwaters of the Everglades. For restoration and monitoring purposes, these wetlands were assigned to 24 hydrologic units (HUs), each of which has an instrumented well for continuously measuring water level. Initial monitoring data included “organic soil depth” determined by pushing a probe into the soil to resistance along multiple transects of each HU. We used this existing monitoring data to test the hypothesis that long-term (12-20 year) relative average water level (defined as water elevation relative to ground elevation at specific sampling locations), measured at a single point within each HU, would be strongly correlated with organic soil depth measured along the sites’ original monitoring transects. 

Results/Conclusions

There was a significant (p < 0.001) positive correlation between organic soil depth and relative average water level across all sites, but the model had poor explanatory power (R² = 0.089). When HUs were analyzed individually, all models yielded significant positive correlations but R²  values ranged from 0.010 to 0.695. There was a significant (p = 0.014) negative correlation between model R² values and HU size (R² = 0.267). Models also more accurately predicted organic soil depth in wetlands with narrower ranges of relative average water level and organic soil depth.  Overall, this analysis revealed that single point measurements of water level within and across wetlands correlated positively with organic soil depth, but the predictive strength of this relationship decreased with increasing wetland size and topographic variation.  How well organic soil depth relates to, or can serve as a proxy for, actual soil carbon storage and accumulation is unknown. Determining soil carbon density  along the established transects not only will provide a better understanding of carbon storage in these systems, but also may reveal that relative water level has a stronger relationship to soil carbon storage than it does to physically-based measurements of organic soil depth.