Quantifying hydrologic controls on local- and landscape-scale indicators of coastal wetland loss

Abstract

Background and Aims Coastal wetlands have evolved to withstand stressful abiotic conditions through the maintenance of hydrologic feedbacks between vegetation production and flooding. However, disruption of these feedbacks can lead to ecosystem collapse, or a regime shift from vegetated wetland to open water. To prevent the loss of critical coastal wetland habitat, we must improve understanding of the abiotic–biotic linkages among flooding and wetland stability. The aim of this research was to identify characteristic landscape patterns and thresholds of wetland degradation that can be used to identify areas of vulnerability, reduce flooding threats and improve habitat quality. Methods We measured local- and landscape-scale responses of coastal wetland vegetation to flooding stress in healthy and degrading coastal wetlands. We hypothesized that conversion of Spartina patens wetlands to open water could be defined by a distinct change in landscape configuration pattern, and that this change would occur at a discrete elevation threshold. Key Results Despite similarities in total land and water cover, we observed differences in the landscape configuration of vegetated and open water pixels in healthy and degrading wetlands. Healthy wetlands were more aggregated, and degrading wetlands were more fragmented. Generally, greater aggregation was associated with higher wetland elevation and better drainage, compared with fragmented wetlands, which had lower elevation and poor drainage. The relationship between vegetation cover and elevation was non-linear, and the conversion from vegetated wetland to open water occurred beyond an elevation threshold of hydrologic stress. Conclusions The elevation threshold defined a transition zone where healthy, aggregated, wetland converted to a degrading, fragmented, wetland beyond an elevation threshold of 0.09 m [1988 North American Vertical Datum (NAVD88)] [0.27 m mean sea level (MSL)], and complete conversion to open water occurred beyond 0.03 m NAVD88 (0.21 m MSL). This work illustrates that changes in landscape configuration can be used as an indicator of wetland loss. Furthermore, in conjunction with specific elevation thresholds, these data can inform restoration and conservation planning to maximize wetland stability in anticipation of flooding threats.

Salt Marsh Elevation Limit Determined after Subsidence from Hydrologic Change and Hydrocarbon Extraction

“…The data were then fit to a log:linear curve where elevation (X) was linear and the percent open water (Y) was transformed to plot log values on a linear scale. The results were graphically compared to Stagg et al's [34]…”

“…Those curve fits have a "break-point" between 0.4 to 0.6 m, which is where open water starts to develop as the salt marsh begins fragmenting. The curves for these two salt marshes overlap the elevation vs. percent open water curve for an S. patens marsh in a coastal marsh in the northwest coast of the Gulf of Mexico which was developed using aerial imagery and a fragmentation index [34].…”

“…A hyperbolic fit of the data is shown. The red line is for a Spartina patens marsh on the Northwest Gulf of Mexico coast[34].The natural marshes with no open water in 2002 gained open water by 2019 (Figure 6). No site (n = 16) with an elevation above 0.6 m in 2002 had open water in 2019, whereas all sites with less than 0.6 m (n = 11) gained open water areas by then.…”

“…A hyperbolic fit of the data is shown. The red line is for a Spartina patens marsh on the Northwest Gulf of Mexico coast[34].Remote Sens 2020, 17, x FOR PEER REVIEW 10 of 15…”

The Life and Death and Consequences of Canals and Spoil Banks in Salt Marshes

“…The lower soil strength and higher hydraulic conductivity are consistent with the anticipated consequents arising from the fewer but longer flooding cycles, and less frequent but longer drying cycles behind spoil banks (Swenson and Turner 1987) which Nichols (1959) observed more than 60 years ago: "When water is over the marsh, the surface material is supersaturated and become very weak and almost fluid." The longer flooding reduces the amount of belowground biomass and increases the potential sedimentation of mineral matter (Stagg et al 2019;Alldred et al 2020); the longer drying cycles results in increased oxidation of the soil carbon which lowers the soil surface, increasing flooding duration and causing plant stress. This effect of subsidence is most significant in these coastal salt marshes because they occupy a narrow tidal range (McKee and Patrick 1988).…”