Abstract
Feedbacks among inundation, sediment trapping, and vegetation productivity help maintain coastal wetlands facing sea‐level rise (SLR). However, when the SLR rate exceeds a threshold, coastal wetlands can collapse. Understanding the threshold helps address key challenges in ecology—nonlinear response of ecosystems to environmental change, promotes communication between ecologists and resource managers, and facilitates decision‐making in climate change policies. We studied the threshold of SLR rate and developed a new threshold of SLR acceleration rate on sustainability of coastal wetlands as SLR is likely to accelerate due to enhanced anthropogenic forces. Deriving these two thresholds depends on the temporal scale, the interaction of SLR with other environmental factors, and landscape metrics, which have not been fully accounted for before this study. We chose a representative marine‐dominated estuary in the northern Gulf of Mexico, Grand Bay in Mississippi, to test the concept of SLR thresholds. We developed a mechanistic model to simulate wetland change and then derived the SLR thresholds for Grand Bay. The model results show that the threshold of SLR rate in Grand Bay is 11.9 mm/year for 2050, and it drops to 8.4 mm/year for 2100 using total wetland area as a landscape metric. The corresponding SLR acceleration rate thresholds are 3.02 × 10−4 m/year2 and 9.62 × 10−5 m/year2 for 2050 and 2100, respectively. The newly developed SLR acceleration rate threshold can help quantify the temporal lag before the rapid decline in wetland area becomes evident after the SLR rate threshold is exceeded, and cumulative SLR a wetland can adapt to under the SLR acceleration scenarios. Based on the thresholds, SLR that will adversely impact the coastal wetlands in Grand Bay by 2100 will fall within the likely range of SLR under a high warming scenario (RCP8.5), highlighting the need to avoid RCP8.5 to preserve these marshes.
Highlights
Coastal wetlands are disappearing at an alarming rate in many parts of the world, along with their associated ecosystem services, including carbon sequestration, water quality improvement, flood control, protection from storms, habitat, fishery, recreational opportunities, and cultural values (Costanza et al, 1997; Engle, 2011)
During the present day with intense human activities, changes in each of the dynamic components involved in balancing wetland platform elevation become accelerated, which may lead to reduced sea-level rise (SLR) rate thresholds for coastal wetlands
We explored the thresholds which account for the fertilization effect of the increased concentration of CO2 on vegetation productivity, and those based on the landscape metrics other than total area, such as mesh size and mean patch size which could represent landscape fragmentation (Jaeger, 2000; McGarigal, Cushman, & Ene, 2012; Turner & Gardner, 2015)
Summary
Coastal wetlands are disappearing at an alarming rate in many parts of the world, along with their associated ecosystem services, including carbon sequestration, water quality improvement, flood control, protection from storms, habitat, fishery, recreational opportunities, and cultural values (Costanza et al, 1997; Engle, 2011). During the present day with intense human activities, changes in each of the dynamic components involved in balancing wetland platform elevation become accelerated, which may lead to reduced SLR rate thresholds for coastal wetlands These human impacts include (1) reduced sediment inputs through damming and channelization (Day, Pont, Hensel, & Ibanez, 1995), (2) increased nutrient inputs that likely lower the production of extensive roots and destabilize salt marsh platforms (Darby & Turner, 2008), (3) increased atmospheric CO2 concentration that acts as a photosynthesis stimulant to increase vegetation productivity (Cherry, McKee, & Grace, 2009; Langley, McKee, Cahoon, Cherry, & Megonigal, 2009), (4) increased temperature that could simultaneously increase and decrease salt marsh sustainability by concurrently promoting primary productivity and stimulating decomposition (Kirwan, Guntenspergen, & Langley, 2014; Wu, Huang, Biber, & Bethel, 2017), and (5) accelerated relative SLR that affects vegetation productivity (Morris et al, 2002) and causes more rapid edge erosion resulting in a potential decrease in marsh area.
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