Abstract
BackgroundTidal marshes will be threatened by increasing rates of sea-level rise (SLR) over the next century. Managers seek guidance on whether existing and restored marshes will be resilient under a range of potential future conditions, and on prioritizing marsh restoration and conservation activities.MethodologyBuilding upon established models, we developed a hybrid approach that involves a mechanistic treatment of marsh accretion dynamics and incorporates spatial variation at a scale relevant for conservation and restoration decision-making. We applied this model to San Francisco Bay, using best-available elevation data and estimates of sediment supply and organic matter accumulation developed for 15 Bay subregions. Accretion models were run over 100 years for 70 combinations of starting elevation, mineral sediment, organic matter, and SLR assumptions. Results were applied spatially to evaluate eight Bay-wide climate change scenarios.Principal FindingsModel results indicated that under a high rate of SLR (1.65 m/century), short-term restoration of diked subtidal baylands to mid marsh elevations (−0.2 m MHHW) could be achieved over the next century with sediment concentrations greater than 200 mg/L. However, suspended sediment concentrations greater than 300 mg/L would be required for 100-year mid marsh sustainability (i.e., no elevation loss). Organic matter accumulation had minimal impacts on this threshold. Bay-wide projections of marsh habitat area varied substantially, depending primarily on SLR and sediment assumptions. Across all scenarios, however, the model projected a shift in the mix of intertidal habitats, with a loss of high marsh and gains in low marsh and mudflats.Conclusions/SignificanceResults suggest a bleak prognosis for long-term natural tidal marsh sustainability under a high-SLR scenario. To minimize marsh loss, we recommend conserving adjacent uplands for marsh migration, redistributing dredged sediment to raise elevations, and concentrating restoration efforts in sediment-rich areas. To assist land managers, we developed a web-based decision support tool (www.prbo.org/sfbayslr).
Highlights
Projections of sea-level rise (SLR) range from 18 cm to nearly 2 m over the century [1,2], making low-lying coastal zones vulnerable to climate change
We sought to answer the following key questions: (1) What are the thresholds and sensitivities for marsh sustainability in terms of mineral sediment supply, organic material contribution, SLR rates, and starting elevations? (2) How is the Bay-wide area and composition of intertidal habitats likely to change under varying projections for SLR and sediment availability? (3) How much space exists for new marshes to form, and how much habitat may be expected under these different scenarios? Our goal was to deliver results to land managers in an accessible and interactive web-based map tool, to support conservation planning and restoration activities
Under a high SLR rate, marshes with 50 mg/L suspended sediment concentration (SSC) would not be sustainable for 20 years regardless of organic material (OM) (Fig. 3)
Summary
Projections of sea-level rise (SLR) range from 18 cm to nearly 2 m over the century [1,2] (and recent assessments suggest that as much as 5 m could be possible [3]), making low-lying coastal zones vulnerable to climate change. Tidal marshes provide high-value ecosystem services such as water filtration, flood abatement, protection for infrastructure, and carbon sequestration [13,14,15]. They have high ecological value, supporting a large number of specialized and endemic species [16,17] and have already experienced dramatic historical declines in area and hydrologic integrity [18]. Managers seek guidance on whether existing and restored marshes will be resilient under a range of potential future conditions, and on prioritizing marsh restoration and conservation activities
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