Emergent Wetland Plant Biophysical Characteristics Associated with Wave Attenuation and Sediment Retention

Estuarine wetlands have proven a cost-effective buffer against coastal hazards because they reduce erosive wave energy and enable sediment retention by producing hydraulic friction. Human modifications to coastal hydrology and sediment transport have resulted in loss of wetlands and associated protection. Our study area, the Stillaguamish Delta has experienced a 55% loss in wetlands and significant marsh retreat (Grossman and Curran (in review)). We quantified vegetation characteristics (spatial, vertical and, seasonal) that effect wave attenuation using image analysis, remote sensing and, in-situ measurements. We produced a sediment budget for the northern region and evaluate suspended sediment dynamics. Our elasticity and Side-On Photo analyses indicated dominate bulrush species maintained rigidity and biomass through January. Our classifications of hyperspectral imagery delineated vegetation assemblages with an overall accuracy of 76.9%. From biomass patterns, we predict the highest potential winter wave attenuation to occur within the first 0.5-0.75m of vegetation from the marsh floor and within the first 50m of the marsh edge. The highest winter sediment deposition coincided with highest biomass predictions, up to 300m inland. We observed the Low Marsh had been incorporated into the tidal flat by November and the lower Mid-Marsh (BOMA), the winter marsh edge, to have decreased in biomass to 25%. From these patterns, we hypothesize that the Low Marsh and lower Mid Marsh serve critical roles during the early monsoon season while the upper Mid-Marsh (BOFL) and High-Marsh become more influential during large and late season (Jan-Mar) storm events. Overall deposition is estimated to be 2.6% of the Stillaguamish River’s daily sediment load. Turbidity data indicated a delayed response to the river and showed relationships to regional wave generating winds

Variable marsh resilience to stress offers clues to climate change adaptive management

In Puget Sound’s Stillaguamish estuary, tidal marshes exhibit evidence of multiple stressors that affect their vulnerability and provide insight into adaptive management opportunities to enhance their resilience. Despite high accretion rates, some marsh areas have receded by 10m/yr since 1964. Sources of stress include overgrazing by snow geese, high soil salinities, insect attacks, and changes in flow and inundation patterns. These interact with winter vegetation structure, sediment composition, and wave exposure to result in spatially variable marsh resilience. Some marshes are receding quickly, some slowly, and others are minimally affected. In the context of climate change, with potentially substantial near-term salinity changes due to summer low flow projections, and likely changes in sediment dynamics, it is critical to identify how marshes will respond, and develop adaptive management actions to increase resilience. Geese consume the rhizomes of four dominant bulrushes, and loosen the soil during winter storm season. Each bulrush species has different winter structural characteristics that affect grazing vulnerability, and the ability to trap sediment and attenuate erosive wave energy. Coarser sediments affect grazing intensity, being more difficult for geese bills to probe. Sediment and soil salinity affect plant density and height. During summer 2015, a harbinger for coming decades, twice-normal soil salinities resulted in stunted marsh that failed to flower. Finally, small differences in winter wave exposure affect marsh susceptibility to erosion after heavy grazing. With spatially variable marsh resilience to stress, potential adaptive management responses should similarly vary. Responses could include, among others, restoration to improve freshwater connectivity, sediment addition in restored areas to shift them above erosion thresholds or to target grazing-resistant bulrush species, snow goose population management or behavior modification, manipulation of soil particle size with sediment addition, and strategic use of logjams and sediment addition to reduce wave energy