Spatially integrative metrics reveal hidden vulnerability of microtidal salt marshes

Abstract

Salt marshes are valued for their ecosystem services, and their vulnerability is typically assessed through biotic and abiotic measurements at individual points on the landscape. However, lateral erosion can lead to rapid marsh loss as marshes build vertically. Marsh sediment budgets represent a spatially integrated measure of competing constructive and destructive forces: a sediment surplus may result in vertical growth and/or lateral expansion, while a sediment deficit may result in drowning and/or lateral contraction. Here we show that sediment budgets of eight microtidal marsh complexes consistently scale with areal unvegetated/vegetated marsh ratios (UVVR) suggesting these metrics are broadly applicable indicators of microtidal marsh vulnerability. All sites are exhibiting a sediment deficit, with half the sites having projected lifespans of less than 350 years at current rates of sea-level rise and sediment availability. These results demonstrate that open-water conversion and sediment deficits are holistic and sensitive indicators of salt marsh vulnerability.

Figure 1. Aerial imagery and drainage delineation of eight salt marsh complexes.

(a) Pt. Mugu, California; (b) Ogunquit, Maine; (c) Reedy Creek, New Jersey; (d) Dinner Creek, New Jersey; (e) Schooner Creek, New Jersey; (f) Blackwater, Maryland; (g) Fishing Bay, Maryland and (h) Seal Beach, California. Unvegetated areas shown in blue, vegetated areas in orange. Drainage areas were established through the Hydrology toolbox in ArcGIS, for areas landward of sediment flux measurement location within the tidal channel; open water and marsh delineation was determined through aerial imagery and the National Wetlands Inventory. All images are oriented with north upwards. Imagery from the ArcGIS World Imagery Basemap.

Figure 2. Relationships between sediment transport and geomorphic metrics.

(a) Ratio of unvegetated to vegetated marsh (UVVR) within the tidal channel drainage area versus net sediment budget (sediment flux minus supply needed to offset SLR) and (b) UVVR versus sediment-based lifespan of the marsh complex. Individual points are coloured to represent local sea-level rise. Regression statistics apply to fitted curves to data: (a) y=−0.42 ln x−1.07; and (b) y=48x^−1.66. Error bounds correspond to potential uncertainty in (a) drainage area and UVVR; and (b) bulk density and UVVR. Note that sites with nearly zero sediment flux exhibit smaller errors in net sediment budget (a) due to a greater influence of the sea-level rise term.

Figure 3. Relationships between sediment transport metrics.

(a) Time-averaged suspended-sediment concentration (SSC) versus sediment flux per unit area, with points coloured by flood-ebb SSC differential and (b) flood-ebb SSC differential versus sediment flux per unit area from eight tidal marsh channels coloured by local sea-level rise. SSC is a reliable predictor of sediment flux for neutral-to-importing channels, but high SSC from marsh deterioration can result in large sediment export. The differential between flood and ebb SSC is a more reliable predictor of the integrative sediment budget. Regression statistics apply to fitted curves to data: (a) y=0.017x−0.11; site with large negative differential (BW) excluded; and (b) y=0.039x+0.13.