Vegetation and elevation data were collected by the Partnership for the Delaware Estuary.

Topographic Change: Transects were spaced at 3m intervals creating a 3m2 survey grid covering~0.12 hectares (1,170m2). This area included the marsh platform and mudflat to ~5m waterward of the installed breakwaters.

Fixed Monitoring Plots Analyses: Mean +/- SEM for elevation, vegetation robustness, bearing capacity, and mussel density were calculated as the average of three replicate plots per position per sub-area per year. Bearing Capacity (Substrate Firmness): Measured as the penetrative capacity of a slide hammer after 5 blows.

Vegetation Robustness in Fixed Monitoring Plots: Vegetation robustness integrates the horizontal and vertical obstruction of a parcel of vegetation into an overall unit-less index between 0-1 and is reported as a percentage. A score of 0% indicates no robustness, and a score of 100% indicates full robustness. By integrating the horizontal and vertical obstruction through the marsh canopy, a full picture of the three-dimensional structure of the vegetation with in the parcel is obtained.

The formula for calculation was: 𝑉𝑒𝑔𝑒𝑡𝑎𝑡𝑖𝑜𝑛 𝑅𝑜𝑏𝑢𝑠𝑡𝑛𝑒𝑠𝑠 = 𝐻𝑜𝑟𝑖𝑧𝑜𝑛𝑡𝑎𝑙 𝑉𝑒𝑔𝑒𝑡𝑎𝑡𝑖𝑜𝑛 𝐷𝑒𝑛𝑠𝑖𝑡𝑦+𝑉𝑒𝑟𝑡𝑖𝑐𝑎𝑙 𝑉𝑒𝑔𝑒𝑡𝑎𝑡𝑖𝑜𝑛 𝐷𝑒𝑛𝑠𝑖𝑡𝑦2

• Horizontal Vegetation Density: measured horizontal density by counting the number of bars visible (out of 10; 10cm width each) on a 1m obstruction board from 3m away within the same band of vegetation. The count was conducted at three heights: 0.25m; 0.50m; 0.75m. The height to which data was used for calculations (number of bars available; max=30, 10 at each height) was determined by the max vegetation height as measured by Blade Height below.

Calculations were as follows: 𝐻𝑜𝑟𝑖𝑧𝑜𝑛𝑡𝑎𝑙 𝑉𝑒𝑔𝑒𝑡𝑎𝑡𝑖𝑜𝑛 𝐷𝑒𝑛𝑠𝑖𝑡𝑦 = 𝑛𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑏𝑎𝑟𝑠 𝑎𝑣𝑎𝑖𝑙𝑎𝑏𝑙𝑒 − 𝑛𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑏𝑎𝑟𝑠 𝑣𝑖𝑠𝑖𝑏𝑙𝑒 /𝑛𝑢𝑚𝑏𝑒𝑟𝑜𝑓 𝑏𝑎𝑟𝑠 𝑎𝑣𝑎𝑖𝑙𝑎𝑏𝑙𝑒

• Blade Height: Twenty-five stems were measured, moving from the waterward corner towards the interior. Max blade height was used for the Vegetation Robustness calculation.

• Vertical Vegetation Density (Canopy Cover): Five measurements of ambient light were taken above each plot (corners and center) and at the ground level (penetrative light) beneath canopy using a light meter. Calculations were as follows:

𝑉𝑒𝑟𝑡𝑖𝑐𝑎𝑙 𝑉𝑒𝑔𝑒𝑡𝑎𝑡𝑖𝑜𝑛 𝐷𝑒𝑛𝑠𝑖𝑡𝑦 = 1 − (ratio of penetrative light: ambient light)