Using Landsat satellite images (pixel size of 30 m × 30 m) from the website, we compared coastal sectors spanning a latitudinal range of 5.5 min (ca. 10 km of coastline), centered around the estuaries of the four rivers. For each coastal sector, we selected images for three dates: 1975 (earliest image set available), 1990, and 2010 (most recent set available). Using the color signature identifier of a digital image editor (Adobe Photoshop), we selected in each image all the pixels that corresponded to seawater and coastal unvegetated strands and simplified each image into a binary representation of terrestrial and marine pixels. We then superimposed the binary images and calculated the area of coastal vegetation that was lost or gained during the 1975–1990 and the 1990–2010 periods. Because areal calculations are derived from pixel counts, which are frequency data, we obtained standard errors for each areal estimate using the mean-to-variance properties of the Poisson distribution. To make the more recent satellite images comparable with those of 1975, we used in all cases false-color infrared imagery in which terrestrial vegetation shows up in red, and we took in all images the seaward limit of terrestrial vegetation as the boundary of terrestrial ecosystems.

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