These data represent estimated percent cover of macroalgae, invertebrates, and exposed substrate within fixed quadrats. Benthic cover data were collected in 1 square meter permanent quadrats placed 1 m to the left or right of the 50 m main transect at 10 fixed locations at each of nine permanent subtidal monitoring sites around San Nicolas Island. The number of points out of 20 possible in each quadrat was multiplied by 5 to yield the percent cover listed for each of 158 "species". Total cover for all species in a quadrat usually exceeds 100 percent due to layering. The list of taxa recorded was open and ranged from actual individual species to species groups, for example, “orange encrusting sponge.” Substrate type (bare rock or sand) was also scored if exposed. There are a total of nine sites, each of which has ten 1 square meter quadrats. Six of the sites were first sampled in 1980, one additional was added in 1987, and the other two were added in 2014. The sites have usually been monitored twice per year, most often in October and April.