Abstract: Anthropogenic carbon emissions influence marine systems primarily in two ways, by fuelling temperature changes in many parts of the oceans and by changing seawater carbon chemistry, which lowers seawater pH and drives the process of ocean acidification. Macroalgae are major players in coastal carbon cycling at temperate latitudes where there exists high algal biodiversity and biomass, and also in the tropics where coralline algae are important to reef carbon dynamics. As primary producers that also have calcium carbonate skeletons, corallines are expected to have mixed responses to ocean acidification. Historical baselines in a coastal community of coralline algae provide context for ecological changes since the onset of acidification in the Northeast Pacific. Changes in tissue production have led to shifts in historical baselines and competitive hierarchies, which corroborate observed changes in local species abundances and point to a pervasive change in the populations of some coralline algal species in this region.