Latitudinal gradient in intertidal seaweed composition off the coast of southern Brazil and Uruguay

•We found a latitudinal gradient in intertidal seaweed species turnover and richness along southern Brazil and Uruguayan coast.•Sea surface temperature and salinity are the main drivers of the seaweed species distribution, despite changes in coastal geomorphology.•The lowest seaweed species richness is observed across the latitude of 32 °S and 37 °S, a transitional warm-temperate region.•The study fulfills knowledge gaps in the Southwestern Atlantic Ocean seaweed biogeography.•By integrating historic and recent data, the results provide baselines for conservation and detection of future changes.