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Seasonal changes in growth, biomass, abundance and reproduction of brown algae have been reported from subtropical and temperate regions, but there are very few reports from tropical regions, and none from the Eastern Tropical Pacific (ETP). Sargassum liebmannii is a common brown alga in the ETP at Bahía Salinas, a seasonal upwelling area in the northern Pacific coast of Costa Rica. We measured the length of ∼50 individuals every two months from 2005 to 2007 and collected samples for reproductive analyses from 2008 to 2009. The growth rate of S. liebmannii peaked during the seasonal upwelling events (November to May), when minimum temperatures reached 15.5 °C and nutrient concentrations were high. During upwelling events, S. liebmannii grew from a few millimeters to over 1 m high, with a maximum mean growth rate of 0.68 cm d −1 during the most intense upwelling period. Sexual reproduction occurred in the non-upwelling period. After attaining maximum size, S. liebmannii reproduces (June) and becomes detached from the substrate, floating in the bay and sometimes accumulating in nearby beaches. Sargassum liebmannii at this study site was absent in 2013 following a population outbreak of the black sea urchin, Diadema mexicanum . [ABSTRACT FROM AUTHOR]