ABSTRACT: The surf-zone diatom Asterionellopsis glacialis (Castracane) Round attains extremely high abundances (up to 10⁹ cells l^-1) and chl a concentrations (up to 1.5 mg chl a l^-1), forming dark-brown patches in Cassino Beach, southern Brazil. Several lines of evidence from this study indicate a decoupling between A. glacialis and heterotrophic bacteria. For instance, despite the high abundance of diatoms, associated high levels of chl a and rates of dissolved organic carbon (DOC) production by A. glacialis, bacterial abundance at Cassino Beach was extremely low (0.03 to 0.7 x 10⁶ cells ml^-1) compared to the nearby estuarine waters, adjacent coastal waters and those from the literature. No statistical differences were found between bacterial abundance in- and outside the diatom patches, suggesting that direct association with A. glacialis did not stimulate bacterial growth. Moreover, field and laboratory experiments demonstrated that bacteria take 1 to 4 d to begin growing following the input of A. glacialis cell-free filtrate. Five possible reasons for this decoupling, i.e. (1) viral infection, (2) bacterial grazing, (3) DOC quality, (4) nutrient competition and (5) antibiotic production, are discussed. Because of the decoupling between the surf-zone diatom and heterotrophic bacteria in this high-energy surf-zone ecosystem, a large fraction of A. glacialis primary production is not contemporaneously channeled through the microbial food web. Consequently, much of the surf-zone diatom production may fuel metazoan secondary production in the adjacent nearshore and coastal environment.