ABSTRACT: Understanding the relative importance of ecological processes at different spatial scales is an issue central to both ecological theory and conservation efforts. In this mensurative study, we quantified the role of mesoscale oceanographic variation on the structure of subtidal (~15 m depth) rock wall communities. We used a hierarchical sampling design to survey 18 sites, nested within 5 distinct oceanographic seascapes in the Salish Sea (Northeast Pacific Ocean). Three of the seascapes are waterways, and 2 of the seascapes are restricted inlets. Waterways and inlets are categorically different in their relative levels of water retention and tidal currents; sites in waterways tend to exhibit lower water retention and stronger tidal currents. The most striking variation in diversity was observed between the 5 seascapes, primarily between waterways and inlets. Namely, sites nested in waterways exhibited greater diversity at the quadrat and site scales than did sites within inlets. Multivariate analyses of community composition reflected a similarly conspicuous separation between waterway and inlet sites. Four abiotic correlates (predicted current speed, alabaster dissolution rate, temperature, and sediment cover) of water retention supported the qualitative generalization that waterways and inlets represent distinct abiotic environments and are associated with unique subtidal biota. We hypothesize that reduced larval delivery and increased post-settlement mortality, related to the covarying effects of water flow and quality, are the potential drivers of low diversity in high-retention sounds and fjords.