Sponge reefs on the Northeast Pacific margin: geomorphic and biological variability

Hexactinellid sponges form extensive reefs in coastal and shelf waters of the northwestern margin of North America. The framework-constructed reefs are unique to this region, and create a diverse array of reef structures in water depths of 20–240 m. The reefs develop morphologies on a continuum ranging from thin-bedded biostromal reefs to steep sided bioherms and ridges >20 m in height. The diverse morphologic types likely result from local variations of the seafloor tidal current regime and suspended sediment delivery as well as the ecologic development of the reef controlled by the characteristics of the benthic community dominated by reef-forming sponges. Measured ecological parameters in the various reef forms indicate differences in sponge cover in morphologic end members. Some reef morphologies, such as ridges, have large areas that support sponge growth, while in waveform morphologies the most favorable sponge growing situations are on the top and slope of the waveforms. Biostromal reefs possess gradients of most suitable reef growing habitat where patchiness results in significantly different densities of sponges on the reef surface.