A combination of remotely operated vehicle and SCUBA dives at 40 to 80 m depth seaward of the Florida Keys reef tract revealed extensive areas of the seafloor dominated by communities of large benthic macroalgae. Macroalgal cover exceeded 80% in many areas where the substratum was dominated by coral and shell fragments and calcareous cobbles. Macroalgal cover differed significantly among sites and depths. At 2 sites to the northeast, macroalgal cover was relatively low at 30 m depth close to the reef tract, increased to maximal densities at 50 to 60 m, and then decreased at 70 to 80 m. At 2 sites to the southwest, macroalgal cover ranged from 10 to 80% and was inversely related to the presence of deep shifting sands. The concentration of total inorganic nitrogen in the water column was closely associated with decreased temperature below approximately 26°C. Bottom temperature records showed extensive variability and frequent periods of cooling indicative of short term upwelling, pointing to high nutrient availability and markedly increased duration and magnitude of cooling with increasing depth. Water column measurements of photosynthetically active radiation showed an average vertical light attenuation coefficient of –0.056 m–1 (± 0.011 SE). Thus average light availability is expected to vary from approximately 16% of surface irradiance at 30 m to <1% at 80 m. A 9 mo algal recruitment experiment showed greatest recruitment in areas with high macroalgal cover, but algae did recruit to the artificial surfaces in areas with low adult cover except where shifting sand was abundant. Algal recruitment was low in close proximity to the reefs at 30 m depth. The observed patterns suggest high rates of benthic primary production in this environment and a potentially important role of drift algae and detritus transported onshore as a source of fixed carbon for reef-associated consumers.