The discovery of abundant autotrophic macrophytes living below 200 meters indicates their importance to primary productivity, food webs, sedimentary processes, and as reef builders in clear oceanic waters. Estimates concerning minimum light levels for macroalgal photosynthesis and macrophytic contributions to the biology and geology of tropical insular and continental borderlands must now be revised.

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The deep-water macroalgal community of the continental shelf off the east coast of Florida was sampled by lock-out divers from two research submersibles as part of the most detailed year-round study of a macroalgal community extending below routine SCUBA depths. A total of 208 taxa (excluding crustose corallines) were recorded; of these, 42 (20.2 %), 19 (9.1%), and 147 (70.7 %) belonged to the Chlorophyta, Phaeophyta, and Rhodophyta, respectively. Taxonomic diversity was maximal during late spring and summer and minimal during late fall and winter. The number of reproductive taxa closely followed the number of taxa present; when reproductive frequency was expressed as a percentage of the species present during each month, two peaks (January and August) were observed. Most perennial species had considerable depth ranges, with the greatest number of taxa observed from 31 to 40 m in depth. Although most of the taxa present also grow in shallow water (i.e. <10 m), there were some species whose distribution is hmited to deeper water. The latter are strongly dominated by rhodophytes. This community has a strong tropical affinity, but over half the taxa occur in warm-temperate areas. Forty-two new records (20% of the taxa identified) for Florida were listed; this includes 15 taxa whicl~ previously had been considered distributional disjuncts in this area. The phaeophyte Halopteris filicina is a new addition to the flora of the western Atlantic. Although more studies are needed, it appears that the deep-water flora may be relatively continuous over a large portion of the tropical and subtropical western North Atlantic. The traditional practice of considering Cape Canaveral as a biogeographical boundary may only be appropriate for shallow water. This study demonstrates the need for additional floristic work on macroalgal communities, especially from deep water, before more detailed biogeographical analyses can be performed and suggests that deep-water species could be important in experimental approaches to macroalgal biogeography.

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