| Summary: | Study of the geography of benthic marine algae has traditionally taken the form of descriptions of floras, analyses of floras in terms of floristic components, comparison of the flora and vegetation of one area with that of another area, and delineation of floristic provinces. The concept of genetic diversity transcends floristic analysis and leads to the recognition of geographic diversity patterns related to, but not coincidental with, floristic patterns. Unlike vascular plants, in which the ratio of tropical to nontropical species is 2 : 1, benthic marine algae reach their peak of species diversity on transitional warm-cool temperate coasts. Lowest species diversity, as would be expected, is exhibited by the arctic and antarctic floras. The Mediterranean flora is highly diverse. In the Atlantic, the cold-water flora is richest in the east, while the warm-water flora is richest in the west. In the vastly broader Pacific, the cold-water flora is equally rich on both sides, but again the warm-water flora is richest in the west. Moreover, many warm-water species extend into the Indian Ocean. The Pacific is complicated by the presence of a myriad of islands of various sizes, shapes, structure, ages, and degree of isolation. In the Indian Ocean, the high species diversity of the floras of Natal and southwestern Australia is matched by that of India, while intervening equatorial areas are significantly less rich. Taxonomic diversity is an assessment of the evenness of distribution of the species of a local flora compared to the regional taxonomic spectrum. The highest degree of intrageneric morphological diversity is exhibited by Codium and Caulerpa.
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