Miami The USGS serves the Nation by providing basic, Color Key objective, and informative scientific Bug Light <10% Fowey Rocks knowledge on Earth history and 10% Brewster Reef processes, and on human impact on 20% Star Reef resources. Two of the most effective 30% N Triumph Reef 40% Long Reef Pacific Reef ways are through descriptive and >50% interpretive maps Dominant Grain Turtle Rocks and through the Halimeda Carysfort Reef Key Largo Mollusc USGS marine The Elbow Reef Open-File Report 97-453 Coral home page on the Plate 1 sample sites Molasses Reef World Wide Web Key Largo Conch Reef Davis Reef (http:// Alligator Reef Dominant Grain Moser Tennessee Channel A Reef Key Sombrero Key West Reef Big Pine Shoal Looe Key Reef American, Maryland, Sand Key and Pelican Shoals; Reef Eastern Sambo and Eastern Dry Rocks reefs <10% B upper Keys 10% Halimeda Percent 10% The maps middle Keys <10% show con- toured data obtained 10% senescent reef corals through C lower Keys petrography which is the <10% microscopic inner platform Percent Mollusc examination outer platform of resin-impregnated thin sections of platform margin Percent Algae sediment (or rock) to identify grains and <10-29% determine percentages. (A) Upper Keys sands are dominated by grains of <10-29% 30% Halimeda or mollusc. Middle and lower D Keys sands are dominated by grains of 10-29% 30-65% healthiest reef corals Halimeda or coral. The most important <10% percentages are those of coral grains. 30-49% Percent Coral (B) Halimeda grains are general-ly most 20-65% abundant on the inner platform. (C) Mollusc grains are most major tidal passes abundant near the margin off the upper Keys but on the inner plat- form to the southwest. (D, E) Healthiest reefs are protected by the linear island of Key Largo, where coral sand grains are <10%. The senescent reef corals least healthy reefs lie off the wide tidal passes. Reefs off narrower E passes in the lower Keys are generally perceived as declining. Coral grain percentages are highest along the outer margin of the middle Coral Vitality declining reef corals and lower Keys. There is a direct correlation between reef health, location off islands or tidal passes, and the coral fraction of the sand. .