A patch reef in the platform lagoon off Old Rhodes Key, Florida, was studied to determine the overall contribution of the green algae Halimeda to the local regime of calcium carbonate sedimentation. It was found that whole and fragmented Halimeda segments can be identified to species in the sediments surrounding the reef, and that the distribution of the disarticulated segments can be correlated with the living Halimeda flora. Seven species of Halimeda were found living on and around the reef in two distinct zones separated by a barren sandy zone. The inner zone (patch reef zone) is inhabited by Halimeda discoidea, H. goreauii , H. opuntia and H. tuna . The outer zone (mixed grass zone) is inhabited by Halimeda incrassata , H. lacrimosa , H. monile , H. opuntia , and H. tuna . The distribution of these last two species is controlled by the availability of hard surfaces which occur in both zones. The distribution of the other species appears to be controlled by localized parameters such as water depth, water temperature, and light penetration. Morphological characteristics, life cycles and reproduction rates control the contribution of Halimeda to patch reef sedimentation. In addition to its role as sediment producer, Halimeda indirectly influences sediment accumulation by serving as a substrate for the attachment of other sediment-contributing organisms, binding sediment below the water-sediment interface, and trapping particles in suspension by locally reducing current velocities.