Rates of growth and carbonate production of the calcifying green alga, Halimeda opuntia, were measured from July 1982 to September 1983 at the Marquesas Keys, 29 km (18 mi) west of Key West, Florida. Seasonal fluctuations in growth rate were determined by collecting and analyzing whole colonies of algae that had been previously stained in situ with Alizarin Red-S. Weight increase of individual plants was calculated by recording weights of various H. opuntia colonies harvested at different times during the year. Growth was strongly seasonal with 80-90% of new plates being produced in summer months (May through October). From November through April, colonies appear to be in a semidormant state generating little new growth. Surprisingly, more than 90% of the new plates produ ed by parent plants were the offspring of only a few (usually less than 50) "dominant" plates.

Measured growth rates appear sufficient to explain the 12 m (6.6 ft) thick, 6 km (4 mi) wide, 16 km (10 mi) long, crossbedded Halimeda accumulation located west of the study area.

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