SEAGRASS DIE-OFF IN FLORIDA BAY (USA) - CHANGES IN SHOOT DEMOGRAPHIC CHARACTERISTICS AND POPULATION-DYNAMICS IN THALASSIA-TESTUDINUM

Population dynamics, including demographic characteristics, of short-shoots of Thalassia testudinum were evaluated within 3 Florida Bay (USA) basins experiencing differing severities of seagrass die-off. Annual mean plastochrone intervals (PI) ranged from 14.1 to 20.6 d, and they increased from the basin experiencing the most prolonged and extensive die-off to the basin least affected by die-off, PIs calculated by leaf punching were always shorter than those estimated by cohort analyses. In all 3 basins, overall population age structure changed significantly between April 1989 and April 1990; population half-life, mean shoot age, the age of the oldest shoot, and shoot density declined, whereas rhizome apical density increased. An examination of the population dynamics of the 3 T. testudinum populations revealed that both recruitment and mortality of short-shoots increased between April 1989 and April 1990, but the increase in mortality was proportionately greater. These changes indicated that the T. testudinum populations in Florida Bay were continuing to decline and that the declines corresponded to the increased extent and severity of short-shoot die-off.