Temporal and spatial variation in plasma thyroxine (T4) concentrations in juvenile alligators collected from Lake Okeechobee and the northern Everglades, Florida, USA

We examined variation in plasma thyroxine (T,) in juvenile American alligators (Alligator mississippiensis) collected from three sites within the Kissimmee River drainage basin (FL, USA). Based on historical sediment data, Moonshine Bay served as the low contaminant exposure site, Water Conservation Area 3A served as an intermediate contaminant exposure site, and Belle Glade served as the high contaminate exposure site. In May 1999, alligators (n = 22) from Water Conservation Area 3A exhibited higher T, concentrations than animals from both Belle Glade (n = 22; p = 0.0003) and Moonshine Bay (n = 33; p = 0.001). In May 2000, alligators (n = 29) Water Conservation Area 3A again exhibited higher T, concentrations than those from Belle Glade (n = 49; p = 0.02) but not those from Moonshine Bay (n = 40). No sexual dimorphism was observed among mean T-4 concentrations within any of the sites during either year (p > 0.05). Animals within all sites exhibited higher T-4 concentrations in May 2000 when compared to May 1999. When variance was examined, animals from Water Conservation Area 3A exhibited higher variance in plasma T-4 concentrations than those from either Moonshine Bay or Belle Glade. We concluded that mean plasma T-4 concentrations did not match the sediment contaminant mixture data presently available to us, whereas variance seems to be a more reliable indicator of contaminant exposure.