Seasonal metabolism of juvenile green turtles (Chelonia mydas) at Heron Island, Australia

We investigated seasonal changes in the energetics of juvenile green turtles (Chelonia mydas (L., 1758)) (9.8-23.8 kg) from a resident population on the southern Great Barrier Reef. Doubly labeled water was used to determine water flux and field metabolic rate (FMR) of green turtles during summer (mean water temperature (T-W) = 25.8 degrees C) and winter (mean T-W = 21.4 degrees C), and muscle tissue was collected for analysis of thermal sensitivity of metabolic enzyme activity. There was no significant seasonal difference in total body water (TBW) content (73.7% and 75.1% of body mass in summer and winter, respectively) or water flux (7.5% and 6.1% of TBW per day in summer and winter, respectively). The mean FMR during winter (81 +/- 15 kJ center dot kg(-1 center dot)d(-1)) was 43% lower than that during summer (142 +/- 33 kJ center dot kg(-1 center dot)d(-1)), but the seasonal difference in FMR marginally failed to reach statistical significance (P = 0.17). Thermal dependence of citrate synthase, lactate dehydrogenase, and pyruvate kinase activity was relatively low (Q(10) = 1.20-1.68) over the range of temperatures experienced by turtles during summer and winter, and there were no significant seasonal differences in enzyme activity. In summary, juvenile green turtles experience only moderate changes in metabolism within the temperature range and environmental conditions experienced seasonally at this subtropical site.