Seasonal changes in the diel surfacing behaviour of the bimodally respiring turtleRheodytes leukops

Abstract

The purpose of this study was to determine whether a relationship existed between the diel surfacing trends of the bimodally respiring freshwater turtle Rheodytes leukops and daily fluctuations in specific biotic and abiotic factors. The diel surfacing behaviour of adult R. leukops was recorded over four consecutive seasons (Austral autumn 2000 – summer 2001) within Marlborough Creek, central Queensland, Australia, using pressure-sensitive time–depth recorders. Additionally, diurnal variations in water temperature and aquatic PO2level, as well as the turtle's behavioural state (i.e., active versus resting), were monitored. In autumn and summer, surfacing frequency increased significantly during the daylight hours, with peak levels normally occurring around dawn (0500–0700) and dusk (1700–1900). However, no consistent diel surfacing trend was recorded for the turtles in winter or spring, owing to considerable variation among individual R. leukops. Diurnal surfacing trends recorded for R. leukops in autumn and summer are attributed to periods of increased activity (possibly associated with foraging) during the daylight hours and not to daily variations in water temperature or aquatic PO2level. Turtles generally remained at a depth greater than 1 m throughout the day, where the effect of diel fluctuations in water temperature (<0.5 °C) and aquatic PO2level (<15 mm Hg (1 mm Hg = 133.322 Pa)) was considered to be negligible.