Lakes Barrine and Eacham, ~1.0 and 0.5 km2 area, 67 and
63 m depth respectively, lie at ~740 m a.s.l., ~17°S in north-eastern
Australia. Seasonal changes in their volumes modelled from meteorological data
correspond well with observations at Eacham. Temperature profiles through 6
years show summer stratification with a metalimnion at 20–30 m; in
winter, near isothermy is usually attained. At Barrine, thermal stability
varies between winter and summer (<500 and >4000 g-cm
cm-2 respectively). Mixing is related to low ground
temperatures during periods of generally low thermal stability; exceptionally
it penetrates to >60 m. Oxygen saturation decreases from the surface to
~20% at the base of the euphotic zone (15–21 m) but oxygen is
carried lower by mixing after which anoxia commonly rises to ~40 m. At
Barrine, Fe-reducing redox (<200 mV) usually occurs below 50 m, but during
mixing this boundary falls to within 1 m of the mud–water interface. The
Barrine solution is dilute (total dissolved solids 55–58 mg
L-1), and that of Eacham is more so. A concentrated
monimolimnion has developed in the lowermost 2–3 m at Barrine but not at
Eacham. Sedimentation at the middle of each lake results from the continuous
deposition of open-water products punctuated by the redistribution of coarser
detritus from the ‘shallows’ at times of deep mixing. The
resultant laminations are preserved only at Barrine, protected by the chemical
stability of the monimolimnion.
