Thermal stratification and the distribution of dissolved oxygen in billabongs of the Alligator Rivers Region, Northern Territory (final report)

1982

Open File Record 28
Walker TD, Waterhouse J & Tyler PA
Department of Home Affairs and Environment

Summary

The recognition of stratification in tropical waterbodies poses more problems than in the temperate zone. First, the small temperature changes involved demand careful attention to accurate measurements and, second, the large diurnal variation must be taken into account. Most limnologists have, with difficulty, adopted various arbitrary criteria based on a finite temperature change per unit depth. In this study it has been more useful to consider biological relevance, and recognise a de facto stratification from attendant ecological events, specifically oxygen depletion. Such a notion does not demand lasting rigorous hydraulic separation of water masses, only a sufficient barrier to mixing to allow progressive oxygen depletion. It is consistent with the considerable localised and limited vertical mixing common to tropical lakes, and with the view of a billabong as a heterogeneous and dynamic  mosaic of water cells.

Based on these criteria the billabongs of the Alligator Rivers Region form a continuum of stratification behaviour. At one end of the spectrum lies Kulukuluku, and perhaps Woolwonga, with several months of de facto stratification. Leichhardt has less sustained periods of hypolimnetic anoxia, separated by periods of mixing. The sum of these limited periods means that bottom waters are anoxic for a considerable time during the year. Jabiluka occupies a middle position, with regular alternation of holomixis and brief episodes of stratification.

It is a classical example of a tropical, polymictic lake. At the other extreme of the spectrum are Bowerbird, with its continual flow, and the shallow backflow billabongs. The position any billabong occupies in this continuum depends on such factors as morphometry, shelter and orientation with respect to prevailing winds.

Since heavy metal mobility and the susceptibility of the native fauna to toxins are both enhanced at low oxygen tensions the clear implication for the Magela system, where long term anoxia does not occur, is that the most critical time is the first few weeks of the Dry, the most critical billabong Leichhardt. The implications for the Nourlangie system are moreserious. Not only may anoxia be persistent in bottom waters but also it may extend for periods throughout the whole water column of billabong and floodplain.