Chemical characteristics and nutrient status of billabongs of the Alligator Rivers Region, Northern Territory (final report)
Open File Record 27
Walker TD & Tyler PA
A major feature of tropical Australia is the sharp climatic distinction between the Wet and the Dry, a characteristic climatic pattern which imposes a distinctive hydrological regime on running and standing bodies of water. During the Wet, vast quantities of water pour down rivers from the Arnhem Land escarpment to inundate the lowlands of the Alligator Rivers. All existing bodies of water are flushed. When the rains cease, the floodwaters recede, leaving behind a series of billabongs filled with the dilute water of the Wet. During the ensuing Dry, water levels fall continuously as more than 2 metres of water evaporates, with no replenishment from rain. Some billabongs receive groundwater inflows; a few near the escarpment are spring-fed, perhaps to flow throughout the Dry.
Chemical changes occur as water levels drop. Solutes become concentrated as water evaporates, wind and buffalo resuspend fine sediments, and spring or groundwater inflows may influence the chemical composition. The magnitude and direction of change varies from billabong to billabong, depending on morphometry, position, sediment type, and so on. In December, the floodwaters come again, replacing this individuality with the uniform standards of one big lake.
Then, the water is dilute (K25<20µScm-l), near neutral, and dominated by sodium bicarbonate. Nutrient levels are at their lowest but, on a world scale, total phosphorus levels are high and the billabongs would be regarded as meso-eutrophic to hypereutrophic. N:P ratios are low, and in terms of inorganic nitrogen the billabongs are ultra-oligotrophic. The sheer volume of water, from intense rainfall and rapid runoff, ensures that billabongs over a wide area of the Magela, Nourlangie and Coopers Creek catchments enter the Dry with this common water chemistry. This uniformity is shortlived, and by the end of the Dry three main groups of billabongs can be distinguished by the nature and extent of chemical change they have undergone.
Least changed are the channel billabongs of the three creek systems. They are still dilute, have suffered scant change in pH, and their ionic and nutrient character is still that of the Wet. That they have changed so little can be attributed, in large measure, to the characteristically low surface area to volume ratios of channel billabongs. Under these conditions, the effects of evaporation are at a minimum. Implicit in this is the assumption that any inflowing groundwater is also unchanged from Wet season chemistry.
In all other billabongs a number of changes take place. All become turbid. Prominent amongst the changes in ionic character is the decided move to sodium chloride dominance. Usually, this is accompanied by marked increases in conductivity as the waters concentrate, and a decline in pH as the Dry progresses. Two groups can be recognized. In one, the flood plain billabongs (plus Corndorl), the change to sodium chloride dominance is accompanied by significant enrichment by sulphate from groundwater inflows. In the other, containing most backflow billabongs (plus Kulukuluku and Leichhardt), sulphate plays little part. In both groups, nutrient concentrations increase rapidly during the Dry, earlier on the floodplain than in the backflow billabongs. In the former, inorganic nitrogen constitutes a much higher proportion of total nitrogen than in the latter. Except in floodplain billabongs, N:P ratios are low and limitation of production by nitrogen, not phosphorus, is suspected. Phosphorus levels now indicate hypereutrophy, while inorganic nitrogen levels indicate a range from meso-eutrophic to hypereutrophic.
Throughout the Dry nutrient cycles can be envisaged as a closed, endorhoeic system, with external inputs from biotic and, perhaps, groundwater sources, with evaporative concentration, and with internal loading from resuspended sediments. In marked contrast to temperate lakes, there is no output other than biotic migration.
The classification into 3 groups on the basis of major ion chemistry and on the basis of nutrients is congruent, and coincides, with few exceptions, with the traditional division on geographical, morphological and hydrological criteria into channel, backflow and floodplain billabongs.