Limnological characteristics of some tropical Australian river catchments

With Government approval to mine uranium in the Alligator Rivers Region (APR), Northern Territory, top priority was given to conservation of the rich and diverse aquatic habitats, and a comprehensive limnological program in the Magela, Nourlangie and Coopers Creek catchments was instituted. A major feature of tropical Australia is the sharp climatic distinction between the \Wet\" and the \"Dry\". 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 two metres of water evaporate with no replenishment from rain. Transformation of limnological character may attend falling water levels with changes in light climate stratification behaviour water chemistry nutrient status and primary productivity. Of profound importance is the elevation of suspended loads in some billabongs as wind and buffalo resuspend fine sediments. Despite individualistic seasonal response depending upon morphometry position sediment type etc. classification of billabongs into three types - channel backflow and floodplain - is tenable and appropriate. In December the floodwaters come again replacing the heterogeneity of the Dry with the uniform standards of one big lake. Of the three parameters most likely to affect light conditions dissolved organic colour (gilvin) turbidity and chlorophyll turbidity proves to be the dominant. Gilvin concentrations are always low and rarely does chlorophyll modify the spectral character of the underwater PAR. The presence of finely-divided suspended sediments (turbidity) so characteristic of many of the billabongs specifies a blue and green-deficient spectrum and a shallow euphotic zone. Turbidity exerts its influence through absorption rather than scattering; the greater the turbidity the greater the limitation of PAR to a narrow band of red wavelengths. The clearest billabongs have a transmission peak in the yellow as in oceans and oligotrophic lakes. Recognition of stratification in the tropics poses considerable problems. One alternative is to reject arbitrary criteria of gradient and recognize de-facto stratification from attendant phenomena specifically oxygen depletion. This notion demands not lasting rigorous hydraulic partitioning only sufficient barriers to mixing to permit progressive oxygen depletion. Based on this criteria the billabongs form a continuum of stratification behaviour. At one end lies billabongs with several months of de-facto stratification every Dry; at the other extreme are those which circulate every night. During the Wet all billabongs are inundated and flushed to enter the Dry with a common water which is dilute (K\\(_{25}\\) < 25uScm\\(^{-1}\\)) near neutral and dominated by Na/Mg HCO\\(_3\\). Nutrient levels are at their lowest but on a world scale total phosphorus levels are high and the billabongs mesa - to hyper - eutrophic. Chemical changes occur as water evaporates water levels drop and solutes become concentrated. Spring and ground water inflows may influence some billabongs. The channel billabongs change little through the Dry and retain the ionic and nutrient character of the Wet with scant change in pH. This is attributed to low surface area to volume ratios limiting evaporation. All other billabongs change with most becoming turbid with a decided move to sodium chloride dominance usually accompanied by marked increases in conductivity as the waters concentrate and a decline in pH sometimes severs. In the floodplain billabongs the change to NaCl dominance is accompanied by significant enrichment of sulphate from shallow groundwater aquifers whilst in backflow billabongs sulphate plays little part. In both however nutrients burgeon during the Dry largely because fertile sediments are resuspended into the water column. The high flushing rate and rapid export of phytoplankton in concert with the seasonal low in nutrient status minimise primary production in the Wet. The magnitude of production in the endorrheic billabongs during the Dry is controlled principally by interactions between the underwater light climate and nutrient conditions. Nutrients are generally abundant in the billabongs but in most the massive increase in turbidity during the Dry imposes severe restrictions on available light and prevents phytoplanktonic production attaining levels expected on the basis of available nutrients."