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Enterprise Pit Lake is a water‐filled mining void located in Australia’s wet/dry tropics south of Darwin in the Northern Territory (13°49.6′S, 131°49.8′E). In the mid‐1990s, the void was rapidly filled by the artificial diversion of the nearby Pine Creek watercourse. Profile data and surface water quality data were collected in the lake on a monthly basis from November 1998 to October 1999. Analysis of this data showed that Enterprise Pit Lake was strongly stratified for most of the year with deep mixing occurring once during the middle of the cool, dry season (August, 1999). The relatively small volume of its epilimnion and its low productivity allowed significant oxygen concentrations to remain in the hypolimnion of the lake when it was strongly stratified. Low pH values in a layer of water with a depth of approximately 35 m and located in the lake’s hypolimnion might be associated with several factors, including enhanced sulphide oxidation on the walls of the pit in this region.

1. The light climates of Darwin River Reservoir (DRR) and Manton River Reservoir (MRR), in northern Australia, are compared for an 8‐year period. The reservoirs are subject to the same wet/dry tropical climate and have similar catchment characteristics, but differ in their basin morphology, retention time and trophic status. 2. Median euphotic depths in DRR and MRR were 9.7 and 4.4 m, respectively. Seasonal variation in each reservoir’s euphotic depth was best explained by colour, based on a stepwise linear regression. Turbidity was excluded from the two regressions, while chlorophyll a concentration was significant only for the MRR regression. 3. Both reservoirs showed the same seasonal pattern for colour. Wet season inflow increased reservoir colour, and was followed by a reduction in colour due to photodegradation and microbial decomposition of humic material, reaching a minimum before the next wet seasons inflow. 4. Although the colour of catchment run‐off into both reservoirs was similar, MRR colour was two to three times greater than that measured in DRR. The higher colour and greater light attenuation of MRR is attributed primarily to its shorter retention time, and therefore shorter time for colour removal. Annual retention time accounted for 97% of the variability of average annual colour (measured as absorption at 440 nm), based on a linear regression of log10 transformed data for both reservoirs. 5. Long retention times favour reduced colour, increasing water transparency, particularly in water bodies of low trophic state and inorganic turbidity.

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