In this paper, the dynamics of primary production in the Daly River in tropical Australia are investigated. We used the diurnal-curve method for both oxygen and pH to calculate photosynthesis and respiration rates as indicators of whole-river productivity. The Daly River has maximum discharges during the summer, monsoonal season. Flow during the dry season is maintained by groundwater discharge via springs. The study investigated how primary production and respiration evolve during the period of low flow in the river (April–November). The relationship between primary production and the availability of light and nutrients enabled the role of these factors to be assessed in a clear, oligotrophic tropical river. The measured rate of photosynthesis was broadly consistent with the estimated mass of chlorophyll associated with the main primary producers in the river (phytoplankton, epibenthic algae, macroalgae, macrophytes). A significant result of the analysis is that during the time that plant biomass re-established after recession of the flows, net primary production proved to be ~4% of the rate of photosynthesis. This result and the observed low-nutrient concentrations in the river suggest a tight coupling between photosynthetic fixation of carbon and the microbial degradation of photosynthetic products comprising plant material and exudates.
A model was developed to predict the impact of reduced dry season base flow, due to groundwater and river extraction, on the standing crop of Spirogyra along an 18 km reach of the Daly River, located in the Australian wet/dry tropics. The alga can constitute up to 40% of the primary producer standing crop and is a food source for turtles. Outputs from a two-dimensional hydrodynamic model and the observed maximum biomass of Spirogyra for a range of shear velocities were used to predict the maximum potential standing crop (MPSC) of Spirogyra. With reduced flow, hydraulic conditions became less favourable for the growth of Spirogyra, whilst the area of suitable substrata was not as responsive. The MPSC of Spirogyra was predicted for the minimum dry season flow for a 47 year period of hydrographic record using two hypothetical extraction scenarios. Both scenarios produced MPSCs that were frequently less than the minimum crop under natural flows and underscored the need for a minimum flow to maintain a MPSC that exceeded the historic minimum. The MPSC, however, is unlikely to be attained due to autogenic sloughing, nutrient supply and other factors, thus the model is best considered an index of the impact of reduced river flows. The model communicates a reduction in the standing crop of Spirogyra due to reduced dry season base flows and an inferred impact on its dependent fauna and the river's nutrient dynamics. The model demonstrates the utility of applying a benthic algal model to evaluate the ecological impact of modified flow regimes and contribute to environmental flow recommendations.
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.
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