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1. The cellular nutrient contents of microalgae, when growing at or approaching maximum rates, approximate the Redfield C : N : P (molar) ratio of 106 : 16 : 1. Deviations from this optimal ratio can be used to infer nutrient limitation of microalgal growth. However, this ratio may not be applicable to macroalgae, which are distinguished from microalgae by forming a thallus that is a discrete structure visible to the naked eye. The utility of the Redfield ratio to infer nutrient limitation of the growth of macroalgae was tested for Spirogyra fluviatilis in a field experiment conducted in tropical Australia. 2. The optimal cellular C : N : P ratio for S. fluvialitis was estimated by means of in situ nutrient addition. This was compared with S. fluvialitis cellular ratios determined from eight sites with a wide range of soluble N concentrations (<1-90 lg L )1 ), a smaller range of soluble P concentrations (5-12 lg L )1 ), and soluble molar N : P ratios of 0.11-27. 3. Spirogyra fluviatilis had an optimal molar C : N : P ratio of 1800 : 87 : 1 which differs substantially from the Redfield ratio, and suggests that the latter ratio is not applicable to this macroalga. Concentrations of N and P in the river deviated from the optimal N : P ratio of 87 : 1, inferring nutrient limitation of growth. 4. C : P and C : N ratios of S. fluviatilis varied in accordance with general stoichiometric relationships for autotrophs under nutrient limitation of growth. Ratios of C : P and C : N increased, respectively, with increased severity of P-and N-limitation. Additionally, C : P ratios increased with increased N : P ratios, whilst the C : N ratio increased with decreased N : P ratios. The C : N molar ratio however was an insensitive indicator of nutrient depletion compared with the C : P ratio. Under N-limitation of growth, luxury amounts of P were stored by S. fluviatilis. 5. In aquatic environments where macroalgae are sufficiently abundant to be sampled, their cellular carbon, nitrogen and phosphorus stoichiometry can be used to infer nutrient limitation of growth when their optimal C : N : P ratio is known.

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Comparison of measured dissolved oxygen concentrations and modelled concentrations for the Daly River, November 2008Fig. S1. Time series of oxygen concentration measured (heavy line) and for modelled fits to these data obtained using three values of Ik (µmol photons m -2 s -1 ) in the representation of photosynthesis rate. No photo inhibition is the case Ik = ∞.

Quantitative relationships between river discharge and hydraulic habitat availability for key taxa are important elements of environmental flow assessment. We used radiotelemetry to examine diel patterns of habitat use by tracking the locations of 17 juvenile Sooty grunter (Hephaestus fuliginosus) over a 10‐day period during the late dry season in a river in the wet–dry tropics of northern Australia. Habitat use data were integrated with a hydrodynamic model to identify preferred hydraulic habitat and explore different river discharge scenarios to assess the potential effects of water abstraction on habitat availability. Sooty grunter exhibited a strong preference for shallow, fast flowing mesohabitat (riffles and runs). Hydraulic microhabitat preference was modelled using generalised additive mixed‐effect models (GAMMs) and showed no significant difference in microhabitat selection between day and night. Habitat criteria developed from a combined day‐night GAMM were defined as locations with velocities of 0.26–1.42 m s−1 and depths <0.69 m. Hydrodynamic modelling of river discharge scenarios in the study reach showed that the area of preferred habitat was highest at 8 m3 s−1, with large declines in habitat area under low flows (61% decline in habitat area at 0.5 m3 s−1 compared to the discharge of 2.8 m3 s−1 at the time of radio‐tracking). While the study focusses on a single species, our findings demonstrate the broad applicability of radiotelemetry as a means of quantifying the diel hydraulic habitat requirements of riverine fish to support the objective determination of environmental flow regimes.