Stream metabolism is a fundamental ecosystem process that links organisms to their environment through the transformation of organic matter. This process focuses on the production and consumption of carbon in an ecosystem, sustaining both the food and energy resources of most aquatic food webs. Measurements of stream metabolism are increasingly used to assess aquatic ecosystem health and detect responses to human disturbance. Rivers in the northern Murray Darling Basin can have exceptionally high turbidity and nutrient concentrations, even when compared to other rivers in the Basin. This study aims to explore the key drivers of stream metabolism in response to flow in the northern Murray Darling Basin.
Water quality, water column nutrients and stream metabolism indicators were measured in ten sampling sites in the Warrego and Darling Rivers in 2015-17 and linked to flow data. The higher flow and hydrological connectivity in the Warrego and Darling Rivers increased phosphorus loadings across the systems and boosted algal productivity, resulting in higher dissolved oxygen concentrations in 2015-16. The increase in rates of gross primary production and ecosystem respiration corresponding to the higher phosphorus availability revealed that phosphorus availability was a limiting factor controlling metabolism rates. However, despite high nutrient concentrations driving gross primary production, all sites were net heterotrophic and therefore a carbon sink, recording rates up to 7.23 mg/ O2/ L/ day net oxygen consumption. We propose that the lower rates of gross primary production are driven by turbidity, regardless of high nutrient loading, suggesting light and not nutrients are regulating rates of production in turbid rivers of the northern Murray Darling Basin.