A novel control volume methodology to constrain ecosystem nutrient cycling within a tidal freshwater river

Tidal freshwater systems are dynamic biogeochemical hotspots that play a crucial role in nutrient cycling and the attenuation of catchment derived material. Traditional biogeochemical analysis typically relies on estimation techniques such as interpolation, regression, and remote sensing based on limited data sets, which can result in significant uncertainties. This study applies a novel sampling approach to a tidal freshwater system located on the far northeast coast of New South Wales, Australia, to quantify nutrient fluxes within a “control volume” (CV) by combining high-resolution discrete time series and hydrodynamic measurements in combination with a traditional benthic flux methodology. The study found high NH 4 consumption and high NO x production reflective of nitrification within the aerobic water column. The total oxygen demand within the control volume was very close to the aggregate of the sediment oxygen demand and nitrification oxygen demand, highlighting the importance of sediment biogeochemical processes within the TFZ. Control volume NOx consumption and P release were orders of magnitude larger than those obtained from traditional sediment core incubations, suggesting that NOx and P dynamics in the Richmond River TFZ are driven by processes other than sediment dynamics and/or sediment incubations underestimated in situ fluxes. The novel control volume methodology offers a high-resolution sampling technique with fewer sampling artifacts and an enhanced understanding of biogeochemical trends over the tidal and diel cycles. By integrating biogeochemical measurements with hydrodynamic processes, this method effectively constrain nutrient cycling within a defined reach, providing detailed insights into nutrient processing during the measurement period. • Novel control volume methodology developed for the tidal freshwater zone (TFZ). • High-resolution flux time series over the tidal and diel cycle. • Method integrates all physical and biogeochemical processes. • Improves understanding of processes within the TFZ with fewer sampling artifacts. • The TFZ is a dynamic zone along the aquatic continuum for nitrogen and phosphorus cycling. A novel high-resolution control volume sampling methodology is presented to constrain nutrient cycling within the tidal freshwater zone of an Australian river.