Drivers of alkalinity cycling in a high nitrate tidal freshwater river: insights from a novel mass balance control volume experiment

Riverine alkalinity exports play a crucial role in modulating carbonate chemistry in estuarine waters. While often overlooked, groundwater discharge can be a significant source of alkalinity, nutrients, and other dissolved constituents to tidal river systems, however, the overall net effect on alkalinity budgets remains poorly constrained. Here, we quantified the net seasonal alkalinity fluxes and associated nutrient cycling in the tidal freshwater zone (TFZ) of the Hawkesbury-Nepean River (Australia). This was achieved using a novel control volume mass balance approach that integrated high-resolution timeseries measurements, groundwater endmembers, and benthic flux estimates spanning three distinct reaches of the TFZ. The approach enabled the estimation of overall fluxes from the TFZ while additionally highlighting potential areas of enhanced biogeochemical processing that may have been overlooked if using a more traditional study design. The TFZ displayed hydrologically modulated alkalinity source and sink behavior, acting as a net source of alkalinity under dry conditions (296.4 mmol m −2 d −1 ) and shifting to a net sink under wet conditions (−108.9 mmol m −2 d −1 ), which highlights the need to incorporate hydrological variability into global riverine alkalinity budgets. We further examine the possible drivers of the hydrologically driven switch between the net alkalinity source to net sink observed, including hydrologically and tidally-modulated residence times, groundwater inputs, and biogeochemical processes. Groundwater contributed significant amounts of alkalinity to the TFZ, 117.1 and 52.8 mmol m −2 d −1 , under wet and dry conditions, respectively. Upscaled to an annual export for 2023, biogeochemical processing in the TFZ increased the system-wide net alkalinity export roughly 4.8-fold relative to upstream inputs, from 1.4 × 10 8 mols yr. −1 upstream of the TFZ to 6.6 × 10 8 mols yr. −1 upon discharge. This study has determined that the control volume methodology employed is a powerful tool for estimating fluxes while simultaneously highlighting areas of unexpected biogeochemical processing to guide further research.