Oral Presentation Society of Environmental Toxicology and Chemistry Australasia 2023

Anthropogenic dissolved inorganic nitrogen loads discharging from Great Barrier Reef catchments, are they ecologically relevant? (#139)

Ryan DR Turner 1 , Cath A Neelamraju 1 , Michael StJ Warne 1 , Reinier Mann 2 , David Orr 2 , Ben Ferguson 2
  1. Reef Catchments Science Partnership, University of Queensland, St Lucia, Queensland, Australia
  2. Queensland Department of Environment and Science, Water Quality and Investigations, Brisbane, Queensland, Australia

Like many receiving waters around the world, the Great Barrier Reef lagoon is being exposed to multiple stressors. The key compounds causing the decline in one of those stressors, water quality, are sediment, nutrients (N and P) and pesticides. The Australian and Queensland governments have been addressing the decline in water quality since 2003 through the Reef Water Quality Improvement Plan (Reef Plan). One of the monitoring and evaluation components of Reef Plan is the GBR Catchment Loads Monitoring Program, which has been measuring water quality in the GBR catchments since 2006. Loads based runoff has been the focus of the monitoring and evaluation since 2009. In 2017 the pesticide loads target was reviewed and modified to be based on risk, as this was more ecologically relevant.   This presentation will propose that the dissolved inorganic nitrogen (DIN) target should be investigated in terms of concentrations as opposed to loads. Furthermore, that the current DIN target should be split into both nitrate and ammonium, as typically these two nitrogen compounds can be in different ratios and have different ecological effects. This presentation will discuss measured nitrate concentrations compared to the ANZG (i.e., PC95) and demonstrate that a hazard assessment of the concentrations does not align with the Reef Plan loads DIN targets. In brief, the rank of basin load reductions for Reef Plan targets, (in tonnes) are Haughton(640), Herbert(620), Johnstone(350), Mulgrave-Russell(300), Plane(260), Tully(190), Mary(180). However, using the rank of Hazard Quotient for nitrate (HQ = [max]/PC95 guideline 2.4mgL-1) the order is different; Burrum~6.9, Haughton~3.8, Kolan~2.3, O’Connell~2.1, Herbert~2.0, Proserpine~1.8, Plane~1.6. While loads may be a relevant measure and target for the near future, we suggest that more focus should be placed on smaller catchments that discharge high concentrations of nitrate and in the medium to long term consider changing the nitrogen target.