Water quality trends in river catchments draining to the Great Barrier Reef: an analysis of ∼40 years of data

Overview

Analysis of water quality trends in river catchments provides critical insights into the influence of climate, landscape/land use change and management drivers thereby informing evidence-based policy interventions. Long-term water quality datasets from nine monitoring sites across the Great Barrier Reef (GBR) catchment of northeastern Australia were carefully curated and analysed using Generalised Additive Mixed Models (GAMMs) to identify both long-term (∼linear) and episodic (non-linear) trends in dissolved inorganic nitrogen (DIN) and suspended sediment (SS) concentrations. The GAMMs approach addresses common shortfalls in long-term time series including temporal autocorrelation and data gaps. Most sites exhibited episodic behaviour, with periods of increasing and decreasing concentrations over monitoring periods ranging from 18 to 37 years. For DIN, one site showed a significant linear increase in DIN concentrations, while seven sites displayed significant linear declines. These trends largely align with changes in nitrogen fertiliser application, although interpretation is complicated in catchments with mixed upstream land uses. For SS, seven sites exhibited significant downward linear trends, and one site showed increasing concentrations reflecting the influence of multiple drivers including land use changes, climate cycles, variable ground cover and management interventions. Although the underlying causes of these changes are not yet fully resolved, the observed trends are nevertheless encouraging. Further work is needed to evaluate how constituent loads delivered to the GBR may be changing under current and projected climate conditions.