Declining water quality is implicated in the degradation of near-shore Great Barrier Reef (GBR) ecosystems. The goal of this project is to provide a definitive answer to the question of how GBR water quality has changed since European arrival (pre- 1860). Using novel geochemical proxies in long-lived coral cores and innovative remote sensing techniques, we will develop quantitative histories of water quality and mangrove distribution change. This project will deliver the first integrated assessment of how coastal water quality and associated ecosystems have varied historically, which will be immediately applicable for long-term management of coastal ecosystems lining the GBR.