Impact of European settlement and land use changes on Great Barrier Reef river catchments reconstructed from...

The increase in sediment and nutrient loads entering the coastal waters of the Great Barrier Reef (GBR) and the associated degradation of water quality represents a major threat to coral reefs. Although the strengthening of preventative management strategies remains a priority, there is a general lack of terrestrial runoff baseline information with respect to the spatial and temporal severity of disturbances associated with ongoing European-style land use practices. Here we use new and existing high-resolution coral Ba/Ca and luminescence records from the central Cairns region to the southern GBR shelf to reconstruct sediment fluxes discharged into the GBR from before European settlement in the 1860s to the present-day. Since the commencement of European settlement in the 1860s we document a tripling of flood-plume suspended sediment loads delivered by the Burdekin River to the GBR lagoon relative to 'natural' pre-European baseline levels. We show that this is indicative of a much more extreme degradation of the river catchments than hitherto appreciated with intensified discharge events particularly from the central and southern catchments carrying higher sediment loads. More-over from the 1930s onwards the Burdekin River, the largest source of both sediment and freshwater to the GBR, has also exhibited a progressive northwards expansion of its flood plume. This, together with increased variability of freshwater inputs indicated by coral luminescence records, now shows that the inner GBR not only continues to be impacted by increasing sediment/nutrient loads but is also subject to higher intensity river discharge events due to the loss of ground cover causing increased overland runoff and erosion.