The origin of suspended particulate matter in the Great Barrier Reef

Abstract

River run-off has long been regarded as the largest source of organic-rich suspended particulate matter (SPM) in the Great Barrier Reef (GBR), contributing to high turbidity, pollutant exposure and increasing vulnerability of coral reef to climate change. However, the terrestrial versus marine origin of the SPM in the GBR is uncertain. Here we provide multiple lines of evidence (13C NMR, isotopic and genetic fingerprints) to unravel that a considerable proportion of the terrestrially-derived SPM is degraded in the riverine and estuarine mixing zones before it is transported further offshore. The fingerprints of SPM in the marine environment were completely different from those of terrestrial origin but more consistent with that formed by marine phytoplankton. This result indicates that the SPM in the GBR may not have terrestrial origin but produced locally in the marine environment, which has significant implications on developing better-targeted management practices for improving water quality in the GBR.

Tracing sources of inorganic suspended particulate matter in the Great Barrier Reef lagoon, Australia

“…In both the 2017 and 2018 flood events the terrigenous-mineral component dominates the SPM export from the Burdekin catchment (> 85%) and it remains the dominant component (> 70%) in flood plume waters 115 km from the Burdekin River mouth, despite the addition of marine organic matter. While the organic material in the offshore plume waters is rapidly processed and recycled (and more closely resembles a marine composition) 10 – 12 , our findings unequivocally show that the terrigenous mineral particles retain their key characteristics and can be traced back to the sub-catchments from which they were derived. Changes in these SPM characteristics between events indicates that, in each case, the sediments were derived from a different sub-catchment source.…”

“…These plumes can extend hundreds of kilometres offshore and may reduce water quality over large areas of the GBR lagoon 3 , 8 , 9 . The excess SPM comprises terrigenous mineral particles and organic materials originating from either the river or produced within the plume 3 , 7 , 10 . While the organic material is rapidly processed and recycled in the marine environment 10 – 12 , the terrigenous mineral particles often form the nucleus of sediment flocs 7 , 13 and hence ‘control’ water clarity in the GBR.…”

“…The excess SPM comprises terrigenous mineral particles and organic materials originating from either the river or produced within the plume 3 , 7 , 10 . While the organic material is rapidly processed and recycled in the marine environment 10 – 12 , the terrigenous mineral particles often form the nucleus of sediment flocs 7 , 13 and hence ‘control’ water clarity in the GBR. Identifying the dominant sources of this material is critical to prioritise remediation programs, worth $100 M’s, aimed at protecting the reef ecosystems 1 , 14 .…”

“…As SPM moves offshore within riverine flood plumes it is subjected to major transformations including flocculation, preferential deposition (i.e. particle size/mineralogy related), desorption/adsorption of elements and the addition of new components such as microbes (bacteria/fungi) and algae (phytoplankton/zooplankton) 7 , 10 , 13 , 15 , 27 – 31 . For the larger rivers that deliver the highest SPM loads to the GBR (e.g.…”