What’s really damaging the Reef? Tracing the origin and fate of the environmentally detrimental sediment and associated bioavailable nutrients

Stephen Lewis

Led by: Dr Stephen Lewis, JCU


Project Summary

Recent research has shown that only a portion of the fine sediment fraction delivered from rivers draining into the Great Barrier Reef Lagoon (GBRL) reaches coral reefs and seagrass meadows. The specific sources of this sediment, which affects the health of corals and seagrasses, are as yet unresolved. This project will, for the first time characterise and trace the origin(s), fate and nutrient bioavailability of this environmentally detrimental sediment using samples collected in flood plumes and during resuspension events. This will enable direct spatial targeting of the sources of this material to achieve maximum benefits to GBR health from catchment management actions.


Project Publications






Project Description


Recent research tracing the sources of sediments exported from the mouth of the larger rivers draining into the Great Barrier Reef Lagoon (GBRL) including the Burdekin, Normanby, Fitzroy, Johnstone and Herbert Rivers, have identified both the dominant erosion processes and sources of the sediment. This work has provided important knowledge on specific areas contributing higher sediment yields; however the transport, fate and impacts (including nutrient bioavailability) of recently delivered catchment sediments within the GBRL have received less attention. The data show that most (>80%) of the sediment load from the Burdekin River (less for other rivers like the Tully) is deposited and largely retained near the river mouth with a much smaller proportion transported further afield; this sediment fraction has certain properties (i.e. particle size, clay mineralogy, organic content) which allow it to be transported greater distances in the marine environment where it can reach coral reefs and seagrass meadows. Furthermore this sediment is likely to be more easily resuspended and influence water clarity along the inner shelf of the GBR over both short (flood plume) and longer (months following plume) timeframes. With respect to bioavailable nutrients, preliminary data from NESP 2.1.5 indicates that plume sediments derived from grazing lands generate significant amounts of dissolved inorganic nitrogen in the marine environment. Unfortunately, little data exists on this ‘environmentally detrimental sediment’, its within-catchment origin is unknown and its risk to the marine environment in terms of nutrient input has not been assessed. If its origin were known targeted catchment works could be used to reduce these specific sediment sources.

How Research Addresses Problem

We will characterise the physical, biogeochemical and isotopic composition of suspended sediment samples from flood plumes and resuspension events. Selected samples from severe erosion source areas in the Burdekin catchment associated with gully remediation trials will also be analysed for elemental and mineral composition to aid in source identification. This characterisation will enable us to trace the within-catchment source of the sediments and give insight into the various sediment and nutrient transformations that occur as fine clay-sized sediment moves from ‘catchment to reef’. This project will test our hypothesis that the composition of newly delivered clay-sized sediment to the GBRL changes during transport, deposition and resuspension, with increasing importance of the biological component. While project results from the first 3 years have provided strong evidence of the influence of newly delivered sediment from the Tully River on the turbidity regimes off nearby Dunk Island, the results on the Burdekin River’s influence on Cleveland and Halifax Bays are so far inconclusive. This is largely because of the lack of a moderate to major event in the Burdekin River over this time as well as the movement of the 2017 plume away from our sediment trap sites. The project has closely aligned with the Queensland Government/NQ Dry Tropics Burdekin Landholders Driving Change Major Integrated Project (BMIP) in the Bowen-Broken-Bogie sub-catchment of the Burdekin Basin and has obtained samples for sediment characterisation and tracing from other projects working in this area including NESP project 2.1.4, NESP 3.1.7 and the Bioavailable Nutrient project under the Queensland Reef Water Quality Science Program. The outcomes of this additional collaboration will allow sediment tracing across the catchment to marine continuum and result in a more targeted investment of on-ground remediation activities in the region where sediment particle size, mineralogy and soil type can be taken into account.

For the first time sediment traps have allowed the collection and detailed characterisation of sediment in resuspension events in the Great Barrier Reef Lagoon. Particle size, clay mineralogy, biogeochemical and isotopic characteristics of the analysed samples will be used to identify the ‘most environmentally detrimental sediment’ in terms of transportability and nutrient bioavailability. The organic component of the floc aggregates in flood plumes and sediment traps will also be characterised using 13C-NMR spectroscopy and microbial composition analysis to determine the liability (i.e. bioavailability) of organic matter and to quantify the contributions of terrestrial and marine sources. Combined with existing sediment tracing data from the catchments, and new data from the gully remediation sites this new information will then be synthesized to identify the origin of this problematic sediment so that catchment management priorities can be further refined. In addition to providing critical information for targeting catchment sources of environmentally detrimental sediment, this work also has significant applicability to port management and dredging as it examines the changing behavior of sediments in resuspension events throughout the year. Importantly, the project has collected a sizable baseline dataset from established sites to evaluate the influence of the dredging associated with the Townsville Port expansion project.

Current Project Outcomes

  • Our preliminary in situ continuous turbidity, light and wave pressure loggers and sediment trap accumulation dataset provides the most comprehensive documentation on the influence of new sediment delivered to the Great Barrier Reef (GBR).
  • Our results show that particles < 20 µm travel furthest in the marine environment, and newly delivered sediment from the Tully (and possibly Murray/Herbert/Hull) River influenced turbidity regimes surrounding Dunk Island for ~5 months following 2017 and 2018 flood events.
  • Preliminary research has demonstrated the potential considerable contribution of dissolved inorganic nitrogen in the Burdekin flood plume that has desorbed from sediment (ammonium ions) and the mineralisation of particulate organic matter.
  • Several novel sampling techniques and methods have been developed to characterise and trace the origin(s) and fate of environmentally detrimental sediment within the GBR delivered from the Burdekin, Tully and Johnstone basins using sediment and floc samples collected in flood plumes and during resuspension events. A novel method of combining organic tracing with trace element data has also been developed to trace land use sources of the particulate nutrients exported from river basins.

Strong stakeholder engagement and feedback. Project results have attracted considerable interest across a wide range of stakeholders (see table page 2) and the project team have put considerable effort into the dissemination and extension of the preliminary project findings,  with significant contributions to: (1) OGBR Bioavailable Nutrients workshop/ concept paper; (2) the creation of a “fluffy” sediment layer within the eReefs model; (3) the Burdekin Landholders Driving Change Major Integrated Project (grazier presentations, advice); (4) Scientific Consensus Statement and additional presentations to (4) GBRMPA Outlook Report staff; (5) Port of Townsville Community Liaison Group; (6) CANEGROWERS Policy Council Meeting; (7) attendance at Queensland Government OGBR stall at Beefweek 2018 (Rockhampton) and (8) Sediment Working Group.

Research Objectives, Activities and Expected Outcomes

  1. Trace the environmentally detrimental sediment that reduces photic depth and water clarity during both flood plume and resuspension events at key locations in the GBRL (i.e. coral reefs and seagrass meadows) back to a specific catchment source (core objective). (Lead proponent James Cook University)
  • The characterisation of sediment from both flood plumes and resuspension events using various tracing techniques will allow data to be compared to pre-existing catchment datasets to identify the key catchment source(s) of this environmentally detrimental sediment. Such identification will allow for targeted investment for catchment remediation. In particular, gully remediation is expensive and this project will refine priority areas for gully management at a finer spatial scale enabling maximum water clarity improvement in the GBRL with available funds.
  • The proposed extension of the project will specifically focus on the Burdekin River floodplume and sediment trap sites to continue the collection of the high resolution logger (turbidity, wave pressure, light) and sediment trap data and further document the influence, impact and risk of floods from different catchment sources, resuspension events and capital dredging. Such data will help better understand catchment-to-marine processes, better quantify the risk of newly delivered sediment and provide measured data for model (Source Catchments, eReefs) validation and improvement. In addition, more limited sampling of the Johnstone catchment (Wet Tropics catchment) and floodplume (1 event) will be undertaken to finalise data collection for this project component.
  1. Improve understanding of key sources of sediment generating DIN in the marine environment. (Lead proponent: Department of Environment and Science)
  • To achieve this further plumes will be sampled to quantify how much DIN is generated from particulate sources in the plume including the desorption of ammonium and mineralisation of organic matter (incubation experiments). These results will then be combined with sediment and organic matter tracing results to determine the dominant source of sediment producing DIN in the marine environment.
  1. Quantify changes in chemical nature of organic carbon (using 13C NMR spectroscopy) and microbial community composition and shifts across the catchment-to-reef continuum, to explain flood plume nitrogen and carbon biogeochemical cycling and and subsequent availability within the GBR lagoon. (lead proponent: Griffith University)
  • A detailed understanding of the organic processes occurring to form sediment flocs in flood plumes provides critical information to help explain nutrient fluxes and bioavailability of particulate nutrients. The data will also help describe the transformation of microbial communities from the catchment to reef continuum (e.g. end-of-river to sediment trap) and notably the transition from terrestrial to marine forms. The data collected will also be used as biological fingerprints, together with 13C and 15N isotopic signatures to identify the terrestrial or marine origin of organic matters. 
  1. Conduct pilot study to characterise the sediment causing persistent turbidity issues in the Whitsunday Islands. (lead proponent: James Cook University)
  • The SediPump™ developed to sample sediments in flood plumes (and used for the first time in the current project) can be used to obtain four-five representative samples from selected sites in the Whitsunday Islands region. The sediment collected can be characterised using the same techniques from this current research to examine if the sediment causing the persistent turbidity issues is derived from the same source.
  1. Synthesise the tracing data collected under the current NESP project with other recent datasets (including catchment tracing and marine cores: Bainbridge PhD research, Queensland Reef Water Quality Science Program projects RP65G and RP128G,Bartley NESP 2.1.4, Brooks NESP 3.1.7) to capture and integrate the Bowen-Burdekin catchment-to-reef continuum. (lead proponent: James Cook University)
  • The project will continue to collaborate with soil sampling programs funded by the State and Australian Governments with a particular focus on the gully remediation sites. This information will help evaluate the effectiveness of remediating sites in terms of the reduction of the most damaging sediment as well as the release of bioavailable nutrients (N&P) to the GBR.
  • The existing tracing data (i.e. particle size, mineralogy, trace element geochemistry) across the sub-catchments of the Burdekin and the new tracing data from the gully remediation sites will be synthesised with the end-of-catchment and sediment trap data from the Burdekin which will allow a more complete story of sediment tracing across the catchment to marine continuum. The paddock to reef modelling and monitoring teams will be continually engaged and data provided for further model validation and calibration, particularly related to sediment and nutrient loadings.

Analysis of such a rich dataset will provide a more complete understanding of the key catchment sediment (and associated bioavailable nutrient) sources and how these sources have changed over time. This synthesis has direct relevance and linkages with the BMIP in the Bowen-Broken-Bogie catchment to best prioritise the considerable investment (i.e. BMIP, Reef Trust, Greening Australia etc.) for the gully remediation sites that contribute the most damaging sediment to the GBR. Conceptual diagrams can also be developed to capture the most current understanding of the sources, transport, fate and impact of sediment and particulate nutrients in the GBR. Overall, one of the key project outcomes will be to identify locations where catchment remediation will be most beneficial for the GBR


NESP 2017 Research Priority Alignment

The project closely aligns with Theme 1 of the 2017 NESP Research Priorities including:

  • “Improved understanding of the impacts, including cumulative impacts, and pressures on priority freshwater, coastal and marine ecosystems and species (Theme 1)” and
  • “Improve our knowledge of cumulative pressures on environmental and social values of the Great Barrier Reef to determine more effective management actions (1.4).”

The project not only aims to examine the influence and impact of newly delivered sediments and associated particulate nutrients on ecosystems of the GBR but also traces their source in the catchment so that more effective management actions can occur.


Project Keywords

Suspended sediment; Bioavailable nutrients, Sediment tracing; Organic fingerprinting; Resuspension.


Project Funding

This project is jointly funded through JCU, DES (OGBR), DES (Landscape Sciences), GU, DES (Advance Queensland) and the Australian Government’s National Environmental Science Program.