Benthic light as ecologically-validated GBR-wide indicator for water quality: Drivers, thresholds and cumulative risks

Barbara Robson

Led by: Dr Barbara Robson / Dr Katharina Fabricius, AIMS


Project Summary

The project is developing a water quality indicator based on benthic PAR (light penetrating to the seafloor). We have experimentally estimated irradiance thresholds for ecosystem health for the Great Barrier Reef and have developed a new remote sensing algorithm to estimate and map benthic irradiance throughout the GBR. This will feed into Reef Plan report cards, RIMReP monitoring, and assessments of drivers of cumulative risks. This proposed extension will allow us to: work with stakeholders to embed the products into their workflows; improve the remote sensing algorithm with seasonal and regional cloud corrections; and evaluate an additional benthic light product based on the eReefs models to complement the remote sensing product.

Project Description


The availability of sufficient light (irradiance) is essential for photosynthesis, the foundation of all food webs and the dominant source of energy for corals and seagrasses. The most important part of the light spectrum for marine systems is photosynthetically active radiation (PAR), and the amount of light reaching the sea floor is “benthic PAR” (bPAR). bPAR is reduced by terrestrial run-off and nutrient-fueled plankton production, and by subsequent resuspension of fine particles due to vertical mixing by currents, winds and tides, and due to activities such as dredging. Until now, it has been not been possible to estimate bPAR over a wide area, nor to relate it to the requirements of coral, seagrasses and inter-reefal communities. This project is developing ecologically relevant, regionally specific light thresholds for health of corals and benthic communities, and is applying these thresholds to produce maps for the whole GBR to provide a cost-effective marine water quality indicator (WQI).

Research – Activities, outputs and outcomes from the current project (NESP 2.3.1)

All original objectives of the current project will be achieved by March 2019. Outputs include:

  • Light thresholds for corals and seagrass: SeaSIM experiments have been conducted to assess coral and seagrass responses to low and to variable light (accounting for resuspension). One manuscript has been submitted for journal publication and a second will be completed by March 2019.
  • Light thresholds for inter-reefal communities: By March 2019, we will have an additional manuscript assessing the role of bPAR in structuring inter-reefal communities and the usefulness of bPAR thresholds in determining maximum colonisation depths, using the CSIRO-AIMS ‘Seabed Biodiversity’ data.
  • In situ PAR data: We now have high-quality open-water PAR data from eight loggers positioned at two depths at each of four stations. These data are already being requested by various parties to support other projects.
  • Benthic light from satellite observations: In cooperation with NASA, we have developed a benthic light (bPAR) algorithm and validated it against our in situ PAR data. This is now being used to produce maps of benthic PAR for the whole GBR. The work will be presented at the international Ocean Optics conference in October and a manuscript describing the algorithm will be submitted by the end of the current project.
  • Data available through eAtlas: We will produce maps for the whole GBR showing where habitat for coral and seagrass communities is compromised by water clarity. WQI maps and data sets from this project will be made available to other researchers and stakeholders through eAtlas.

Outcomes have so far included:

  • Improved understanding of how much light corals and seagrasses need to thrive in the GBR, allowing bPAR to be used as water quality indicator (WQI) in condition maps.
  • Light response thresholds determined from our SeaSim experiments have provided input to setting of light thresholds used in the GBRMPA-led Fortunate Reefs project.
  • Benthic light products from this project are being used by the NESP Cumulative Impacts Project (2.1.6) to assess how light interacts with other pressures that affect coral and inter-reefal (including seagrass) habitats, and by the NESP Bleaching Project to assess how water quality co-determined bleaching susceptibility of specific reefs in the GBR.

Research – Proposed new work

We propose to:

  1. Develop an additional bPAR reporting tool for managers, suitable for use within RIMRep and Outlook, that uses eReefs modelled bPAR. The eReefs marine models are capable of producing maps of simulated bPAR, water clarity and spectral light quality that could complement the satellite bPAR and photic depth maps developed through this project and previous work. While satellite-derived bPAR can provide a longer historical perspective, eReefs bPAR, if demonstrated to be sufficiently accurate, can provide a more flexible product that will allow evaluation of the impacts of future change on bPAR and improve our understanding of drivers of variability.
    We will validate eReefs PAR against our observational data, evaluate the utility of eReefs bPAR as a complement to satellite-derived bPAR estimates, work with the eReefs project team to incorporate information from this project about seagrass and coral light responses into the next generation of eReefs models, and produce annual light anomaly maps from eReefs and/or satellite bPAR estimates, suitable for incorporation into Outlook and RIMReP products, delivered via eAtlas.
  2. Work with NESP 3.2.1 (desired state project) to develop maps of bPAR suitability for diverse benthic ecosystems by relating bPAR maps to existing data describing in situ seagrass and coral distributions and benthic community structure. This would also allow us to estimate bPAR thresholds for maintenance of seagrass biodiversity and thresholds to support seagrasses of particular significance as for maintenance of Aboriginal and Torres Strait Islander Sea Country values.
  3. Incorporate regional and seasonal corrections for cloud cover into the satellite-derived bPAR estimates, enhancing their accuracy and utility.
  4. Conceptually evaluate the degree to which drivers of spatial and temporal variations and trends in bPAR in the GBR can be influenced by management actions, and produce a journal paper discussing this.
  5. Complete a peer-reviewed publication on the use of benthic light as a water-quality indicator and thresholds for key ecological assets, to facilitate uptake of the results by RIMRep and GBRMPA Outlook Reports.
  6. Work with NESP 3.2.1 (desired state project) to develop maps of bPAR suitability for diverse benthic ecosystems by relating bPAR maps to existing data describing in situ seagrass and coral distributions (beyond those species for which we have laboratory-derived PAR responses) and other pressures. This would also allow us to estimate bPAR thresholds for maintenance of seagrass biodiversity and thresholds to support seagrasses of particular significance as for maintenance of Aboriginal and Torres Strait Islander Sea Country values.
  7. Work with Geoscience Australia to explore incorporation of products from this project into the Australian Geoscience Datacube.

Informing decision-making

We will work with stakeholders to incorporate the new bPAR Water Quality Indicator into their workflows and reporting tools, including RIMRep reporting, GBRMPA 2050 and Outlook Report Cards, the Geoscience Australia Datacube, e-Atlas, and the Fitzroy River Health Report Card, providing a clear means to assess how this WQI for ecosystem health is changing from year to year and how it is affected by events such as cyclones. Our satellite bPAR products will allow evaluation of 16 years of historical variations in bPAR and its effects on corals, seagrasses and benthic communities. Validation of the new eReefs model bPAR products will allow them to be used with confidence for evaluation of change scenarios, including the effects of on-ground management actions.

We will also develop plain English fact sheets for policy agencies, describing why bPAR is important for Great Barrier Reef Ecosystem Health and how it is influenced by human activities.

Links with other projects and hubs

eAtlas: The bPAR exceedance maps will be developed in collaboration with the eAtlas.

Marine Biodiversity Hub Project C3; Australian Geoscience Datacube (Sagar et al.): Working with Geoscience Australia, we will explore the possibility of the datacube as an additional, more interactive mechanism for processing and delivering the satellite bPAR product.

NESP 2.1.6 (Uthicke, Fabricius et al): This project will provide data layers to NESP 2.1.6 and contribute to the analysis of cumulative impacts.

NESP 3.2.1 (Collier et al.): This project will work with NESP 3.2.1 to establish in situ light thresholds for seagrasses and other benthic habitats.

NESP 4.5 (Mumby et al.): This project will provide bPAR products as shapefile or netCDF data layers for input to the tools being developed by NESP 4.5.

eReefs: This project will use and evaluate eReefs benthic light model outputs and feed into improved algorithms for the next iteration of the models.

IMOS: Light loggers have been deployed and data processed by the IMOS team at four sites (Yongala, Rib, Myrmidon and Palm Passage). These data are freely available through IMOS.

Related prior research

NESP Project 2.3.1 Benthic Light

MTSRF Project 3.7.1 (Fabricius et al.): “Marine and estuarine indicators and thresholds of concern” (Fabricius et al. 2012), and Water Quality Guidelines for the GBR (GBRMPA 2009, De’ath and Fabricius 2010).

ARC-Linkage Project LP100100342 – collaboration UQ and NASA (Weeks, Werdell and McKinna): Development of the Shallow Water Inversion Model (SWIM; McKinna et al (2015)) and Photic Depth algorithms (Weeks et al. 2012) for the GBR, demonstrating the strong historical collaboration with NASA Ocean Biology Processing Group.

NERP 4.1 (Fabricius et al): “Tracking coastal turbidity over time and demonstrating the effects of river discharge events on regional turbidity in the GBR”

NESP Round 1: 3.3 (Collier): Light thresholds for seagrasses of the GBR: a synthesis and guiding document for managing seagrass

MMP, LTMP and RIMReP: Field validation of ecological data and development of a reporting tool.

WAMSI Dredging project will contribute to informing on light requirements in corals.

NESP 2017 Research Priority Alignment

This project aligns with NESP Theme 3, “Natural resource management improvements based on sound understanding of the status and long term trends of priority species and systems”. In particular, it aligns with priority 3.3, “Combine existing indicators and monitoring programmes to develop a cost-effective integrated monitoring programme to support natural resource management, evaluate results and communicate trends” and 3.5, “Develop and implement better tools, including spatial information, to support the prioritisation of onground investments and interventions and asses their success.” This will provide information needed for Theme 2, “Maximise the resilience of vulnerable species to the impacts of climate change and climate variability by reducing other pressures, including poor water quality” by improving the connection between marine water quality and the responses of vulnerable species.

Project Keywords

Water quality indicator; PAR; Light; Benthic biota; Remote sensing.

Project Funding

This project is jointly funded through AIMS and the Australian Government’s National Environmental Science Program.

Project Publications