Secchi depth, vertical attenuation at 490nm & light intensity above seagrass (GBR4 BGC v4.2 baseline)
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Data gap notice
Some dates in the 4km eReefs BioGeoChemical model v4.2 dataset have been removed because the model output for those dates was found to be inaccurate, as shown in this sample video.
The following date ranges have been removed:
| Start date | End date |
|---|---|
| 2 Jan 2011 | 31 Jan 2011 |
| 2 Feb 2012 | 2 Mar 2012 |
| 9 Mar 2013 | 7 Apr 2013 |
| 1 Mar 2014 | 30 Mar 2014 |
| 6 Apr 2015 | 5 May 2015 |
| 10 May 2016 | 8 Jun 2016 |
| 14 Jun 2017 | 13 Jul 2017 |
| 19 Jul 2018 | 17 Aug 2018 |
| 23 Aug 2019 | 21 Sep 2019 |
| 26 Sep 2020 | 25 Oct 2020 |
| 31 Oct 2021 | 29 Nov 2021 |
| 19 May 2022 | 17 Jun 2022 |
Please use caution when interpreting the videos in the player above.
Although the obviously inaccurate dates have been removed, the model may take some time to recover after each disruption. As a result, data in the days and weeks following the removed periods may also be affected. The 30-day buffer used here is a visual estimate and not scientifically validated, so some remaining inaccuracies may still be present in the dataset.
We will update this portal when corrected model output becomes available.
Secchi depth
Secchi depth is an estimate of water clarity, which is traditionally measured by lowering a secchi disk into the water until the disk cannot be seen. The model estimates the optical properties of the water based on the sediment and nutrients in the water. These all affect the scattering and absorption of light as it passes through the water allowing the model to estimate the attenuation of the light through the water. The Secchi depth is calculated from the vertical integral of attenuation of light at 488nm.
Vertical attenuation at 490nm
As light passes through the water column its intensity decreases due to a combination of absorption and scattering. The amount of light decreases exponentially as a function of the depth. The amount of light at a given depth can be estimated by scaling the surface irradiance by e(-Kd*depth), where Kd is the vertical attenuation coefficient, and e is the base of the natural logarithm.
In practice the vertical attenuation varies with the wavelength of light as colours are absorbed at different rates due to the inherent properties of sea water (blue penetrates more than red light) and the amount of sediment and organic material in the water. The vertical attenuation also varies with depth if the water column is not fully mixed. Stratification of coloured dissolved organic matter (CDOM) or suspended sediment can result in variation in the clarity of the water, and thus the vertical attenuation, with depth.
In the BGC model, the vertical attenuation is a depth dependent variable that corresponds to the light attenuation in the model depth layer at a wavelength of 490 nm (blue/cyan). It is calculated based on the optical model of the water, which considers the wavelength dependent absorption and scattering of the light through the water column depending on the amount of modelled coloured dissolved organic matter (CDOM) and sediment in the water column. Further details of the vertical attenuation are outlined in section 3.2.2 of the BGC Appendix B: Scientific description of the optical, carbon chemistry and biogeochemical models.
Light intensity above seagrass
The variable EpiPAR_sg represents the light intensity above seagrass measured in daily light integral (mol photons per square meter per day). Please note that the eReefs BGC model is using a 4km resolution and is developed for large-scale predictions. This limits the possibility of using this variable for a detailed analysis of processes relevant to seagrass. The NESP project 3.2.1 (Deriving ecologically relevant load targets to meet desired ecosystem condition for the Great Barrier Reef: a case study for seagrass meadows in the Burdekin region) is currently investigating the possibility of using this model to derive and test ecologically relevant targets.
Source data
The videos/images on this page are based on the 4km eReefs BioGeoChemical model (v4.2) run with SOURCE Catchments using Baseline catchment conditions. The model builds on the CSIRO Environmental Modelling Suite (EMS), described in the paper: Scientific description of the optical and biogeochemical models (vB3p0). The dataset metadata is available from the NCI GeoNetwork: eReefs GBR4 Biogeochemistry and Sediments v4.2 baseline catchment scenario. The raw model data is available from the NCI THREDDS server (daily, in curvilinear NetCDF format).
Data span
These results are based on a fixed time period (Dec 2010 - Apr 2019) hind-cast analysis developed for comparing changes in land practices. The river runoff used to drive the BGC model was provided by the SOURCE Catchments modelling.