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Secchi depth, vertical attenuation at 490nm & light intensity above seagrass (GBR4 BGC v3.1 baseline)

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    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 can not 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 descreases due to a combination of absorption and scattering. The amount of light descreases 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 colour 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 attenutation 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 colour dissolved organic matter (CDOM) and sediment in the water column. Further details of the vertical attenuation is 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.