GBR spectral diffuse attenuation coefficient (Kd) of downwelling irradiance (Ed) from ocean colour, 2002-2019 (NESP TWQ 5.3, AIMS and JCU)

This dataset contains estimates of diffuse attenuation coefficient (Kd) of downwelling irradiance at different wavelengths (412, 443, 469, 488, 531, 547, 555, 645, 667 and 678 nm) in the Great Barrier Reef Marine Park and Coral Sea for each day from July 2002 through to December 2019. Data are provided on a nominal 1km grid for each wavelength.

The spectral diffuse attenuation coefficient (Kd) of downwelling irradiance (Ed) determines the amount of light that penetrates the water column and reaches at depth. When this parameter is known along with water depth and estimates of surface irradiance at corresponding spectral wavelength, equivalent benthic irradiance (or the amount of light reaching the seafloor) can be estimated for any location within the GBR. Information on the available light at depth is important since light is a critical requirement to sustain growth and maintain health of benthic ecosystems via photosynthesis. In coastal marine environment, light is a limiting factor that determines the distribution of benthic organisms including seagrasses and corals. 

This data product was developed through NESP TWQ projects 2.3.1 and 5.3 as part of the benthic irradiance model development for the generation of an ecologically relevant water quality indicator based on benthic light. 

Methods:

The diffuse light attenuation coefficient (Kd) of downwelling irradiance is calculated as part of the benthic irradiance model (Magno-Canto et al., 2019; Magno-Canto et al., 2020) using the semi-analytical approach of Lee et al. (2005) and estimates of inherent optical properties (total absorption coefficient, at, and total backscattering coefficient, bb) obtained from the SWIM algorithm of McKinna et al. (2015). Kd is calculated at each of the 10-MODIS wavelength band, hence, a spectrally-resolved estimate of light attenuation at each location within the GBR. 

Technical details of the Kd model is available in:
Lee, Z.-P., Du, K.-P., Arnone, R., 2005. A model for the diffuse attenuation coefficient of downwelling irradiance. Journal of Geophysical Research: Oceans 110.

and some relevant details in the application of the Kd model to the GBR region available in:
Magno-Canto, M.M., McKinna, L.I., Robson, B.J. and Fabricius, K.E., 2019. Model for deriving benthic irradiance in the Great Barrier Reef from MODIS satellite imagery. Optics express, 27(20), pp.A1350-A1371.

Limitations of the data:
Similar to other remote sensing data products, this data set has gaps where atmospheric conditions such as cloud cover precluded clear ocean colour observation at the time of the daily MODIS satellite over-pass.
The holes between the data points are due to a long-standing issue with the degradation of the sensors on the MODIS satellites.
The estimates inherit the limitation of the IOP model used as inputs which assumes that the total absorption and total backscattering coefficients are homogenous within the water column. This Kd data has not also been validated against direct observations (of spectral Kd coefficients) within the GBR.

Format:
This dataset consists of a collection of 18 NetCDF files for each of the 10 MODIS bands/wavelengths (: 412, 443, 469, 488, 531, 547, 555, 645, 667, 678), one for each year between 2002 and 2019, inclusive. Each NetCDF files contain a variable, “Kd_”, which is the diffuse attenuation coefficient of the downwelling irradiance at specific wavelength (i.e., ) in m-1, along with dimensions “latitude”, “longitude” and “time” (to the nearest day). 

References:

Lee, Z.-P., Du, K.-P., Arnone, R., 2005. A model for the diffuse attenuation coefficient of downwelling irradiance. Journal of Geophysical Research: Oceans 110.

Magno-Canto, M.M., McKinna, L.I., Robson, B.J. and Fabricius, K.E., 2019. Model for deriving benthic irradiance in the Great Barrier Reef from MODIS satellite imagery. Optics express, 27(20), pp.A1350-A1371.

Magno-Canto, M.M., McKinna, L.I.W., Robson, B.J., Fabricius, K.E., Garcia, R., 2020. Model for deriving benthic irradiance in the Great Barrier Reef from MODIS satellite imagery: erratum. Opt. Express 28, 27473-27475.

McKinna, L.I., Fearns, P.R., Weeks, S.J., Werdell, P.J., Reichstetter, M., Franz, B.A., Shea, D.M. and Feldman, G.C., 2015. A semianalytical ocean color inversion algorithm with explicit water column depth and substrate reflectance parameterization. Journal of Geophysical Research: Oceans, 120(3), pp.1741-1770.

Reichstetter, M., Fearns, P., Weeks, S., McKinna, L., Roelfsema, C., Furnas, M., 2015. Bottom Reflectance in Ocean Color Satellite Remote Sensing for Coral Reef Environments. Remote Sensing 7, 15852.

Data Location:

This dataset is filed in the eAtlas enduring data repository at: data\nesp5\5.3_Benthic-light

Organization: Australian Ocean Data Network