GBR photosynthetically active radiation (PAR) at 8 m depth from ocean colour, 2002-2019 (NESP TWQ 5.3, AIMS, Go2Q Consulting and JCU)

This dataset contains estimated daily integrated photosynthetically active radiation (PAR) at 8 m depth in the Great Barrier Reef Marine Park and Coral Sea for each day from April 2002 through to December 2019. Data are provided on a 1km grid.

The amount of light available for photosynthesis (photosynthetically active radiation, or PAR) is an important determinant of ecosystem health. PAR at 8 m depth is relevant to many coral and seagrass habitats.
This data product was developed through NESP TWQ projects 2.3.1 and 5.3 as a first step in developing an ecologically relevant water quality indicator based on benthic light.

Methods:
Daily benthic light (PAR) at 8 m below the water surface is derived from NASA MODIS Ocean Color data in combination with bathymetric data from Beaman (2010) using the algorithm described by Magno-Canto et al. (2020).

In brief:
Ocean colour observations are used to estimate inherent optical properties (absorption and backscattering) at each of ten MODIS wavelengths, using the SWIM algorithm of McKinna et al. (2015). These in turn are used to calculate spectrally-resolved light attenuation (Kd). The Beer-Lambert equation is then used to propagate surface PAR down to a depth 8 m below the surface of the water. This yields an estimate of instantaneous PAR at 8 mon a 1km grid scale across the study region. 

Instantaneous PAR is then used to calculate daily integrated PAR by considering the path and angle of the sun over the course of each day, assuming constant atmospheric conditions within each day.

A full description of the method can be found in this publication: 
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 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 PAR data has not also been validated against direct observations of PAR at 8 m below the water surface within the GBR.

Format:
This dataset consists of a collection of 18 netcdf files, one for each year between 2002 and 2019, inclusive. Each netCDF files contain a variable, “par8”, which is daily integrated downwelling photosynthetically available radiation at a depth of 8 m below the water surface in Einsteins per square metre per day, 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