GBR benthic light (bPAR) from ocean colour, 2002-2019 (NESP TWQ 5.3, AIMS, Go2Q Consulting and JCU)

This dataset contains estimated daily integrated benthic photosynthetically active radiation (bPAR) 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 1km grid.

The amount of light available for photosynthesis (photosynthetically active radiation, or PAR) is an important determinant of ecosystem health. PAR reaching the bottom of the water column is known as benthic PAR (bPAR). Where there is sufficient light reaching the bottom, seagrasses and corals may thrive. bPAR varies seasonally as a function of surface PAR, but also varies function of both water depth and water quality. 
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 (bPAR) 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. (2019).

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 the depth of the bottom of the water column using the bathymetry (i.e. depth map) provided by Beaman (2010). This yields an estimate of instantaneous bPAR on a 1km grid scale across the study region. 
Instantaneous bPAR is then used to calculate daily integrated benthic light 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:
This data set has gaps where atmospheric conditions such as cloud cover and sun angle 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.
While the data have been evaluated against in situ observed PAR at four sites within the Great Barrier Reef with good results, its accuracy has not been evaluated in muddy nearshore waters or deeper offshore waters.

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, “parb”, which is daily integrated downwelling photosynthetically available radiation at the sea floor in Einsteins per square metre per day, along with dimensions “latitude”, “longitude” and “time” (to the nearest day). 

References:
Beaman, R., 2010. Project 3DGBR: A high-resolution depth model for the Great Barrier Reef and Coral Sea, Marine and Tropical Sciences Research Facility (MTSRF) Project 2.5i.1a, Final Report, Reef and Rainforest Research Centre.

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.

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.

Robson, B., Canto, M., Collier, C., di Perna, S. and Logan, M., 2019. Benthic light as ecologically-validated GBR-wide indicator for water quality. NESP Tropical Water Quality Research Hub.

Data Location:

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

Organization: Australian Ocean Data Network