Particulate Backscattering Ratio as an Indicator of Changing Particle Composition in Coastal Waters: Observations From Great Barrier Reef Waters

Abstract

Particle scattering is an important process that determines both the light penetration through the water column and water‐leaving light. Backscattering, in combination with absorption, determines the remote‐sensing reflectance that is used in ocean color algorithms. Additionally, the wavelength dependence of the backscattering ratio can be related to the particle composition in seawater. Here, we examine the magnitude and the spectral behavior of the backscattering ratio against other bio‐optical properties based on a comprehensive set of continuous measurements collected in coastal waters of the Great Barrier Reef over 3 years. The site is located offshore of a major river, and close to the cross‐shelf transition of bottom sediments from terrigenous muds to marine carbonates. The backscattering ratio measured at 650 nm clearly clustered the data into two types. Type 1, which we identified as terrigenous mud with a low organic fraction, is characterized by backscattering ratio below 0.011 with a mean value of 0.005. Type 2, which we identified as marine carbonate with a higher organic fraction, has backscattering ratio above 0.011 with a mean value of 0.02. Within 3 years, study site was exposed to type 1‐dominated particles 13% of the time, and type 2 87%. The observed changes in the backscattering ratio at this one coastal site are as large as the variability seen throughout the global ocean. This work provides a better understanding of processes determining the optical characteristics and insights into optical parameterizations that can be used in process‐based optical modeling of the Great Barrier Reef.

First Evaluation of PRISMA Level 1 Data for Water Applications

“…The sites represent a rather wide range of water types and trophic conditions but also atmospheric turbidity. In particular: (1) the Zeebrugge site in the North Sea is representative of turbid nearshore waters [ 40 ]; (2) the Lucinda site is located in tropical coastal waters in Eastern Australia, dominated by non-algal particulates and CDOM [ 41 , 42 ]; (3) the Casablanca site in the Western Mediterranean Sea is representative of open ocean chlorophyll-a dominated waters [ 43 ]; (4) the Bahia Blanca site in the southern Atlantic coast is representative of high suspended loads induced by widespread erosion and strong tidal currents [ 44 ]; (5) the Acqua Alta Oceanographic Tower (AAOT) of Venice in the northern Adriatic Sea is representative of moderately sediment dominated waters [ 45 , 46 ]; (6) finally, Lake Trasimeno has shallow turbid waters with recurrent sediment resuspension and moderate to high bloom of phytoplankton, including cyanobacteria species [ 47 ].…”

“…The sites represent a rather wide range of water types and trophic conditions but also atmospheric turbidity. In particular: (1) the Zeebrugge site in the North Sea is representative of turbid nearshore waters [40]; (2) the Lucinda site is located in tropical coastal waters in Eastern Australia, dominated by non-algal particulates and CDOM [41,42]; 3 Whenever PRISMA scenes were acquired within ± 1 day of a corresponding Sentinel-2 overpass, the corresponding L1C Multi Spectral imager (MSI) images were downloaded for a direct comparison with PRISMA. Sentinel-2/MSI images were processed in SNAP toolbox (http://step.esa.int/main/) to transform the dimensionless apparent reflectance into physical units of TOA radiances and for spatially resampling the multispectral cube to 30-m pixel size-hence comparable to the PRISMA GSD.…”

“…The sites represent a rather wide range of water types and trophic conditions but also atmospheric turbidity. In particular: (1) the Zeebrugge site in the North Sea is representative of turbid nearshore waters [40]; (2) the Lucinda site is located in tropical coastal waters in Eastern Australia, dominated by non-algal particulates and CDOM [41,42];…”

CSIRO Environmental Modelling Suite (EMS): scientific description of the optical and biogeochemical models (vB3p0)

“…where E above,λ for macroalgae is E d,bot,λ , the downwelling irradiance of the bottom water column layer, A λ is the leaf- Figure 10. Spectrally resolved energy distribution of sunlight, clear-water absorption and clear-water scattering (Smith and Baker, 1981).…”

“…The awkwardness of the variety of benthic communities (corals, seagrass, kelp, etc.) and their prime role in shallow water, has meant that estuarine and coastal models have, like ERSEM and EMS, typically chosen the functional model approach (Madden and Kemp, 1996;Spillman et al, 2007).…”