Extended abstract, Ocean Optics XXII __________________________________________________________________________________ Ocean Optics XXII conference, Portland, Maine. October 26-31, 2014. Influence of suspended particulate concentration, composition and size on the variability of light backscattering in the Great Barrier Reef region, Australia. Nagur Cherukuru, Janet Anstee, Kadija Oubelkheir, Thomas Schroeder, Lesley Clementson CSIRO Oceans and Atmosphere, email: Nagur.cherukuru@csiro.au 1. Introduction The Great Barrier Reef (GBR) region in Australia hosts one of the most bio diverse and natural habitats in the world, yet this ecosystem is under stress due to climate and anthropogenic induced changes (Brodie et al., 2012). A general decline in the overall ecosystem of the GBR has partially been linked to an increase in suspended loads. However the dispersal of particulate matter has received little attention (Bainbridge et al., 2012). Optical remote sensing (ORS) has the potential to support the monitoring and management requirements in such a remote and ecologically sensitive region. But, to take advantage of the large volumes of remote sensing (RS) data it is essential to understand the relationship between the total suspended sediments (TSS) and the backscattered optical signal. In this study we present our observations of distribution of TSS and the variability in light backscattering due to particulate matter (bbP), made during the wet season of 2013. An understanding of bio-optical relationships is expected to improve the performance of remote sensing algorithms and support the application of remote sensing tools in the monitoring and management of optically complex and environmentally sensitive areas (Brando et al., 2012). 2. Study area The GBR region is located between 10°S and 24°S with a coastal zone area of 30,000 km 2 along the northeastern coast of Australia (Fig 1). An extensive catchment area containing multiple river basins drains to the GBR coastal waters. This catchment includes both large dry tropical catchments as well as smaller wet tropical catchments. Wet Tropic catchments, north of 19°S, have frequent storm and runoff events and in the Dry Tropic catchments, the major flow events may occur at intervals of years (Brodie et al., 2009). The frequency and duration of high river flow periods influence the GBR through the wet season (Devlin et al., 2012).Generally the water quality conditions are associated with low concentrations of dissolved and particulate substances. In contrast, the water quality parameters during the wet season are higher by one to two orders of magnitude (Shaffelke et al., 2012). A comprehensive bio-optical study covering GBR was presented by Blondeau-Patissier et al.,(2009), which mainly detailed the dry season optical properties. They summarised non- algal particulate (NAP) matter to be the dominant optical component in GBR waters during the dry season. 3. Methods In total 77 stations were sampled along the coastline between 13.9 °S and 20.2 °S. We sampled 42 stations between Townsville and Princess Charlotte Bay (PCB) during 26/January/2013 to 04/February/2013. Coastal stations (35) between Townsville to Bowen was sampled during 13-18/March/2013 to understand the extent and influence of Burdekin