Satellite-Derived Photic Depth on the Great Barrier Reef: Spatio-Temporal Patterns of Water Clarity

Abstract

Detecting changes to the transparency of the water column is critical for understanding the responses of marine organisms, such as corals, to light availability. Long-term patterns in water transparency determine geographical and depth distributions, while acute reductions cause short-term stress, potentially mortality and may increase the organisms' vulnerability to other environmental stressors. Here, we investigated the optimal, operational algorithm for light attenuation through the water column across the scale of the Great Barrier Reef (GBR), Australia. We implemented and tested a quasi-analytical algorithm to determine the photic depth in GBR waters and matched regional Secchi depth (Z SD ) data to MODIS-Aqua (2002 and SeaWiFS (1997SeaWiFS ( -2010 satellite data. The results of the in situ Z SD /satellite data matchup showed a simple bias offset between the in situ and satellite retrievals. Using a Type II linear regression of log-transformed satellite and in situ data, we estimated Z SD and implemented the validated Z SD algorithm to generate a decadal satellite time series (2002)(2003)(2004)(2005)(2006)(2007)(2008)(2009)(2010)(2011)(2012) values during the austral summer, most likely due to river runoff and increased vertical mixing, and a decline in water clarity between 2008-2012, reflecting a prevailing La Niña weather pattern. The decline in water clarity was most pronounced in the inshore area, where a significant decrease in mean inner shelf Z SD of 2.1 m (from 8.3 m to 6.2 m) occurred over the decade. Empirical Orthogonal Function Analysis determined the dominance of Mode 1 (51.3%), with the greatest variation in water clarity along the mid-shelf, reflecting the strong influence of oceanic intrusions on the spatio-temporal patterns of water clarity. The newly developed photic depth product has many potential applications for the GBR from water quality monitoring to analyses of ecosystem responses to changes in water clarity.

A semianalytical ocean color inversion algorithm with explicit water column depth and substrate reflectance parameterization

“…The aim of these programs is to halt and reverse the decline in the quality of water flowing into the GBR, supported by continual monitoring of water quality conditions in the region. Because of its synoptic-scale spatial coverage and near-daily over-pass frequency, ocean color remote sensing has become an integral part of monitoring and reporting spatiotemporal trends in water quality for the GBR region [Brando et al, 2011;Devlin et al, 2012;Devlin and Schaffelke, 2009;Fabricius et al, 2014;Schroeder et al, 2012;Weeks et al, 2012].…”

Water Quality and River Plume Monitoring in the Great Barrier Reef: An Overview of Methods Based on Ocean Colour Satellite Data

“…The frequency of ocean colour products has been linked to gradients in water quality [28,30], measures of photic depth linked to river flow and water quality [59,61], and CDOM utilised as a proxy for freshwater as a layer in a GBR vulnerability assessment [89]. The CDOM analysis has also been used as part of a GBR wide vulnerability assessment [104].…”

“…The effect of lower clarity in large river discharge years is driven by the river plume-delivered fine material, which contains large amounts of organic material in flocs [76] being resuspended in periods of strong winds and large tides (a characteristic in the central-southern GBR). This work is an integral part of recent work on modelling the influence of river flow and suspended sediments on the dry season turbidity of the GBR [59,61].…”

“…Recent work also includes the development of a regional bio-optical algorithm for determining regional Secchi depth (ZSD) related to turbidity, clarity, and TSS in coastal waters in GBR waters over wet and dry seasons [59,61]. This algorithm has been used to calibrate MODIS time series of photic depth in GBR waters using inshore water quality collected through long-term monitoring programs [74,75]).…”

Validation of the semi-analytical algorithm for estimating vertical underwater visibility using MODIS data in the waters around Korea

“…Long-term monitoring of vertical underwater visibility based on Secchi depth has been conducted since nineteenth century for monitoring the light availability related to marine organism (Lorenzen, 1970(Lorenzen, , 1972Morel, 1988;Morel and Berthon, 1989;Falkowski and Wilson, 1992;Park et al, 1998;Doron et al, 2011;Weeks et al, 2012). With in-situ Secchi depth data, various satellite ocean color data have been used to estimate vertical and horizontal vertical underwater visibility on a global scale (Doron et al, 2011;Weeks et al, 2012;Barnes et al, 2013). The vertical underwater visibility is affected by the optics of the detector, the Inherent Optical Properties (IOPs) and Apparent Optical Properties (AOPs) of seawater (Zaneveld and Pegau, 2003).…”