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

Scarla Weeks,John Wilding,Gene Feldman,P Werdell,Marites Canto,Britta Schaffelke,Zhongping Lee

doi:10.3390/rs4123781

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 (ZSD) data to MODIS-Aqua (2002–2010) and SeaWiFS (1997–2010) satellite data. The results of the in situ ZSD/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 ZSD and implemented the validated ZSD algorithm to generate a decadal satellite time series (2002–2012) for the GBR. Water clarity varied significantly in space and time. Seasonal effects were distinct, with lower 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 ZSD 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.

Highlights

The quantification of environmental variables in space and time is essential to understand the ecology of marine organisms and their responses to a changing environment
The primary objectives of this study were: (1) to develop a photic depth product for the Great Barrier Reef (GBR); (2) to detect changes in GBR water clarity in space and time based on the optimal, available algorithm for light attenuation through the water column; and (3) to determine the dominant modes of variation and identify the physical drivers that influence the spatio-temporal patterns of water clarity across the
The results of the satellite-to-in situ “matchups” for the Australian Institute of Marine Science (AIMS)/Queensland Department of Primary Industries and Fisheries (QDPI) ZSD dataset were far better than expected, given that there had been no regional refinement of the inherent optical properties (IOPs) photic depth algorithm

Summary

Introduction

The quantification of environmental variables in space and time is essential to understand the ecology of marine organisms and their responses to a changing environment. Temperature, salinity and the availability of light and nutrients are key parameters controlling their distribution and productivity. Ocean color satellites, such as the NASA Sea-viewing Wide Field-of-view Sensor (SeaWiFS) and the NASA Moderate Imaging Spectroradiometer onboard Aqua (MODIS-Aqua), provide oceanographic data records on spatial and temporal scales unattainable using conventional in situ sampling schemes. The current state of operational satellite remote sensing is that algorithms for measuring geophysical parameters, such as chlorophyll concentration or water clarity, are reasonably reliable over deep water, but still limited in optically shallow regions. Recent progress in remote sensing applications includes the development of bio-optical algorithms for determining the photic depth (Z%; units of m) from the observed satellite radiances. Z% is a measure of water clarity [4] with, for example, Z1% reflecting the depth where only

Objectives

Results

Conclusion