Preliminary Assessment of Turbidity and Chlorophyll Impact on Bathymetry Derived from Sentinel-2A and Sentinel-3A Satellites in South Florida

Isabel Caballero,Andrew Meredith,Richard Stumpf

doi:10.3390/rs11060645

Abstract

Evaluation of the impact of turbidity on satellite-derived bathymetry (SDB) is a crucial step for selecting optimal scenes and for addressing the limitations of SDB. This study examines the relatively high-resolution MultiSpectral instrument (MSI) onboard Sentinel-2A (10–20–60 m) and the moderate-resolution Ocean and Land Color instrument (OLCI) onboard Sentinel-3A (300 m) for generating bathymetric maps through a conventional ratio transform model in environments with some turbidity in South Florida. Both sensors incorporate additional spectral bands in the red-edge near infrared (NIR) region, allowing turbidity detection in optically shallow waters. The ratio model only requires two calibration parameters for vertical referencing using available chart data, whereas independent lidar surveys are used for validation and error analysis. The MSI retrieves bathymetry at 10 m with errors of 0.58 m at depths ranging between 0–18 m (limit of lidar survey) in West Palm Beach and of 0.22 m at depths ranging between 0–5 m in Key West, in conditions with low turbidity. In addition, this research presents an assessment of the SDB depth limit caused by turbidity as determined with the reflectance of the red-edge bands at 709 nm (OLCI) and 704 nm (MSI) and a standard ocean color chlorophyll concentration. OLCI and MSI results are comparable, indicating the potential of the two optical missions as interchangeable sensors that can help determine the selection of the optimal scenes for SDB mapping. OLCI can provide temporal data to identify water quality characteristics and general SDB patterns. The relationship of turbidity with depth detection may help to enhance the operational use of SDB over environments with varying water transparency conditions, particularly in remote and inaccessible regions of the world.

Highlights

Updated and accurate coastal bathymetry is becoming a crucial source of information required for a wide range of purposes, including navigation, resource exploitation, fisheries, aquaculture, coastal management, spatial planning, benthic research, and tourism
Employing widely available and free Sentinel-2 and Sentinel-3 imagery, accurate and consistent bathymetry could be retrieved when calibrated with limited available chart data
Chlorophyll algorithm can be proxies for turbidity and pigment impacts on satellite-derived bathymetry (SDB) estimations. This is an important step toward addressing the influence of turbidity on SDB

Summary

Introduction

Updated and accurate coastal bathymetry is becoming a crucial source of information required for a wide range of purposes, including navigation, resource exploitation, fisheries, aquaculture, coastal management, spatial planning, benthic research, and tourism. According to International Hydrographic Office (IHO) publication, an estimated 70% of the world’s littoral zone is not mapped to modern standards [1,2] One reason for this limited coastal coverage is that traditional survey methods for mapping water depth are extremely expensive. Different approaches have been applied to characterize satellite-derived bathymetry (SDB) from the physics of light in the water. These include semi-analytical equations [6,7] and approximation equations [8] with empirically tuned coefficients. When multi-temporal approaches are used, the retrieved water reflectances need to be consistent, even with different atmospheric conditions [8,14]

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Discussion

Conclusion