Abstract
Despite being recognized as a key component of shallow-water ecosystems, submerged aquatic vegetation (SAV) remains difficult to monitor over large spatial scales. Because of SAV’s structuring capabilities, high-resolution monitoring of submerged landscapes could generate highly valuable ecological data. Until now, high-resolution remote sensing of SAV has been largely limited to applications within costly image analysis software. In this paper, we propose an example of an adaptable open-sourced object-based image analysis (OBIA) workflow to generate SAV cover maps in complex aquatic environments. Using the R software, QGIS and Orfeo Toolbox, we apply radiometric calibration, atmospheric correction, a de-striping correction, and a hierarchical iterative OBIA random forest classification to generate SAV cover maps based on raw DigitalGlobe multispectral imagery. The workflow is applied to images taken over two spatially complex fluvial lakes in Quebec, Canada, using Quickbird-02 and Worldview-03 satellites. Classification performance based on training sets reveals conservative SAV cover estimates with less than 10% error across all classes except for lower SAV growth forms in the most turbid waters. In light of these results, we conclude that it is possible to monitor SAV distribution using high-resolution remote sensing within an open-sourced environment with a flexible and functional workflow.
Highlights
Published: 7 January 2022The ecological importance of submerged aquatic vegetation (SAV; syn. hydrophytes or macrophytes) has been recognized for a long time in both marine and freshwater sciences
With sufficient understanding of the radiometric challenges and acknowledging some limitations, large-scale SAV distribution data can be obtained from optical remote sensing using object-based image analysis (OBIA) in an open-source environment
Absolute radiometric calibration performed with Equation (1) generates radiance values very similar for both images, which translates into top-of-atmosphere (TOA) reflectance that are high in the lower wavelengths due to aerosol scattering and lower in the higher wavelengths due to aerosol absorption
Summary
Published: 7 January 2022The ecological importance of submerged aquatic vegetation (SAV; syn. hydrophytes or macrophytes) has been recognized for a long time in both marine and freshwater sciences. Because of the interactions between their engineering capabilities and their sensitivity to environmental stress, SAV has been identified as a good functional group of candidates for sentinel species [10]. On one hand, their role as ecological engineers is related to their ability to regulate water quality through density-dependent feedback mechanisms acting as sediment traps, nutrient sinks, and biodiversity hotspots, all of which modulate the availability of resources to other species in the ecosystem [6,12,13,14,15]. At lower densities of SAV, individuals become more exposed to the impact of multiple ecological stressors and their distribution starts to depend on environmental factors related to ecological integrity, including nutrient and light availability, hydrology, and climate [10,16,17,18]
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