River Radii: A Comparative National Framework for Remote Monitoring of Environmental Change at River Mouths
Shane Orchard, François Thoral, Matthew H. Pinkerton, Christopher N. Battershill, Rahera Ohia, David R. Schiel
Abstract
River mouths are important indicators and mediators of interactions between rivers and the sea that mark the dispersal point for catchment-based stressors and subsidies. Satellite remote sensing data products and algorithms present many new possibilities for monitoring these dynamic and often inaccessible environments. In this study, we describe a national-scale comparative framework based on proximity to river mouths and show its application to the monitoring of coastal ecosystem health in Aotearoa New Zealand. We present results from light attenuation coefficient (Kd) analyses used to develop the framework considering data products of differing resolution and the effects of coastline geometries which might obscure the influence of catchment-derived stressors. Ten-year (2013–2022) Kd values from the highest-resolution product (500 m) showed significant differences (p < 0.01) in successively larger radii (1–20 km) despite the confounding influence of adjacent river mouths. Smaller radii returned a high variability that dropped markedly > 5 km. Tests of a 10 km radius showed that coastline geometry had a significant influence on Kd (p < 0.001), which is also likely for other water quality indicators. An analytical approach stratified by coastline geometry showed significant effects of stream order on open (p < 0.01) but not enclosed coasts, differences between marine bioregions (p < 0.05), and a degradation trend in the 90th percentile of Kd on enclosed coasts, which is indicative of extreme events associated with catchment erosion or sediment resuspension. We highlight applications of the framework to explore trends across many other meaningful scales (e.g., jurisdictions and ecosystem types) in addition to tracking changes at individual river mouths.