Assessing intertidal sediment photopigment content from spectral reflectance with an UAV-mounted 10-band multispectral sensor

Abstract

Remotely sensed multispectral information can be leveraged to quantify microphytobenthic (MPB) biofilm biomass in intertidal mudflats and overcome some of the limitations of traditional field campaigns with sediment collection. Satellite-based approaches are well established, however moderate spatial resolution (≥ 10 m) is not suitable for investigating fine-scale MPB spatial heterogeneity. Multispectral sensors mounted to unoccupied aerial vehicles (UAVs) can fill this gap by providing data products with cm-scale pixels, but no standard protocol currently exists for calibrating UAV-acquired spectral information to sediment MPB content. Here, we present a new protocol for calibrating data from a UAV-mounted multispectral sensor (MicaSense RedEdge-MX dual) to sediment MPB biomass as measured by photopigment content. To do so, we (1) built an adaptable methodology for acquiring and analyzing UAV imagery and sediment photopigment field data, then (2) implemented a protocol in the Fraser River Estuary, Canada, to build a statistically valid calibration equation, testing the effectiveness of several spectral indices and photopigment measurements. Calibrated spectral index values from MicaSense RedEdge-MX data can provide a very accurate measurement of MPB biomass, able to achieve 90 % correlation between the normalized difference vegetation index (NDVI) and sediment chlorophyl-a (chl-a) concentration. This high performance was achieved by (1) closely pairing georeferenced sediment samples to corresponding multispectral imagery, (2) selecting an accessible section of mudflats with a wide range of sediment photopigment concentrations, and (3) minimizing the lag between sediment sample collection and MicaSense imagery acquisition. This newly developed protocol can facilitate the use of calibrated UAV-acquired MicaSense RedEdge-MX multispectral imagery of mudflats for investigating ecologically-important fine-scale spatial heterogeneity and short-term temporal dynamics of MPB biomass, and can bridge the gap between UAV- and satellite- acquired imagery of intertidal ecosystems.