Quantifying intertidal macroalgae stocks in the NW Iberian Peninsula using unmanned aerial vehicle (UAV) multispectral imagery

Abstract

Intertidal macroalgae play a fundamental role in providing various ecosystem services, and they must be managed effectively for conservation purposes. Using remote sensing techniques to monitor macroalgae is a promising way of optimizing efforts in relation to logistics and personnel. This study aimed to quantify the cover and biomass of the predominant macroalgae in a rocky intertidal zone (of area 5.9 ha) in NW Spain. Drone monitoring with multispectral sensors, in situ photography and biomass sampling were conducted in June and July, 2021 (i.e. in summer, when macroalgae are most abundant in the study area). The macroalgae were classified using Minimum Distance, Spectral Angle Mapping and Maximum Likelihood algorithms, the latter of which proved the most accurate. The fresh weight of macroalgae determined in the quadrat sampling was regressed on the cover of July to enable subsequent estimation of biomass from the classification maps. The area occupied by brown algae (phylum Ochrophyta) in July 2021 was estimated to be 5574 m2 with a biomass of 28.5 t. The remote sensing method, which did not require in-depth knowledge of programming, proved to be a rapid, effective means of estimating macroalgal cover. The method may be useful for managing commercial macroalgal stocks and monitoring protected marine areas.