Developing an integrated approach to seaweed resource assessment

Abstract

Increasing interest in the sustainable management of Irish macroalgal resources requires the development of a cost-effective and efficient methodology for quantifying the distribution of key species. Remote sensing provides a mapping solution that allows for large areas to be covered and is increasingly being applied to a range of macroalgal mapping research questions. Of interest to this research were the commercially and ecologically important intertidal brown fucoid, Ascophyllum nodosum and subtidal kelp communities (often dominated by Laminaria hyperborea). Using a spectroradiometer, the spectral reflectance signatures of common canopy-forming intertidal macroalgae were sampled across four seasons during 2018. Classification and regression tree (CART) analysis showed that it was possible to discriminate between the three macroalgal groups and also between all sampled spectrally similar brown species in all seasons, aside from in winter. Intra-specific variation in spectral response of A. nodosum thalli was observed across the seasons and should potentially be accounted for in the creation of a spectral library. A pushbroom hyperspectral drone survey showed that, using a Maximum Likelihood Classifier (MLC), it was possible to accurately map A. nodosum distribution ((Overall Accuracy (OA) 94.7 %) along with other dominant canopy-forming species. The accurate mapping of multiple species corroborated the results found using the spectroradiometer and highlighted the potential of this technology for intertidal resource mapping. Further work was undertaken at a separate site to compare the ability of two multispectral remote sensing platforms (drone and plane) to accurately map A. nodosum. Using MLC, the drone was found to produce a more accurate (OA 92 %) and higher taxonomic resolution map than the plane (OA 78.9 %) which could only identify a mixed A. nodosum and fucoid class. Experience gained from this research contributed to the creation of a comprehensive guide for using drones to map intertidal macroalgae which detailed the current technology and key challenges associated with mapping within the intertidal zone. Vessel-mounted multibeam sonar was used to map a subtidal kelp bed. Three different acoustic frequencies (200, 300, 400 kHz), each logging water column data, were used to determine whether there was an optimum frequency for the accurate estimation of canopy height and extent. Each of the three frequencies provided slightly different estimates of canopy height and extent. A drop-down camera validated the presence of the kelp bed (dominated by L. hyperborea) but further research is required to determine the source of the variation between the three survey frequencies.