A High-Resolution Global Map of Giant Kelp (Macrocystis pyrifera) Forests and Intertidal Green Algae (Ulvophyceae) with Sentinel-2 Imagery

Alejandra Mora-Soto,Erasmo Macaya,Mary Young,Marc Macias-Fauria,Mohammad Yaqub,Mauricio Palacios,Pirjo Huovinen,Neil Golding,Alejandro Pérez-Matus,Iván Gómez,Martin Toro

doi:10.3390/rs12040694

Abstract

Giant kelp (Macrocystis pyrifera) is the most widely distributed kelp species on the planet, constituting one of the richest and most productive ecosystems on Earth, but detailed information on its distribution is entirely missing in some marine ecoregions, especially in the high latitudes of the Southern Hemisphere. Here, we present an algorithm based on a series of filter thresholds to detect giant kelp employing Sentinel-2 imagery. Given the overlap between the reflectances of giant kelp and intertidal green algae (Ulvophyceae), the latter are also detected on shallow rocky intertidal areas. The kelp filter algorithm was applied separately to vegetation indices, the Floating Algae Index (FAI), the Normalised Difference Vegetation Index (NDVI), and a novel formula (the Kelp Difference, KD). Training data from previously surveyed kelp forests and other coastal and ocean features were used to identify reflectance threshold values. This procedure was validated with independent field data collected with UAV imagery at a high spatial resolution and point-georeferenced sites at a low spatial resolution. When comparing UAV with Sentinel data (high-resolution validation), an average overall accuracy ≥ 0.88 and Cohen’s kappa ≥ 0.64 coefficients were found in all three indices for canopies reaching the surface with extensions greater than 1 hectare, with the KD showing the highest average kappa score (0.66). Measurements between previously surveyed georeferenced points and remotely-sensed kelp grid cells (low-resolution validation) showed that 66% of the georeferenced points had grid cells indicating kelp presence within a linear distance of 300 m. We employed the KD in our kelp filter algorithm to estimate the global extent of giant kelp and intertidal green algae per marine ecoregion and province, producing a high-resolution global map of giant kelp and intertidal green algae, powered by Google Earth Engine.

Highlights

Kelp forests provide a variety of ecosystem services: they protect and modify the substrate and surrounding water column and act as a habitat for several species, increasing the complexity of coastal trophic networks [1,2]
Difference Vegetation Index (NDVI [21], an index used in remote sensing to detect vegetation based on the differential reflectance of vegetation between wavelengths within the photosynthetically active radiation and the near infrared); the Floating Algae Index (FAI [44], an index designed to detect floating algae); and the Kelp Difference (KD) we introduce in this paper
Using the original training data of 90 grid cells per Regions of Interest (ROI) (720 grid cells in total), the applied protocol preserved a fraction of Giant kelp grid cells, which were concentrated at the centre of the observed kelp canopies and represented the purest grid cells or at least the grid cells most dominated by kelp canopy, and Green algae grid cells on rocky intertidal areas (Table 4)

Summary

Introduction

Kelp forests provide a variety of ecosystem services: they protect and modify the substrate and surrounding water column and act as a habitat for several species, increasing the complexity of coastal trophic networks [1,2]. The term ‘kelp’ is used to refer to all large brown algae [3], the focus of this study is the commonly called giant kelp [3], Macrocystis pyrifera (Laminariales, Phaeophyceae), Earth’s most widespread kelp species, which forms one of the most productive and diverse ecosystems on the planet [4,5,6]. Mapping kelp’s global distribution and biomass dynamics is a necessary first step in understanding its contemporary distribution and identifying possible threats in a context of global change [11]. This information can contribute to the sustainable development agenda of the UN by 2030, goal 14: “Conserve and sustainably use the oceans, seas and marine resources for sustainable development” [12]

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Conclusion