The biogeography of giant kelp submarine forests at the sub-Antarctic latitudes of South America

This doctoral thesis presents the first work on the extent, spatial dynamics, and long-term trends of the giant kelp (Macrocystis pyrifera) ecosystem in southern South America and sub-Antarctic latitudes. This work was based on an interdisciplinary approach of remote sensing using satellite images, unmanned aerial vehicles (UAV), geographical information systems, underwater exploration, and a collection of historical sources and secondary information from international collaborations. The first research paper of this thesis (Chapter 3) presents a mapping methodology of giant kelp derived from filtering values from the satellite Sentinel-2. This method was validated with secondary sources (previous kelp surveys) and primary sources of data, such as detailed orthomosaics made with UAV. Because of the high overall accuracy of this method, it was possible to map and measure the surface of the complete distribution of the giant kelp ecosystem by ecoregion and province. The second paper (Chapter 4) is based on the mapping methodology from Chapter 3, developing a spatial analysis of the sub-Antarctic kelp from South America, Falkland Islands (Malvinas) and South Georgia to understand the abiotic patterns that could predict the typical size of a giant kelp forest. Additionally, these patterns were compared in a temporal analysis stretching as far back as the 19th century to assess the persistence of this ecosystem. The results of this paper indicate that sub-Antarctic giant kelp is one of the least modified and most persistent marine ecosystems on Earth. Chapter 5 analyses the spatial distribution of marine sea surface temperature (SST) heatwaves and marine cold spells in the Channels and Fjords ecoregion in past decades, with a focus on understanding the climatological trends as they can affect giant kelp ecosystems. This paper established that, in contrast to the main global trend pointing towards SST warming, the frequency of marine cold spells in the nearshore coastlines of southernmost South America has in fact increased in the decade of 2010. We identified two main mechanisms as possible causes for this trend: increased frequency of storms driven by the Antarctic Circumpolar Current and glacier melting in the Patagonian fjords. Cold waters favour the persistence of giant kelp and associated biodiversity, hence the lack of SST warming highlights the region as a potential refugium for the ecosystem. However, more studies are needed to understand these new dynamics, and we conclude the paper with a summary of potential stressors to giant kelp ecosystems in the study area. Finally, Chapter 6 provides an overview of the main findings of this thesis and recommendations for future research, highlighting the need for further interdisciplinary studies on the Magellanic marine province as a climatic refugium for the giant kelp ecosystem.