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Background The macroalgal flora of the Island of São Miguel (eastern group of the Azores archipelago) has attracted the interest of many researchers in the past, the first publications going back to the nineteenth century (see summary in Neto et al. 2014). Initial studies were mainly taxonomic, resulting in the publication of species lists, which were compiled by Neto (1994) in the first checklist of the Azorean benthic marine algae. Later, the establishment of the University of the Azores on the Island permited the logistic conditions to develop both temporal studies and long-term research, and this resulted in a significant increase on research directed at the benthic marine algae and littoral communities of the Island and consequent publications (see revision in Neto et al. 2014 and Haroun et al. 2019). Prior to the present paper, the known macroalgal flora of São Miguel Islandcomprised around 260 species. Despite this richness, a significant amount of the research was never made public, notably Masters and PhD theses encompassing information regarding presence data recorded at littoral and sublittoral levels down to a depth of approximately 40 m around the Island, and the many collections made, which resulted in vouchers deposited in the AZB Herbarium Ruy Telles Palhinha and the LSM- Molecular Systematics Laboratory at the Faculty of Sciences and Technology of the University of the Azores. The present publication lists the macroalgal taxonomic records together with information on their ecology and occurrence around São Miguel Island, improving the knowledge of the Azorean macroalgal flora at local and regional scales. New information A total of 12,781 specimens (including some identified only to genus) belonging to 431 taxa of macroalgae are registered, comprising 284 Rhodophyta, 59 Chlorophyta and 88 Ochrophyta (Phaeophyceae). Of these, 323 were identified to species level (212 Rhodophyta, 48 Chlorophyta and 63 Ochrophyta), of which 61 are new records for the Island (42 Rhodophyta, 9 Chlorophyta and 10 Ochrophyta), 1 an Azorean endemic (Predaea feldmannii subsp. azorica Gabriel), 5 are Macaronesian endemisms (the Rhodophyta Botryocladia macaronesica Afonso-Carrillo, Sobrino, Tittley & Neto, Laurencia viridis Gil-Rodríguez & Haroun, Millerella tinerfensis (Seoane-Camba) S.M.Boo & J.M.Rico, Phyllophora gelidioides P.Crouan & H.Crouan ex Karsakoff and the Chlorophyta Codium elisabethiae O.C.Schmidt), 19 are introduced species (15 Rhodophyta, 2 Chlorophyta and 2 Ochrophyta), and 32 are of uncertain status (21 Rhodophyta, 5 Chlorophyta and 6 Ochrophyta). Introduction Research on the marine algae from the Azores started in the mid nineteenth century (1838) when Guthnick and the two Hochstetters, father and son, visited the archipelago (Neto 1994). Since then, many other researchers and naturalists have visited the archipelago, resulting in several publications on the marine algal flora of this region (see summary in Neto 1994; 1997). Most initial studies were taxonomic focusing on the production of species lists. Almost a century later, the German botanist Otto Christian Schmidt visited several islands, including São Miguel, and initiated a more comprehensive ecological approach describing species associations and their spatial organization (Schmidt, 1931). Ever since the first half of last century, several studies have focused more widely on intertidal and shallow subtidal communities providing information on the vertical distribution of macroalgae and invertebrates and their trophic relations (see Neto 1992, 2000, 2001 for a review on this subject). Taxonomic investigations have continued and the first checklist of the Azorean benthic marine algae published by Neto (1994) brought together the existing published information, provided distributional records within the archipelago and reported 307 species, indicating a moderately rich flora given its isolated mid-Atlantic position. A revision of this first checklist was made by Parente (2010), increasing the number of algae species to 327, but without providing their distributional information on the archipelago. Later, Rosas-Alquicira et al. (2011) published a catalogue of non-fossil geniculate coralline red algae (Corallinales, Rhodophyta) of the Macaronesia in which they made both a critical review of species and infraspecific taxa as ell as an assessment of species diversity in the region. Research by local teams was also dedicated to the Azorean littoral communities and biota conservation (see, for example, Abecasis et al. 2015, Amorim et al. 2015, Chainho et al. 2015). Taxonomic, ecological and biotechnological investigations have continued generating knowledge on the Azorean macroalgae flora, its biotechnological potential and also on the structure and functioning of littoral communities (see revisions on Neto et al. 2014 and Haroun et al. 2019). Recently, several additional studies have been published with important information on the Azorean algae biodiversity, biogeography, conservation, ecology, and taxonomy (see, for example, Bruno de Sousa et al. 2019, Cacabelos et al. 2019, 2020, Freitas et al. 2019, Kellaris et al. 2019, Martins et al. 2019, Parente et al. 2019, 2020, Patarra et al. 2017, 2019, 2020, Sousa et al. 2019, Faria et al. 2020a-b, Vieira et al. 2020). The paper by Freitas et al. (2019) increased the number of macroalgae species occurring in the Azores to 405 and reported that amongst the mid-Atlantic archipelagos, the Azores is second in species richness after the Canary Islands, with 689 species, and followed by Madeira (396), Cabo Verde (333) and Selvagens (295 species). For some species the Azores archipelago forms a boundary in their distribution. Codium effusum (Rafinesque) Delle Chiaje, for example, is as its western distribution limit in the archipelago (León-Cisneros et al. 2012), whereas for Dudresnaya crassa M.Howe, a western Atlantic warm-water species, the Azores extends its known distributional range to the east. Some northern species such as the red alga Schizymenia dubyi (Chauvin ex Duby) J.Agardh and Lomentaria orcadensis (Harvey) Collins come close to their southern limit of distribution in the Azores while some southern warm-water species such as green alga Anadyomene stellata (Wulfen) C.Agardh and the red alga Sebdenia rodrigueziana (Feldmann) Codomier ex Athanasiadis reach their Atlantic northern limit of distribution on the islands (Neto et al. 2005, León-Cisneros et al. 2012). Some species, relatively common in the region a few years ago, have become uncommon or even very rare, e.g. Scytosiphon lomentaria (Lyngbye) Link, Schimmelmannia schousboei (J.Agardh) J.Agardh. In contrast, there has been an increase of unexpected macroalgae in the Azores, with the arrival and establishment of several non-native species (see Cardigos et al. 2006, Micael et al 2014, Vaz-Pinto et al. 2014, Parente et al. 2019, Cacabelos et al. 2019, 2020, Martins et al. 2019). Within the spread of the archipelago there are no marked differences between floras of individual Islands or Island groups, and biogeographically the Azores algal flora reveals to have a mixed nature, with species shared with Macaronesia, North Africa, the Mediterranean Sea, Atlantic Europe and America (Tittley 2003, Tittley & Neto 1995, 2005, 2006, Wallenstein et al. 2009b). This nature of the Azorean marine algal flora was reinforced by the work of Freitas et al. (2019), who using an extensive analysis encompassing data on coastal fishes, brachyurans, polychaetes, gastropods echinoderms and macroalgae, suggested that the Azores should be a biogeographical entity on its own and proposed a redefinition of the Lusitanian biogeographical province, in which they included four ecoregions: the South European Atlantic Shelf, the Saharan Upwelling area, the Azores ecoregion, and a new ecoregion they named Webbnesia, which comprises the archipelagos of Madeira, Selvagens and the Canary Islands. Not all the Azorean Islands have received the same attention regarding the studies on macroalgae. Furthermore, many species may have been overlooked due to their small size, opportunisctic nature or ephemeral life span. To overcome this and gain a better and up to date knowledge of the archipelago’s macroalgae flora, an effort was made by resident teams to undertake a considerable amount of research over the past three decades on several Islands. The present paper is the last one of a series and presents physical, occurrence data, and information gathered from macroalgal surveys undertaken on São Miguel Island between 1989 and 2019 mainly by the Island Aquatic Research Group of the Azorean Biodiversity Centre of the University of the Azores (Link: https://ce3c.ciencias.ulisboa.pt/sub-team/island-aquatic-ecology), the BIOISLE, Biodiversity and Islands Research Group of CIBIO-Açores at the University of the Azores (Link: https://cibio.up.pt/research-groups-1/details/bioisle), and the OKEANOS Centre of the University of the Azores (Link: http://www.okeanos.uac.pt). In these surveys particular attention was given to the small filamentous and thin sheet-like forms that are often short-lived and fast-growing, and usually very difficult to identify in the field, without the aid of a microscope and specialised literature in the laboratory. This paper aims to provide a valuable marine biological tool to aid research on the systematics, diversity and conservation, biological monitoring, climate change, ecology and more applied studies, such as biotechnological applications, which will be of assistance to a wide range of focal groups including academics, students, governments, private organizations and the general public. Purpose: This paper presents taxonomic records of macroalgae for São Miguel Island and provides general information on their occurrence and distribution. By doing this, it will contribute to address several biodiversity shortfalls (see Cardoso et al. 2011, Hortal et al. 2015), namely the need to catalogue the Azorean macroalgae (Linnean shortfall) to improve current information on their local and regional geographic distribution (Wallacean shortfall), as well as to provide a better understanding of species abundance and dynamics in space (Prestonian shortfall).