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A comprehensive checklist of the marine flora from Faial Island, mid-north Atlantic, is still unavailable, despite the numerous works that gathered data around its coasts for over 150 years. This study brings together and harmonises all macroalgae collections and taxonomical records from multiple projects and teams over the last thirty years. The result is a list of taxonomic records and georeferenced species occurrence data that substantially increases our knowledge of the coastal Azorean marine biodiversity while providing key information for future ecological studies and to support local, marine management and conservation. A total of 2283 specimens belonging to 118 taxa of macroalgae are registered here, comprising 61 Rhodophyta, 26 Chlorophyta, 30 Ochrophyta (Phaeophyceae) and one Cyanophyta (Cyanobacteria). From these, 26 taxa are identified only at the genus level and 92 to species level (47 Rhodophyta, 21 Chlorophyta and 24 Ochrophyta). The list comprises 75 native species, 12 records of uncertain origin and five introductions ( Caulerpa prolifera (Forsskål) J.V.Lamouroux, Caulerpa webbiana Montagne, Hydroclathrus tilesii (Endlicher) Santiañez & M.J.Wynne, Asparagopsis armata Harvey and Symphyocladia marchantioides (Harvey) Falkenberg). Codium elisabethiae O.C.Schmidt represents the only Macaronesian endemism known on the Island. Twenty-three species are newly recorded to the algal flora of the Island.

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. Initial studies were mainly taxonomic, resulting in the publication of a checklist of the Azorean benthic marine algae. Later, the establishment of the University of the Azores on the Island permitted 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. Prior to the present paper, the known macroalgal flora of São Miguel Island comprised 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. 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), one an Azorean endemic (Predaea feldmannii subsp. azorica Gabriel), five are Macaronesian endemisms (the red algae 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 green alga Codium elisabethiae O.C.Schmidt), 19 are introduced species (15 Rhodophyta, two Chlorophyta and two Ochrophyta) and 32 are of uncertain status (21 Rhodophyta, five Chlorophyta and six Ochrophyta).

The algal flora of the western group of the Azores archipelago (Islands of Flores and Corvo) has attracted the interest of many researchers on numerous past occasions (such as Drouet 1866, Trelease 1897, Gain 1914, Schmidt 1929, Schmidt 1931, Azevedo et al. 1990, Fralick and Hehre 1990, Neto and Azevedo 1990, Neto and Baldwin 1990, Neto 1996, Neto 1997, Neto 1999, Tittley and Neto 1996, Tittley and Neto 2000, Tittley and Neto 2005, Tittley and Neto 2006, Azevedo 1998, Azevedo 1999, Tittley et al. 1998, Dionísio et al. 2008, Neto et al. 2008). Despite this interest, the macroalgal flora of the Islands cannot be described as well-known with the published information reflecting limited collections preformed in short-term visits by scientists. To overcome this, a thorough investigation, encompassing collections and presence data recording, has been undertaken for both the littoral and sublittoral regions, down to a depth of approximately 40 m, covering a relatively large area on both Islands (approximately 143 km 2 for Flores and 17 km 2 for Corvo). This paper lists the resultant taxonomic records and provides information on species ecology and occurrence around both these Islands, thereby improving the knowledge of the Azorean macroalgal flora at both local and regional scales. For the Island of Flores, a total of 1687 specimens (including some taxa identified only to genus level) belonging to 196 taxa of macroalgae are registered, comprising 120 Rhodophyta, 35 Chlorophyta and 41 Ochrophyta (Phaeophyceae). Of these taxa, 128 were identified to species level (80 Rhodophyta, 22 Chlorophyta and 26 Ochrophyta), encompassing 37 new records for the Island (20 Rhodophyta, 6 Chlorophyta and 11 Ochrophyta); two Macaronesian endemics ( Laurencia viridis Gil-Rodríguez & Haroun and Millerella tinerfensis (Seoane-Camba) S.M.Boo & J.M.Rico); six introduced (the Rhodophyta Asparagopsis armata Harvey, Neoizziella divaricata (C.K.Tseng) S.-M.Lin, S.-Y.Yang & Huisman and Symphyocladia marchantioides (Harvey) Falkenberg; the Chlorophyta Codium fragile subsp. fragile (Suringar) Hariot; and the Ochrophyta Hydroclathrus tilesii (Endlicher) Santiañez & M.J.Wynne and Papenfussiella kuromo (Yendo) Inagaki); and 14 species of uncertain status (10 Rhodophyta, two Chlorophyta and two Ochrophyta). For the Island of Corvo, a total of 390 specimens distributed in 56 taxa of macroalgae are registered, comprising 30 Rhodophyta, nine Chlorophyta and 17 Ochrophyta (Phaeophyceae). Whilst a number of taxa were identified only to the genus level, 43 were identified to species level (22 Rhodophyta, eight Chlorophyta and 13 Ochrophyta), comprising 22 new records for the Island (nine Rhodophyta, four Chlorophyta and nine Ochrophyta), two introduced species (the Rhodophyta Asparagopsis armata and the Chlorophyta Codium fragile subsp. fragile and seven species of uncertain status (five Rhodophyta and two Ochrophyta).

The IUCN Red List of Threatened Species is an authoritative tool in biodiversity conservation. Whilst IUCN criteria have been applied successfully to groups such as birds and mammals, a Red List assessment of British seaweeds in 2021 revealed that the categories to which seaweed species were assigned were dependent on how the criteria were applied. Here, this seaweed assessment is used as a case study with which to evaluate the IUCN methodology for use with ‘non-standard’ groups of organisms. A data-driven assessment of red (Rhodophyta), green (Chlorophyta) and brown (Phaeophyceae) seaweeds, which applied three (A, B and D) of the five IUCN criteria (A–E), categorized 13% of 617 British species as threatened. Following peer review, only 7% of species were categorized as threatened (1% Critically Endangered—CR, 3% Endangered—EN, 3% Vulnerable—VU), and 55% as Data Deficient. This reduction in species categorized as threatened suggests that strict application of the IUCN criteria may, at least for the seaweeds, over-estimate threat. As a result of this assessment, recommendations include the need for a more unified monitoring system and a review of the suitability for/application of the IUCN assessment criteria to some types of organisms. For example, in clonal populations, it is not possible to count individuals, and complex life histories cause additional complications. IUCN criteria must be applicable to a wide range of organisms, including seaweeds.

We isolated 17 strains of Chrysotila stipitata and 10 strains of Chrysotila lamellosa from samples collected at exposed chalk surfaces, Westgate, Kent, England, which is the type locality for C. stipitata (generitype) and C. lamellosa. The nuclear-encoded small subunit and large subunit rRNA were used in a molecular phylogenetic analysis. We also examined the cells using light microscopy. Chrysotila stipitata had two chloroplasts per cell, as originally described, and it was positioned within the Coccolithales (Prymnesiophyceae) in a RAxML tree. Chrysotila stipitata gene sequences formed a clade with Pleurochrysis, and because Chrysotila has priority, Pleurochrysis was placed in synonymy. New nomenclatural combinations were also made. Chrysotila lamellosa was located within the order Isochrysidales, and like other members of the order, C. lamellosa had one chloroplast per cell. We reinstated the generic name Ruttnera to accommodate this species (Ruttnera lamellosa comb. nov.). We also showed that several other taxa described from the chalk cliff type locality were alternate life stages for C. stipitata, and we assigned these as synonyms of C. stipitata.

The misspelling of Chrysotila carterae is corrected. Two new combinations (Chrysotila dentata comb. nov., Chrysotila roscoffensis comb. nov.) and a new replacement name (Chrysocapsa wetherbeei sp. nov.) are proposed.

Understanding long-term persistence and variability in species populations can help to predict future survival, growth and distribution; however, sustained observations are exceedingly rare. We examine and interpret a remarkable record of the calcareous brown alga Padina pavonica (Phaeophyceae) at its northern limit on the south coast of England (50°N, 1–3°W) from 1680 to 2014, which is probably the longest compilation and review of any marine algal species. Over this period, which extends from the middle of the Little Ice Age to the present, there has been considerable variability in temperature and storminess. We identified a significant number of site extinctions in the second half of the nineteenth century, which coincided with cooler conditions and stormier weather. To interpret these changes, we measured recruitment, growth and production of tetraspores at sheltered and exposed sites in 2012–2014, years which had low and high spring temperatures. Potential spore production was greater at the sheltered site due to a longer growing period and survival of larger fronds. Delayed growth in the cooler spring resulted in smaller fronds and lower potential production of tetraspores by early summer. Yet in the warmer year, rapid initial growth caused higher sensitivity to damage and dislodgement by summer storms, which also limited potential spore production. Antagonistic responses to multiple stressors and disturbances make future predictions of survival and distribution difficult. Fronds of Padina pavonica are sensitive to both temperature and physical disturbances, yet vegetative perennation appears to have enabled population persistence and explained the longevity of remaining populations.

Nineteen of the 57 littoral rocky shore biotopes and 4 of the 26 variants as well as 1 sublittoral fringe biotope classified for Britain and Ireland have been recorded on Helgoland in this baseline study. Most of the wave-cut platform is at sublittoral and lower littoral levels and most high eulittoral biotopes are confined to narrow zones on seawalls. Large areas of gently sloping lower eulittoral and also boulder areas adjacent to seawalls are characterised by a mixture of two or more biotopes. Only 4 of the 23 littoral biotopes and variants are characterised by faunal species. Comparison with past descriptions of intertidal communities suggests continuing presence of most of the autochthonous biotopes over the past 80 years but also change due to the invasion of the macroalgae Mastocarpus stellatus and Sargassum muticum over the past 20 years. One previously recorded cave biotope and a sublittoral seagrass site have become extinct due to habitat loss while other biotopes probably have extended their range due to habitat increase. The presence of 4 intertidal biotopes (20%) considered rare or scarce in Britain supports the recognition of Helgoland as a site of special conservation in a regional and international context.

We isolated 17 strains of Chrysotila stipitata and 10 strains of Chrysotila lamellosa from samples collected at exposed chalk surfaces, Westgate, Kent, England, which is the type locality for C. stipitata (generitype) and C. lamellosa. The nuclear-encoded small subunit and large subunit rRNA were used in a molecular phylogenetic analysis. We also examined the cells using light microscopy. Chrysotila stipitata had two chloroplasts per cell, as originally described, and it was positioned within the Coccolithales (Prymnesiophyceae) in a RAxML tree. Chrysotila stipitata gene sequences formed a clade with Pleurochrysis, and because Chrysotila has priority, Pleurochrysis was placed in synonymy. New nomenclatural combinations were also made. Chrysotila lamellosa was located within the order Isochrysidales, and like other members of the order, C. lamellosa had one chloroplast per cell. We reinstated the generic name Ruttnera to accommodate this species (Ruttnera lamellosa comb. nov.). We also showed that several other taxa described from the chalk cliff type locality were alternate life stages for C. stipitata, and we assigned these as synonyms of C. stipitata.