| Phylum Myxomycota – Slime Molds | |
| Lycogala epidendrum | Wolf’s Milk Slime |
| Phylum Phaeophyta – Brown Algae | |
| Family Ectocarpaceae | |
| Botrytella micromora Bory | uncommon summer annual; not reported since 1900 |
| Ectocarpus fasciculatus Harv. | aseasonal annual commonest in winter and spring |
| Ectocarpus siliculosus (Dillwyn) Lyngb. | annual in quiet waters |
| Hincksia granulose (J. E. Smith) P. C. Silva | year-round |
| Hincksia ovata (Kjellm) P. C. Silva | shallow sublittoral; not reported since 1900 |
| Hincksia sandriana (Zanardini) P. C. Silva | season in fall and winter; unreported since 1900 |
| Hincksia secunda (Kütz.) Batters | annual in shallow sublittoral |
| Pilayella littoralis (L.) Kjellm. | common in spring and early summer |
| Spongonema tormentosum (Huds.) Kütz. | epiphyte on other species |
| Family Lithodermataceae | |
| Petroderma maculiforme (Wollny) Kuck. | common crustose species |
| Pseudolithoderma paradoxum Sears and R. T. Wilce | perennial crustose species |
| Pseudolithoderma subextensum (Waern) S. Lund | perennial crustose species in sublittoral |
| Sorapion kjellmani (Wille) Rosenv. | perennial crustose species |
| Sorapion simulans Kuck. | perennial crustose species in sublittoral |
| Symphyocarpus strangulans Rosenv. | perennial crustose species |
| Family Myrionemataceae | |
| Myrionema balticum (Reinke) Foslie | epiphyte; first record south of Cape Cod (Sears 1971) |
| Myrionema strangulans Grev. | uncommon |
| Family Ralfsiaceae | |
| Ralfsia fungiformis (Gunnerus) Setch. & N. L. Gardner | perennial in tide pools |
| Family Acrothricaceae | |
| Acrothrix gracilis Kylin | summer annual in quiet bays |
| Family Chordariaceae | |
| Chordaria flagelliformis (O. F. Müll.) C. Agardh | year-round in lower intertidal zone |
| Eudesme virescens (Carmich. ex Berk.) J. Agardh | annual in spring and summer |
| Sphaerotrichia divaricata (C. Agardh) Kylin | summer annual |
| Family Corynophlaeaceae | |
| Leathessia difformis (L.) Aresch. | common epiphyte |
| Family Elachistaceae | |
| Elachista fucicola (Velley) Aresch. | common year-round on open coasts |
| Leptonematella fasciculata (Reinke) P. C. Silva | common epiphyte |
| Family Spermatochnaceae | |
| Stilophora tenella (Esper) P. C. Silva | common in quiet bays |
| Family Tilopteridaceae | |
| Haplospora globosa Kjellm. | common winter-spring annual |
| Tilopteris mertensii (Turner in Sm.) Kütz. | winter-spring annual in semiprotected bays |
| Family Cladostephaceae | |
| Cladostephus spongiosus (Huds.) C. Agardh | perennial in sandy areas |
| Family Sphacelariaceae | |
| Sphacelaria cirrosa (Roth) C. Agardh | common perennial on rocks and an epiphyte |
| Sphacelaria fusca (Huds.) Gray | perennial, on open coasts |
| Sphacelaria plumigera Holmes ex Hauck | perennial |
| Sphacelaria racemosa Grev. | perennial in tide pools |
| Sphacelaria radicans (Dillwyn) Harv. | perennial in tide pools |
| Family Dictyosiphonaceae | |
| Dictyosiphon foeniculaceus (Huds.) Grev. | summer annual in sublittoral pools |
| Family Giraudiaceae | |
| Giraudia sphacelaroides Derbés & Solier in Castagne | rare; last recorded in 1892; needs confirmation |
| Family Myriotrichiaceae | |
| Myriotrichia clavaeformis Harv. | uncommon epiphyte |
| Family Pogotrichaceae | |
| Pogotrichum filiforme Reinke | rare annual; last reported in 1895 |
| Family Punctariaceae | |
| Asperococcus fistulosus (Huds.) Hook | late winter-summer annual |
| Punctaria latifolia Grev. | seasonal annual in shallow sublittoral |
| Punctaria plantaginea (Roth) Grev. | common spring-early summer annual |
| Punctaria tenuissima (C. Agardh) Grev. | seasonal annual in spring and summer |
| Rhadinocladia farlowii Schuh | uncommon epiphyte |
| Family Striariaceae | |
| Stictyosiphon tortilis (Rupr.) Reinke | uncommon annual |
| Striaria attenuata (Grev.) Grev. | uncommon annual in spring |
| Family Scytosiphonaceae | |
| Petalonia fascia (O. F. Müll.) Kuntze | winter-spring annual |
| Petalonia zosterifolia (Reinke) Kuntze | uncommon epiphyte from spring to early winter |
| Scytosiphon dotyi M. J. Wynne | aseasonal annual in tide pools |
| Scytosiphon lomentaria (Lyngb.) Link | winter-spring annual |
| Family Arthrocladiaceae | |
| Arthrocladia villosa (Huds.) Duby | summer annual |
| Family Desmarestiaceae | |
| Desmarestia aculenta (L.) J. V. Lamour. | perennial |
| Desmarestia viridis (O. F. Müll.) J. V. Lamour. | winter-spring annual |
| Family Halosyphonaceae | |
| Halosiphon tormentosus (Lyngb.) Jaasund | common especially spring to fall |
| Family Chordaceae | |
| Chorda filum (L.) Stackh. | common summer-fall annual |
| Family Laminariaceae | |
| Laminaria digitata (Huds.) J. V. Lamour. | common |
| Laminaria saccharina (L.) J. V. Lamour. | common perennial in deep tide pools |
| Family Fucaceae | |
| Ascophyllum nodosum (L.) Le Jol. | common perennial |
| Fucus evanescens C. Agardh | year-round in sheltered areas |
| Fucus spiralis L. | moderately common in upper intertidal zone |
| Fucus vesiculosus L. | common perennial |
| Family Sargassaceae | |
| Sargassum filipendula C. Agardh | common perennial in shallow warm areas |
| Sargassum fluitans (Børgesen) Børgesen | occasionally brought by the Gulf Stream |
| Phylum Rhodophyta – Red Algae | |
| Family Porphyridiaceae | |
| Stylonema alsidii (Zanardini) K. M. Drew | uncommon on stones or as an epiphyte |
| Family Erythrotrichiaceae | |
| Erythrotrichia carnea (Dillwyn) J. Agardh | aseasonal annual in quiet waters |
| Porphyropsis coccinea (J. Agardh ex Aresch.) Rosenv. | spring-summer annual |
| Family Bangiaceae | |
| Porphyra leucosticta Thur. in Le Jol. | annual winter-spring epiphyte |
| Porphyra umbilicalis (L.) Kütz. | variably abundant annual |
| Family Acrochaetiaceae | |
| Audouinella dasyae (Collins) Woelk. | uncommon annual in shallow sublittoral |
| Audouinella efflorescens (J. Agardh) Papenf. | uncommon fall-early winter annual |
| Audouinella endozoica (Darbishire) P. S. Dixon | uncommon in sublittoral |
| Family Palmariaceae | |
| Palmaria palnmata (L.) Kuntze | perennial |
| Family Rhodophysemataceae | |
| Rhodophysema elegans (P. Crouan & H. Crouan ex J. Agardh) P. S. Dixon | perennial crust |
| Rhodophysema georgii Batters | annual crust on eelgrass |
| Family Liagoraceae | |
| Nemalion helminthoides (Velley in With.) Batters | sporadic annual |
| Family Galaxauraceae | |
| Scinaia furcellata (Turner) J. Agardh | uncommon summer annual |
| Family Bonnemaisoniaceae | |
| Bonnemaisonia hamifera (Hariot) Okamura | common summer annual; includes Trailliella intricata (J. Agardh) Batters |
| Family Gelidiaceae | |
| Gelidium pusillum (Stackh.) Le Jol. | intertidal zone perennial |
| Family Ahfeltiaceae | |
| Ahnfeltia plicata (Huds.) Fr. | moderately common perennial |
| Family Gracilariaceae | |
| Gracilaria tikvahiae McLachlan | sheltered water perennial; includes G. foliifera (Forsskål) Børgesen and G. verrucosa (Huds.) Papenfuss. |
| Family Hildenbrandiaceae | |
| Hildenbrandia rubra (Sommerf.) Menegh. | perennial crust; includes H. prototypes Nardo |
| Family Coralinaceae | |
| Corallina officinalis L. | perennial on open coasts |
| Melobesia membranacea (Esper.) J. V. Lamour. | annual on open coasts |
| Phymatolithon laevigatum (Foslie) Foslie | perennial coralline crust |
| Family Cystocloniaceae | |
| Cystoclonium purpureum (Huds.) Batters | perennial in quiet waters |
| Family Dumontiaceae | |
| Dumontia contorta (S. G. Gmel.) Rupr. | late winter to spring annual |
| Family Gigartinaceae | |
| Chordrus crispus Stackh. | abundant perennial |
| Family Gloiosiphoniaceae | |
| Gloiosiphonia capillaris (Huds.) Berk. | sporadically present in tide pools |
| Plagiospora gracilis Kuck. | possibly perennial crustose species |
| Family Haemeschariaceae | |
| Haemescharia hennedyi R. T.Wilce & Maggs | invader from Europe |
| Family Hypneaceae | |
| Hypnea musciformis (Wulfen in Jacq.) J. V. Lamour. | epiphyte on coarse algae |
| Family Kallymeniaceae | |
| Callophyllis cristata (C. Agardh) Kütz. | sublittoral perennial |
| Family Nemastomataceae | |
| Tsengia bairdii (Farl.) K. C. Fan & Y. P. Fan | rare; known from Gay Head 5 times from about 1880 to 1961 |
| Family Peyssonneliaceae | |
| Peysonnelia dubyi P. Crouan & H. Crouan | perennial crust |
| Family Phyllophoraceae | |
| Coccotylus truncatus (Pallas) M. J. Wynne & J. M. Heine | perennial |
| Phyllophora pseudoceranoides (S. G. Gmel.) Newroth & A. R. A. Taylor | common perennial in shallow sublittoral; includes P. membranifolia |
| Family Polyidaceae | |
| Polyides rotundus (Huds.) Greville | common perennial |
| Family Solieraceae | |
| Agardhiellia subulata (C. Agardh) Kraft & M. J. Wynne | common perennial in quiet water |
| Family Champiaceae | |
| Champia parvula (C. Agardh) Harv. | abundant annual in summer |
| Family Lomentariaceae | |
| Lomentaria baileyana (Harv.) Farl. | abundant summer annual |
| Lomentaria clavellosa (Turner) Gaillon | invader from Europe |
| Lomentaria orcadensis (Harv.) Collins ex W. R. Taylor | abundant perennial |
| Family Ceramiaceae | |
| Aglaothamnion roseum (Roth) Maggs & L’Hardy-Halos | summer annual epiphyte |
| Anotrichium tenue (C. Agardh) Nägeli | summer annual in shallow sublittoral |
| Antithamnion cruciatum (C. Agardh) Nägeli | pseudo-perennial epiphyte |
| Callithamnion corymbosum (Sm.) Lyngb. | common summer annual |
| Callithamnion tetragonum (With.) S. F. Gray | summer annual |
| Ceramium cimbricum H. E. Petersen in Rosenv. | usually an epiphyte on eelgrass, other algae |
| Ceramium deslongchampii Chauv. ex Duby | late spring-fall annual |
| Ceramium diaphanum (Lightf.) Roth | late spring-fall annual |
| Ceramium strictum Harv. | perennial common in summer |
| Ceramium virgatum Roth | late spring-fall annual |
| Griffithsia globulifera Harv. ex Kütz. | common in summer |
| Pleonosporium borreri (Sm.) Någeli ex Hauck | summer annual |
| Plumeria plumosa (Huds.) Kuntze | perennial in warmer waters |
| Pterothamnion plumula (J. Ellis) Någeli | perennial in warmer bays |
| Ptilota serrata Kütz. | uncommon perennial |
| Scagelia plylaiaei (Mont.) M. J. Wynne | perennial epiphyte |
| Seirospora interrupta (Sm.) F. Schmitz | perennial in sublittoral |
| Spermothamnion repens (Dillwyn) Rosenv. | abundant summer annual |
| Spyridia filamentosa (Wulfen) Harv. in Hook | spring-summer annual |
| Family Dasyaceae | |
| Dasya baillouviana (S. G. Gmel.) Mont. | late summer annual in quiet waters |
| Family Delesseriaceae | |
| Grinnellia americana (C. Agardh) Harv. | late summer in warm water |
| Phycodrys rubens Batters | common perennial in deep cool water |
| Family Rhodomelaceae | |
| Chondria baileyana (Mont.) Harv. | common summer annual |
| Chondria dasyphylla (Woodw.) C. Agardh | summer annual |
| Polysiphonia denudata (Dillwyn) Grev. ex Harv. in Hook | common perennial in littoral and sublittoral |
| Polysiphonia elongata (Huds.) Spreng. | common in littoral and sublittoral |
| Polysiphonia fibrillosa (Dillwyn) Spreng. | summer annual |
| Polysiphonia flexicaulis Harv. Collins | summer annual |
| Polysiphonia fucoides (Huds.) Grev. | common pseudo-perennial in sublittoral |
| Polysiphonia harveyi Bailey | summer annual |
| Polysiphonia nigra (Huds.) Batters | summer annual |
| Polysiphonia stricta (Dillwyn) Grev. | winter annual |
| Polysiphonia subtilissima Mont. | annual is estuaries and salt marsh |
| Rhodomela confervoides (Huds.) P. C. Silva | common perennial in tide pools; includes R. virgata Kjellm |
| Rhodomela lycopodioides (L.) C. Agardh | uncommon perennial in sublittoral |
| Phylum Chlorophyta – Green Algae | |
| Family Prasiolaceae | |
| Prasiola stipulata Suhr ex Jess. | local very near its southern limit |
| Family Monostromataceae | |
| Blidingia minima (Nägeli ex Kütz.) Kylin | common year-round |
| Family Ulvaceae | |
| Enteromorpha clathrata (Roth) Grev. | rare; one 1880 specimen |
| Enteromorpha intestinalis (L.) Nees | common |
| Enteromorpha linza (L.) J. Agardh | common year-round |
| Ulva lactuca L. | common in warms bays |
| Family Ulvellaceae | |
| Bolbocoleon piliferum Pringsh. | annual |
| Family Codiolaceae | |
| Urospora wormskjoldii (Mert. in Hornem.) Rosenv. | uncommon winter-spring annual |
| Family Chaetosiphonaceae | |
| Blastophysa rhizopus Reinke | perennial |
| Family Cladophoraceae | |
| Chaetomorpha aerea (Dillwyn) Kütz. | year-round on open coasts |
| Chaetomorpha linum (O. F. Müll.) Kütz. | aseasonal annual in shallow water |
| Chaetomorpha melagonium (F. Weber & D. Mohr) Kütz. | littoral tide pools |
| Cladophora albida (Nees) Kütz. | year-round in estuaries and bayas |
| Cladophora flexuosa (O. F. Müll.) Kütz. | uncommon; specimen from 1946 |
| Cladophora rupestris (L.) Kütz. | uncommon |
| Cladophora sericea (Huds.) Kütz. | annual in late fall and winter |
| Family Bryopsidaceae | |
| Bryopsis plumosa (Huds.) C. Agardh | year-round, upper sublittoral |
| Derbesia marina (Lyngb.) Solier | year-round |
| Family Codiaceae | |
| Codium fragile (Suringar) Harv. subsp. tormentosoides (Goor) P. C. Silva | common |