In total, 63 epiphytic macroalgae were found on leaves of Zostera marina in the northwestern Iberian Peninsula (Table 1). Diversity was higher than the recorded in other areas of the Iberian Atlantic where eelgrass meadows hosted up to 38 species (Cullinane & al. 1985; Novaczek 1987; Johnson & al. 2005). By contrast, it was higher in Gran Canaria, where up to 79 species were recorded in seagrasses (González 1976).

Red algae—40 species (figs. 3–4)—were much more abundant than brown algae—16 species (fig. 5)—and green algae—7 species (fig. 6)—. The distribution of red, brown, and green algae is similar to the observed in eelgrass meadows from other Iberian regions, and it is proportional to the number of recorded species for these three groups in the northwestern Iberian Peninsula (Bárbara & al. 2005). However, the diversity of epiphytes was lower than the observed in other benthic habitats of the study region, such as maërl beds (Peña Freire 2010) or canopies of species of the genus Cystoseira L. (García-Fernández & Bárbara 2016). This is probably related to the short life span of the leaves of Zostera marina—88 days (Hemminga & Duarte 2000)—, the physical and chemical changes that occur in the leaves during their growth or the stressful environmental conditions of eelgrass meadows—v. gr., influence of sediments or emersion time—. Interestingly, the diversity of epiphytes varies among seagrass species and the studied regions. Posidonia oceanica (L.) Delile has a longer life span than Zostera marina—170 days (Hemminga & Duarte 2000)—but the diversity recorded on its leaves in the Mediterranean is even smaller—51 spp. (Nesti & al. 2009)—. By contrast, in the Canary Islands Cymodocea nodosa (Ucria) Asch. has a similar diversity of epiphytes—53 spp.—than Posidonia oceanica, despite the life span of the former species is shorter—68 vs. 170 days (Reyes & al. 1995; Reyes & Sansón 1996).

Two functional forms, filamentous—23 spp.—and filiform—23 spp.—, accounted most of the functional diversity of the epiphytic flora of Zostera marina. Foliose species, both thin and intermediate, were also common—seven and six species, respectively—, while the corticated filiform and crustose species were rare—three and one species, respectively—. Unicellular, corticated foliose and articulated calcareous functional groups included in García-Fernández & Bárbara (2016) were never observed among epiphytes. This pattern is the expected considering the short life span that the leaves of Zostera marina provide for epiphytes, as mentioned above. Consequently, species with high growth rates thrive better in this particular habitat, while perennial species cannot persist. Filiform, filamentous, and foliose are the functional forms characterized by having the highest growth rates (Littler & Littler 1984). Pneophyllum fragile Kütz. is the only crustose species that occur on the leaves. It is a thin calcareous coralline algae with only a few cell layers (Irvine & Chamberlain 1994), so it can grow quickly and is adapted to eelgrass habitat. It is also a common epiphyte on the seagrass Cymodocea nodosa, in which Pneophyllum fragile is one of the first epiphytic species colonizing the young leaves (Reyes & Sansón 1996). Probably, the pioneer character of this species also applies in Zostera marina. Mean covering of epiphytic macroalgae on the leaves of Zostera marina is low. Table 1 shows that the category 2—0.007–0.033%—is the most common, while category 5—10–73% cover—is rare. Some species were occasionally observed covering a high percentage of leaves, such as Pneophyllum fragile—up to 73%—, Colaconema daviesii (Dillwyn) Stegenga—up to 16.23%—, Ectocarpus siliculosus (Dillwyn) Lyngb.—up to 29.60%—or Rhodophysema georgei—up to 5.23%.

Ten species were more abundant and common, as were found in practically all the sampling dates and sites. Among them, Colaconema daviesii, Porphyrostromium ciliare (Carmich.) M.J.Wynne, Erythrotrichia bertholdii Batters, and Erythrotrichia carnea (Dillwyn) J.Agardh are small—< 5 mm—red algae with filamentous or foliose morphologies. Ceramium secundatum Lyngb., Gayliella flaccida (Harv. ex Kütz.) T.O.Cho & L.J.McIvor, and Polysiphonia fibrillosa (Dillwyn) Spreng. are filiform red algae whose thallus can be up to 1 cm in length. The brown algae Ectocarpus fasciculatus Kütz. and Ectocarpus siliculosus are filamentous species that can be up to 2 cm in length.

The diversity of epiphytes greatly varied among sampling sites (Table 1). Ferrol—FER—had the highest diversity and Sálvora—SAL—the lowest, 36 and five species, respectively. Interestingly, the low diversity found in Sálvora contrasts with the highest covering observed in this study—73%, Pneophyllum fragile—. Some of the species here recorded were only found in one of the areas (Table 1), and, for example, Litosiphon laminariae (Lyngb.) Harv., Apoglossum ruscifolium (Turner) J.Agardh, Champia parvula (C.Agardh) Harv., and Cladophora albida (Nees) Kütz. were only observed in Ferrol. Thus, it is important to study several meadows in order to achieve a comprehensive view of the epiphytic flora at a regional scale.

The present work, focused on the epiphytic macroalgae, was developed in the framework of a broader study on the flora associated with Zostera marina. This allows us to establish comparisons among the flora associated to the different habitats within the meadows. Nine species have been found in Zostera marina exclusively as epiphytes on leaves: Cladosiphon zosterae (J.Agardh) Kylin, Ectocarpus fasciculatus, Ectocarpus siliculosus, Feldmannia globifera (Kütz.) Hamel, Litosiphon laminariae, Myriotrichia clavaeformis Harv., Pneophyllum fragile, Polysiphonia fibrillosa, and Rhodophysema georgei. However, other epiphytic species can be found also growing on the adjacent sedimentary substrate of meadows—V. García-Redondo, pers. comm.—, such as Cutleria multifida (Turner) Grev., Hincksia spp., Sargassum muticum (Yendo) Fensholt, Aglaothamnion spp., Ceramium spp., Dasya spp. or Ulva spp. Most of the species here recorded were only found as juvenile stages—43%—. For example, specimens of the brown algae—Cutleria multifida, Desmarestia ligulata (Stackh.) J.V.Lamour., Dictyota dichotoma (Huds.) J.V.Lamour., Taonia atomaria (Woodw.) J.Agardh, Sargassum muticum—as well as red algae—Apoglossum ruscifolium, Callithamnion tetragonum (With.) Gray, Compsothamnion thuyoides (Sm.) Nägeli, Dasya sessilis Yamada, Dasysiphonia japonica (Yendo) Hy.S.Kim, Hypoglossum hypoglossoides (Stackh.) Collins & Hervey—and green algae—Cladophora hutchinsiae (Dillwyn) Kütz. and Ulva australis Aresch.—were found on leaves of Zostera marina, but they were less than 1.5 cm in length and lack reproductive structures.

Regarding non-native seaweeds, nine species have been recorded: Sargassum muticum, Anotrichium furcellatum (J.Agardh) Baldock, Antithamnionella ternifolia (Hook.f. & Harv.) Lyle, Dasya sessilis, Dasysiphonia japonica, Lomentaria hakodatensis Yendo, Melanothamnus harveyi (J.W.Bailey) Díaz-Tapia & Maggs, Pyropia leucosticta (Thur.) Neefus & J.Brodie, and Ulva australis. The high diversity of introduced species might be facilitated by the placement of most of the eelgrass meadows in sheltered areas, which are subjected to a high incidence of the most relevant vectors for introduction and spread of non-native seaweeds, i.e., harbors or aquaculture facilities (Williams & Smith 2007).

Some of the species here recorded are unusual or scarcely known in the northwestern Iberian Peninsula. Rhodophysema georgei (fig. 7) had been only recorded before by Miranda (1934) and remained unnoticed up to now. The lack of information on this species is probably explained because it is an obligate epiphyte on Zostera marina and its flora has been scarcely studied in the Iberian Peninsula. Morphological characters of Rhodophysema georgei from the northwestern Iberian Peninsula agree with the descriptions for other regions (Saunders & Bird 1989; Saunders & McLachlan 1989). Other scarcely known species are Asperococcus scaber Kuck., which is here recorded for the second time in the Iberian Atlantic—first report in Bárbara & al. (2015)—and for the first time in the province of Pontevedra—Ría de Vigo—; and Gayliella mazoyerae T.O.Cho, Fredericq & Hommers.that is recorded for the first and second time in Pontevedra—A Toxa—and Galicia, respectively—first report in Bárbara & al. (2016).

In sum, our study shows that the epiphytic flora on the leaves of Zostera marina is a diverse assemblage, considering the continuous environmental variations associated with this habitat. The composition of the flora was highly variable in frequency and abundance among the studied regions, as it depends on the interaction of several factors and processes that operate at different spatial and temporal scales (Borowitzka & al. 2006). The supply of propagules is a key process that influences the epiphytic assemblage (Borowitzka & al. 2006). They can come from the meadows or from adjacent communities, which explain the high number of species that were observed only as juvenile stages because leaves are an unsuitable substrate for mature stages.

Key to the NW Iberian Peninsula epiphytic macroalgae on Zostera marina

Identification of epiphytes is sometime challenging because most part of specimens were found as juvenile and immature stages. Thus, some of the key vegetative and reproductive characters needed for morphological identifications were not properly developed. In order to facilitate the identification of epiphytic macroalgae of the leaves of Zostera marina, we provide the following identification key that can be successfully used independently of the maturity of specimens. Pictures showing the main characters of these species are provided in figs. 3–7.