Boletín de la RSEHN. Sección Biológica

Se aporta el inventario de los moluscos y anélidos poliquetos encontrados en los fondos colonizados por Zostera marina L. y Zostera noltei Hornemann de la Ensenada de O Grove (Galicia, NW España). Los muestreos incluyeron un estudio espacial, en el que se muestrearon 10 estaciones con una draga Van-Veen (noviembre 1996), y un estudio temporal, en el que se realizaron inmersiones bimensuales con escafandra autónoma (mayo 1998-marzo 1999) en una estación. Durante el estudio espacial, se contabilizaron 158 taxones, siendo dominante el grupo de los poliquetos (91 taxones) frente al de los moluscos (67 taxones). Con el estudio temporal (136 taxones) se incrementó el número de taxones presentes en la ensenada, ascendiendo el total a 188 taxones. Los resultados reflejan la importancia de las praderas de fanerógamas para la preservación de la biodiversidad y refuerzan el valor ecológico de una ensenada ya protegida por la legislación vigente y donde hay una gran actividad socio-económica (cultivo de mejillón, marisqueo y pesca). Este estudio representa el primer inventario en el que se caracterizan cualitativa y detalladamente los moluscos y poliquetos asociados a las praderas de fanerógamas de la Ensenada de O Grove.

Grove, located in the inner part of Ria de Arousa (Galicia, NW Spain) (Fig. 1), is presented in this study. This work is part of a bigger cartography of the benthic fauna associated to the soft bottoms of the inlet. The studied bottoms were mostly muddy and colonized predominantly by Zostera marina L., Zostera noltei Hornemann or by mixed meadows Z. marina and N. noltei. Molluscs and annelid polychaetes of these seagrass meadows were studied both at spatial and temporal scales. The spatial study was carried out in November 1996 and samples were collected in 10 intertidal and subtidal sites with a Van-Veen grab in order to compare our results with the remaining samples of the Ensenada de O Grove. Five replicated samples were collected from each site accounting for a total area of 0.28 m2. For the temporal study, samples were collected bimonthly between May 1998 and March 1999 in a muddy sand bottom colonized exclusively by Z. marina. This site (Site 27, Fig.1) was selected because of its subtidal position, the high diversity and the good state of the seagrass meadow. Five replicate samples were collected by scuba diving with a quadrat of 0.0625 m2 (Total area: 0.31 m2). Blades, shoots and rhizomes and sediment were kept separately in plastic bags. In both, spatial and temporal study, samples were sieved through a 0.5 mm mesh and retained material was fixed in 10% buffered formalin for later sorting and identification of the fauna. Samples of surface and bottom water were taken at each site for salinity (psu) and temperature (ºC) measurements. An additional sample of sediment was collected from each site in order to analyze particle-size composition and carbonate (%) and organic matter contents (TOM; %).

The studied seagrass meadows were muddy bottoms characterized by moderate to high content of silt/clay (6- 62%) (Table I). Sand content was generally greater in subtidal sites and the sedimentary types ranged from muddy sand to mud. Carbonate content ranged from 5 to 10 % and content in organic matter ranged from low values in subtidal sites (1.3%) to higher ones (10.7-15.5%) in intertidal inner areas. Water salinity was low (<33), particularly in St. 37 (S: 21-20), site subjected to strong changes of salinity due to the influence of freshwater inputs from a river. Sediment characteristics of the muddy sand bottom of site 27 during the temporal study were summarized in Table II for each sampling time.

As result of spatial study, a total of 158 taxa were collected. Polychaetes were a diverse and abundant group with 91 taxa mainly belonging to Syllidae, Capitellidae, Spionidae, Sabellidae and Maldanidae families. Among molluscs, 67 taxa were collected. Bivalves and gastropods had the greater number of species (38 and 26 respectively). Veneridae (7 spp.) and Lucinidae (3 spp.) families within bivalves and Trochidae, Nassaridae (5 spp.), Rissoidae and Littorinidae (3 spp., respectively) families within gastropods contained most of species. Polyplacophorans were represented by 3 species.

During the dynamic study, a total of 136 taxa were collected. Again, annelid polychaetes were the most representative group (75 taxa and 23 families) in front of molluscs (40 taxa and 25 families). It is remarkable, with the temporal study, the increment of polychaetes and molluscs in 14 and 16 species, respectively, accounting for a total number of 189 taxa (105 polychaetes and 84 molluscs).

Similar diversity of polychaetes has been recorded in other studies focussed on benthic fauna associated to seagrass meadows (Somaschini et al., 1994; Gambi et al., 1998; Hutchings, 1998; Brito et al., 2005; Box et al., 2010). Similarly, some authors studying Zostera meadows recorded similar numbers of mollusc species to those inhabiting the seagrasses of the Ensenada de O Grove (Jacobs, 1980; Jacobs & Huisman, 1982; Curr ás et al., 1993; Junoy, 1996; Çinar et al., 1998; Glemar ec & Gra ll, 2000; Arr oyo et al., 2006; Cacabelos et al., 2008).

Seagrass meadows influence certain processes such as predation, through the protection of some preys from their predators (Heck & Thoman, 1981; Orth et al., 1984; Hovel et al., 2002; Heck et al., 2003; Orth et al., 2006), accumulation and retention of sediment (Terra dos & Duar te, 2000; Hovel et al., 2002) and food availability for many organisms (Kitting et al., 1984; Bostrom & Matilla, 1999; Hovel et al., 2002; Hily et al., 2004; Fredriksen et al., 2005).

Due to the great socio-economic activity (mussel culture on rafts, bivalve collection, fishing, etc.) and its high ecological value, the inlet had been protected by current legislation from different perspectives. This study represents the first complete and detailed check-list as qualitative characterization of molluscan and annelid polychaetes colonizing the seagrass beds in the Ensenada de O Grove. The presence of these benthic invertebrates in the Z. marina and Z. noltei meadows remarks their importance for the preservation of its biodiversity, and contributes to reinforce the ecological importance of the inlet.