Biological importance of an Atlantic European maerl bed off Benencia Island (northwest Iberian Peninsula)

We studied the conservation status of a maerl bed off Benencia Island ( NW Spain) over a 2-year period using SCUBA and dredging. The maerl bed, which includes a maerl beach, extended from the intertidal to subtidal zones ( 18 m), and occupied an area of 215 ha. It was composed of a pure unattached coralline algal deposit occasionally mixed with broken shells. The area estimated for the highest maerl cover (76-100%) was greater than reported in previous research. The living maerl layer reached 15 cm in depth and the living/dead maerl ratio was high (80-100% living maerl cover). The dominant maerl-forming species was Phymatolithon calcareum, although unattached plants of Mesophyllum sp. reaching 10 cm in diameter were found in two sites. Both maerl species are fruticose. The proportion of discoidal specimens of P. calcareum was related to depth. The associated flora comprised 137 species whose seasonal variations were very marked, with high floristic richness in summer ( 71 species) and low in winter ( 35 species). Crustose and turf-like species were the dominant floral components throughout the year, whereas the presence of other species was restricted to specific periods. We propose that Benencia Island be included in future Atlantic Iberian maerl bed conservation schemes.