Structure and dynamics of the community of Rissoella verruculosa (Bertoloni) J. Agardh (Gigartinales, Rhodophyceae) in the northwestern Mediterranean
Research on community structure and dynamics has been carried out in the mediolittoral Rissoella verruculosa assemblages from Tossa de Mar (Northwestern Mediterranean). Rissoella verruculosa is the dominant species in the community. Other red algae such as Porphyra species, Nemalion helminthoides and Polysiphonia sertularioides seasonally develop between the leafy thallus of Rissoella. Encrusting algae are mainly represented by the brown crusts of Ralfsia verrucosa and some colonial cyanophytes (Brachytrichia quojii and Rivularia atra being the most conspicuous). Epiphytic coverage of Rissoella is scarce and it is mainly constituted by the green alga Blidingia chadefaudii. Algal biomass ranges from 27 to 748 g dw m-2 and percentage coverage ranges between 16 and 863 % . Species richness and species diversity are usually higher than other upper mediolittoral Mediterranean communities but lower than lower mediolittoral communities. The relatively high number of species developing in the community may be related to the growth of small infralittoral algae during winter and spring protected from desiccation by the well developed thallus of Rissoella. Two stages can be distinguished in the annual cycle of the community. A developed stage occurs in spring while a diversified stage occurs in autumn. Both are related to the growth and decline of the Rissoella verruculosa populations. Growth of Rissoella is maximal in early spring but sharply declines at the end of May, coinciding with depletion of nitrates in surface waters and with the existence of low sea level periods coupled with high irradiance. The annual production of Rissoella verruculosa attains 376 g C m-2 with a maximal daily production near 3 g C m-2 d-l (April-May) and undetectable levels between June and September. These production rates are really high if we take into account the low nutrient concentrations found in Mediterranean waters, suggesting a high degree of habitat adaptation of Rissoella verruculosa to the unpredictable Mediterranean mediolittoral zone.