Estructura i dinàmica del poblament algal de les fulles de "Posidonia oceanica" (L.) Delile als herbeis de Tossa de Mar (Girona)

Research on leaf-epiphyte community structure and dynamics was carried out in Posidonia oceanica beds from Tessa de Mar (Western Mediterranean). Three sampling stations
where chosen at 2, 9 and 23 m depth. Seasonal community structure was studied along the year 1982 in the 9 m station. Species composition, species covering, species
richness, specific distribution, homogeneity (Kulczynski's similarity index), species diversity,
pattern diversity and the qualitative and quantitative minimal areas were obtained from each sample. These parameters were estimated from the species/number of shoots curves, the diversity (Shannon index) /number of shoots curves and the similarity/number
of shoots curves as indicated by Ballesteros (1986). Structural changes in the epiphyte community have been observed; there is an increase in species richness and a decrease in the specific distribution and pattern-diversity from autumn to summer. Two different
phases can be distinguished: a settling phase, where different shoots have a rather different species composition, that begins in October, just after the fall of senescent Posidonia leaves; and a canalization phase, where there is a progressive concurrence in the
qualitative and quantitative composition of leaf-epiphytes between shoots. Comparison with other Mediterranean algal communities shows the miniaturization, the great homogeneity and the scanty structure of the Posidonia leaves epiphytic community.

Dynamics were studied in the three sampling stations. Species composition of Posidonia leaves is well characterized by the abundance of Fosliella species and the encrusting phaeophycean Myrionema magnusii. Other species such as Giraudia sphacelarioides, Castagnea
irregularis, C. cylindrica, Giffordia mitchelliae, Myriactula gracilis, Ectocarpus siliculosus v. confervoides and Feldmannia globifera have a spring maxima in the shallow stations. Zooepiphytes are more abundant, in relative terms, in the deepest station (basically the hydrozoan Sertularia perpusilla and bryozoans Electra posidoniae and Fenestrulina joannae).

Mean phytoepiphyte biomass varies from 12 to 495 mg dw/shoot (11-470 g dw/m2) in the shallow stations and 4 to 335 mg dw/shoot (1-82 g dw/m2) in the deepest station. Zooepiphytes biomass varies from 5 to 188 mg dw/shoot (2-180 g dw/m2). A time delay between the shallowest and the deepest stations has been found for all leaf features and phytoepiphyte composition and biomass, but not for zooepiphyte biomass. A time delay in terms of production and biomass maxima between shallow and deep phytobenthos communities is something general in the Mediterranean (Ballesteros, 1984). This appears to be an adaptation
of different species and communities in order to couple their life-cycle to the seasonal pattern of the limiting factor (or factors), usually light (in deep communities) and nutrient availability (in shallow ones); this could be also the case of Posidonia and its epiphytes.
Production estimates of leaf-epiphytes vary from 70 g C/m2 year in shallow stations to 12 g C/m2 year in the deepest one, that is, between the 10 and the 20 % of the whole
Posidonia meadow production.