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Abstract. Investigations were carried out in a Posidonia oceanica meadow at Ischia (Gulf of Naples) along a depth gradient from 1 to 32 m in November 1979 and May 1981. In these two months, different discontinuities were found at the sampling stations (1, 5, 10, 20, 30 m) at 10 m for leaf features, between 1 and 5 m for the algal community and the vagile fauna. The leaf features (length, surface, and biomass) along the transects show a delay in leaf production toward the deeper stations, below the thermocline. The algal community shows in both seasons and in all stations a persistence of an encrusting layer, mostly represented by Corallinaceae and the brown alga Myrionema orbiculare, while at the shallowest station (1 m) the community is characterized by a more developed upright layer. The vagile fauna of the leaf stratum, mostly represented by Polychaeta, Mollusca, and Crustacea (Peracarida and Decapoda), shows a consistent zonation in both seasons along the transect. A superficial community at I m, characterized by a low number of species and individuals and specialized for high environmental energy levels, and a deeper community, persistent in time and more strictly related to the Posidonia oceanica meadows, are identified. The influence of environmental factors and the importance of meadow structure for the zonation of the algal and animal communities are discussed.

The carbon partitioning of the epipelic diatom Cylindrotheca closterium (Ehrenberg) Reiman and Lewin isolated from the Adriatic Sea was studied in the laboratory under varying scenarios of nutrient limitation. Total number of cells, photosynthesis measured at 695 μmol photons·m−2·s−1 irradiance (P695‐μmol), chlorophyll (a+c) content, respiration, extracellular polymeric substances (EPS), total particulate carbohydrate (TPC), and dissolved carbohydrate were evaluated under nitrogen and phosphorus deficiencies in culture. The highest total number of cells was found in the control, whereas the nitrogen‐limited treatment showed the lowest value. During the transition phase of growth, photosynthesis in the nitrogen‐limited treatment was 3‐fold lower than in the phosphorus‐limited treatment and 4‐fold lower than in the control. Differences in respiration rates and chlorophyll (a+c) content were even more marked. Dissolved carbohydrate remained the same in all the treatments, whereas during the transition and stationary phase, EPS presented the highest values under phosphorus limitation and the lowest in the control treatment. The production of EPS was closely linked to the periods of carbon assimilation (transition phase) in the nutrient depleted treatments, especially in the phosphorus‐limited treatment. These results point out the relevance of the nutrient imbalance (nitrogen or phosphorus) in the production of EPS by the benthic or resuspended diatoms and suggest that these diatoms play an important role in nutrient‐unbalanced systems like sediments or marine snow.

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Microsatellite markers were utilized in a study on population genetic diversity of the endemic Mediterranean seagrass Posidonia oceanica (L.) Delile. Five nuclear and one chloroplastic microsatellite markers detected low levels of polymorphism in 6 populations sampled along the coasts of Italy and Corsica (Western Mediterranean). The number of alleles per locus ranged from 1.5 to 2.0 and homozygosity was high within populations (f = 0.314). In the 120 individuals analyzed, only 32.5 % were distinct genotypes. Although gene flow seems to exist between geographically disjunct meadows (Nm -1.55), private alleles were found in some localities. In one population in particular (Lacco Ameno, Gulf of Naples), sampled at 2 different depths, a private allele was present only in the individuals of the shallow stand. Distance analysis identified genetic disjunction between the northern and the centralsouthern populations. This study indicates that (1) clonal growth is important in the maintenance of P. oceanica populations, (2) limited inbreeding occurs in P oceanica populations, which can be composed of clonal patches of different size, (3) gene flow exists, but genetic disjunction between populations can be influenced by local forces, and (4) microsatellites are powerful markers in detecting genetic variability in clonally reproducing species.