The hypothesis that both photosynthesis and pigment pattern are more reflective of size related strategies than taxonomic composition of the assemblages was tested under natural conditions in alpine lakes during fall 1999. The small size fraction (< 10 mm) in Lake Lucerne, Mondsee and Traunsee and in an additional incubator experiment contributed 55 -67 % to the total integral of chlorophyll-a and photosynthesis per m 2 . The photosynthetic depression induced by UV-A in Lake Lucerne, measured during the 7 th GAP-workshop, markedly increased to 65 % with PAR up to 500 mmol m -2 s -1 . At super-saturating light intensities near the surface, UV-A inhibition further increased marginally to 69 %. Effects of light acclimation and pigment adaptation on photosynthetic rates and efficiencies of phytoplankton indicated size related strategies that were more important than the taxonomic composition of the assemblage measured in Traunsee and Mondsee. Algal communities and small size fractions (< 10 mm), that had a high maximum light utilisation coefficient (a*) were adapted to low light by high ratios of chlorophyll-a to photo-protective b-carotene. Algae at high solar radiation and of large size (> 10 mm) were photosynthetically less efficient but high light adapted having low ratios of chlorophyll-a to b-carotene. In contrast to low light adapted small cells the high light adapted large-cells increased their relative proportion of photo-protective carotenoids above saturating light levels. At light limitation in deeper water layers lipophilic accessory photo-synthetic versus photo-protective pigments increased for all fractions.

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ABSTRACT1. This paper is focused on the identification of patterns of variation in phytoplankton abundance and taxonomic composition at the eco-regional scale. In addition, these patterns were evaluated with respect to climatic, hydrological and physiographic drivers.2. A hierarchical sampling design was used to integrate seasonal and spatial variations in taxonomic richness, abundance and taxonomic composition of phytoplankton guilds. Data were collected synoptically during two seasons (autumn and spring) in 12 transitional water bodies in Italy, Albania and Greece.3. The number of taxa and the cell density of phytoplankton guilds varied between seasons and across ecosystems. Overall, physiographic and hydrological components accounted for 61% of the variation in the number of taxa, but just 19% of the variation in cell density. 4. At the univariate level, cell density varied significantly with temperature, whereas the number of taxa was found to vary significantly with depth, sinuosity index and outlet structure, lagoon surface area and geographic location.5. The taxonomic composition of phytoplankton guilds varied markedly among lagoons. More than 61% of the 242 taxa identified overall were detected in only one of the lagoons.6. The average similarity of phytoplankton taxonomic structure among ecosystems was 33.15 AE 15.02. Taxonomic similarity varied substantially across both ecosystems and seasons. Overall, more than 70% of variance in taxonomic similarity was explained by physiographic and hydrological forcing factors. Specifically, differences in taxonomic structure were found in transitional ecosystems showing differences in outlet structure, depth and salinity, as well as geographic location. A recurrent pattern of decreasing taxonomic similarity with increasing Euclidean distance was observed for each factor. 7. The results obtained in this study suggest that certain large-scale driving forces can explain eco-regional scale patterns of species richness and taxonomic composition but not of cell abundance, which are probably more affected by local forcing factors.