While the isolated responses of marine phytoplankton to climate warming and to ocean acidification have been studied intensively, studies on the combined effect of both aspects of Global Change are still scarce. Therefore, we performed a mesocosm experiment with a factorial combination of temperature (9 and 15°C) and pCO2 (means: 439 ppm and 1040 ppm) with a natural autumn plankton community from the western Baltic Sea. Temporal trajectories of total biomass and of the biomass of the most important higher taxa followed similar patterns in all treatments. When averaging over the entire time course, phytoplankton biomass decreased with warming and increased with CO2 under warm conditions. The contribution of the two dominant higher phytoplankton taxa (diatoms and cryptophytes) and of the 4 most important species (3 diatoms, 1 cryptophyte) did not respond to the experimental treatments. Taxonomic composition of phytoplankton showed only responses at the level of subdominant and rare species. Phytoplankton cell sizes increased with CO2 addition and decreased with warming. Both effects were stronger for larger species. Warming effects were stronger than CO2 effects and tended to counteract each other. Phytoplankton communities without calcifying species and exposed to short-term variation of CO2 seem to be rather resistant to ocean acidification.

Three strains of Limnothrix (Cyanobacteria) isolated from Lake Kastoria, Greece, were characterized based on their morphological features and 16S rRNA gene sequences. The Limnothrix isolates 007a, 165a, and 165c can morphologically be assigned to Limnothrix redekei (Van Goor) Meffert. The 16S rRNA gene of the Limnothrix strains showed a 99% similarity to the 16S rRNA gene of Planktothrix sp. FP1. Limnothrix redekei strains 165a, 165c, 007a and Planktothrix sp. FP1 formed a separate cluster in the cyanobacterial 16S rRNA gene tree. It was distinct from the Pseudanabaena cluster, which included the other Limnothrix strains isolated from northern temperate lakes. This is the first report on the phylogeny of L. redekei strains originating from a Mediterranean lake (southern Europe) and provides new data about the genus Limnothrix.

Global warming has revitalized interest in the relationship between body size and temperature, proposed by Bergmann's rule 150 years ago, one of the oldest manifestations of a 'biogeography of traits'. We review biogeographic evidence, results from clonal cultures and recent micro-and mesocosm experiments with naturally mixed phytoplankton communities regarding the response of phytoplankton body size to temperature, either as a single factor or in combination with other factors such as grazing, nutrient limitation, and ocean acidification. Where possible, we also focus on the comparison between intraspecific size shifts and size shifts resulting from changes in species composition. Taken together, biogeographic evidence, community-level experiments and single-species experiments indicate that phytoplankton average cell sizes tend to become smaller in warmer waters, although temperature is not necessarily the proximate environmental factor driving size shifts. Indirect effects via nutrient supply and grazing are important and often dominate. In a substantial proportion of field studies, resource availability is seen as the only factor of relevance. Interspecific size effects are greater than intraspecific effects. Direct temperature effects tend to be exacerbated by indirect ones, if warming leads to intensified nutrient limitation or copepod grazing while ocean acidification tends to counteract the temperature effect on cell size in non-calcifying phytoplankton. We discuss the implications of the temperature-related size trends in a global-warming context, based on known functional traits associated with phytoplankton size. These are a higher affinity for nutrients of smaller cells, highest maximal growth rates of moderately small phytoplankton (ca. 10 2 μm 3 ), size-related sensitivities for different types of grazers, and impacts on sinking rates. For a phytoplankton community increasingly dominated by smaller algae we predict that: (i) a higher proportion of primary production will be respired within the microbial food web; (ii) a smaller share of primary production will be channeled to the classic phytoplankton -crustacean zooplankton -fish food chain, thus leading to decreased ecological efficiency from a fish-production point of view; (iii) a smaller share of primary production will be exported through sedimentation, thus leading to decreased efficiency of the biological carbon pump.

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We studied the seasonal dynamics of phytoplankton, bacterioplankton, heterotrophic nanoflagellates, ciliates, and metazoan plankton in the highly eutrophic polymictic Lake Kastoria (Greece), which has a history of toxic cyanobacterial blooms. An acute increase in the flushing rate of the lake during spring inhibited cyanobacterial biomass accumulation. During this transient oligotrophic period, which was characterized by abundant lake snow particles, the plankton food web was an inverted biomass pyramid (low autotrophic biomass and high heterotrophic biomass). Prokaryotes played a key role in these changes (cyanobacteria during periods of autotrophy and bacteria during periods of heterotrophy). In summer and autumn, toxic cyanobacterial blooms developed, and the microbial loop was weak. The microbial loop was weak because the heterotrophic nanoflagellates and nanociliates decreased to undetectable densities during the summer, when larger bacterivores (rotifers and small cladocera) were abundant. Toxic blooms may have a dual effect on heterotrophic nanoplankton: negative during the first bloom and postbloom period and positive during a following toxic bloom. Different species (Cylindrospermopsis raciborskii, Aphanizomenon spp., and Microcystis aeruginosa) and succession phases of toxic blooms may differentially affect the microbial food web structure.In pelagic systems, phytoplankton and bacterioplankton constitute the complementary functional components that primarily produce new particulate matter by autotrophy and heterotrophy. Their carbon pool represents the base of grazing food chains and the microbial loop. Thus, the relative dominance of each functional component has significant implications for food web structure and the function and bio- AcknowledgmentsWe would like to thank the two anonymous reviewers and Associate Editor R. Bachmann for their constructive criticisms and suggestions. We are grateful to U. Sommer for his critical comments and suggestions on drafts of this manuscript and to U. Christaki for her helpful comments. We thank L. Economou for linguistic suggestions and C. M. Cook for the critical reading of sections of the manuscript concerning cyanotoxins. This work was partially funded by the Municipality of Kastoria, Research Committee, Aristotle University of Thessaloniki, project 7468. We thank all participating members of this project.