For unicellular organisms, a lack of effects of local species richness on ecosystem function has been proposed due to their locally high species richness and their ubiquitous distribution. High dispersal ability and high individual numbers may enable unicellular taxa to occur everywhere. Using our own and published data sets on uni- and multicellular organisms, we conducted thorough statistical analyses to test whether (1) unicellular taxa show higher relative local species richness compared to multicellular taxa, (2) unicellular taxa show lower slopes of the species:area relationships and species:individuals relationships, and (3) the species composition of unicellular taxa is less influenced by geographic distance compared to multicellular taxa. We found higher local species richness compared to the global species pool for unicellular organisms than for metazoan taxa. The difference was significant if global species richness was conservatively estimated but not if extrapolated, and therefore higher richness estimates were used. Both microalgae and protozoans showed lower slopes between species richness and sample size (area or individuals) compared to macrozoobenthos, also indicating higher local species richness for unicellular taxa. The similarity of species composition of both benthic diatoms and ciliates decreased with increasing geographic distance. This indicated restricted dispersal ability of protists and the absence of ubiquity. However, a steeper slope between similarity and distance was found for polychaetes and corals, suggesting a stronger effect of distance on the dispersal of metazoans compared to unicellular taxa. In conclusion, we found partly different species richness patterns among uni- and multicellular eukaryotes, but no strict ubiquity of unicellular taxa. Therefore, the effect of local unicellular species richness on ecosystem function has to be reanalyzed. Macroecological patterns suggested for multicellular organisms may differ in unicellular communities.

Competition between benthic cyanobacteria and diatoms as influenced by different grain sizes and temperaturesFrank W a t e r m a n n l . ABSTRACT An experimental laboratory set-up was used to study the influence of different grain size compositions and temperatures on the growth of benth~c cyanobacteria and diatoms, and on the competition between these 2 groups. Monospecific cultures of 3 species of cyanobacteria (Merismopedia punctata, Microcoleus chthonoplastes, Oscillatoria limosa), and of 2 species of benthic diatoms (Phaeodactylum Oicornutum and Nitzschia sp.) were used. The organisms were cultured in 100 m1 flasks filled with medium and 3 different kinds of sediment.(1) Sand (fine sand, 63 to 200 pm), (2) Mud-I (mixed fine sand and mud <63 pm in the ratio 80:20 wt %), ( 3 ) mud-I1 (mixed fine sand and mud in the ratio 5050 wt %). Experimental temperatures were 10, 15 and 25°C. At 10°C and 15"C, both diatom species achieved the highest biomass on the sediments of the finest grain size (50 wt % < 63 pm) while cyanobacteria achieved low biomass levels. Coarsening of sediments at the same temperature levels revealed a gradually lower biomass of the diatoms. Particularly on sand, the diatoms never reached the same concentrations of chlorophyll a as on mud. The cyanobacteria, on the other hand, had the highest biomass on sand at 15'C. In the competition experiments the benthic diatom species Njtzschla sp. dominated all types of sediments at 10°C and 15°C. The experiments at 25'C were dominated by the filamentous cyanobacterium M. chthonoplastes. This indicates the importance of abiotic conditions for the distribution and abundance of benthic phototrophic micro-organisms.

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