Competition between benthic cyanobacteria and diatoms as influenced by different grain sizes and temperatures

An experimental laboratory set-up was used to study the influence of different grain size compositions and temperatures on the growth of benthic 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 tricornutum and Nitzschia sp.) were used. The organisms were cultured in LOO mi flasks filled with medium and 3 different kinds of sediment: (1) Sand (fine sand, 63 to 200 mu m), (2) Mud-I (mixed fine sand and mud <63 mu m in the ratio 80:20 wt %), (3) Mud-II (mixed fine sand and mud in the ratio 50:50 wt %). Experimental temperatures were 10, 15 and 25 degrees C. At 10 degrees C and 15 degrees C, both diatom species achieved the highest biomass on the sediments of the finest grain size (50 wt % < 63 mu m) 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 degrees C. In the competition experiments the benthic diatom species Nitzschia sp, dominated all types of sediments at 10 degrees C and 15 degrees C. The experiments at 25 degrees 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.