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 differ ent grain size compositions and temperatures on the growth of benthic cyano bacteria and diatoms, and on the competition between these 2 groups. Monosp ecific cultures of 3 species of cyanobacteria (Merismopedia punctata, Micro coleus chthonoplastes, Oscillatoria limosa), and of 2 species of benthic di atoms (Phaeodactylum tricornutum and Nitzschia sp.) were used. The organism s were cultured in LOO mi flasks filled with medium and 3 different kinds o f sediment: (1) Sand (fine sand, 63 to 200 mu m), (2) Mud-I (mixed fine san d and mud <63 mu m in the ratio 80:20 wt %), (3) Mud-II (mixed fine sand an d mud in the ratio 50:50 wt %). Experimental temperatures were 10, 15 and 2 5 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 % < 6 3 mu m) while cyanobacteria achieved low biomass levels. Coarsening of sedi ments at the same temperature levels revealed a gradually lower biomass of the diatoms. Particularly on sand, the diatoms never reached the same conce ntrations 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 experi ments the benthic diatom species Nitzschia sp, dominated all types of sedim ents at 10 degrees C and 15 degrees C. The experiments at 25 degrees C were dominated by the filamentous cyanobacterium M. chthonoplastes. This indica tes the importance of abiotic conditions for the distribution and abundance of benthic phototrophic micro-organisms.