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

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 25C. At 10C 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 10C and 15C. 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.