Spring-phytoplankton of the River Danube in the year 1990
Stoyneva, M. P.
The paper provides data on the composition and abundance of the spring (1990) Danube phytoplankton between Linz and Izmail (kilometres 2132-93). In total more than 332 algal taxa (except diatoms) were determined. The number of species slightly increased downstream. Number of species per site varied between 32 and 127. Most of the algae (60 %) were rarely distributed in the Danube river and had frequency quotients (FQ) between 2 and 20 %. Monoraphidium arcuatum (Kors.) Hind., M. contortum (Thur.) Kom.-Legn., M. komarkovae Nyg. and Scenedesmus communis (Turp.) Hegew. were the broadly distributed (FQ = 100 %) algae. According to the values of Sorensen's index most of the sites (47 %) show similarity between 51 and 60 %. Species of Stephanodiscus hantzschii Grun. - group and Nitzschia acicularis W. Smith + Diatoma elongatum (Lyngb.) Ag. dominated the phytoplankton community and pennate diatoms prevaled over centrics at the sites located downstream. Phytoplankton cell numbers and biomass varied from 10 73 x 105cells · 1-1 to 384.10 x 105 cells · 1-1 and from 0.18 mg · 1-1 to 4.87 mg · 1-1, respectively. A general slight decrease in algal abundance along the Danube river flow, as well as some differences between the stretches located above and below the Iron Gate reservoirs were established. Variations in the species composition and in the relative quantitative participation of different algal groups occurred from site to site. These fluctuations were due to the multiplicity of possible causative factors. In the absence of nutrient limitation the spatial changes of the community structure were connected mainly with the amplitudes of physico-chemical parameters and biotic interactions.