Net-phytoplankton in the monitoring of Drava River system

During seven year period (1999-2005) phytoplankton assemblages of reservoir systems in Croatian part of Drava River (input/output waters and inner waters) was investigated. According to Regulation on water qualification in Republic of Croatia the one of the relevant water quality factors are abundance (cells per liter) of net-phytoplankton and Pantle-Buck saprobity index. Samples were collected seasonally four times a year at eight different sites of three reservoirs. Phytoplankton samples were taken by filtrating 10L of water through 25 μ m phytoplankton net. Index of saprobity was calculated for each sample. To explore the value of such monitoring procedure, on phytoplankton data set several more indexes were calculated: Trophic Diatom Index, Generic Diatom Index, Shannon-Wienner index and Pielou's index. All five indexes showed increase in eutrophication and pollution. During whole investigation period the most abundant group was Bacillariophyceae. Changes in species dominance, same as accompanying species in assemblages, were compared to physical, chemical and biological characteristics of the River. Species with high abundance and low frequency were also discussed. Differences in species dominance between input and output river waters were observed. During seven years of research, quality of Drava River changed from oligotrophic (I category) to fairly eutrophic (II-III category), characterized by oligo to  (beta)- mesosapobic and α (alpha)-mesosapobic species, respectively. In general, during investigation primary dominance of Astrionella formosa Hass./Fragilaria crotonensis Kitt. later was replaced by Melosira varians Ag. The study can be base of species occurring in Croatian part of Drava River basin and in future should be extended to nanophytoplankton and used as a testing tool for developing monitoring system according to Water Framework Directive.