Diatoms in monitoring of Drava River, Croatia
Plenković-Moraj, Andelka; Gligora, Marija; Kralj, Koraljka; Mustafić, Perica
During a seven year period (1999-2005) phytoplankton assemblages of reservoir systems in the Croatian part of Drava River (input/output waters and inner waters) were investigated. According to the Regulation on Water Qualification in Republic of Croatia the relevant water quality factors are abundance (cells per liter) of net-phytoplankton and Pantle-Buck saprobity index. Phytoplankton and water chemistry samples were collected seasonally four times a year at six different sites of three reservoirs. Phytoplankton samples were taken by filtrating 10 l of water through a 25 mm phytoplankton net. Index of saprobity was calculated for each sample. To explore the value of such a monitoring procedure, on phytoplankton data set several more indices were calculated: Trophic Diatom Index that measures the effect of nutrients (mostly phosphorus) on the communities, Generic Diatom Index (GDI) as a general measure of water quality taking into account more factors along with nutrients, calculation of evenness (expressed as Pielou's evenness index) and Shannon-Wiener index of diversity. All five indices and nutrients showed increase in eutrophication and pollution. During the whole investigation period the most abundant group was Bacillariophyceae. During seven years of research, the quality of Drava River changed from oligotrophic (category I) to fairly mesotrophic (category II), characterized by oligo- to 𝛽-mesosaprobic and 𝛼-mesosaprobic species, respectively. In general, during the investigation primary dominance of Asterionella formosa Hass./Fragilaria crotonensis Kitt. was gradually replaced by Melosira varians AG. The study, as database of species occurring in the Croatian part of Drava River basin, should be in the future extended to nanophytoplankton and than be used as a testing tool for developing a monitoring system according to the Water Framework Directive.