Effect of floods on phytoplankton communities in aspect of river monitoring: a case of the Middle Daugava River (South-east Latvia)
The role of the floods in variation of major characteristics of phytoplankton communities of the Middle Daugava at Daugavpils (South-east Latvia) was investigated from March 2005 till October 2006. Two different reaches of the river (the upland and the lowland one) were selected for the sampling and field measurements. Samples of water and phytoplankton were collected 1-4 times per month, along with the measurements of water level height, temperature, turbidity and other parameters. The four distinct flood pulses caused by different factors were observed during this study. More than 100 taxa at each sampling site were found, mostly of Bacillariophyceae and Chlorophyta. All taxa were grouped in relation to their unit volume. 17 variables of phytoplankton communities were analysed by the Spearman's rank correlation method and Principal Component Analysis in order to determine the significance of the floods in their seasonal variation. Only 4 parameters at the upland reach and the one in the lowland reach indicated statistically significant impact of the water level fluctuation. In both sites, the relative proportion of the Group III (unit volume >10,000 𝛍m3 in total biomass was negatively related to the height of the water level or the rate of its change. Some other factors, which partly explain total variance of selected biotic parameters, included the underwater light intensity, water temperature and trophic interactions between the phyto- and zooplankton communities.