The invasion and spread of non-native species of many different kinds of organisms is of increasing interest to researchers. Invasions by microscopic organisms, however, are poorly understood, and their impact on the environment is probably underestimated. We collected available data on nonnative and invasive/expansive algae and cyanobacteria in the Czech Republic; we mapped their distribution and assessed their impact on local species and other real or potential risks resulting from their spread. The list of alien species in the Czech Republic contains 10 species of cyanobacteria, 9 species of Bacillariophyceae, 1 species of Dinophyta, 1 species of Ulvophyceae, 2 species of Chlorophyceae, and 1 species complex of Zygnematopyceae. The literature on the worldwide occurrence of these taxa is also reviewed.

Parallel determinations of phytoplankton biomass and chlorophyll-a concentration were carried out on five reservoim. Highly significant positive correlations between phytoplankton biomass values and chlorophyll-a values were found. The possibility of finding a conversion factor linking these two parameters is discussed. The chlorophyll content per unit phytoplankton biomass encountered during the period of study in all reservoirs varied between 0.14 and 3.41% of fresh weight. The influence of light intensity, nutrient concentration and species composition on chlorophyll content per unit phytoplankton biomass is considered.

a b s t r a c tThe results are presented of an intensive study of phytoplankton assemblage carried out in the Berounka River above its confluence with the Vltava River (Czech Republic) in the period 2002-2007. The annual and interannual changes of phytoplankton development (based on high frequency of sampling) and their relation to hydrological conditions and concentrations of main nutrients are analysed. A marked decline of nutrient concentrations was observed during the period 1996-2007. The annual mean values of total P decreased from 0.43 mg L −1 to 0.16 mg L −1 , those of N-NO 3 from 4.6 mg L −1 to 1.5 mg L −1 and N-NH 4 from 1.9 mg L −1 to 0.04 mg L −1 . Despite this, the phytoplankton biomass remained at a high level. The seasonal mean values of chlorophyll-a ranged from 51.0 g L −1 to 116.8 g L −1 in the same time period. An obviously stronger relationship was found of the phytoplankton biomass and pattern of its development to the variation of flow rates than to the existing level of nutrient concentrations. A significantly decreasing relationship (R 2 = 0.384, P < 0.001) of chlorophyll-a to flow rates and a significantly increasing relationship (R 2 = 0.359, P < 0.001) of chlorophyll-a to water temperatures were found, based on monthly mean values for the seasonal period 2002-2007. The results obtained indicate a remarkable increase of phytoplankton biomass and its prolongated occurrence in watercourses, which can be expected due to the consequences of the predicted climate change (i.e. higher occurrence of summer droughts and low precipitation amounts accompanied by a substantial drop of flow rates, increase of air and water temperatures), as described in the respective scenarios for the territory of the Czech Republic. Simulations by the regional climate models HIRHAM and RCAO and emission scenario SRES indicated the increase of air temperature by 2.5-5 • C, decrease of precipitation amount by 6-25% and decline of flows by 14-43% in the Berounka River for the scenario period 2071-2100.

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