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Background and Purpose: The primary objective of this investigation was to examine the effects of nutrient enrichment, by using miniaturized algal bioassay, on the growth of the unicellular green alga C. kessleri in order to make a comparative evaluation of the trophic level and productivity of water ecosystems of the Danube (1426-1388 r.km) and its wetland areas. Comparative algal growth potential (AGP) of original water samples was done in inoculum concentrations of 400.000 and 800.000 cells/ml of C. kessleri. Materials and Methods: Water samples were collected on September 12th and November 14th 2002, and March 27th 2003 at 13 localities in the Danubian region. The miniaturized bioassay method was used to establish the algal growth potential of C. kessleri. Results: The results of AGP in nutrient enrichment bioassay indicate that algal growth was generally limited by nitrogen. Based on the results of chemical analysis the highest nitrate concentrations were measured at locations of the Danube main-arm and the smallest nitrate concentrations were measured at locations of the flood protected area of Kopački Rit Nature Park and the Stara Drava dead arm. Ratio of total nitrogen to total phosphorus (N/P) varied in September from 3.945 to 18.268, in November from 0.747 to 8.97 and in March from 0.670 to 14.176. According to AGP results (46.566 - 100.288 mg/l DW), the waters in the wetland area of the Danube region were oligomesotrophic to mesotrophic and highly productive. Conclusions The bioassay method is more suitable for the trophic evaluation with inoculum concentration of 800.000 cell/ml of Chlorella kessleri than with inoculum of 400.000 cell/ml. Nitrogen was the limiting nutrient for growth potential of C. kessleri in 79% of samples and phosphorus in only 2% of the samples, while in 19% of samples there was no significant influence of added nutrients on the growth of C. kessleri. A greater trophic potential of the investigated localities was established on the basis of the N/P ratio than on the basis of the AGP of C.kessleri.

The algal growth potential (AGP) of C. kessleri in water samples from the Danube River (1388 rkm) and the Hulovo Channel, as well as the influence on the AGP due to adding nitrates and phosphates was investigated by miniaturized bioassay method. According to the AGP results, P was a growth-limiting factor in the water of the Danube River and in the Hulovo Channel. The adding of nitrates to the water samples had a stimulating effect on the growth of the algal biomass. According to the classification for waters of Middle Europe, the conditions for the growth of C. kessleri in the investigated waters were mostly oligomesotrophic. According to the US classification, these waters belonged to the category of highly productive waters.

The unicellular green alga Chlorella kessleri FOTT et NOV. is suitable for the evaluation of the AGP by the miniaturized bioassay method in order to evaluate the trophic level and the productivity and toxicity of water ecosystems. The aim of this paper is to compare in situ phytoplankton growth with the influence of the addition of nitrates and phosphates into the original waters on the growth potential of C. kessleri in Lake Sakadaš. Samples were collected once a month from March to December 1999, from a site in the central part of Lake Sakadaš. The "tubular-chamber" method was used for the quantitative analysis of phytoplankton. Fresh weight phytoplankton biomass was calculated from the volume of phytoplankton units. The miniaturized bioassay method was used to establish the algal growth potential of C. kessleri. The total microphytoplankton biomass in situ varied 6.82 - 115.62 mg/l. The following sequences in the phytoplankton successions were evident: Bacillariophyceae community (March) ; Dinobryon community (April) ; depression of phytoplankton (May) ; algal bloom of blue greens (July) ; Synura bloom (September) ; Euglenophyceae community (October-December). Results of the AGP of C. kessleri showed that the addition of nitrates has caused a statistically significant increase of AGP. The addition of phosphates did not have an influence on the AGP only in June, September and October. The in situ microphytoplankton biomass dynamics as well as the qualitative composition and seasonal succession of phytoplankton indicate eutrophic conditions of Lake Sakadaš. According to the results of the AGP of C. kessleri, the water of Lake Sakadaš can be classified as highly productive and the algal growth was N-limited.

The influence on the algal growth potential (AGP) due to the addition of nutrients (nitrates and phosphates) to the original water samples was investigated by the method of bioassay. The results of the AGP were compared with in situ limiting nutrients. The case study was done in two different ecosystems - Stara Drava (part of the hydromelioration system) and Lake Sakadaš (part of the wetland ecosystem of Kopački rit Nature Park). The obtained results showed that in cases when nitrates were the limiting nutrient in in situ conditions (at site I of the Stara Drava), the adding of higher concentrations of nitrates (N2 and N3) in experimental conditions stimulated the growth of the AGP of Chlorella kessleri. In cases when phosphates were the limiting nutrient in in situ conditions (Lake Sakadaš), the adding of all three concentrations of nitrates did not influence on the growth of the dry weight (DW), but the adding of phosphates stimulated the AGP.

Comparative evaluation of the trophic state of Lake Sakadaš, the deepest lake in the wetland ecosystem of the Nature Park Kopački Rit, was done using the miniaturized bioassay method, as well as according to physical and chemical water parameters and seasonal succesions of phytoplankton. Sampling was done at monthly intervals from March till Dec. of 1999. The algal growth potential (AGP) of culture Chlorella kessleri FOTT et NOV. was investigated. AGP in original water samples from Lake Sakadaš and the influence on the AGP due to adding nitrates (final conc. of 0.180 ; 0.360 ; 0.723 g l-1) and phosphates (final conc.of 0.028 ; 0.056 ; 0.169 g l-1) were determined. Water dynamics is the main ecological factor in the Lake Sakadaš. According to dynamics of flooding, during 1999 two periods can be distinguished: inflow of water to the wetland from the Danube with a high water level (March-July) and outflow of water from the lake to the Danube with decreasing of water level (August- December). High Secchi depth (1.1-3.6 m), low amount of phytoplankton (2.43-45.02 &#956 ; ; g l-1 chl-a) with predominance of Dinobryon and diatoms (Asterionella, Melosira) was the characteristic of the period of low water level and indicated oligo-mezotrophic and low eutrophic conditions. Results of AGP based on the maximal biomass of C. kessleri in original water samples (32-63 mgl-1DW) proved the oligotrophic till mesoeutrophic conditions. Results of AGP in conditions of increasing phosphates (all three conc. stimulated the growth of C. kessleri) proved the low level of phosphates in Lake Sakadaš. The algal blooms of Peridinium and Euglenophyceae (69.88 - 86.85 &#956 ; ; g l-1 chl-a) and low Secchi depth (0.59-1.03 m) was the main characteristic of the eutrophic conditions in the period of low water level. Results of AGP based on the maximal biomass of C. kessleri (till 92 mgl-1 DW) in original water samples proved the mezo-eutrophic conditions while AGP was unchanged in conditions with increasing of phosphates. According to statistical analysis it can be concluded that phosphates were not a limiting factor of C. kessleri growth. Based on the AGP results in conditions of increasing nitrates, there is an evident increase in the C. kessleri growth. It can be concluded that nitrates controlled the eutrophication of Lake Sakadaš during the whole investigation period. Regarding the given results, the method of bioassay will be introduced and tested in the long-term period for the monitoring of eutrophication of Lake Sakadaš.