Bioassay Method in Evaluation of Trophic Conditions and Nutrient Limitation in the Danube Wetland Waters (1388–1426 r. km)

Abstract

The algal growth potential (AGP) in water samples of the Danube wetland waters (1388–1426 r. km) as well as the effect of nitrogen (in final concentration of 0.16 g l−1) and phosphorus enrichment (in final concentration of 0.02 g l−1) on the AGP was investigated by miniaturized bioassay method. Values of the total biomass of Chlorella kessleri up to the 14th day of incubation were suitable for the evaluation of trophic conditions according to the classification of AGP. On the basis of the AGP results, trophic conditions in 55% of the samples were oligo-mesotrophic and in 46% of the samples meso-eutrophic. A statistically significant correlation (r = 0.34) was established between the AGP of C. kessleri in original water samples and NO3 concentrations in situ. The TN/TP ratio in the wetland waters indicated a greater limitation due to nitrogen than phosphorous. Significantly lower TSITN than TSISD, TSIChla and TSITP indicated nitrogen limited conditions. In order to quantify established nutrient limitation by the bioassay method, the effect of added N and P concentrations on the growth rate of C. kessleri was expressed as the degree of nutrient limitation (Δr d−1) during 7 days of incubation. In the Danube wetland waters only N limitation was established in June and July; N and P limitation in May and September while in August and October 2003 neither of the tested nutrients were limiting. From May to October 2003 the significantly highest degree of nitrogen (Δr = 0.736 d−1) and phosphorus limitation (Δr = 0.474 d−1) was determined in Lake Sakadas.