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Changes in N and P limitation induced by water level fluctuations in Nature Park Kopački Rit (Croatia) and growth of C. kessleri in bioassay

The paper presents the results of the influence of water level fluctuations (low waters from May to October 2003 and high water in the same period 2004) on changes in nutrient limitation and phytoplankton Chla concentration in situ and the growth of C. kessleri in water samples from Lake Sakadaš and Stara Drava (NP Kopački rit) in laboratory conditions. The influence of nutrient enrichment with N (final concentrations of 0.16 g l-1) and P (final concentrations of 0.02 g l-1) on the growth of C. kessleri was investigated by miniaturized bioassay method. During 2003, values of the TN/TP and DIN/PO4 ratio suggested that sufficient nutrients were available for phytoplankton growth. An increase in the water level in 2004 caused a significant decrease (fourfold) in ammonium and nitrate (twofold) concentrations at the Stara Drava site, indicating N limited conditions. A significant correlation was observed between ammonium and Chla (r=0.716). A higher total biomass of C. kessleri in water samples diluted 1:1 in relation to the original water samples from the Stara Drava Channel site in 2003 indicated the possible presence of substances that inhibit the growth of C. kessleri. The results of the nutrient enrichment bioassay confirmed that sufficient nutrients were present in 2003. In 2004 N limited conditions were determined for the growth of C. kessleri with the maximal degree of N limitation (Dr = 0.595 d-1) in June. A significant positive correlation was established between the Danube water level and the TN/TP ratio (r=0.706) in Lake Sakadaš. With an increasing water level in 2004, there was a shift from N to P limitation, caused by increased TN concentrations and decreased TP concentrations. A positive correlation (r = 0.785) was determined between phytoplankton Chla and PO4 concentrations. Bioassay experiments were conducted and N limitation was determined during both years, with a higher degree of N limitation in 2004.

Danube water level; N and P limitation; phytoplankton chlorophyll a; bioassay; C. kessleri

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