Changes in N and P limitation induced by water level fluctuations in Nature Park Kopački Rit (Croatia): nutrient enrichment bioassay

Abstract

The importance of nutrients as limiting factors might vary in environments with different limnological characteristics. In this article we evaluate the effect of frequency and duration of flooding on nutrient limitation in a riparian floodplain. Variations in N and P limitations were studied in the period of low (2003) and high (2004) water level in two different floodplain habitat types in the Nature Park Kopacki Rit (Croatia), a floodplain area of the Danube River. In 2003 and 2004, the limnological characteristics of floodplain lake (Lake Sakadas) and the channel (Stara Drava) differed due to their hydrological regimes. Potential for nutrient limitation was determined by DIN:TP and TN:TP ratios, while the actual nutrient limitation was assessed by nutrient enrichment bioassay. A change from non-limited to N-limited conditions in the channel, and consistent actual N limitation in the lake was determined by the nutrient enrichment experiment. Of the two ratios, DIN:TP matched better with the bioassay data. Phosphorus limitation was only occasionally evident. Changes in trophic conditions from hypereutrophy to eutrophy (between low and high water levels) reflected the importance of the hydrologic regime as a factor which can modify the trophic state of Lake Sakadas. Compared with 2003, the increase of total nitrogen concentrations in 2004 calls attention to the importance of nitrogen inputs from the Danube to the system with excessive phosphorus concentrations.