Vesna Peršić

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

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 Kopački Rit (Croatia), a floodplain area of the Danube River. In 2003 and 2004, the limnological characteristics of floodplain lake (Lake Sakadaš) 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 Sakadaš. 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.

Spatial Distribution of Nutrient Limitation in the Danube River Floodplain in Relation to Hydrological Connectivity

In this paper, we describe the influence of flooding on nutrient conditions and phytoplankton Chl-a concentrations in a hydrologically dynamic Danube floodplain in North–Eastern Croatia. Based on hydrological typology, the connectivity gradient extends from the main river stream (Eupotamal) to the backwater channels and side-arms (Parapotamal) to the edge of the floodplain (Paleopotamal). A spatial gradient of phytoplankton Chl-a occurred during low water levels, while uniformity of Chl-a across wetland habitats was observed during high water levels. Nutrient enrichment assays indicated that inorganic nutrients in Eupotamal waters (N/P ratio 16–22) could support a higher growth rate of the assay alga Chlorella kessleri, while potential N limitation was found in Parapotamal and Paleopotamal waters (both with N/P ratios < 14). Spatial expansion of N limitation toward the main river was observed as inundation continued. Lateral flood pulse connectivity along with the duration of inundation period affects the input and the retention of nutrients, especially of nitrogen. Understanding of the relationship between spatial (location of the floodplain waterbodies regarding the river) and temporal (hydrological connectivity between the river and its floodplain) dimensions contribute to the importance of healthy floodplains for the maintenance of water quality and environmental conditions in the Danube River.

Evaluation of equivalence between different methods for enumeration of fecal indicator bacteria before and after adoption of the new Bathing Water Directive and risk assessment of pollution

The quality of bathing water is of considerable public importance due to the possibility of fecal contamination. In 2009, Croatia implemented the new European Bathing Water Directive (BWD, 2006/7/EC) establishing stricter microbiological standards for new parameters with new reference methods. This study aims to evaluate the equivalence of different methods according to the old and revised BWD and to provide the possibility of data comparison. Furthermore, the directive requires the establishment of the bathing water profile (BWP) for pollution risk assessment. The estimation of consistency of pollution risk assessment with obtained microbiological results was also performed. Six marine beaches of the Municipality of Rijeka (Croatia) were examined during the 2009 season. Statistical analysis showed equivalence between determination methods for fecal contamination indicators. Based on the current water classification results, the need for correction of estimated pollution risks and recommendations for inclusion of historical microbiological data during BWP enactment was noticed.

Trophic State and Water Quality in the Danube Floodplain Lake (Kopački Rit Nature Park, Croatia) in Relation to Hydrological Connectivity

The investigated floodplain lake is located in the marginal part of the Kopacki Rit Nature Park, Croatia, a floodplain area of the Danube. In this chapter, we have assessed the influence of hydrological connectivity with the main channel on the trophic state and water quality in the Lake Sakadas. The evaluation of trophic state combines a number of physical, chemical and biological parameters with the objective to select the relevant indicators for the floodplain lakes. The following environmental parameters have been considered: water temperature, nutrients, dissolved oxygen, water transparency, chlorophyll-a concentration, phytoplankton primary productivity, benthic and periphytic communities and laboratory experiments (nutrient enrichment bioassay). An integrative approach, which includes in situ research and laboratory experiments, indicated strong correlations of abiotic and biotic components with a hydrological regime. Also, during the biological monitoring, occurrence and spread of invasive species was noted as an indication of ecosystem disturbance (hydrological and/or anthropogenic). Therefore, our results contribute to the perception of importance of the hydrological connectivity (by inundation) on the eutrophication process in the investigated floodplain lake.