Distribution of selenium: A case study of the Drava, Danube and associated aquatic biotopes.

This paper presents the results of the research on the overall distribution of selenium (Se) in various aquatic compartments (water, sediment, plankton and macrophytes) at six selected sites of the Croatian part of the Drava and Danube rivers, the connected floodplain lake and the melioration channel system carried out in two sampling periods (flooding in June and the drought period in September). In addition, the physicochemical water properties, plankton composition and biomass were analysed. Our study revealed low mean Se contents in sediments and water, indicating Se deficiency in the studied freshwater systems. The physicochemical environment, including Se distribution, was primarily influenced by hydrology rather than site-specific biogeochemical and morphological characteristics. The flooding period was characterised by higher Se content in water and higher transparency, nitrate and total nitrogen concentrations than drought conditions. At the river sites, sediment Se content was the highest during the flood period, while at all other sites, higher concentrations were found during the drought, reaching the maximum in the lake. Although Se concentrations were below the threshold for aquatic ecotoxicity, they increased in the following order: water (0.021-0.187 µg Se L-1) < sediments (0.005-0.352 mg Se kg-1) < macrophytes (0.010-0.413 mg Se kg-1) < plankton (0.044-0.518 mg Se kg-1) indicating its possible biomagnification at the bottom of the food chain. Species known for high Se accumulation potential dominated the biomass of the main plankton groups and the composition of the macrophyte community, which may provide a more sensitive and accurate steady-state compartment monitor for Se assessment in freshwater biotopes.

Removal of nitrogen and phosphorus by aboveground biomass of Phragmites australis in Constructed Wetland System under the conditions of temperate continental climate.

In this paper, aboveground biomass and basic nutrients removal, nitrogen (N) and phosphorus (P), was analyzed by the use of reed as the main component of Constructed Wetland System (CWS) "Glozan". In almost ideal conditions of temperate continental climate, with favorable substrate humidity, due to the constant inflow of municipal wastewater, reed populations reach a high density, on average 217 ind/m2. The reed produces significant aboveground biomass, fresh weight (FW) of 144.21 g/plant and dry weight (DW) of 77.04 g/plant, with the largest share being per tree (87.49 g FW/plant, 48.17 g DW/plant), then leaf (49.45 g FW/plant, 24.89 g DW/plant) and the smallest inflorescence (7.27 g FW/plant, 3.99 g DW/plant). The results obtained in this way indicate that the largest amount of nitrogen was removed by leaves, then by stems and, the smallest by inflorescences, 181.07 g/m2, 97.73 g/m2, 23.41 g/m2, respectively. Thus, an average of 302.21 g/m2 of nitrogen was removed by the entire aboveground part of the reed. Also, the largest amount of phosphorus was removed by leaves, then by stems, and the smallest by inflorescences, 5.72 g/m2, 4.82 g/m2 and 2.57 g/m2, respectively, while the entire aboveground part of the reed is on average about 13.11 g/m2.

The contribution of this paper is reflected in the obtained results for population density, reed biomass, and reed efficiency in the process of accumulation and removal of nitrogen and phosphorus as the main factors of accelerated eutrophication of aquatic ecosystems, the recipients of municipal wastewater. These results are significant because there is not enough data concerning this topic in the temperate continental climate of Southeast Europe. In addition to this, Constructed Wetland System is the first system of this kind, developed in our country that has been functioning for past 18 years. Due to this, the results are encouraging the application of CWS for a large number of smaller settlements in Southeast Europe as well as in other similar areas. Also, the results obtained in this paper can be useful to all those who are committed to the environmental approach and are engaged in research related to the use of reed in the process of municipal wastewater treatment.