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.

Plastisphere development in relation to the surrounding biotic communities.

To study the early colonization processes, polyethylene terephthalate (PET) microfragments were immersed in Lake Sakadas and the Drava River and sampled weekly together with the surrounding biotic communities - phytoplankton, zooplankton, epixylon in the lake and epilithon in the river. At the end of the study, a rise in water level occurred in the river, which altered the environmental conditions and plankton communities. In studied environments, all of the sampled biotic communities were diverse and abundant. Plastispheres formed in both waters by the seventh day of incubation and developed rapidly, reaching a peak in abundance on the last day of the study. Initial colonization was supported equally by planktonic and periphytic taxa in both environments, but after initial settlement, plastisphere assemblages were affected differently in the river and lake. This study suggests that PET microfragments are a suitable substrate for microphyte settlement and may provide an important pathway for their transport in dynamic freshwater floodplains and river systems.

Impact of habitat heterogeneity on zooplankton assembly in a temperate river-floodplain system.

Dissimilar life features of Rotifera, Cladocera and Copepoda enable these organisms to respond differently to changes in the hydrological regime which influence alterations in environmental characteristics. We investigated the effect of habitat heterogeneity (e.g. eupotamal, parapotamal, palaeopotamal) on individual zooplankton group assemblages and biodiversity indices (α, ß and γ diversity) during hydro regime change in floodplain waterbodies. Dissolved oxygen and organic nitrogen concentrations changed significantly among hydrological states while water depth was affected by both site and hydro regime replacement. Each studied site supported different zooplankton assemblage that highly depended on species-specific responses to hydro regime change. Also, individual zooplankton groups exhibited different correlations with specific environmental parameters regarding site change. Throughout the study, rotifers' local (α) and among-community (ß) diversities were susceptible to the site and inundation change while the microcrustacean biodiversity pattern diverged. Copepods highly discriminated different habitat types and hydrological phases at the regional scale (γ diversity), while we found a complete lack of biodiversity dependence on both site and hydrology for Cladocera. Our results show that heterogeneous environments support the development of different zooplankton assemblages that express the within-group dissimilarities. They also point to the importance of identifying processes in hydrologically variable ecosystems that influence biodiversity patterns at an individual zooplankton group level. Our results suggest the use of appropriate zooplankton groups as biological markers in natural habitats and stress the importance of proper management in preserving biodiversity in floodplain areas.