Melita Mihaljević

Cyanobacterial blooms in a temperate river-floodplain ecosystem: the importance of hydrological extremes

In the past decade, extreme hydrological events were expressed with extreme droughts and floods in temperate regions. The aim of this paper is to explain how such changes in hydrology can influence cyanobacterial populations in floodplain ecosystems. We therefore analyzed a 6-year (2003–2008) study of the phytoplankton in the Kopački Rit floodplain, one of the largest natural floodplains in the middle section of the Danube River (Europe). During the studied period, the shallow floodplain lake shifted between a state of turbid water, characterized by high phytoplankton biomass and regular appearance of cyanobacteria blooms, to a state of clear water with very low phytoplankton biomass and absence of cyanobacteria, and back to the turbid state. Apparently, the major forces driving the cyclic shift were closely related to extremely high and long-lasting flood events. Significant increase in water level, low hydraulic residence time of water, decrease in transparency and low-light climate, together with mass developed aquatic macrophyte vegetation in the whole inundated floodplain were unfavorable conditions for growth and proliferation of cyanobacteria. With the establishment of the flood regime characterized by long-lasting periods without flooding, in-lake processes prevailed leading to cyanobacterial bloom. The most successful were filamentous non-N-fixing cyanobacteria tolerant to mixed and low-light conditions (Planktothrix and Limnothrix) and invasive species Cylindrospermopsis raciborskii. Their massive development led to the establishment of a phytoplankton steady state. All our results demonstrate that the altered intensity and frequency of flood events will have pronounced effects on the appearance of cyanobacterial blooms and generally on alternative stable states in the floodplain. Relating to this, management objectives should be focused on qualifications of changes in hydrology and projecting those effects for potential floodplain restoration.

Changes of phytoplankton functional groups in a floodplain lake associated with hydrological perturbations

During the past decade, basic hydrological conditions of a floodplain lake in the middle Danube section have been altered with long-lasting extremely high flooding. The objective of the paper is to show the effectiveness of the functional approach to explain phytoplankton changes associated with hydrological events. Intensity and duration of flooding were qualified as the primary cause for the changes of functional groups. Flooding phase was characterised by diatoms (B, C, D, P, TB) tolerant to water column mixing. Due to the dilution and washout effect their biomass was low during the long-lasting flooding despite their input from the river. Co-occurrence of coccoid green algae (X1, J, F) was associated with turbid and mixed waters. High-nutrient concentrations and water column stability during the long-term dry conditions led to the dominance and high biomass of cyanobacteria. Low-nitrogen H1 group was particularly sensitive to stress caused by flooding, while filamentous N2-fixing (SN) and non N2-fixing species (S1) showed tolerance to short-term flooding. The development of euglenoids and dinoflagellates (W1, W2, LO) was also associated with dry conditions and seasonal changes in autumn. The functional classification allows representing of the hydrological phases which characterise the phytoplankton succession in highly disturbed river-floodplain systems.

Assessment of flood-induced changes of phytoplankton along a river–floodplain system using the morpho-functional approach

In this research, we aimed to find out how the differences in hydrological connectivity between the main river channel and adjacent floodplain influence the changes in phytoplankton community structure along a river–floodplain system. The research was performed in the River Danube floodplain (Croatian river section) in the period 2008–2009 characterised by different flooding pattern on an annual time scale. By utilising the morpho-functional approach and multivariate analyses, the flood-derived structural changes of phytoplankton were analysed. The lake stability during the isolation phase triggered the specific pattern of morpho-functional groups (MFG) which were characterised by cyanobacterial species achieving very high biomass. Adversely, the high water turbulence in the lake during the frequent and extreme flooding led to evident similarity between lake and river assemblages. Besides different diatom species (groups of small and large centrics and pennates), which are the most abundant representatives in the river phytoplankton, many other groups such as cryptophytes and colonial phytomonads appeared to indicate altered conditions in the floodplain driven by flooding. Having different functional properties, small centric diatom taxa sorted to only one MFG cannot clearly reflect environmental changes that are shown by the species-level pattern. Disadvantages in using the MFG approach highlight that it is still necessary to combine it with taxonomical approach in monitoring of phytoplankton in the river–floodplain ecosystems.

Early spring flora of the Sub-Pannonic steppic grassland (NATURA 2000 site) in Bilje, northeast Croatia

Abstract The diversity of early spring vascular flora was studied in the Sub-Pannonic steppic grassland in the village of Bilje, north eastern Croatia. In all, 109 plant taxa within 35 families were found. The highest number of taxa belongs to the families Poaceae, Fabaceae, Asteraceae, Caryophyllaceae, Lamiaceae and Rosaceae. Specific habitat conditions, characterized by moderately wet and moderately acidic soil with intermediate fertility and the effects of the continental climate favour the development of different plant life forms. Out of the total recorded plant taxa, hemicryptophytes make up 59.6%, followed by therophytes (22.0%) and geophytes (13.8%). Chorological analysis shows that the most numerous are plants of Eurasian (33.9%), Pontic-Central-Asian (21.1%) and Central European (21.1%) floral elements. According to their status in the Red List, three critically endangered (CR), one vulnerable (VU) and three nearly threatened (NT) plant species were found. Altogether, the steppe-like grassland in Bilje is a unique habitat rich in valuable plants of the Croatian flora, including the critically endangered Doronicum hungaricum, therefore it is of great importance to preserve it. Important management tools include mowing and controlling the spreading of cultivated and invasive plant species.

The importance of microphytic composition on coarse woody debris for nematode colonization: a case study in a fluvial floodplain environment

Abstract Coarse woody debris (CWD) are an important structural feature of many aquatic ecosystems. This study focused on the relationships between nematode colonization and microphytic community structure on CWD. In situ investigations were performed using white willow branches introduced into the floodplain lake, a part of the large natural floodplain in the middle section of the Danube River (Europe) impacted by strong sandy and silt accumulations. Our results showed that CWD support rapid colonization and high species richness of microphytes, regardless of seasonality and flood-induced environmental changes. Successional stages during the accrual of microphytic communities were expressed. Nematode functional feeding group compositions closely followed microphytic composition. The epistrate feeder Chromadorina bioculata over-dominated in diatom-dominated communities with adnate/prostrate and stalk-forming species in spring and autumn. The complex structure of cyanobacterial mats associated with green algae and diatom assemblages in summer attracted suction feeders, represented by the Dorylaimidae family. The obtained results indicated that nematodes are involved in strong trophic coupling with a microbial habitat on CWD. The retention of CWD in aquatic environments or addition of CWD to systems with little or no wood may have important implications for the primary and secondary production in these ecosystems.