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
,
T
B
) 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 N
2
-fixing (
S
N
) and non N
2
-fixing species (
S1
) showed tolerance to short-term flooding. The development of euglenoids and dinoflagellates (
W1
,
W2
,
L
O
) 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.
Lake Jošava (Croatia) is a shallow reservoir surrounded by agricultural land. In the present study, the trophic cascade was tested by examining the effects of stocking with common carp on plankton ...and periphytic microphytes. Before stocking, the phytoplankton community was dominated by the chrysophyte
Synura uvella
. In the epilithon and epiphyton, the predominant diatoms were prostrate, stalk-forming, and motile taxa representing an important food source for adult copepods. After stocking, phytoplankton biomass declined and the community shifted towards small centric diatoms, allowing the small-bodied zooplankton to exploit them. The lower biomass of adult copepods allowed rotifers to proliferate and exploit phytoplankton, while small cladocerans and nauplii fed primarily on epilithon. One month after stocking, phytoplankton was dominated by cryptophytes, small centric diatoms and chlorophytes, which were an important food for rotifers, while none of the zooplankton groups showed a significant relationship with the epilithic and epiphytic communities. By the end of the experiment, food was scarce due to reduced biomass of autotrophs, and zooplankton possibly began to feed on other sources. Our results add to the knowledge about the trophic cascade hypothesis in small shallow reservoirs.
To study the early colonization processes, polyethylene terephthalate (PET) microfragments were immersed in Lake Sakadaš 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.
Display omitted
•The PET fragments support the rapid development of three-dimensional plastispheres.•Initial colonization is supported by planktonic and periphytic taxa equally.•Following succession, the plastispheres in the lake resemble an epixylic community.•Progression of plastispheres in the river favours the settlement of planktonic taxa.
Changes in the functional groups of zooplankton were studied in autumn in a temperate floodplain lake (Lake Sakadaš, Kopački Rit Nature Park, Croatia) and in the Drava River (in the Croatian part of ...the river). Various abiotic parameters as well as available food sources (phytoplankton and microphytes (algae and cyanobacteria) developing on epixylon, epilithon and artificially introduced microplastics called “plastisphere”) were also studied. The lake was hydrologically isolated from the main river during the study, while the water level of the Drava River fluctuated, resulting in larger variations in limnological parameters. Due to stable conditions in the lake, zooplankton abundance, biomass, and species richness were higher than in the Drava River. In both environments, zooplankton species feeding on bacteria, detrital suspensions, and small algae were most abundant, with predators and microfilter-feeders being more abundant in the lake. Microphytes were diverse and mostly small and medium-sized in phytoplankton and all substrate types. Stable lake conditions promoted higher abundance of the zooplankton group, which effectively uses larger algae as a food source. The lower abundance of zooplankton feeding on larger algae and predatory species in the river suggests that the epilithon and plastisphere community was a less mature community compared to the lake, and the heterotrophic component with ciliates and/or other small heterotrophs was not well developed. The importance of plastispheres was particularly evident under the turbid hydrologic conditions that prevailed in the river at the end of the study, when phytoplankton biomass decreased and zooplankton abundance steadily increased, suggesting that microphytes colonised on microplastics were an additional food source for higher trophic levels.
•Northern aquatic ecosystems are most threatened by the presence of cyanobacteria species that produce neurotoxins.•Periods of heat waves could favour an increase in toxic cyanobacterial blooms, ...especially in northern cool lakes.•Nutrients are also a very important factor directly enhancing blooms across latitudes in central Europe.•The cyanobacteria species that dominate in blooms might be recognized as ecological indicators of eutrophication, but also of climate change.
Climate change has increased the frequency, duration and intensity of heatwaves in Europe. These extreme events result in alterations of physical, chemical, and biological properties of lakes that may synergistically promote cyanobacterial dominance. In our study we focused on cyanobacterial blooms in lakes distributed over a longitudinal gradient in Central Europe during one of the “top ten European heat waves” in summer 2015. 92 lakes were included in the study, located across three climatic subregions: cool northern lakes, situated in Lithuania, temperate northern lakes in Poland, and warm northern lakes in Croatia. The objective of the study was to determine if cyanobacterial biomass, predominant species, and cyanotoxin concentration differed, across the south-north gradient, as a function of water temperature, total phosphorus, and total nitrogen. Statistical significance of observed patterns was tested using the Kruskal-Wallis rank sum test and the generalized linear model. We found the lowest average epilimnion temperature, but the highest average cyanobacterial biomass in the northern, ‘cool’ lakes while the highest average temperature with the lowest average cyanobacterial biomass in the southern, ‘warm’ lakes. The concentration of cyanotoxins was also the highest in the ‘cool’ lakes. Total phosphorus and total nitrogen correlated significantly with cyanobacterial biomass, cyanotoxins concentration and biomass of some cyanobacterial species (mainly Planktothrix agardhii), regardless of the latitude. Only in the ‘cool’ lakes concentration of cyanotoxins (microcystins and anatoxin-a) correlated significantly with cyanobacterial biomass and the biomass of some dominant cyanobacterial species (P. agardhii). Our results emphasized the differences of heat weaves impact on lakes of various latitudes, with the strongest increase in toxic cyanobacterial blooms in northern ‘cool’ lakes, situated in high latitudes. On the other hand, nutrients directly enhanced blooms across all the studied latitudes of Central Europe. The cyanobacteria species dominating in blooms might be recognized as ecological indicators of climate change, especially in the north-eastern part of Europe.
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.
There are several conflicting hypothesis that deal with the influence of flooding in the natural river-floodplain systems. According to the Flood Pulse Concept, the flood pulses are not considered to ...be a disturbance, while some recent studies have proven that floods can be a disturbance factor of phytoplankton development. In order to test whether flooding acts as a disturbance factor in the shallow Danubian floodplain lake (Lake Sakadaš), phytoplankton dynamics was investigated during two different hydrological years--extremely dry (2003) without flooding and usually flooded (2004). A total of 18 phytoplankton functional groups were established. The sequence of phytoplankton seasonality can be summarized P/D rightward arrow E (W1, W2) rightward arrow C/P (only in potamophase) rightward arrow S2/H1/SN/S1 rightward arrow W1/W2 rightward arrow P/D. The canonical correspondence analysis (CCA) demonstrated that the water level was a significant environmental variable in 2004. Due to the higher total biomass of Bacillariophyceae established under potamophase conditions, floodings in the early spring seem to be a stimulating factor for phytoplankton development. On the other hand, the flood pulses in May and June had dilution effects on nutrients, so that a significantly lower phytoplankton biomass was established indicating that flooding pulses can be regarded as a disturbance event. Such conditions supported diatom development (D, P, C species) and prolonged its dominance in the total phytoplankton biomass. A long-lasting Cyanoprokaryota bloom (various filamentous species--S1, S2, SN and H1 representatives) with very high biomass characterized the limnophase (dry conditions) in summer and autumn of both years. In-lake variables (lake morphology, internal loadings of nutrients from sediments, light conditions) seem to be important for the appearance of Cyanoprokaryota bloom. The equilibrium phase was found during the Cyanoprokaryota bloom only in the extremely dry year. This study showed that depending on the time scale occurrence, flood pulses can be a stimulating or a disturbance factor for phytoplankton development in Lake Sakadaš.
Intensive human activities have resulted in a critical reduction of grasslands in Croatia, one of the richest Europian countries in terms of biodiversity. Sub-Pannonic steppic grasslands are very ...rare, only few occur in the continental part of Croatia and most of them are protected as NATURA 2000 sites. We studied vascular flora of the Sub-Pannonic stepic grassland with the aim to examine the changes in plant communities 15 years from the mowing application. Results showed that hand mowing once per year at the end of vegetation season increased the species diversity as well as the number of medium and low growth taxa. The critically endangered, nearly threatened and vulnerable species remained preserved. The occurrence of trees, shrubs and invasive species as a consequence of succession and anthropogenic influences could be important threat for steppe-like grassland flora. From a conservation point of view infrequent mowing regime could be an effective management tool, although it needs to be adapted to regional and local circumstances for maintaining high biodiversity of steppe-like grasslands and valuable plant species.
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.
