A Decade of Change in the Floodplain Lake: Does Zooplankton Yield or Resist?

Abstract

Natural ecosystems, especially those regulated by floods, are sensitive to prolonged temperature fluctuations that affect hydrology and the lateral connection between the river and its floodplain. Here, we analyzed a series of zooplankton data collected monthly from 2007 to 2016 during the ice-free period in Kopački Rit Nature Park in the Middle Danube, an area important as a food source and nursery area for fish stocks in the Danube. The aim was to find out how the long-term change in temperature and fluctuating environmental parameters affect the succession of zooplankton in the warmer (from April to September) and colder parts of the year (from October to March). Throughout the decade, total nitrogen concentrations showed significant differences between years, with an increase since 2012. Despite the increase in nitrogen levels and the expected increase in primary production, the higher nitrogen levels were accompanied by lower zooplankton biomass. A significant difference was found between the values of the zooplankton geometric mean index, with 73% of the variance explained by the difference between groups. In general, a trend toward a significant decrease in zooplankton biomass, with a simultaneous increase in the number of species and high turnover rates, was observed throughout the decade.

1. Introduction

Climate change has led to predictions of an increase in the global mean near-surface temperature of between 1.1 °C and 1.9 °C above the annual average from 2024 to 2028 [1]. Due to the stronger warming of the land masses compared to the ocean, faster warming is expected over the continents of the northern hemisphere, where extreme weather events such as heavy rainfall and droughts will occur more frequently [2]. In recent years, the number of extreme events in Europe including droughts, severe storms, floods, heatwaves, forest fires and heavy rainfall has increased by 60% since the beginning of the 21st century [3]. Natural ecosystems are particularly sensitive to these stress factors, which influence the seasonal fluctuations of the water cycle and affect their stability [4]. Changes in precipitation, snow cover and air temperatures, which have a significant impact on snowmelt [4,5] and river discharge, alter the hydrological regime (water retention and flow) of aquatic ecosystems, especially those regulated by floods [6]. However, not all floodplains are equally affected, as they are strongly linked to the specific land morphology, which can affect the lateral connectivity of the river and its floodplain and lead to a reduction in habitat areas and their quality, in addition to impacts on groundwater level [7].

In the last century, more than 70% of European floodplains have been lost [8], and the remaining portion is now home to 12% of the European population [7]. One of the best-preserved floodplains on the continent is the Danube Delta [7], while the Middle Danube is one of the last-remaining lowland river corridors in Europe. Kopački Rit Nature Park is part of the middle section of the Danube on Croatian territory, which forms an inland delta at the confluence of the Danube and Drava rivers. This area is an important source of food and a nursery ground for the Danube’s fish stocks, as it is the most important spawning area in the middle and upper Danube region [9]. As elsewhere, a clear trend towards an increase in extreme events over the course of the year can be observed in the Danube region as well [4].

Aside from economically and socially valuable ecosystem services, floodplains are one of the most productive ecosystems on Earth [10]. They are often referred to as internal retention zones, as they have a longer water retention compared to rivers [11,12]. Taking into account the biogeochemical processes (the retention, transformation and production of substances), these storage zones are referred to as “hot spots” in the river catchment. Lower water flow and lower turbulence intensity lead to the sedimentation of seston [13] and to higher sediment stability, a more stable water temperature, more uniform light intensity and the accumulation of nutrients [11]. In these transitional waters, there is a great diversity of benthic species as well as periphyton, plankton and fish species [11]. Although a considerable amount of primary and secondary production takes place in the floodplain, the nutrient concentration depends on the quantity and quality of dissolved and suspended sediments from the main river [14]. Therefore, the timing of flooding in temperate latitudes is a very important factor [15] due to seasonal variations in light and temperature as well as the decomposition of organic material, which can be faster or slower depending on the season. The varying frequency, amplitude and duration of floods determine the extent and intensity of changes in the floodplain [16] and thus influence the productivity of the system and the development of species [17].

The comparison of the phytoplankton in Kopački Rit Nature Park from the 1970s shows that the changing intensity and timing of floods influence the alternative stable conditions in the floodplains and the known phytoplankton succession in a naturally eutrophic system [18,19]. Prolonged periods without flooding usually lead to unfavorable conditions in floodplain lakes, and the occurrence of cyanobacterial blooms [18]. In contrast to drought, runoff pulses and floods favor diatoms and diatom-dominated functional groups, the abundance of which is influenced by the duration and intensity of floods as well as nutrient input (mainly nitrogen) from the main river [20,21]. In addition to flooding, the global rise in temperature also has an important influence on the development of phytoplankton [22]. In warmer winters, a greater amount of algae can be found in the river, so the number of algae in winter was comparable to that in summer in some years [23]. These results indicate the importance of temperature increases for the overall functioning of the food web throughout the year.

The amount of available phytoplankton is an important factor for the development of zooplankton [22], which also depends on the habitat type. If the conditions in a floodplain are stable, a diverse zooplankton community can be found there, whereas a less mature and complex zooplankton community is usually found in rivers due to the higher flow velocity and the dilution effect [24,25]. Water level fluctuations also influence the influx of zooplankton recruits from the floodplain [22]. Zooplankton also show a high affinity to environmental parameters within the floodplain, and rotifers and copepods have been found to be particularly sensitive to site-specific characteristics and hydrology of the area [26]. As with phytoplankton, increases in temperature in the winter also reflect deviations from the usual zooplankton succession, and food webs have been shown to be more unstable in warmer compared to colder winters [27]. As zooplankton is important for feeding fish during the growing season [22], a destabilized food web might have a prolonged effect on zooplankton support for various ecosystem services [28].

The aim of this study was to identify how long-term temperature change and fluctuating environmental parameters affect the succession of zooplankton in surface waters in the warmer (from April to September) and colder seasons (from October to March) in a temperate floodplain lake over a ten-year period. We hypothesized that the succession would be more strongly influenced by temperature changes in the colder months and by nitrogen and phosphorus inputs in the warmer months of the year.

2. Materials and Methods

2.1. Study Site

Kopački Rit Nature Park in northeastern Croatia (45°36′ N; 18°48′ E; 80.5 m a. s. l.) is one of the largest floodplain areas in the Central European part of the Danube and part of the NATURA 2000 network of nature reserves. On Croatian soil, it extends from 1410 to 1383 river km (Figure 1) and covers a total of 177 km². There are 25 different habitat types in the area of the Nature Park, 5 of which are endangered at European level and protected by the Habitats Directive, including rivers and lakes, wetlands, grasslands, meadows and tall grasslands, thickets, forests, and habitats with weed and ruderal vegetation. Lake Sakadaš (Figure 1) is the deepest permanent water body in the Kopački Rit floodplain, and its ecological status has been monitored for over a decade. The lake depth ranges from approximately 3 m to 12 m with an average depth of 4–5 m. The total lake surface area is of about 0.15 km², with relatively steep banks. The Danube determines the height of the water fluctuations of Lake Sakadaš [29]. Below +167 cm, at the Apatin gauge (1401.4 river km), the lake is completely isolated from the adjacent river (Figure 1A), whereupon a flow pulse starts [30], which lasts until +300 cm (Figure 1B), when the flooding of the area begins [29]. Lake Sakadaš is characterized as a eutrophic–hypereutrophic system [31] and is thermally stratified (usually from May to August), based on long-term data [32].

The sampling station at Lake Sakadaš was located in the southwestern part of the lake (Figure 1) at the deepest part of the lake, and sampling was carried out monthly from 2007 to 2016 during the ice-free period.

2.2. Physical and Chemical Analyses

Water samples for the analysis of limnological parameters were taken at the surface (about 20 cm below the water surface) during each sampling, and a total of 98 samples were taken. The water depth (WD) was measured with a weighted rope, while the water transparency (SD) was measured with the Secchi disk. The portable instruments WTW Multi 340i (Wissenchaftlich-Technische Werkstätten, Weilheim, Germany) and HQ30d Flexi (Hach Company, Loveland, CO, USA) were used to measure the dissolved oxygen concentration (DO), water temperature (WT), and the pH and conductivity (Cond). The chemical variables of water were analyzed in the laboratory according to the standard methods for ammonium (NH₄-N) [33], nitrites (NO₂-N) [34], nitrates (NO₃-N) [35], Kjeldahl (orgN) [36] and total nitrogen (TN) [37], and total phosphorus (TP) [38]. To measure chlorophyll a (Chl-a) concentration (which was used as a proxy for phytoplankton biomass), water samples were filtered through Whatman GF/C filters, pigments were extracted with acetone, and Chl-a concentrations were calculated according to standardized methods, described in [39,40]. The hydrological measurements were carried out daily at the Apatin gauge (river km 1401.4, Republic of Serbia) and were provided by the Croatian Waters, a legal entity for water management.

2.3. Biotic Community Analysis

Zooplankton samples were taken from the lake surface (approximately 20 cm below the water surface) for each sampling. For the analysis of rotifers, 10 L of lake water was filtered through a 25 μm net, while for the analysis of microcrustaceans (cladocerans and copepods), 25 L of lake water was filtered through a 65 μm net. Both samples were preserved in situ in a formaldehyde solution with a final concentration of 4%. In the laboratory, at least 500 rotifer individuals were counted in each sample and identified to the species level using the standardized literature [41,42], while the entire sample was counted for microcrustacean abundance estimation. The adult microcrustaceans were subsequently dissected and identified to the species level using specialized keys [43,44,45], while nauplii and copepodites were only counted. Literature data were used to calculate the biomass of rotifers, while a maximum of 20 individuals of the same species were used for the measurements to calculate the biomass of microcrustaceans [46]. The data were calculated using species-specific biomass equations [47,48,49,50].

2.4. Statistical Analyses

Statistical data processing was performed using the R programming language [51]. A Spearman rank correlation test was performed to analyze the zooplankton communities over a ten-year period (2007–2016). Due to the location of Lake Sakadaš in a temperate climate zone, different conditions prevail in the system depending on the season in terms of physico-chemical parameters and characteristics of the analyzed communities. Accordingly, the data were divided into two data sets, one belonging to the autumn and winter months (from October to March) and the other to the spring and summer months (from April to September). A Spearman rank correlation test was performed on these two data sets to determine the relationships between the ecological factors and their influence on the biomass of phytoplankton and zooplankton. Finally, a Spearman rank correlation test was performed on the entire data set to determine the change in nutrient concentrations during the study period. The Spearman rank correlation test was calculated using the package “stats” [51] and the results were visualized using the package “corrplot” [52]. As a value for community diversity, the geometric mean index was calculated separately for the autumn and summer zooplankton communities using the package “OnomasticDiversity” [53]. To determine whether there is a difference between the values of the geometric mean index between years, the Kruskal–Wallis test was performed separately for summer and autumn, excluding the year 2007 for the autumn data set due to missing data. The Dunnet post hoc test was performed to determine which years differ in the values of the geometric mean index. For the autumn data set, the Benjamini–Hochberg p-adjustment method was used, as the Holm method was unable to detect differences between years, although the Kruskal–Wallis test revealed differences between groups. For the summer data set, the p-adjustment Holm method (Holm–Bonferroni) was used. The Kruskal–Wallis test, the Dunnet post hoc test, the graphical representation of the geometric mean and statistical processing were performed using the package “ggstatsplot” [54]. Other graphical representations were created with the package “ggplot2” [55]. The species turnover number was calculated using the “codyn” package [56], where the three values represented the rate of change in the community for each year. For the zooplankton community, the number of new species, the number of lost species and the number of total turnover, defined as the sum of new and lost species, were calculated.

3. Results

3.1. Environmental Parameters

The water level fluctuated throughout the year (Figure 2A), with an overall increase in the median water level since 2012 onwards compared to earlier years (accept 2009) (Figure 2B).

The WT also fluctuated between a minimum of 3.3 °C (October 2010) and a maximum of 30.6 °C (August 2012); the median average water temperature did not differ significantly between the years (Figure 3A). The DO values oscillated between 0.94 mg/L (September 2007) and 16.40 mg/L (March 2013), but the median values were lower from 2013 onwards. The concentration of TN followed the floods (Figure 2B and Figure 3C) and showed significant increases throughout the years (r = 0.39, p < 0.001). Its concentrations also increased from 2012 onwards, as did the median Danube water level (Figure 3C). An increase in TP concentration over the years can also be observed (Figure 3D), and although the change was slow, it was significant (r = 0.24, p = 0.015). Organic nitrogen also showed a significant change (r = 0.39, p < 0.001) over the years, with a clear trend towards an increase since 2012. Although the changes in nutrient concentrations were visible, the Chl-a concentration remained stable over the ten years.

The results of the Spearman rank correlation test showed that for zooplankton, WT had an influence on the biomass of the community. In the warmer months, the concentrations of organic N and TN and the conductivity values were found to be particularly important (Figure 4A), while in the colder months of the year, this influence was found to diminish and WT was found to remain the only environmental variable important for the development of the zooplankton community (Figure 4B).

3.2. Community Diversity

A total of 135 species were identified, of which 112 belonged to the rotifers, 15 to the cladocerans and 8 to the copepods in juvenile stages (Table S1). In terms of biomass, Synchaeta (phytoplankton feeders) and Asplanchna (predators) were the predominant genera of rotifers throughout the year. The biomass of Brachionus (bacteria-detritus suspension feeders) and Trichocerca (net algae feeders) increased in the warm seasons, while a shift in rotifers was observed in the colder months, with an increase in the biomass of Polyarthra (phytoplankton feeders) and Keratella (bacteria–detritus suspension feeders) genera. Among the cladocerans, Bosmina longirostris (bacteria and algae feeders), the genus Daphnia (phytoplankton feeders) and Moina affinis (phytoplankton feeders) dominated during the warm season, while Bosmina longirostris and Chydorus sphaericus (phytoplankton feeders) dominated during the cold season. Megacyclops gigas (predators) dominated the copepod community during the warm season, and Thermocyclops crassus individuals (herbivores/omnivores) were present in almost all samples. Colder periods favored the presence of Cyclops vicinus (omnivores) [57]. Rotifers and copepods dominated the zooplankton community in terms of biomass throughout the study period (Figure S1). However, it should be noted that rotifers were more abundant, and, in the case of copepods, their developmental stages were more abundant than those in adults. From 2012 onwards, a significant decrease in zooplankton biomass was observed, with a simultaneous increase in the number of species (Figure S2).

Overall, in colder months, the Kruskal–Wallis test showed a significant difference in the geometric mean index values of zooplankton between years (χ²_{Kruskal–Wallis} = 27.00, p < 0.001) and that 73% of the variance can be explained by the difference between groups ( $\hat{\epsilon }$²_ordinal = 0.73) (Figure 5A). In warmer months, a similar pattern was observed, where the Kruskal–Wallis test showed a strong significant difference in the geometric mean index values of zooplankton between years (χ²_{Kruskal–Wallis} = 42.39, p < 0.001) and that 74% of the variance can be explained by the difference between groups ($\hat{\epsilon }$²_ordinal = 0.74) (Figure 5B). The medians of the geometric mean index in both periods were lower from 2012 to 2016 than in previous years. This trend is consistent with the trend in biomass, which has decreased since 2012, indicating the sensitivity of the index to biomass values.

The period from 2007 to 2016 showed a relatively high turnover rate of over 50% for zooplankton species (accept in 2015 and 2016) (Figure 6). The rate of appearance of species was almost always lower than that their disappearance, which is consistent with the changing hydrological conditions at the investigated site (Figure 6).

4. Discussion

As for the rest of the northern hemisphere, a temperature increase of up to 0.6 °C in winter and up to 1 °C in summer is also predicted for Croatia for the period 2011–2040 [58]. In the period we analyzed (2007–2016), the water temperatures did not differ significantly. An increase in mean seasonal or annual water temperature could have similar effects on freshwater zooplankton to accelerated eutrophication [27]. Our study showed an increase in the water level of the Danube since 2012, whereby a significant difference in nitrogen concentration was also observed over the same period. The results show that the decrease in dissolved oxygen levels could be related to the increased nitrogen input, as eutrophication and organic pollution cause oxygen deficits in freshwater habitats, which can further affect decomposition rates, leading to a further decrease in oxygen levels [59].

Like other freshwater invertebrates, zooplankton are sensitive to variations in dissolved oxygen concentrations [60], and changes in zooplankton biodiversity and abundance could affect species at higher trophic levels, particularly planktivorous fish [61]. Although some simulations of climate warming progress suggest that cladocerans might predominate and copepods might decline in the future scenarios [61], our results show that throughout the decade, alongside rotifers, copepods dominated the community. These results are consistent with previous studies in the waters of the Danube, where the zooplankton community has been dominated by rotifers and juvenile copepod stages [26,46,62].

It was found that the total biomass of zooplankton in floodplains increases continuously with water age [63]; this was also observed in Lake Sakadaš [64]. With the introduction of high water levels, water transports organisms throughout floodplain sections, and at low water levels, the community is limited, so species richness shows a better match between organisms and the environment in the absence of high amounts of water [65]. However, the different life characteristics of rotifera, cladocera and copepoda allow these organisms to respond differently to changes in the hydrological regime [26]. This is particularly important in the context of recurrent flooding in temperate floodplains, where efficient resource utilization may be limited by the hydrology of the area and impact the turnover rates of the community.

Temperature emerges as another integrative driver affecting planktonic organisms. Zooplankton responses to warming gradients are taxon-specific; while some taxa exhibit reduced body size in warmer conditions, others show complex or contradictory trends [66]. Our results corroborate the influence of temperature on community structure in the development of zooplankton [67]. Also, during warmer months, temperatures generally enhance the growth of cyanobacteria, which are often thermophilic and can dominate under eutrophic, stratified, and low-flow conditions [68] that are found in Lake Sakadaš as well. These changes alter the structure of the phytoplankton community, which is a source of high-quality food for zooplankton [69], and the presence of cyanobacteria and reduced production of high-quality phytoplankton can lead to zooplankton starvation [70].

The highest overall occurrence of cladocerans in Lake Sakadaš was observed in 2013, when extreme flooding was recorded and the invasive cyanobacteria Raphidiopsis raciborskii was abundant and blooming among the phytoplankton [71]. In the absence of preferred food, large cladocerans can ingest filamentous cyanobacteria [72] and are able to ingest and tolerate microcystin [73]. Among the cladocerans, Daphnia, the biomass of which peaked in 2013, is often considered a suitable zooplankton for the propagation of cyanobacterial blooms [74]. Small cladocerans such as Chydorus sphaericus, the species commonly found in eutrophic lakes, can also feed on cyanobacteria, especially colonial ones, which can be an important food source, supporting their coexistence with cyanobacterial blooms [75]. For this reason, cladocerans are considered the most important trophic link that transfers cyanobacterial carbon into the food web of a highly eutrophic lake [75]. On the same occasion, the biomass of copepods decreased significantly and was the lowest in the study period. As juvenile stages predominated in the copepod community, this is to be expected as juveniles do not feed on cyanobacteria; they are able to detect secondary metabolites of cyanobacteria and thus avoid harmful food [76].

The opposite annual hydrological event occurred in 2012 and 2015, when widespread droughts affected large parts of the EU territory [71] and the highest copepod biomass was observed in these years. In the phytoplankton community of Lake Sakadaš, R. raciborskii was present in low numbers [71], and the lake was in a clear state. The community was characterized by the low phytoplankton biomass of Crypthophyta and Bacillariophyceae [77]. These phytoplankton groups are a high-quality food for copepods that can be easily ingested and are rich in highly unsaturated fatty acids [78], which favors good copepod development. In addition, low water levels and relatively stable hydrological conditions in Lake Sakadaš favored the development of adult copepods, which contributed to the increase in copepod biomass in the total biomass of the zooplankton community.

Our results show that the zooplankton community is also affected by conductivity in the warmer months of the year (from April to September), as has been observed in other studies as well [79]. High conductivity values combined with an increased trophic state of the water body can lead to a reduction in zooplankton size and their grazing efficiency [79]. Rotifers are a common group in eutrophic waters that tolerate increased conductivity and resuspended sediments in the Danube waters.

Nutrient stoichiometry also plays a crucial role in mediating phytoplankton–zooplankton interactions. High levels of organic and total nitrogen had a significant impact on the zooplankton community in our study. Increased nitrogen concentrations were associated with increased N:P and C:P ratios in phytoplankton and reduced their nutritional value for zooplankton [80]. This shift corresponded with a decrease in cladoceran and total zooplankton biomass and an increasing dominance of rotifers—smaller-bodied zooplankton taxa that are more tolerant of poor food quality and fluctuating environments [81]. Despite the observed increase in nitrogen content and increased phytoplankton biomass, zooplankton biomass is not necessarily increasing [82], as shown in our study, where higher nitrogen levels coincided with lower zooplankton biomass and the increased dominance of rotifers. The Danube brings a high concentration of nutrients to the studied area, but the results suggest that fluctuations in TP may be related to phosphorus loading and cycling within the lake [83]. Although TP is an important nutrient for plankton development, the lack of correlation with Chl-a concentrations in the colder seasons suggests that other factors for phytoplankton development are at play, e.g., light and temperature [84].

In zooplankton, the genera Synchaeta and Asplanchna dominated the biomass of rotifers throughout the year. Their populations were maintained by a sufficient food supply, as both species can feed on larger organisms compared to other rotifers [85,86]. The zooplankton community in the warmer months was characterized by the presence of bacteria-detritus suspension feeders (Brachionus genus), which usually predominate during the isolation of the water body or a flow pulse, as previously noted [87]. At the same time, there was an increase in the abundance of the Trichocerca genus; these individuals can alternately switch to animal food [57]. Although the functional groups of rotifers remained the same during the colder seasons, the dominant species in the community shifted to Polyarthra and Keraltella genera. This shift in dominance could indicate a change in the lower trophic levels. The presence of Polyarthra indicates that larger species dominate the phytoplankton community compared to the case during warmer months, as Polyarthra feeds on phytoplankton up to 30 µm in size [57]. This genus also has a low probability of being attacked as it has an effective escape response [86], as do Keratella individuals, who have the ability to develop longer spines in the presence of predators [85]. This may suggest that in the colder season, when large floods are less likely and waters are calmer, predation pressure from higher trophic levels may have increased. In the warm season, Bosmina longirostris, which competes with rotifers for the same resources, and larger cladocerans (genera Daphnia and Moina), which can feed on larger phytoplankton and thus utilize the niche not available to smaller rotifers, were well developed. Interestingly, in addition to the bacterivorous and detritivorous Bosmina, Chydorus sphaericus, which can feed on algae, detritus and various microorganisms, also dominated during the colder months [88]. These zooplankton species are an important food source for larger predators such as Megacyclops gigas, which was also recorded here during the warm seasons. The shift in the dominance of cyclopoid copepods from the warm to the colder season, where Thermocyclops crassus was replaced by Cyclops vicinus, is part of a natural cycle, as T. crassus goes into winter diapause while C. vicinus goes into diapause during the warming periods [89]. Our results show that despite the strong influence of hydrology on the food web in floodplain systems, general seasonal succession is a strong determinant of plankton development.

Since many species in the upper trophic levels, such as fish, rely on zooplankton as an important food source, there are significant interactions between trophic levels associated with changes in zooplankton dynamics. The study area of Kopački Rit harbors numerous indigenous fish populations, and the park’s fish fauna includes more than 50 freshwater species, of which the European carp, northern pike, wels and zander are common. Their larvae feed on zooplankton and later change their diet. Carp larvae, for example, feed mainly on zooplankton, initially rotifers and nauplii, and, as they grow older, they feed on larger organisms, such as copepods and cladocerans (Daphnia hyalina and Simocephalus spp., etc.). Although they turn to benthic feeding as they mature, zooplankton remains an important part of their diet [90,91]. The same applies to northern pike and zander, which are generalists but also feed on cyclopoid copepods and cladocerans [92,93]. Based on their occurrence in Kopački Rit, we may assume the predatory influence of these species on the investigated zooplankton community.

Species diversity and turnover were relatively high in the zooplankton community during the study period, as expected, since the turnover rate is indicated to represent community dynamics, and a flux in species richness is characteristic of hydrologically connected systems. The introduction of species and their disappearance in floodplain areas is influenced by hydrology, and the disappearance of a species at a certain time does not mean that the species has disappeared forever; it may reappear later under certain environmental conditions. Turnover enhances the functional complementarity and resilience of the ecosystem, especially under fluctuating environmental conditions. Greater prey diversity can increase resistance to predation by increasing the likelihood of including inedible species and reducing the efficiency of specialized predators faced with diverse prey [94]. In predators, altered diversity can have a cascading effect on lower-trophic-level biomass, but the strength of this interaction is complex and depends on prey behavior and the proportion of omnivores. Although rotifers dominated the entire community in our study, all foraging niches were present at all times and were closely related to prey biomass. The results of our study show that in the warmer months, zooplankton biomass is significantly related to that of phytoplankton, suggesting that it is an important food source, while in the colder months of the year, the zooplankton community shifts to species that feed on other food sources (bacterivores and omnivores), as has been observed in other studies as well [95]. The functional composition of phytoplankton in floodplains is increasingly dominated by taxa with traits that are favorable under climate-altered conditions: buoyancy regulation, nitrogen fixation, tolerance to low light or high temperatures and resistance to grazing [96]. The increasing dominance of these groups often occurs at the expense of diatoms and other groups that depend on turbulent, nutrient-rich conditions. In our study, the number of zooplankton species fluctuated, with rotifers dominating in terms of species and individual abundance, as also reported elsewhere [63,65]. In general, the highest values occurred in years with increased hydrological fluctuations. Interestingly, zooplankton biomass decreased in years with high species abundance, indicating the introduction of smaller species.

5. Conclusions

Our decade-long study emphasizes that the zooplankton community in dynamic floodplain systems responds to hydrological changes and fluctuations environmental parameters. Although the occurrence of extreme floods increased during the decade under study, we found no statistically significant changes in the water level of the Danube. Water temperature was a significant parameter influencing zooplankton development throughout the year, with concentrations of organic N and TN and conductivity values also being particularly important for zooplankton development, but in the warmer months. Higher nitrogen levels were associated with lower zooplankton biomass and a greater dominance of rotifers, except in years when cyanobacteria were more abundant. In general, a trend towards a significant decrease in zooplankton biomass with a simultaneous increase in the number of species was observed. The strong correlation between zooplankton and the changing environmental parameters emphasizes the importance of integrated management approaches for the conservation of biodiversity and the functioning of ecosystem services under future climate scenarios.