Contracaecum larvae are geographically widely distributed, utilizing many animal species as hosts; and fish represent an important paratenic host in their life cycle. Their presence in Prussian carp (Carassius gibelio) was studied in Lake Sakadaš (Croatia) in 2017 and 2018. Two gill nets of different sizes submerged during a 12-h period were used to collect the fish. Contracaecum larvae were recorded in the stomach, slightly coiled or elongated on the intestine serosa or encapsulated in a gut wall of 20 individuals. The effect of Contracaecum sp. on the health of their host was determined by estimating the effect of the parasites’ presence, number, and biomass on fish length, weight, and the Fulton’s condition factor (CF). Infected fish showed negative (b < 3; p < 0.05) and uninfected fish positive allometric growth (b > 3; p < 0.05). Fish weight and CF in infected individuals were significantly low in comparison to the uninfected ones (Mann–Whitney U test: U = 1078.00, U = 423.50, respectively; p < 0.004). These results emphasize the importance of evaluating parasitic nematode presence in economically important fish species. Even more, if this endoparasite has a detectable negative impact on a resilient species such as the Prussian carp, it is important to monitor its occurrence in other fish species.

Different types of water bodies in lowland river floodplains represent vital biodiversity havens and encompass diverse microhabitats, which are essential for structuring different macroinvertebrate communities. Chironomidae larvae (Diptera) are an inseparable part of these communities, with their high richness and abundance. In three water body types within the Danube floodplain Kopački Rit in Croatia, over the course of four sampling campaigns, we recorded 51 chironomid taxa in periphyton on macrophytes, twigs, and glass slides. The most diverse were chironomid communities on macrophytes, whilst month-old periphyton on twigs supported the least taxa. Cricotopus gr. sylvestris, Dicrotendipes lobiger, Dicrotendipes spp., Endochironomus albipennis, Glyptotendipes pallens agg., Polypedilum sordens and Polypedilum spp. were present in all studied microhabitats. The type of substrate is a very important factor influencing Chironomidae diversity and abundance, which was evident in the presence and dominance of Corynoneura gr. scutellata and Monopelopia tenuicalcar in the dense macrophyte canopy epiphyton. Finding pristine floodplains such as Kopački Rit can be very challenging, as such areas are increasingly altered by human activities. Studies of resident species and the extent to which changes in the parent river influence floodplain communities are important for the protection and restoration of the floodplains.

Periphytic ciliate communities of riverine floodplains have hardly been studied, although they play an important role in aquatic food webs and contribute to the overall ecosystem functioning. In this study we analyzed the taxonomic and functional composition and diversity of these communities across all seasons and hydrological phases. The study was conducted in a floodplain lake, a part of the large natural Danube floodplain, from February 2015 to September 2016. We found that higher temperature and hydrologically stable conditions during the lake isolation phase, when a high amount of suspended food is available, result in the highest ciliate abundances and dominance of relatively large suspension-feeding ciliates, mainly peritrichs, which could serve as good bioindicators for detecting disturbances in river-floodplain ecosystems. During the flow pulse phase, associated with lower temperatures, and during the phase of extreme floods, when the availability of suspended food was largely reduced, small surface-feeding ciliates prevailed in the periphyton. Further, while the total ciliate abundance was the lowest, the highest taxonomic and functional diversity was found, especially during an intermediate level of hydrological connectivity (flow pulse). Our results confirm the importance of different levels of hydrological connectivity for maintaining biodiversity in riverine floodplains and add to a growing awareness of the need to preserve the natural hydrological regimes of large rivers.

We investigated the colonization process and community dynamics of periphytic ciliates in a floodplain lake. To elucidate the functional role of ciliates, we investigated variations in their functional traits during colonization. Our research revealed that ciliate colonization process differs significantly between spring and summer, mainly due to differences in water temperature and hydrological regime. The most pronounced influence of temperature-initiated differences was observed during the initial phase of colonization, during which faster colonization with different pioneer species, and more diverse and abundant ciliate community characterized the summer initial phase. Hydrological changes significantly affected periphytic communities during the intermediate and late phases, resulting in different patterns of the colonization process. Mitigation of suspended food particles during flooding periods led to a decrease in ciliate abundance and the dominance of mobile benthivore ciliates (raptorial and filter feeders), whilst during more stable periods of low water, sessile planktivores (filter feeders) that consume suspended particles, dominated. They are considered to be main representatives of periphytic ciliates in floodplain lakes, playing a significant role in the pelagic-benthic coupling. The results of the present study offer new insights into the ecology and functional role of periphytic ciliates in a floodplain ecosystems of temperate regions.

When studying the well-being and fitness of ichthyofauna in a specific area, vital information comes from length–weight relationships (LWRs) and condition factor analyses and calculations. Data were collected from 2015 to 2019 in an area important for fish shelter, feeding, and spawning. Twenty four species belonging to 10 different families were identified from the total 16,895 of caught individuals, with the most abundant species being Blicca bjoerkna. The calculation of LWRs was possible for 19 species, (R² = 0.7049–0.9998 (p < 0.05)), with the values of the mean coefficient b ranging from 2.6831 to 3.5747, indicating an overall positive allometric growth in the fish population. The results of Fulton’s and the relative condition factors showed that a total of eight species were in a relatively good condition, although the species in question varied between the two condition factors. Fulton’s condition factor showed the highest average value for Lepomis gibbosus (1.4956), while Cobitis elongatoides had the lowest (0.4739). Contrarily, the relative condition factor showed the highest average value for Silurus glanis (1.0524), and the lowest for Gymnocephalus cernua (1.0023). The lower values of calculated condition factors in some species could be attributed to sampling being conducted during the colder months of autumn and winter. Future studies should include seasonal sampling in order to achieve a better understanding how various abiotic factors impact the condition factors of these fish species. Nevertheless, this research provided valuable data on the status of the fish population in the floodplain and could be used as a guideline for designing future ichthyofauna studies in this area.

The Danube in Croatia serves as an important transport route but also favors the spread of invasive species, especially in the floodplain areas. Many of them originate from the Ponto-Caspian region and influence European ecosystems with their migrations. One of these species, Limnomysis benedeni, a mysid shrimp, thrives in shallow waters and plays a crucial role as a food source for fish. L. benedeni was first recorded in Croatia in 2004 in Lake Sakadaš (Kopački Rit). Prior to the study on aquatic macroinvertebrates in Kopački Rit Nature Park, conducted from July 2020 to July 2023, there had been no documented records in recent years. Sampling was carried out seasonally for macroinvertebrates and monthly for environmental parameters at 15 sites within the park or in the immediate vicinity. Samples were collected according to standard AQEM methodology. A total of 21 macroinvertebrate groups (407 taxa), out of which the most diverse were Diptera with 20 families, were identified in this study, including nine allochthonous species in addition to L. benedeni. The most abundant populations of L. benedeni were found in the Danube, the Petreš channel, and Vemeljski Dunavac channel, which supply the floodplain with water from the Danube. Most individuals were collected in summer and spring, with the highest density being 741 individuals per square meter. Environmental parameters such as water level, type of habitats, pH values, chemical oxygen demand, and phosphorus content, were statistically significant for the distribution of species. The dominant microhabitat for L. benedeni in Kopački Rit was argyllal in combination with coarse particulate organic matter and wood debris, and the composition of these microhabitats remained consistent throughout the seasons. L. benedeni was the only crustacean species to establish a stable population in the floodplain area, excluding Asellus aquaticus (water louse), a cosmopolitan species. The ongoing influence of L. benedeni on the native community still remains to be determined.