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Article

Spatio-Temporal Distribution of Limnomysis benedeni Czerniavsky, 1882 and Other Invaders in the Danube Floodplain Kopački Rit Nature Park, Croatia

by
Viktorija Ergović
1,
Miran Koh
1,
Natalija Vučković
2,
Mario Rumišek
2,
Dubravka Čerba
3,
Barbara Vlaičević
1 and
Zlatko Mihaljević
2,*
1
Department of Biology, University of Osijek, Cara Hadrijana 8/a, 31000 Osijek, Croatia
2
Department of Biology, Faculty of Science University of Zagreb, Horvatovac 102a, 10000 Zagreb, Croatia
3
National Water Reference Laboratory of Slovakia, Department of Assessment and Aquatic Ecosystems Research, Water Research Institute, Nábr. arm. gen. L. Svobodu 5, 81249 Bratislava, Slovakia
*
Author to whom correspondence should be addressed.
Diversity 2025, 17(9), 630; https://doi.org/10.3390/d17090630
Submission received: 31 July 2025 / Revised: 26 August 2025 / Accepted: 4 September 2025 / Published: 8 September 2025
(This article belongs to the Special Issue Wetland Biodiversity and Ecosystem Conservation)
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Abstract

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.

1. Introduction

With a course length of around 140 km, the Danube is an important transport route in Croatia. It forms the border between Croatia and Serbia along almost the entire course, from the village of Batina upstream (1425 rkm) to the town of Ilok downstream (1298 rkm) [1]. However, this advantage of being a transport route also carries the risk of transferring allochthonous species, many of which become invasive in their new environment [2]. The main characteristics of invasiveness, such as a high reproductive rate of species, dominance over habitat, predation, and the spread of disease within the native community [3] emphasize the urgent need to address this issue.
A large number of allochthonous macroinvertebrate species that are widely distributed in European freshwaters belong to crustaceans, which are represented with 30 allochthonous species from class Malacostraca–superorder orders Amphipoda, Mysida, and Isopoda (21, 5, and 4 species, respectively) [4]. The brackish-water fauna of the Ponto-Caspian region has shown remarkable success in expanding its range northwards and westwards through the use of canals, rivers, or human transport. It is estimated that 40 different species have spread from the Ponto-Caspian region across Europe [5]. As many as 70% of the invasive species reported in inland waters of North America originate from this region [6,7]. On both continents, these invasions have been shown to have detrimental ecological and economic implications; in some cases, the newcomers altered entire populations and took over food chains [7,8,9].
The species Limnomysis benedeni Czerniavsky, 1882, commonly known as Caspian slender shrimp, Danube mysid or opossum shrimp, is the only described species of the genus Limnomysis from the family Mysidae. The natural habitat of this mysid shrimp is the shallow waters of the Ponto-Caspian region. It is found in the littoral zone of shallow, still waters at depths from 0.5 to 1 m [10,11]. It prefers habitats with densely developed submerged and emergent macrophytes or filamentous algae and wood debris. However, it is also frequently found on sheltered shores, in sandy sediment or in empty mussel shells [11,12,13]. Such habitat conditions also characterize the Kopački Rit floodplain [14,15].
L. benedeni is an active feeder during the day [12]. Its diet is diverse and consists of small organic particles, phytoplankton, epiphytes, detritus, and biofilm [11,16], and, therefore, it is considered omnivorous species [17,18]. This mysid crustacean plays an important role in the diet of omnivorous and carnivorous fish [10,19], which further emphasizes their ecological importance. L. benedeni is considered an invasive species because it can rapidly establish dense populations in newly colonized habitats, often outcompeting native macroinvertebrates (through abundance or biomass, alteration of habitat structure, disruption of food webs). Its ability to tolerate a wide range of environmental conditions [20,21] and to spread via interconnected waterways further increases its invasive potential [22].
Together with several other crustacean species, this mysid shrimp was cultivated in the former Soviet Union to be used as supplementary food in fisheries. Until the mid-1930s, there was no significant spread from the autochthonous area [23]. There are several theories about the possible routes of spread. One theory is that they traveled upstream by ship from the Danube Delta towards the North and Baltic Seas and then spread freely across rivers and lakes in Western Europe [12,22,24]. In the 1950s and 1960s, some specimens were introduced into Lake Balaton in Hungary [25,26], into reservoirs along the Dnieper in Ukraine and Belarus [27], and into the Kaunas Reservoir in Lithuania [28,29] as fish food. From the genome analysis of individuals, Audzijonyte et al. [30] concluded that there are at least three waves of spread across Western Europe via the Danube River. All three waves were triggered by human activities [12,24,31]. The first wave of spread started with the introduction of individuals into Lake Balaton, from where they spread via waterways to Slovakia, Austria [32], and Germany [22]. The population to which the individuals found in Kopački Rit belong has yet to be investigated, but, according to the aforementioned study of fragments of the mitochondrial COI gene, the population from the Danube sampled in the town of Vukovar (in the immediate vicinity of the sampling area) has lineages consistent with individuals from the Danube and Dniester deltas in NW Black Sea coast [30].
The first studies on the impact of this species on the ecosystem in which it was introduced began in the mid-1980s [8,33,34], leading to the first conclusions about its invasiveness. Each new finding provides valuable data for understanding the distribution and invasive potential of this freshwater invertebrate. In European waters, it is considered a species with a moderately high detrimental impact on the native aquatic community [35].
Its first occurrence in Croatia was recorded in 2004 in Lake Sakadaš in the Kopački Rit Nature Park during a weekly sampling of epiphytic communities on the Eurasian watermilfoil (Myriophyllum spicatum Linnaeus, 1753) and common hornwort (Ceratophyllum demersum Linnaeus, 1753), in the summer months, using a plastic cylinder. Four individuals of L. benedeni were found in the community on C. demersum, and one individual was found on M. spicatum [36]. According to Lindén [37], M. spicatum and C. demersum act as a kind of repellent for the population of Mysidae species by secreting toxins. Bogut et al. [36] therefore concluded that a larger population could be expected in association with other macrophytes in Lake Sakadaš; however, no conclusion was drawn about the population size of L. benedeni. After recording the first occurrence, monitoring of the macrozoobenthos in Kopački Rit continued through various projects; however, there were no new occurrences recorded until 2020. The lack of detections in this period could be related to the low population density, the limited spatial coverage of the sampling area, or the possibility that the sampling methods used were not optimal for the detection of L. benedeni.
This study focuses on the distribution and population dynamics as well as the definition of the environmental conditions under which the invasive shrimp L. benedeni is present within Kopački Rit Nature Park floodplain. By linking these patterns to physical and chemical parameters, this research contributes to the assessments of habitat suitability that are of direct relevance to the management of invasive species in the wider Danube basin. The objective is also to identify the route of species introduction into the Kopački Rit area and assess its distribution, estimate the size of the population and determine the limits of its spread within the protected area. This research is part of a broader initiative that aims to manage and protect the biodiversity of the Kopački Rit Nature Park.

2. Materials and Methods

2.1. Study Area

The study area is located in the Kopački Rit, a floodplain created by the interaction of two large rivers, Drava and the Danube (Figure 1). Due to its exceptional natural value, it was included in the Ramsar List of Wetlands of International Importance in 1993. It consists of two permanent lakes, Lake Sakadaš and Kopačko lake, which are connected by the Čonakut channel. Kopačko lake is the largest lake in the floodplain with a depth ranging from 1.5 to 5 m, depending on the water level of the Danube, and surface area ranging from 2 to 2.5 km2 [38]. The floodplain is an area of high biological productivity, and the flood intensity in the park depends on the water level of the Danube. According to Schwarz [14], floods are classified into three categories: at the Danube water levels between 3 and 3.5 m, the water begins to flow into the Kopački Rit; floods of medium intensity are at water levels between 3.5 and 4 m; and, above a water level of 4 m, at the village of Batina, the largest area of the park is underwater, and seasonal flooding occurs. The Hulovo channel, Vemeljski Dunavac, and Petreš are the main channels that are connecting the Danube with the Kopački Rit floodplain. Kopački Rit acts as a biodiversity hotspot, especially for invertebrate species [39,40]. The hydrological diversity of the park is reflected in the variety of water bodies that have formed in the floodplain, creating a mosaic of habitats that support high species richness [41,42]. However, the same hydrological connectivity, combined with anthropogenic pressures such as river regulation and nutrient input [43], facilitates the introduction and spread of invasive species, that alter native community structures and ecosystem functioning [44]. The anthropogenic impacts in the Kopački Rit floodplain are relatively high. Hydrotechnical activities, especially in the last century, such as the construction of embankments for flood protection [45], have significantly reduced the area of natural habitats. The largest embankment has divided the floodplain so that one part is in a near-natural state and the other part is disconnected from the Danube and dependent on precipitation [46]. In addition, three pumping stations, controlled by an extensive channel network, manage the water regime by diverting water for agricultural use. These interventions have changed the hydrological regime in the floodplain and affected its natural habitats [38,46]. In this study, the samples were taken in both sampling areas, the near-natural sites (sites with minimal human impact and where the water supply is dependent on the Danube water level), and the anthropogenically disturbed sites (e.g., dams and channels built by human intervention where the water supply is dependent on rainfall and agricultural runoff) (Table 1).

2.2. Sampling

This study was conducted from July 2020 to July 2023. Macroinvertebrates were sampled seasonally, while environmental parameters were determined monthly at 14 sampling sites within the park and at one site in the Danube, next to the Petreš channel, which marks the entrance from the Danube to the Kopački Rit area (Table 1, Figure 1). Macroinvertebrate sampling and the collection of environmental parameters of water were usually carried out within the same week. Invertebrate samples were collected at all sites except lakes following the protocols [47]. The sampling method for lakes (Lake Sakadaš and Kopačko lake) is described in detail in Pozojević et al. [48]. Each composite sample consisted of subsamples corresponding to the proportion of different microhabitat types at each site. Sampling was carried out using a hand net with a frame diameter of 25 cm × 25 cm and a mesh size of 500 µm. Each sample was preserved in 96% ethanol. In the laboratory, all macroinvertebrates were isolated and identified. Individuals from the Mysidae family were separated and preserved in 70% ethanol. Isolation and identification were performed using a binocular microscope (Motic SMZ–171 (Motic, Hong Kong)). Each mysid shrimp was photographed with a Moticam 5 MP camera (Motic, Hong Kong) and identified according to the identification key of Eggers et al. [49].
The water level of the Danube indicates the water level in the floodplain. Due to the severe drought in 2022, no samples were taken at the Četverokut channel site during this period. Neither sampling was carried out during the flood phase, and the first sampling date was three weeks after the flood.

2.3. Environmental Conditions

Environmental parameters were measured in situ (water temperature and pH) at the time of invertebrate sampling using an HQ30d multimeter (Hach, Germany), and water transparency was assessed using the Secchi disk. In addition, surface water samples were taken monthly for laboratory analysis of the chemical properties of water and analyzed at the Croatian Institute of Public Health (chemical and biological oxygen demand, alkalinity and water hardness, chlorophyll a, chloride, silicate, ammonium, nitrite, nitrate, total nitrogen, total phosphorous, orthophosphate, total organic carbon, arsenic, chromium, copper and zinc content) using standard analytical procedures [50]. The water level data were obtained by Hrvatske vode, a legal entity for water management in Croatia, at the Batina measuring point.

2.4. Data Analysis

Data analyses were performed using Primer 6.0 [51], CANOCO 4.5 [52], and PAST 4.13 [53]. Both biological and environmental data were log-transformed, and environmental data were normalized prior to analysis. Bray–Curtis similarity was applied to form Resemblance matrices of abundance data, and Euclidean distance was used for the environmental data. Bray–Curtis incorporates species abundance, minimizes artificial similarities, and emphasizes differences between communities. In contrast, Euclidean distance captures absolute differences and reflects straight-line dissimilarities, making it suitable for detecting clear environmental gradients [51]. After transformation of data and creating Resemblance matrices, we applied BEST (Bio-Environmental Similarity Tool) with Spearman’s rank correlation analysis [54] to calculate the best subset of the environmental data that correlates with the abundance data of individuals. The strength of Spearman’s rank correlation and the statistical significance of the observed correlation were validated by a Monte Carlo permutation test with 999 permutations under the full permutation model.
A hierarchical cluster analysis using the group average linkage method [51] was performed to examine the pattern of similarities between samples of crustacean abundance. The method was used to merge the most similar samples by including percentages.
In addition, redundancy analysis (RDA) and distance-based redundancy analysis (dbRDA) were performed to identify the environmental variables explaining the variation in abundance, followed by a Monte Carlo permutation test with 999 permutations under the full permutation model. The collinearity between the environmental variables was considered by applying forward selection in CANOCO 4.5, which ensures that only non-redundant and statistically significant predictors are retained for the RDA [52].
To analyze the statistical differences between the different study sites and between the seasons, a Normality test was performed, followed by a Kruskal–Wallis H-test and a Dunn’s post hoc test, which were performed using the PAST 4.13 software. For all analyses, the significance level was set at p < 0.050 [52].

3. Results

3.1. Alien Aquatic Macroinvertebrates

A total of 21 macroinvertebrate groups (407 taxa) were identified in 176 samples collected during the study. Dipterans were the most diverse group, represented with 20 different families. Macroinvertebrate allochthonous taxa, listed in the European Commission list of invasive species in Europe [55], were present in 96 samples. These were Mollusca: Corbicula fluminea (O.F. Müller, 1774), Dreissena polymorpha (Pallas, 1771), Ferrissia californica (Rowell, 1863), Physella acuta (Draparnaud, 1805); Crustacea: Jaera istri Veuille, 1979, Chelicorophium curvispinum (G.O. Sars, 1895), Dikerogammarus villosus (Sowinsky, 1894), L. benedeni; and Oligochaeta species: Branchiodrilus hortensis (Stephenson, 1910) and Branchiura sowerbyi Beddard, 1892.
The bivalve C. fluminea was present only in the Danube channel Petreš and Vemeljski Dunavac channel during the entire study period. D. polymorpha was found in the first sampling period, in the summer of 2020, at several sites, two studied lakes, and the Kopačevo dam, as well as at the Danube channel Petreš, together with the Vemeljski Dunavac channel. From this season until the end of the research, individuals were only found at the Kopačevo dam, with the exception of the Tikveš dam in the summer of 2021. Most individuals of the species F. californica were found in all sampling seasons at the Kopačevo dam and in the Podunavlje channel, both anthropogenically disturbed sites. P. acuta was present at almost all sites, with the highest abundance at the Kopačevo dam (Supplement Table S1).
The Oligochaeta species B. sowerbyi was found only in the summer period of the first sampling year, at the Hulovo and Semenča channel sites, while B. hortensis had a wide distribution, with most individuals found at the Kopačevo and Tikveš dam sites, in spring and summer rather than in winter (Supplement Table S1).

3.2. Spatial and Seasonal Distribution of Crustaceans

The abundance of allochthonous crustacean species individuals was higher at near-natural sites (with minimal human impact) than at the anthropogenically disturbed sites (with man-made facilities) (Figure 2). The allochthonous species C. curvispinum and D. villosus were detected only once during the winter sampling period in 2022, at the Zlatna Greda dam in the floodplain, while they were present at the Danube channel Petreš and Vemeljski Dunavac channel for almost the entire study period. The species J. istri was only present at the Danube channel Petreš site. Synurella ambulans (F. Müller, 1846) was found sporadically at several sites but only in the spring and summer sampling periods. Niphargus sp. appeared only in spring 2021. Species Asellus aquaticus (Linnaeus, 1758) commonly known as water louse, was present at all sites in the floodplain area except Semenča channel and Kopačko lake sites, and no individuals were found at Vemeljski Dunavac channel and at the Danube channel Petreš sites.
The native species Gammarus fossarum Koch, 1863, commonly known as mountain riverine amphipod, was found only three times, at Šuma Siget and Vemeljski Dunavac channel in winter of the first-year sampling (in small numbers) and at the Četverokut channel during autumn sampling of the following year. Niphargus sp. and G. fossarum are the only crustacean species found in the Četverokut channel.
The results of the hierarchical cluster analysis (Figure 3) revealed three main clusters based on the crustacean abundances, and the sites were grouped on the basis of their similarity. The clusters were defined at approximately 60% similarity (p < 0.050). Notably, the Četverokut channel (sampled in the autumn) was separated from all other sites, indicating a distinct community composition. The observed groupings reflect differences in species composition, which are likely due to habitat characteristics, with a clear distinction between anthropogenically disturbed and near-natural sites.

3.3. Spatial and Seasonal Distribution of Limnomysis benedeni

During sampling at 15 sites in the Kopački Rit Nature Park from July 2020 to July 2023, individuals of L. benedeni were found in 51 samples at 13 sites. Sites where no individuals were found were the Tikveš dam and Četverokut channel (Figure 1).
The site with the highest number of individuals collected per m2 was the Vemeljski Dunavac channel with 741 ind./m2, followed by the Danube channel Petreš with 540 ind./m2. The lowest number of individuals collected was at Kopačko lake, with only two ind./m2. L. benedeni individuals were most frequently found at the Danube channel Petreš and Vemeljski Dunavac channel sites, both with a frequency of occurrence of 11 times, a total of 12 samplings. At the following sites, L. benedeni was sampled only once in different sampling seasons: Čarna channel, Zlatna Greda channel, Zlatna Greda dam, and Podunavlje dam (Table 2).
Division of the sampling sites into near-natural and anthropogenically disturbed revealed that 79% of the sampled individuals of L. benedeni were found in near-natural sites (Figure 2).
In terms of seasonality, the highest abundance was recorded in summer and spring (2116 and 1532 ind./m2, respectively), rather than in autumn and winter (486 and 413 ind./m2, respectively) (Table 2).

3.4. The Relationship Between Environmental Parameters and Crustacean Species

The result of the RDA and Monte Carlo permutation test, habitat type (F = 40.46, p = 0.001), chemical oxygen demand values (F = 8.87, p = 0.001), pH values (F = 8.69, p = 0.011), phosphorus content (F = 3.78, p = 0.016), and total Danube water level (F = 3.61, p = 0.009) were statistically significant environmental parameters for the distribution of crustaceans in the Kopački Rit floodplain. The RDA plot (Figure 4) shows that invasive crustacean species were more abundant at sites with higher total phosphorus content, pH values, and specific habitat type, with the occurrence of D. villosus being most strongly influenced by habitat type.
The dominant substrate type in the Kopački Rit and the most frequently sampled substrate at the sites where L. benedeni was found is argyllal (inorganic particle size of < 6 µm) in combination with CPOM (coarse particulate organic matter), woody debris (dead parts of terrestrial plants), or phytal (algae, mosses, and macrophytes including living parts of terrestrial plants) components. At the study sites Danube channel Petreš, Vemeljski Dunavac channel, and Hulovo channel, the dominant substrates were argyllal and psammal (particle sizes of 6 µm–0.2 cm). The composition of the sampled substrates remained unchanged from year to year.
The highest water level of the Danube measured at the village of Batina was 398 cm at the time of sampling in spring 2023, while the lowest was −18 cm at the time of sampling in autumn 2021. As a result of the Kruskal–Wallis H-test and Dunn’s post hoc test, there was no statistical difference between the study sites in terms of the abundance of Crustacea individuals, nor was there a statistically significant difference between the seasons. The Vemeljski Dunavac channel site differed statistically the most from the other sites (Table 3).
The Spearman correlation of the best subset of environmental data with the abundance of crustaceans yielded a result of 0.524 (p < 0.050), with significant relationships found for habitat type (anthropogenically disturbed or near-natural), pH, total organic carbon (TOC), and nitrate ion (NO3−) content. Together, these environmental variables can explain 99.7% of species abundance (Figure 5).

4. Discussion

All invasive species recorded in the present study originated from the Ponto-Caspian region [56]. L. benedeni was the only crustacean species that could be considered a permanent inhabitant of the studied floodplain, apart from A. aquaticus, a cosmopolitan species [57]. A. aquaticus was the dominant autochthonous species in the entire study.
Alien species become invasive through a combination of biological characteristics, adaptability, and human impact [58]. Although the first record of L. benedeni in the nature park was in 2004, no new data on the populations’ size was available, and the ecological consequences of its invasiveness in Kopački Rit are still unclear. It does not necessarily pose a threat to the autochthonous fauna, but, on the other hand, it can become a major problem. The trophic impact of L. benedeni could be context-dependent. Mesocosm experiments has shown that it significantly suppresses zooplankton biomass in Cladocera-dominated communities, especially by predation on Daphnia sp., while it has little effect on Copepoda [59]. In eutrophic systems, a decrease in zooplankton grazing relaxes top-down control, which typically increases phytoplankton biomass, prolongs the duration of bloom, reduces water clarity, and increases the deposition of labile organic matter that can intensify anoxic conditions [60]. Meanwhile, the introduction of another mysid shrimp, Mysis diluviana, Audzijonyte and Väinölä, 2005 [61], to oligotrophic Flathead Lake restructured the food web by reducing large zooplankton grazers, increasing phytoplankton biomass, and altering energy transfer, which facilitated the spread of non-native lake trout (Salvelinus namaycush (Walbaum, 1792)) while driving the decline of native species such as zooplanktivorous kokanee salmon (Oncorhynchus nerka (Walbaum, 1792)); westslope cutthroat (Oncorhynchus clarkii lewisi (Girard, 1856)); and bull trout (Salvelinus confluentus (Suckley, 1859)) [61].
From a broader ecological perspective, according to Vuić et al. [62], there are 24 fish species from 10 fish families in the floodplain. The species found in that study [62], such as pumpkinseed sunfish (Lepomis gibbosus Linnaeus, 1758), which only occurs in small numbers, and the bleak (Alburnus alburnus Linnaeus, 1758), the bream (Blicca bjoerkna Linnaeus, 1758), and the Prussian carp (Carassius gibelio Bloch, 1782), which occur in large numbers in Kopački Rit, are considered omnivorous and insectivorous fish, and we assume that individuals of L. benedeni could be part of their diet. The dense populations of fish species that may feed on L. benedeni individuals may result in the fish population keeping the mysid shrimp population at a relatively low abundance [63]. Observed ecological interactions with other invertebrates show what a threat this mysid can pose. For example, in the estuaries of rivers in the Mediterranean area, there is potential competition between L. benedeni and species of the genus Diamysis, leading to the displacement of native species [16]. In similar studies in the Rhine, it was observed that many native representatives of the macrozoobenthos disappeared with the appearance of invasive invertebrates from the Crustacea subphylum, including L. benedeni [64].
The Kopački Rit floodplain is a well-studied protected area [15,40,62,65,66,67,68], which has been partly shaped by dams and embankments built for flood protection in the last century. Despite these modifications, part of the Kopački Rit is still connected to the Danube, and regular spring floods occur due to the snowmelt [44,69]. During floods, riverine species have open passages to enter the protected area. Some of these species are alien species that have the potential to spread throughout the nature park via channels. The bivalve D. polymorpha has been present in the Čonakut and Hulovo channels since 2003 continuously to the present day [36,70,71]. The diatom Didymosphenia geminata (Lyngbye) Mart.Schmidt, 1899, was first observed in 2006 during a major flood in the Kopački Rit. It is an oligotrophic species that is sometimes known to inhabit meso- and eutrophic lowland rivers and reaches the Kopački Rit also via the Danube [72,73]. The invasive cyanobacterium Raphidiopsis raciborskii (Wołoszyńska, 1912) Aguilera et al., 2018 (synonym for Cylindrospermopsis raciborskii (Wołoszyńska) Seenayya and Subba Raju, 1972) [74], which has been present since 2003, is associated with the global increase in water temperature in lowland lakes, including Lake Sakadaš [75].
It is well known that, when alien species invade a new area, they disturb the substrate. By actively moving, they disturb the upper part of the sandy sediment and make the substrate unfavorable for the native fauna [76]. A high tolerance of alien species to changes in climatic conditions is crucial for their ability to alter the community of native species composition and reduce biodiversity [3]. Their ability to survive in new environments often has a significant negative impact on the environment, biodiversity loss, and the economy, as they disrupt regional ecosystems and displace native species [77,78]. But not every alien species is invasive. High reproduction rates, the absence of natural predators, efficient dispersal mechanisms, competition for available habitats, and even the transmission of diseases to native species [79] are characteristics of invasive species. Some invasive species, such as freshwater gastropods, can serve as intermediate hosts for various parasitic larvae that can infect fish and cause economic problems [80]. Others, such as C. fluminea and D. polymorpha, can even cause problems by contaminating the water channels of factories and power plants with their overgrowth which has a significant impact on factory revenues [81]. To control and mitigate the impact of invasive species, prevention and control systems need to be improved. In the European Union, EUR 12.5 billion are spent annually to reduce the damage caused by alien plant and animal species [82].
Four pumping stations were built in the study area as a protection against flooding. The purpose of all stations is to pump water from the Kopački Rit into channels in the surrounding area, and only one of them can work into two ways (pumping water in the Kopački Rit area and from the Kopački Rit area into the Danube). All four pumping stations were sampled during this study, and only one of them (Tikveš dam) was found to be free of L. benedeni. According to Hrvatske vode, a legal entity for water management, only the pumping station at the Kopačevo dam was opened and closed several times during the sampling period. This information is valuable for the clarification of the spread and definition of the influence of L. benedeni in this area. From this information, we can conclude that invasive species L. benedeni is present in all areas of Kopački Rit, with the exception of Tikveš. The Tikveš dam is located in the area supplied with water that must first pass-through previous pumping stations (either from the north or from the south), and none of the pumping stations were open during the sampling period. L. benedeni is known to spread throughout the Danube [12,22,24] and has entered the floodplain in this way [36]. However, no correlation with the water level of the Danube was found, but the water level was a statistically significant parameter, leading us to assume that L. benedeni originated from the Danube and became a permanent inhabitant of the floodplain, invading deeper and deeper into the protected area together with other allochthonous species. Furthermore, L. benedeni was detected at most sites and during the entire sampling period, making it the only crustacean species that can be considered a permanent inhabitant of the Kopački Rit floodplain.
The sporadic occurrence of the taxa S. ambulans and Niphargus sp., only during the spring and summer, is most likely related to the increased water level of the Danube and the subsequent flooding. These species are known as inhabitants of both the groundwater and surface water; therefore, it is likely that shifts in water level are the reason of their presence in the surface water. The native species G. fossarum was found only on one sampling occasion in the two near-natural sites, at the Četverokut channel and Semenča channel sites. The Četverokut channel was the shallowest site in the entire study, and it dried out due to a severe drought in the floodplain.
Isopod (J. istri) and amphipod species (D. villosus and C. curvispinum) co-occurred with the mysid L. benedeni in the Danube, channel Petreš and the Vemeljski Dunavac channel. These two main channels supply floodplain with water from the Danube, and these species are known to inhabit the Danube littoral zone [83,84].
The correlation between high phosphorus content and invasive crustacean species can be explained by the fact that invasive species thrive better under nutrient-rich conditions, especially Bivalvia species [85,86]. The area surrounding the nature park is partly covered by agricultural fields and drainage channels. The influence of drainage channels on the nutrient intake in the nearest lakes or rivers has been well studied [38,87,88]. Most of the nutrient input in floodplain lakes is due to drainage from agricultural fields, which is why lakes are in a eutrophic state [89]. The RDA plot shows that the invasive crustacean species clustered in the same direction, with habitat type and pH, as well as with increase in phosphorus levels, indicating a positive correlation with nutrient enrichment and specific habitat and chemical conditions. Furthermore, the results of hierarchical clustering supported the hypothesis that invasive species in the area of the floodplain came from the Danube, and in the floodplain are clustered depending on the type of habitats, anthropogenically disturbed or near-natural which is connected with nutrient content. This suggests that such environmental gradients facilitate the establishment and spread of invasive taxa, possibly due to their ecological flexibility and tolerance to eutrophic habitats [42]. According to Vilenica et al. [68] anthropogenically disturbed sites are sites with more densely developed macrophytes compared to near-natural habitats with no or only scarcely developed vegetation. This finding indicates that the presence of vegetation is not necessarily a habitat preference for Mysidae species in new environments.
The seasonal dynamics of Oligochaeta species can be attributed to several ecological factors. Temperature appears to be a key driver, as both B. sowerbyi and B. hortensis are warm-water adapted species that exhibit accelerated growth and reproduction at elevated temperatures [90,91]. This explains their higher abundance in spring and summer and their near absence in the colder winter months. In addition to thermal dependence, organic matter availability also has a strong influence on their distribution. Oligochaetes typically thrive in fine sediments enriched with detritus, and, in this study, the dominant substrates were argyllal in combination with coarse organic matter, conditions that favor deposit-feeding taxa [91]. Increased nutrient concentrations, particularly phosphorus and total organic carbon, likely increased food availability and promoted higher densities at dam sites where organic accumulation is greater. Despite its tolerance to low-oxygen conditions, B. sowerbyi was only detected during the first summer sampling at two sites, suggesting that extreme hypoxia during stratification or decomposition events may have limited its persistence. Hydrological connectivity also plays a role, as channels such as Hulovo and Semenča are directly connected to the Danube and provide pathways for dispersal during high-water periods. However, fluctuating hydrological regimes in subsequent years may have restricted the colonization and prevented B. sowerbyi from maintaining stable populations [91].
In the Croatian sections of the Drava river, which is a major tributary of the Danube in Croatia, 10 invasive species were recorded, while, in the Danube, there are 13 invasive species, with the most numerous being the amphipods D. villosus and C. curvispinum and the most widespread Mollusca being D. polymorpha [44,71,92]. These species have been recorded in the current research at the site Danube channel Petreš. In the study by Žganec et al. [44], which was carried out along the course of the Sava River from Slovenia to Serbia at 61 sites, the occurrence of L. benedeni was found in the middle and lower sections of the Sava River, along with a recorded presence in the Danube in the city of Vukovar [30]. Due to its invasive potential, species L. benedeni can significantly alter the structure of native zooplankton communities, and mysid introductions can affect entire aquatic ecosystems [17].

5. Conclusions

The presence of allochthonous species in the Kopački Rit Nature Park is inevitable due to the Danube regime, and the continued presence of some species indicates successful integration in local communities. This study provides new data on the distribution and populations’ size of the allochthonous species L. benedeni in the Kopački Rit floodplain, in the eastern part of Croatia. The presence of the species is not seasonally influenced, and we can conclude that this invasive shrimp is now a permanent resident of the Kopački Rit Nature Park. The new data obtained from this study indicate the need for future monitoring of the macrozoobenthos diversity and abundance in order to determine the interactions between L. benedeni and native fauna, as well as potential for this species to alter food webs and ecosystem health and functioning in this Danube floodplain.

Supplementary Materials

The following supporting information can be downloaded at https://www.mdpi.com/article/10.3390/d17090630/s1, Table S1: List of allochthonous species in the floodplain Kopački Rit recorded in research.

Author Contributions

Conceptualization, Z.M. and D.Č.; methodology and investigation, V.E., M.K., N.V., M.R., D.Č., B.V. and Z.M.; data curation, V.E., M.K., N.V. and M.R.; writing—original draft preparation, V.E. and Z.M.; writing—review and editing, V.E., M.K., N.V., M.R., D.Č., B.V. and Z.M.; project administration, Z.M. and D.Č. All authors have read and agreed to the published version of the manuscript.

Funding

This study was part of a project called “NATURAVITA—Monitoring of water status, groundwater, recent sedimentation, habitats and fauna; Monitoring of ecological status of surface waters and monitoring of additional biological indicators” funded by Hrvatske vode, a legal entity for water management in Croatia; project number: 22-163/20.

Data Availability Statement

Data are available upon request from the authors.

Acknowledgments

The research was funded by the European Structural and Investment Funds, as part of the Croatian Waters monitoring project and systematic testing of biological quality elements under the Naturavita project in Kopački Rit Nature Park. The authors would like to thank the Croatian Institute of Public Health for providing data on chemical water analysis. Special thanks to colleagues from the Public Institution Kopački Rit Nature Park and the Faculty of Science, Department of Biology, University of Zagreb, for their help in field and laboratory work.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Research area, Kopački Rit Nature Park (locations within the park where Limnomysis benedeni was found are marked in red, and locations where L. benedeni is absent are marked in black). Sampling sites: 1—Tikveš dam; 2—Šuma Siget; 3—Čarna channel; 4—Podunavlje channel; 5—Podunavlje dam; 6—Kopačevo dam; 7—Zlatna Greda dam; 8—Semenča channel; 9—Vemeljski Dunavac channel; 10—Lake Sakadaš; 11—Danube, channel Petreš; 12—Kopačko lake; 13—Hulovo channel; 14—Četverokut channel; and 15—Zlatna Greda channel.
Figure 1. Research area, Kopački Rit Nature Park (locations within the park where Limnomysis benedeni was found are marked in red, and locations where L. benedeni is absent are marked in black). Sampling sites: 1—Tikveš dam; 2—Šuma Siget; 3—Čarna channel; 4—Podunavlje channel; 5—Podunavlje dam; 6—Kopačevo dam; 7—Zlatna Greda dam; 8—Semenča channel; 9—Vemeljski Dunavac channel; 10—Lake Sakadaš; 11—Danube, channel Petreš; 12—Kopačko lake; 13—Hulovo channel; 14—Četverokut channel; and 15—Zlatna Greda channel.
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Figure 2. The abundance of recorded crustacean species expressed as a percentage ratio of two types of sites, near-natural and anthropogenically disturbed.
Figure 2. The abundance of recorded crustacean species expressed as a percentage ratio of two types of sites, near-natural and anthropogenically disturbed.
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Figure 3. Hierarchical cluster analysis of the group average of crustacean species abundance in two types of sites. Abbreviations: AD—anthropogenically disturbed sites, NN—near-natural sites; site ID given in Table 1.
Figure 3. Hierarchical cluster analysis of the group average of crustacean species abundance in two types of sites. Abbreviations: AD—anthropogenically disturbed sites, NN—near-natural sites; site ID given in Table 1.
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Figure 4. RDA plot indicating statistically significant environmental variables for Crustacea species. Abbreviations: wl—water level of Danube (cm), P—phosphorus content (mg P/L), type—habitat type (anthropogenically disturbed (AD) and near-natural (NN)), and COD—chemical oxygen demand content (mg O2/L).
Figure 4. RDA plot indicating statistically significant environmental variables for Crustacea species. Abbreviations: wl—water level of Danube (cm), P—phosphorus content (mg P/L), type—habitat type (anthropogenically disturbed (AD) and near-natural (NN)), and COD—chemical oxygen demand content (mg O2/L).
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Figure 5. dbRDA of the most strongly correlated environmental variables with the abundance of Crustacea in the entire study. Abbreviations: TOC—total organic carbon, NO3−—nitrate ion concentration, type—habitat categories, AD—anthropogenically disturbed sites, and NN—near-natural sites.
Figure 5. dbRDA of the most strongly correlated environmental variables with the abundance of Crustacea in the entire study. Abbreviations: TOC—total organic carbon, NO3−—nitrate ion concentration, type—habitat categories, AD—anthropogenically disturbed sites, and NN—near-natural sites.
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Table 1. Geographical location of the sampling sites in the Kopački Rit Nature Park, using HTRS 96 coordinates.
Table 1. Geographical location of the sampling sites in the Kopački Rit Nature Park, using HTRS 96 coordinates.
SiteSite NumberSite IDLongitudeLatitude
Anthropogenically disturbed sites
Tikveš dam1Td683237.315061340.25
Šuma Siget2SS678706.315063622.38
Čarna channel3Cch682218.065067439.25
Podunavlje channel4Pch680277.185060124.63
Podunavlje dam5Pd682411.945057101.75
Kopačevo dam6Kd679203.195054567.25
Zlatna Greda dam7ZGd685015.695065153.63
Near-natural sites
Semenča channel8Sch685830.065059549.09
Vemeljski Dunavac channel9VDch683915.425061136.08
Lake Sakadaš10Sl679480.155054685.72
Danube, channel Petreš11DPch690628.755059687.00
Kopačko lake12Kl683582.605054400.12
Hulovo channel13Hch682918.695053705.88
Četverokut channel14Ctch682516.565056586.69
Zlatna Greda channel15ZGch685373.145064995.20
Table 2. Number of Limnomysis benedeni individuals at the sampling sites in Kopački Rit during different seasons (numbers represent the number of individuals per square meter of area). Abbreviations of the sites are listed in Table 1: AD—anthropogenically disturbed sites, NN—near-natural sites. *—Sampling was not conducted.
Table 2. Number of Limnomysis benedeni individuals at the sampling sites in Kopački Rit during different seasons (numbers represent the number of individuals per square meter of area). Abbreviations of the sites are listed in Table 1: AD—anthropogenically disturbed sites, NN—near-natural sites. *—Sampling was not conducted.
Site/SeasonSummer AutumnWinterSpringSummerAutumnWinterSpringSummerAutumnWinterSpring
AD2020202120222023
Td000000000000
SS00101900000000
Cch0003800000000
Pch0001727002600013
Pd0000000028000
Kd250394013000380064
ZGd00000024000000
NN
Sch00024200000000
VDch741102162402032226464580179
Sl8233130000000332
DPch520627443234610138288111324
Kl402000000000
Hch088164870000000
Ctch000000****00
ZGch0001900000000
Table 3. Values of Kruskal–Wallis H test (H-value) and Dunn’s post hoc test among groups of different sites and groups of different seasons. Abbreviation: df—degrees of freedom; p-value—statistical significance.
Table 3. Values of Kruskal–Wallis H test (H-value) and Dunn’s post hoc test among groups of different sites and groups of different seasons. Abbreviation: df—degrees of freedom; p-value—statistical significance.
H-Valuedfp-ValueDunn’s Post Hoc Test
sampled sites9.919140.517Sch ≠ VDch; Kl ≠ VDch
sampled seasons0.25730.968not significant
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Ergović, V.; Koh, M.; Vučković, N.; Rumišek, M.; Čerba, D.; Vlaičević, B.; Mihaljević, Z. Spatio-Temporal Distribution of Limnomysis benedeni Czerniavsky, 1882 and Other Invaders in the Danube Floodplain Kopački Rit Nature Park, Croatia. Diversity 2025, 17, 630. https://doi.org/10.3390/d17090630

AMA Style

Ergović V, Koh M, Vučković N, Rumišek M, Čerba D, Vlaičević B, Mihaljević Z. Spatio-Temporal Distribution of Limnomysis benedeni Czerniavsky, 1882 and Other Invaders in the Danube Floodplain Kopački Rit Nature Park, Croatia. Diversity. 2025; 17(9):630. https://doi.org/10.3390/d17090630

Chicago/Turabian Style

Ergović, Viktorija, Miran Koh, Natalija Vučković, Mario Rumišek, Dubravka Čerba, Barbara Vlaičević, and Zlatko Mihaljević. 2025. "Spatio-Temporal Distribution of Limnomysis benedeni Czerniavsky, 1882 and Other Invaders in the Danube Floodplain Kopački Rit Nature Park, Croatia" Diversity 17, no. 9: 630. https://doi.org/10.3390/d17090630

APA Style

Ergović, V., Koh, M., Vučković, N., Rumišek, M., Čerba, D., Vlaičević, B., & Mihaljević, Z. (2025). Spatio-Temporal Distribution of Limnomysis benedeni Czerniavsky, 1882 and Other Invaders in the Danube Floodplain Kopački Rit Nature Park, Croatia. Diversity, 17(9), 630. https://doi.org/10.3390/d17090630

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