Introduction

Research on zooplankton during cold and unvegetated periods has mostly been conducted on high altitude mountain lakes (Manca & DeMott, 2009; Obertegger et al., 2022) or high latitude permanently ice-covered Arctic and Antarctic lakes (Iakovenko et al., 2015; Dvoretsky & Dvoretsky, 2021; Shmakova et al., 2021). Recently, more literature data has shown that winter-targeted studies have become more frequent and they have even spread to the temperate climate (Kalinowska & Grabowska, 2016; Kalinowska et al., 2023; Socha et al., 2023). However, most zooplankton research in the northern temperate zone excludes the winter season (Kuczyńska‐Kippen, 2014; Špoljar et al., 2016; Pardo et al., 2023). This is probably owing to the difficult conditions for field work, the impossibility of examination due to the complete freezing of some water bodies to the bottom or the assumption that the winter is not a limnologically significant period since biological processes are suppressed. Climate warming is expected to affect the ecological state of lakes (Merz et al., 2023), especially in winter conditions where its consequences are evidenced in a shortening of the winter period, higher average temperatures, shortening of ice cover duration, thinner ice cover or even its absence during the winter (Virro et al. 2009; Woolway et al., 2020). Climate-related changes, both direct, which are a result of climatic factors such as temperature increase, altered precipitation or extreme weather events, and indirect, which are secondary effects caused by climate factors, influence biotic interactions, community assemblage, life cycles, feeding patterns, winter survival and behaviour (Jeppesen et al., 2010; Tian & Benton, 2020). Comparing different regions within Europe in the latitudinal aspect it may be expected that some pronounced effects are visible in the case of temperate regions where generally a great gradient of environmental features exists in reference to the cold and warm seasons (Gołdyn & Kowalczewska-Madura, 2008; Kim et al., 2020; Shchapov et al., 2021). It is well known that climate and hydrological conditions differently affect lake communities in warmer, subtropical, tropical and Mediterranean regions, compared to colder, temperate parts of North Europe and America (Meerhoff et al., 2007; Moustaka‐Gouni et al., 2014; Maberly et al., 2020).

Apart from higher annual water temperatures than in temperate lakes, Mediterranean shallow water bodies also have significant seasonal water level fluctuations (Beklioglu et al., 2007). An extensive study of over 80 European lakes from North Sweden to Spain showed an increase in the fish:zooplankton biomass ratio, and a decrease in the zooplankton:phytoplankton biomass ratio in the same direction (Jeppesen et al., 2010). In the mesocosm study conducted by Meerhoff et al., (2007) a considerably higher abundance of larger-bodied taxa (frequently including Ceriodaphnia and Daphnia spp.) occurred in temperate (207 ± 39 ind l−1) in comparison to subtropical (10 ± 2 ind l−1) lakes. Small-bodied cladoceran taxa (e.g. Diaphanosoma 12 ± 2 ind l−1) characterised the open water of subtropical lakes (Meerhoff et al., 2007). The same pattern was noted by Lacerot et al. (2010) in the established mesocosm set. Higher temperatures in the Mediterranean climate (annual average around 15 °C, in the temperate climate around 10 °C) enhance the spawning of omnivorous and benthivorous fish which could significantly affect large-bodied zooplankton. Namely, according to Fernando (1994), domination of small-bodied zooplankters in subtropical and tropical lakes can be explained, besides differences in predation, by the physiologically better adaptation of small individuals to higher temperatures.

In winter, resource control of zooplankton is usually assumed to be more important than predator control due to lower irradiance and lower food concentrations (Sommer et al., 1986), which in conjunction with lower temperatures may lead to prolonged development time, postponed reproduction, higher longevity, diminished grazing rates, reduced turnover rate and maximum growth (Adrian & Deneke, 1996; Zhang et al., 2022). Turbid lakes often experience relatively improved light conditions, affecting their water transparency during the ice cover period compared to the rest of the year because particles settle out under significantly reduced turbulence. In a comparative study of numerous shallow lakes in Denmark, Jeppesen et al. (2004) found that the size of cladocerans and Daphnia did not differ between summer and winter, except in lakes with a high coverage of macrophytes during the summer. In these lakes size was smaller in winter at low macrophyte coverage than in summer when the macrophytes provided a refuge. This suggests that predation rates are consistently high in both summer and winter, with even higher rates during winter in lakes that are dominated by macrophytes in the summer.

Latitudinal variation in freshwater ecosystems, and especially in shallow water bodies, has rarely been studied (e.g. Hessen et al., 2006). As the longitudinal distance between two groups of water bodies is reflected by different climate conditions, particularly referring to a ca. 3 °C lower mean annual temperature in the northern region (Poland) compared to the southern region (Croatia), it was expected that the functioning of shallow water bodies, including their rotifer communities, will differ too. Water bodies rarely freeze in Croatia during the winter season and the ice cover extends only for a short time (Branković et al., 2009). Contrariwise, water bodies in Poland have ice cover practically every year (Błażejczyk, 2006). We have, therefore, hypothesized that the latitudinal variation of climate conditions in Poland (long winters with ice cover and shorter vegetative period) and Croatia (long summer period with higher temperatures and resulting longer vegetative period) will have an impact on prevailing environmental conditions and will thus determine the dissimilarity of rotifer communities. Since no distinct patterns with respect to zooplankton ecological and trophic functional traits indices were observed (Kuczyńska-Kippen et al., 2020) we decided that only the taxonomic approach, referring to certain species, would be a sensitive tool.

To minimize the effect of extreme conditions that are usually attributed to the summer and winter seasons, e.g. lowering of the water level and/or freezing to the bottom, we chose for our examination two seasons: 1/ spring—characterized by the post-winter lack of vegetation, and autumn—with the after-summer decaying mode of vegetation. The present study, aimed at a comparison of 10 shallow water bodies located in two parts of Europe, differing with respect to the presence (Poland) or absence (Croatia) of ice cover, will help us to answer the following questions (Q) and verify the corresponding hypotheses (H): Q1/Do environmental factors reveal similarity between the shallow water bodies located in Poland and Croatia in the case of the spring and autumn season? (H1: During the spring and autumn seasons, shallow water bodies in Poland and Croatia will exhibit environmental similarities despite differences in ice cover); Q2/ Will the main triggers of rotifer structure differ during the transitional periods (spring and autumn) in the latitudinal aspect? (H2: The primary drivers of rotifer community composition will vary between the transitional periods (spring and autumn) when considered from a latitudinal perspective); Q3/Does the species composition of rotifers change with latitudes? (H3: The species composition of rotifers is expected to exhibit significant changes with varying latitudes); Q4/Do rotifer communities exhibit distinct characteristics during the two transitional seasons (spring and autumn)? (H4: Rotifer communities will display distinctive characteristics during the two transitional seasons (spring and autumn), indicating seasonal variations in their composition and structure); Q5/What are the main factors from among abiotic and biotic elements that impact the distribution of rotifers between two European regions? (H5: Variations in environmental conditions and ecological interactions significantly shape the distribution of rotifers between two European regions, driven by specific abiotic and biotic factors). Furthermore, the use of such comparative analyses of the responses of certain rotifer species to changing environments will be highly valuable for assessing the trend of environmental changes, especially those caused by climate change.

Method

Study sites

The studied shallow water bodies are located in two parts of Europe differing in yearly average temperature: the northern water bodies in Poland and the southern in Croatia (Fig. 1). Four of the studied water bodies resulted from clay extraction near the city of Poznan (P-GL1, P-GL2, P-GL3, P-GL4), while two of them are situated in an agricultural landscape near the city of Żnin (P-BR1, P-BR2). The research was also carried out in the temperate continental region of Croatia on two oxbows of the Krapina River (C-KR1, C-KR2) and two reservoirs within the Jankovac flow through system in Papuk Nature Park (C-PA1, C-PA2). The distance between the two groups of investigated water bodies amounted to ca. 700 kms. General characteristics of ponds are given in Table 1.

Fig. 1
figure 1

Locality of the studied shallow water bodies in two regions—Poland (Poznań area—GLI1, GLI2, GLI3, GLI4; Żnin area—BRZ1, BRZ2) and Croatia (Zagreb area—KRA1, KRA2; Papuk Nature Park area—PAP1, PAP2)

Full size image
Table 1 Locality, main morphological features, and the range of values of environmental parameters of the investigated water bodies (average and ranges)
Full size table

In each water body the presence of fish was noticed. All ten water bodies were typically shallow and polymictic with differences in the trophic level, physicochemical parameters and macrophyte cover during the vegetated season (Table 1).

Methods

Samples were collected once a month in transition from 2008 to 2009, including winter/spring (PRE: pre-vegetation season—spring; January–March in the case of Croatia and March–April in the case of Poland) and autumn/winter (POST: post-vegetation season—autumn; October–December in the case of Croatia and October–November in the case of Poland). The slight differentiation in the sampling seasons between Croatia and Poland was a result of the ice cover occurring in Poland, which shortens the potential sampling period of spring and autumn.

In total 98 samples of zooplankton were collected, 44 in the case of Croatia and 54 in Poland. Sampling procedures were described in our previous papers (Kuczyńska-Kippen, 2001, 2003; Špoljar et al., 2012a). All zooplankton samples were fixed in 4% formalin and taxonomic identification of rotifers was performed using identification keys (Radwan et al., 2004; Voigt & Koste, 1978). Representatives of Bdelloidea were counted but not identified to particular species. A single category Polyarthra spp. comprised the densities of Polyarthra dolichoptera Idelson and Polyarthra vulgaris Carlin. The dominating species among rotifers and crustaceans in Croatia and Poland were determined as those whose density exceeded 10% of the total abundance.

Environmental analysis included in-situ measurement of basic abiotic features, such as temperature, pH, electrolytic conductivity and dissolved oxygen content. Water transparency was measured in the deepest part of each water body using a Secchi disc. Macrophyte cover was also established by calculating the percentage participation of macrophytes overgrowing the bottom of each water body. The percentage of trees, which appears as a factor—% of trees surrounding a water body, was evaluated at each examination site within the studied water body. Due to the fact that the analysed water bodies were mostly private ponds and we did not obtain permission to catch fish, the stocking level assessment referred to the absence or presence of fish. Chemical analyses and chlorophyll a concentration were conducted in the laboratory. Methods for nutrient content and for chlorophyll a (Chl a) have already been described in the paper by Joniak et al. (2009) and Špoljar et al. (2012b).

In order to identify the pattern of abiotic parameters distribution within all water bodies in Croatia and Poland, both in the spring and autumn, we applied a multivariate analysis of similarities (ANOSIM), which were performed with PRIMER (version 6; PRIMER-E, Plymouth, UK). Moreover, an analysis to discern differences/similarities in rotifer composition, based on abundance, between the two countries and across two distinct seasons was performed. ANOSIM is known to generate a value of R which is scaled to lie between − 1 and + 1, a value of zero indicating no difference among a set of samples (Clarke and Warwick, 2001). In this analysis we interpreted R-values > 0.75 as well separated; R > 0.5 as overlapping, but clearly different and R < 0.25 as barely separable. A SIMPER analysis indicates the contribution (%) of each species between the dissimilarity between two groups, based on the Bray–Curtis dissimilarity matrix (Primer v.6). In this study rotifer composition dissimilarity was analysed during each season (spring and autumn) between two ecoregions (Poland and Croatia).

Prior to the statistical analysis, all environmental and rotifer data were logarithmically transformed [log(x + 1)] (with the exception of pH) and their normality was checked using Shapiro–Wilk’s test. As this test suggested that the data did not follow a normal distribution, even after transformation (P > 0.05), the nonparametric Mann–Whitney U test (the comparison between two independent samples) was used to test differences in rotifer richness and abundance between water bodies in Poland and Croatia in the spring and autumn.

Canonical correspondence analysis (CCA) was applied (Lepš & Šmilauer, 2003) in order to explore the distribution of the rotifer species abundance with the environmental parameters and sampling sites in Poland and Croatia, separative for the pre-vegetated season (spring) and post-vegetated (autumn) period. The CANOCO software program (version 4.5, Biometris, Wageningen, The Netherlands) was used. The Monte Carlo permutation test (499 permutations) was applied to reveal the effect of the obtained explanatory variables. For these analyses we chose species with an overall frequency of equal to or exceeding 30% in the case of Poland. For Croatian water bodies, where generally the species frequency was much lower than in Poland, species that reached at least 15% frequency were chosen. However, additional species that reached a level over 50% in the case of one country (Croatia or Poland) were also included in these analyses.

Results

Environmental factors between northern and southern temperate sub-climates: Poland and Croatia in the spring and autumn seasons

The analysis of similarities (Anosim) of a group of environmental factors, including temperature, dissolved oxygen concentration, conductivity, pH, content of nitrates, phosphates and chlorophyll a, showed a distinctiveness between both countries, particularly in the autumn. However, in the case of the spring season a higher level of similarity between Croatian water bodies and the group of Polish ponds was observed. Contrary to this was a significant variation of environmental factors in the autumn, which resulted in the clear segregation of both groups of water bodies (Fig. 2).

Fig. 2
figure 2

The results of the Anosim analysis illustrating the distribution of particular water bodies based on environmental features of the studied water bodies

Full size image

Rotifer community features between between northern and southern temperate sub-climates: Poland and Croatia in the spring and autumn seasons

There were 118 rotifer taxa identified in total (Poland—90, Croatia—64) in both studied seasons. The rotifer taxocenosis of both countries was characterized by a prevalence of species of the genera Brachionus (11 species), Cephalodella and Lecane (8 species each), Lepadella (7), Euchlanis (6), Synchaeta (5) as well as Colurella, Keratella, Mytilina and Notholca (4 species each).

Out of 88 rotifer taxa identified in total both in Croatia and Poland in the spring there were 20 common species, among which Asplanchna priodonta Gosse, representatives of Bdelloidea, Brachionus angularis Gosse, Filinia longiseta (Ehrenberg), Keratella cochlearis (Gosse), Polyarthra spp. and also Lepadella patella (O.F. Müller) were among the most frequent taxa.

In the case of the autumn season, out of 91 rotifer taxa 23 belonged to the community common for both countries, with A. priodonta, Bdelloidea, B. angularis, K. cochlearis, Lecane closterocerca (Schmarda), Lecane lunaris (Ehrenberg), L. patella, Polyarthra spp. and Synchaeta pectinata Ehrenberg, being among the most frequent taxa.

Looking at the common taxa for spring and autumn in particular shallow water bodies in Poland and Croatia it was noticed that they made up maximally 33% species richness. Almost 2/3 of the taxonomic list was created by species that differed in both seasons (Table 2; App. 1).

Table 2 The level of species number similarities (N species—the number of common species for both seasons; % of species—% participation of species common in both seasons to the total number of species in a certain water body) in both seasons in particular shallow water bodies
Full size table

The mean number of rotifer taxa was significantly higher in Poland compared to Croatia for both the spring (M-W test—Z = 3.31, P < 0.001) and autumn (M-W test—Z = 3.76, P < 0.001) seasons (Fig. 3). In the case of abundance, the differences were not significant (P > 0.05), however, in the spring the abundance of rotifers was higher in Croatia. But generally, for the spring collection rotifer abundances remained at a low level in both countries. In the autumn the abundance was higher, particularly in Poland, where the mean abundance reached a level of 3726 ind l−1 (Fig. 3).

Fig. 3
figure 3

The mean number of rotifer species (N species) and abundance (ind l−1) in Poland and Croatia in the spring (A/B) and autumn (C/D) seasons

Full size image

Distinct rotifer species in shallow water bodies of northern and southern temperate sub-climates: Poland and Croatia

There were some taxa distinct for each country; 28 for Croatia and 54 for Poland (App. 1). Among distinct taxa a number of rare species were noted, both for the Polish fauna [Brachionus falcatus Zacharias, Cephalodella mus Wulfert, Euchlanis contorta (Wulfert), Euchlanis oropha Gosse, Mytilina trigona (Gosse)] and Croatian fauna (Asplanchnopus hyalinus Harring, Epiphanes macroura (Barrois & Daday), Mytilina bicarinata (Perty), Notholoca labis Gosse, Squatinella mutica (Ehrenberg), Squatinella rostrum (Schmarda), Trichocerca cavia (Gosse), Trichocerca musculus (Hauer), Trichocerca relicta Donner).

In the spring, species such as: Cephalodella catellina (O.F. Müller), Filinia terminalis (Plate), Keratella testudo (Ehrenberg) and Synchaeta lakowitziana Lucks, were distinct for Poland and the following species: Synchaeta oblonga Ehrenberg, Cephalodella forficata (Ehrenberg), Mytilina bicarinata (Perty) and Trichocerca porcellus (Gosse) were distinct for Croatia.

In the autumn, however, species such as Brachionus diversicornis (Daday), Keratella tecta (Lauterborn), Pompholyx complanata Gosse, Trichocerca pusilla Lauterborn and Trichocerca similis (Wierzejski) were distinct for Poland, while Ploesoma hudsoni (Imhoff), Trichocerca bicristata (Gosse) and Trichocerca longiseta (Schrank) for Croatia.

Dominating rotifer species in shallow water bodies of northern and southern temperate subclimates: Poland and Croatia

Out of a total of 18 rotifer dominating taxa, only 7 dominated in both countries (Bdelloidea, B. angularis, F. longiseta, K. cochlearis, Keratella quadrata (O.F. Müller), Polyarthra spp., S. pectinata), however, only K. cochlearis and Polyarthra spp. dominated in both countries in both seasons.

There was a group of species that dominated exclusively in Croatia (Colurella uncinata (O.F. Müller), Gastropus stylifer Imhof, L. patella, T. porcellus) or only in Poland (Brachionus quadridentatus (Hermann), C. catellina, Eosphora ehrenbergi Weber, K. tecta, Notholca acuminata (Ehrenberg), Notholca squamula (O.F. Müller), S. lakowitziana).

Keratella cochlearis had the highest frequency; it dominated in over 60% of samples in Croatia and in ca. 30% of samples in Poland (Table 3).

Table 3 Dominating species in Croatia and Poland with the frequency level (in bold frequency level ≥30%) of their dominance in the spring and autumn
Full size table

Dissimilarity of rotifer composition between the seasons in Poland and Croatia was markedly high, and it was more pronounced during spring (83%) compared to autumn (76%). In spring, the higher prevalence of K. cochlearis in Croatia accounted for 15% of the dissimilarity in rotifer composition, and half as many of two species with higher abundance in Poland, S. lakowitziana (8%) and B. quadridentatus (7%). During autumn, the high abundance of K. tecta in Poland contributed to 10% of the dissimilarity in rotifer composition, with a comparable impact of K. cochlearis, which had higher abundance in Croatia (Table 4). Overall, the results of ANOSIM indicated a slight but significant difference in the rotifer composition between Pl S and Cr A (r = 0.31, P = 0.002) (Table 4).

Table 4 Summary of SIMPER analysis (abundance transformation log(x+1)) indicates particular species which contributed mainly to the dissimilarity in rotifer composition across spring (S) and autumn (A) in each ecoregion: Poland (Pl) and Croatia (Cr)
Full size table

Relationship between rotifer species and environmental factors in shallow water bodies of northern and southern temperate sub-climates: Poland and Croatia

In the first CCA analyses conducted regarding data from the spring season a clear separation between Polish and Croatian shallow water bodies was observed. Monte Carlo analyses extracted six environmental factors (depth, % of trees surrounding a water body, conductivity, pH, phosphates, abundance of copepods) to be significantly responsible for the distribution of rotifer species in both regions (Fig. 4, Table 5). The first two CCA axes (CCA1 and CCA2) explained 74.64% of the total variance. Three groups of species were observed. The first group, associated especially with Croatian water bodies, contained taxa of pelagic origin (e.g. A. priodonta, K. cochlearis, K. quadrata, Polyarthra spp., P. complanata), whose occurrence correlated with the depth of water bodies and decreasing levels of PO4 and pH. The second group of species was attributed to Polish water bodies (e.g. B. quadridentatus, K. tecta, N. squamula, S. lakowitziana) and was associated with increasing pH of water and elevated levels of phosphates. At the same time, these species negatively correlated with the depth of water bodies. The third group consisted of species (e.g. Colurella adriatica Ehrenberg, L. patella, Lepadella quadricarinata (Stenroos), N. acuminata) occurring both in Poland and Croatia. These predominately littoral rotifers selectively chose water bodies with high shading caused by surrounding trees and co-occurred with increasing abundance of crustaceans, particularly copepods, irrespective of the country (Fig. 4).

Fig. 4
figure 4

Canonical correspondence analysis (CCA) of zooplankton species (with high frequency) with environmental factors in the spring season in Poland (blue circles) and Croatia (orange circles). Legend—Environmental factors: Depth, Temp—water temperature, O2—Oxygen, Cond—conductivity, pH, SD—water transparency, Chl a—chlorophyll a, NO3—nitrates, PO4—phosphates, M-cover—% of macrophytes at the surface of a water body, Tree—% of trees surrounding a water body; Biotic factors: n Cla—abundance of cladocerans, n Cop – abundance of copepods. Rotifer species: Afis—Anuraeopsis fissa, Apri—Asplanchna priodonta, Bdel—Bdelloidea, Bang—Brachionus angularis, Bqua—Brachionus quadridentatus, Ccat—Cephalodella catellina, Cadr—Colurella adriatica, Flon—Filinia longiseta, Kcoc—Keratella cochlearis, Ktec—Keratella tecta, Kqua—Keratella quadrata, Lclo—Lecane closterocerca, Lpat—Lepadella patella, Lqua—Lepadella quadricarinata, Nacu—Notholca acuminata, Nsqu—Notholca squamula, Pspp—Polyarthra spp., Pcom—Pompholyx complanata, Slak—Synchaeta lakowitziana, Spec—Synchaeta pectinata, Tsim—Trichocerca similis

Full size image
Table 5 Results of Monte Carlo test for the significance of environmental characteristics in explaining variation in the abundance of zooplankton species (Legend as in Fig. RDA). %—% variance explained; significant variables in bold. Legend as in Fig. 4
Full size table

In the second CCA analyses conducted with data from the autumn season there were four environmental factors (depth, conductivity, water transparency, pH) that significantly impacted (Monte Carlo test) the distribution of rotifer species (Fig. 4, Table 5). The first two CCA axes (CCA1 and CCA2) explained 79.35% of the total variance. Rotifer species were divided with respect to the geographic location of the shallow water body and the different responses to environmental characteristics that were observed. A more distinct separation was found in Poland, where both pelagic (e.g. K. tecta, Anuraeopsis fissa (Gosse), F. longiseta, P. complanata) and littoral (e.g. L. closterocerca, L. patella, L. quadricarinata, B. quadridentatus) species were found and they were positively affected by rising pH. These rotifer species chose rather shallower water bodies of low water transparency. In the case of Croatian water bodies only pelagic rotifers were present (e.g. A. priodonta, K. cochlearis, K. quadrata, G. stylifer, B. angularis) and their occurrence was positevely affected by the depth of the water bodies and the rise in water transparency (Fig. 5).

Fig. 5
figure 5

Canonical correspondence analysis (CCA) of zooplankton species (with high frequency) with environmental factors in the autumn season in Poland (blue circles) and Croatia (orange circle). Legend—Environmental factors: Depth, Temp—water temperature, O2—Oxygen, Cond—conductivity, pH, SD—water transparency, Chl a—chlorophyll a, NO3—nitrates, PO4—phosphates, M-cover—% of macrophytes at the surface of a water body, Tree—% of trees surrounding a water body; Biotic factors: n Cla—abundance of cladocerans, n Cop—abundance of copepods. Rotifer species: Afis—Anuraeopsis fissa, Apri—Asplanchna priodonta, Bdel—Bdelloidea, Bang—Brachionus angularis, Bqua—Brachionus quadridentatus, Ccat—Cephalodella catellina, Cadr—Colurella adriatica, Flon—Filinia longiseta, Gsty—Gastropus stylifer, Kcoc—Keratella cochlearis, Ktec—Keratella tecta, Kqua—Keratella quadrata, Lclo—Lecane closterocerca, Lpat—Lepadella patella, Lqua—Lepadella quadricarinata, Nacu—Notholca acuminata, Pspp—Polyarthra spp., Pcom—Pompholyx complanata, Slak—Synchaeta lakowitziana, Spec—Synchaeta pectinata, Tsim—Trichocerca similis

Full size image

Discussion

As we hypothesised, the rotifer communities differed in the latitudinal aspect between shallow water bodies of northern and southern temperate sub-climates i.e. Poland and Croatia, even though we chose two transitional seasons—spring and autumn in order to minimize the effect of more harsh and extreme climate conditions typical for the summer and winter periods. Dissimilarity of rotifer communities was more apparent during the spring, probably due to macrophyte stands enhancing rotifer species diversity (Kuczyńska-Kippen et al., 2020). In autumn, the decay of macrophytes as well as presumably lower fish predation (Špoljar et al., 2012b) may have caused more uniform conditions in both ecoregions, thus equalizing environmental conditions and reducing habitat and species diversity as well as dissimilarity. Moreover, notable variation of abiotic parameters of the studied water bodies between both countries existed. Temperature, oxygen, conductivity, pH, nitrate, phosphate and chlorophyll a revealed higher spring similarity but clear separation between Polish and Croatian water bodies in autumn, partly attributed to trophic conditions. We have observed a higher trophic state, reflected in the increase in chlorophyll a concentration and phosphates, in the case of Polish water bodies in the autumn. Increase in the trophy was also followed by the increase of rotifer abundance, that reached a level of almost 4000 ind l−1 on average, and higher mean number of rotifer taxa. Moreover, the structure of distinct species in Poland, with a large variety of species that are considered to be eutrophy indicators (Karabin, 1985), such as B. diversicornis, K. tecta and T. pusilla, also confirms trophic condition variation in the two water body groups. Many studies show that trophic state can be a very important factor affecting communities of inhabiting organisms, and that rotifer structure can respond to trophic changes very quickly (e.g. Wen et al., 2017; Karpowicz & Ejsmont-Karabin, 2021). But the distinctiveness between both European regions was also underlined by the occurrence of rare species for both Polish and Croatian fauna. Some of the species, e.g. B. falcatus, occurring in Poland may be a symptom of warming climate since this rotifer is warm-stenotherm organism arriving from subtropical and tropical regions (Radwan et al., 2004). We presume that potentially even more such species could be detected in the Polish water bodies if the study were extended to the warm season, when variations in abiotic parameters can be particularly pronounced, and taxonomic diversity is typically more emphasized (Kuczyńska-Kippen et al., 2009; Choi & Kim, 2020). In the summer season warm-stenotherm species would find optimal conditions for development. However, our examination was only restricted to colder seasons, while tropical species would rather occur during the summer period. Generally, the occurrence of rare species contributes greatly to the increase of the overall biodiversity, and shallow water bodies are known to be a source of such unique species (e.g. Pakulnicka et al., 2015; Hill et al., 2021), thus contributing to the high conservation value of this kind of ecosystem (Sahuquillo & Miracle, 2019). The results of our research allow us to conclude that the identification of rare species, which can often be found in the group of distinct species community, may be signals of changes in the environment, such as trophic state or the impact of climate warming.

Despite similar morphometric features (shallow, small, polymictic), fish presence, and macrophyte coverage in water bodies in Croatia and Poland, they differed in environmental conditions, which influenced zooplankton assemblages. Rotifer latitudinal variation may stem from climate differences, especially temperature (over 3 degrees higher on average in Croatia) and ice cover (often freezing the water column in Poland). Southern European water bodies are more susceptible to water level fluctuations and drying out (Beklioglu et al., 2007; Špoljar et al., 2018). Analysing species distinct for each country, some cold stenotherm species (Herzig 1987; Devetter, 2011) were found in the spring season. In Poland F. terminalis and S. lakowitziana, both typical for cold waters were present, while in Croatia S. oblonga, a species preferring cold waters was observed. The prevalence of some rotifer species towards low temperatures has been demonstrated from various aquatic ecosystems. A summer mountain lake study revealed rotifers’ distinct species-specific strategies influenced by food availability, predators, solar radiation and and temperature (Obertegger et al., 2008). Moreover, Diovisalvi et al. (2015), while studying seasonal aspects of shallow lakes of temperate areas, found that rotifer population dynamics are mostly driven by temperature and available food conditions. In our study such cold stenothermal organisms were also noticed among dominating species. Three rotifers, N. acuminata, N. squamula and S. lakowitziana, designated as cold-associated, were in a group of species that dominated exclusively in Poland. This indicates the existence of a temperature gradient that distinguishes two latitudinally separated areas. It also shows that taxonomic-based traits can serve as a sensitive tool for measuring environmental gradients. In a study involving nearly 50 lakes, encompassing both shallow and deep lakes and various trophic-state waters (Karpowicz & Ejsmont-Karabin, 2021) it was suggested that the presence of stenotherm species serves as an excellent indicator of the ecological status of lakes. This phenomenon presumably also applies to small water bodies.

Even though Croatia and Poland are subject to slightly different climate conditions there was a group of species common (> 20% of the taxonomic structure) for both countries. Higher participation of common species was observed in the autumn, which may have been connected with the macrophyte development over the summer period in the water bodies and thus higher participation of littoral species (L. closterocerca, L. lunaris, L. patella) that were observed towards the end of the vegetation season. In the spring, the majority of common taxa were of pelagic origin (A. priodonta, B. angularis, F. longiseta, K. cochlearis, Polyarthra spp.), which indicates a lack of macrophyte cover at the beginning of the vegetative season. All common species found in both Croatia and Poland were cosmopolitan organisms (Radwan et al., 2004; Yin et al., 2018; Stamou et al., 2022). The predominant rotifer genera included Brachionus, Cephalodella, Lecane, Lepadella, Euchlanis, Synchaeta, Colurella, Keratella, Mytilina and Notholca, often found in shallow water bodies with diverse habitats (Meksuwan et al., 2014; Pardo et al., 2023). These habitats can range from open water areas dominated by Brachionus, Synchaeta, Keratella and Notholca, to macrophyte-rich zones supporting littoral-associated rotifers such as Cephalodella, Lecane, Lepadella, Euchlanis, Colurella or Mytilina. Littoral rotifers can persist year-round due to various forms of macrophyte beds that change seasonally (Wu et al., 2021). In spring, dry previous-year plant stems create favourable refuge conditions for rotifers after ice cover melts (Basińska et al., 2014). In autumn, despite a decrease in macrophyte production, remains of macrophytes can still be found, supporting rotifer diversity (Riis et al., 2003; Champion & Tanner, 2000; Hrivnák et al., 2009).

Latitudinal distinctiveness during transitional seasons in shallow water bodies was confirmed in CCA analyses on rotifer species abundance and environmental parameters. Factors such as water depth, conductivity and pH influenced rotifer distribution regardless of the season. In spring, the presence of surrounding trees, PO4 level as well as copepod abundance were also significant, while water transparency mattered in autumn. Species of pelagic origin such as e.g. A. priodonta, K. cochlearis and K. quadrata occurred in accordance with the increasing depth of water bodies in Croatia, irrespective of season. Moreover, in the spring a decreasing level of PO4 affected their distribution, while in the autumn water transparency also had an impact. In the case of Polish water bodies the increase in pH and decrease of depth in water bodies influenced the distribution of both pelagic (e.g. K. tecta) and littoral (e.g. B. quadridentatus) rotifers in both seasons. Furthermore, the increased spring phosphorus concentration and autumn water transparency decline were crucial, affirming the higher trophic state of the studied Polish water bodies. The biotic factors considered in this study did not seem to have a decisive impact on the distribution of rotifers in the latitudinal gradient during the spring and autumn seasons. All water bodies under study had fish, therefore their role in the development of rotifers was alike. Another factor—high habitat heterogeneity of macrophytes—was also of minor impact since we investigated our shallow water bodies during the spring and autumn when aquatic vegetation plays a less important role in shaping the structure of zooplankton. However, in the spring season we observed the evolution of a group of species, with a predomination of littoral rotifers (e.g. C. adriatica, L. patella, L. quadricarinata), who selected water bodies characterized by high shading caused by surrounding trees. These rotifers were associated with the presence of microcrustaceans, particularly copepods, irrespective of the country. Crustaceans coexisting with the littoral rotifers indicate no typical relationship, unlike pelagic rotifers, which can be outcompeted by larger crustaceans both mechanically and exploitatively (Fussmann, 1996; Habdija et al., 2011; Diéguez & Gilbert, 2002).

Conclusions

More pronounced latitudinal variation is expected in summer when abiotic parameters vary further, highlighting taxonomic diversity. Despite limited seasonal data (excluding the summer optimum period), both countries exhibited variable rotifer communities with a substantial presence of rare species in spring and autumn.

Environmental parameter variation was evident in the rotifer community structure, showing a high degree of specificity, contrary to ecological and trophic functional trait indices, which did not reveal any discernible patterns and were not recommended as a sensitive tool for latitudinal-scale analyses in the temperate climate. This clearly indicates that all aspects of the Taxonomic approaches, that focused on dominant species or country-specific species proved to be more responsive in detecting latitudinal specificity. More significant and pronounced latitudinal variation can be expected in the summer season when abiotic parameter variation can be even more striking and when taxonomic diversity is usually much highlighted. Despite conducting our research in only a limited period of season, and in particular omitting the summer optimum period, a variable rotifer community with a high share of rare species in each country was observed in the spring and autumn.

The potential application of such latitudinal comparative studies on the response of particular Rotifera species to different levels of abiotic and biotic features will be important for predicting trends in changing environmental conditions, including those caused by climate change.