Introduction

The term “meiofauna” defines small aquatic invertebrates living in and on substrates of marine and freshwater environments. They are defined by their size, as they can pass a sieve mesh size of 500 µm but are retained by a sieve mesh size of 44 µm (Giere, 2009). Thus, meiofauna is constituted of small insect larvae, oligochaetes, mites, copepods, tardigrades, rotifers and nematodes, among others. In streams and lakes up to 82% and 92%, respectively, and in marine habitats over 99% of the benthic metazoans are represented by the meiofauna, occurring in densities of over 1 million individuals per m2 (Gerlach, 1971; Robertson et al., 2000; Schmid-Araya et al., 2002; Tod & Schmid-Araya, 2009; Majdi et al., 2017; Traunspurger et al., 2019). Nematodes are frequently the most dominant taxa in freshwater meiofauna communities (Traunspurger, 2000; Majdi et al., 2017; Traunspurger et al., 2019). Thus, it is not surprising that these small organisms represent an important component of benthic food webs and provide a standing stock of food for larger organisms like insect larvae, flatworms, crustaceans and even juvenile fish and are subject to strong top-down pressure (Gee, 1989; Coull, 1990; Ptatscheck et al., 2020; Ptatscheck, 2021).

The ability of fish to retain meiofauna (Spieth et al., 2011) is dependent on the morphology of their mouth cavity and the structure of their branchial basket (Sibbing, 1988). Especially within bottom-feeding fish the gill rakers form very fine filters with meshes < 30 µm as shown for young carps (Cyprinus carpio Linnaeus, 1758) and gudgeon [Gobio gobio (Linnaeus, 1758)], while in selective-feeding fish these morphological structures are less pronounced (summarized in Ptatscheck, 2021). With increasing body size the mesh size of their gill rakers increases, which subsequently reduces their efficiency in filtering meiofauna (summarized in Ptatscheck et al., 2020). This is why the ingestion of meiofaunal organisms is mainly attributed to juvenile fish while for larger individuals the feeding on macrofaunal organisms like larger insect larvae or oligochaetes becomes presumably more important.

Predation by fish can significantly reduce meiofaunal abundance and biomass as well as alter their species composition (Spieth et al., 2011; Weber & Traunspurger, 2014b; Weber et al., 2018), but Weber & Traunspurger (2015) reported that after fish were removed, the former community structure was restored within a few weeks. Other studies on the recolonization of azoic sediments in streams or in tidal zones showed similar or even faster effects and the authors highlighted the dispersal of meiofaunal taxa by water drift as a key factor in this effective succession (e.g., Williams & Hynes, 1976; Cross & Curran, 2004; Smith & Brown, 2006). Although meiofaunal organisms are capable of actively moving through the sediment or even swimming, passive dispersal mechanisms like water drift seem to be the most common and effective in aquatic environments (Palmer & Gust, 1985; Palmer, 1988a; Ptatscheck & Traunspurger, 2020).

Dispersal is an essential life history trait that enables organisms to avoid unfavorable environmental conditions like competition, predation, dryness, or lack of food but also enables the establishment of populations in new or undisturbed habitats (Bonte & Dahirel, 2017). Therefore, this process affects both the stability of communities and biodiversity (Leibold et al., 2004; Howeth & Leibold, 2010; Matthiessen et al., 2010). However, dispersal includes not only the actual transmission of organisms between sites, but also their emigration from a source habitat and subsequent immigration (Ronce, 2007). Palmer (1988a) concluded three ways meiofaunal organisms enter the water column: active emergence, passive erosion, or disturbance-induced suspension. Factors that may trigger an active emigration include predation, competition, and disturbance (Ptatscheck & Traunspurger, 2020; Kreuzinger-Janik et al., 2022). It has been shown that even less mobile taxa like nematodes can actively enter the water column (Palmer, 1988a; Ullberg & Ólafsson, 2003a; Peters et al., 2007; Thomas & Lana, 2011), but do so much less frequently than well-swimming taxa such as crustaceans (Armonies, 1988a). However, active dispersal is likely to be slow and inefficient in covering long distances when physical forces are absent (Ullberg & Ólafsson, 2003a) and instead most meiofaunal organisms rely on passive mechanism for their dispersal (Thomas & Lana, 2011).

Passive emigration mechanisms include a strong flow velocity causing sheer stress and erosion of meiofaunal organisms from the upper substrate (Eckman, 1983; Palmer, 1986). Low water flow of 9–12 cm/s is sufficient to wash out meiofaunal organisms, with even lower thresholds of < 3 cm/s having been documented for nematodes (Armonies, 1988b). These figures are below the critical erosion threshold for the sediment, as shown by (Palmer, 1992) and (Thomas & Lana, 2011). Further, dispersal can also be induced by biological disturbances like bioturbation (Palmer, 1988a). Therefore, in addition to their top-down effect on benthic communities, the feeding behaviour of bottom-feeding fish, which causes mechanical disturbance of the sediment, may potentially be an important driver for the dispersal of meiofauna by initiating their entry into the water column. Palmer (1988b) already pointed out that the number of drifting meiofaunal organisms in an artificial flume increased when fish (Leiostomus xanthurus Lacepède, 1802) were present. In contrast, Ullberg & Ólafsson (2003a), who also investigated the colonization phase, found that the presence of amphipods had no effect on the active dispersal of nematodes. To the best of our knowledge, only these two studies have focused on the influence of biological perturbations as triggers for meiofaunal water column dispersal. Accordingly, information is lacking that would allow to quantify this effect, understand its impacts on the benthic community, and compare these dispersal effects to other biotic influences such as predation.

In our study we investigated the impact of the armored catfish Corydoras aeneus (Gill, 1858) on a meiofauna community in two-chamber mesocosms without the possibility of sediment migration and in the absence of water flow. It has already been demonstrated in microcosm studies that C. aeneus exerts strong top-down pressure on nematodes in the sediment (Majdi et al., 2018). Therefore, we hypothesized that the feeding activity of C. aeneus (H1) would reduce meiofauna abundance through predation as well as (H2) result in an intense dispersal of suspended organisms from the disturbed area. Furthermore, while larger taxa like oligochaetes or dipteran larvae are more likely to be retained by the branchial basket of C. aeneus and as their threshold for being transported into the water column by physical forces is higher, we hypothesized that (H3) this dispersal mode would mainly affect small meiofaunal taxa with low biomass (e.g., rotifers or nematodes). In the case of nematodes, we expected (H4) that primarily larger individuals would be consumed by the armored catfish, while smaller specimens would be dispersed, leading to a change in the species composition.

Material and methods

Experimental organisms

Juvenile C. aeneus were obtained from a local aquarium dealer (Steinhagen, Germany), acclimated, and maintained following Jenkins et al. (2014). Eight individuals were accommodated for two weeks in a 125 l aquarium (50 cm × 50 cm × 50 cm), equipped with azoic stream sand (autoclaved at 121 °C for 10 min) from the headwater of the Ems River (Germany), several clay tubes serving as retreats, a filter and an aerating system. Fish were kept at a water temperature of 20 °C and a light:dark cycle of 12:12 h. They were fed with TetraWafer Mix (TETRA, Melle, Germany) and living chironomid larvae collected from ponds on the area of the University of Bielefeld, but were starved for 24 h before the experiment.

Sediment with benthic invertebrates was collected in November 2021 at the Johannis stream (Bielefeld, Germany). To concentrate the density of the organisms, sediment and water were transferred to buckets and mixed, the supernatant filtered through a 10 µm sieve and the retained sediment collected in a sediment container. Additionally, the meiofauna of moss (mites, tardigrades, rotifers, and nematodes, collected on the area of the University of Bielefeld) was extracted using Baermann funnels. This was done to increase the abundance of larger nematode taxa, as we aimed at investigating the importance of size in the dispersal process. The sediment and the organisms in the moss extract were aerated and acclimatized for 6 days at the same temperature and light–dark cycle as the fish.

Experimental setup

The experiments were performed in ten 26 l aquariums (30 cm × 19 cm × 18 cm), each divided into a source chamber (SC) and a dispersal chamber (DC) by a barrier (Fig. 1). The bottom of the barrier was made of a 3 cm high plastic strip (0.5 cm thick) attached with aquarium silicone to prevent migration of meiofauna through the sediment. The remaining barrier consisted of an open metal grid with a grid size of 1 cm, which allowed meiofauna to disperse through the water column but prevented fish from passing. All aquariums were filled in a random order with 300 ml of sediment in each chamber, resulting in a sediment height of approximately 1 cm. The SCs received the living stream sediment, while the DCs received azoic sediment as a refugium and to prevent passive redispersion by water movement upon arrival in the DC. Afterwards, the aquariums were filled with water from the fish aquarium. To minimize the turbulence of the sediment and the meiofauna contained in the SCs, the water was slowly poured into the aquariums along the back wall of the DCs, creating a light trickle. Finally, one single fish (3–3.5 cm body length) was placed in 5 of the aquariums, respectively, leaving the remaining 5 aquariums as the control group. This experimental design allowed us to study the undisturbed sediment (control SC + DC) as well as the effects of predation ((control SC + DC) – (fish SC + DC)) and dispersal (control SC + SC – fish DC) on the meiofaunal organisms.

Fig. 1
figure 1

Experimental setup. Fish were placed in the source chamber with living sediment. After 24 h the sediment of both chambers and the water column were sampled

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Sampling and sample processing

After 24 h, the fish were carefully removed from the aquaria with a net and the water of each aquarium was aspirated with a hose to the rim of the plastic strip. Afterwards, the sediment was removed from each individual chamber with a hose while rinsing the bottom several times. The water column samples and the sediment were sieved (10 µm) and retained particles as well as organisms flushed into PE bottles. All sediment samples were subjected to density centrifugation (Ludox TM 50, Sigma–Aldrich, Munich, Germany; 1.14 g/ml, mesh size = 10 µm) to extract the organisms from the sediment, according to Pfannkuche & Thiel (1988). These samples were stained with Rose Bengal (AppliChem, Darmstadt, Germany) and preserved in 37% formaldehyde (final concentration: 4%).

Organisms (nematodes, rotifers, tardigrades, hydracarina, copepods, dipterans and oligochaetes) were counted in petri dishes with the aid of a Leica L2 stereomicroscope (× 40 magnification; Leica). Additionally, nematodes were assigned to specific size classes by measuring their lengths (< 0.5 mm, 0.5–1 mm, 1–1.5 mm, > 1.5 mm). 60 individuals per sample, if available, were prepared afterwards (Seinhorst, 1959, 1962) and identified to the species level using a Leica Dialux microscope (× 1250 magnification) as well as the classification criteria of Andrássy (2005, 2007, 2009) and Loof et al. (1999, 2001).

Statistics

The statistical evaluation was carried out with R (Version 4.0.3). To detect a significant influence of C. aeneus on the dispersal and composition of organisms, a Wilcoxon-test or a Kruskal–Wallis rank-sum test and subsequent pairwise Wilcoxon rank-sum tests for multiple groups were used. If equal distribution (Shapiro–Wilk-test) and homogeneity of variance (Levene-test) were met, an ANOVA with pairwise T test was performed. A bonferroni correction was applied for the Tukey’s HSD post-hoc tests.

Since different individual numbers of nematodes were determined at species/genus level in the different replicates, a rarefaction analysis was performed using the Package iNEXT to estimate the species richness for 60 individuals.

Differences in the nematode species composition in different chambers were investigated using an analysis of similarity (ANOSIM). The resulting R values (0-1) revealed differences in the nematode species composition, with larger R values suggesting separation between groups. In addition, the percentage dissimilarity was determined using an analysis of similarity percentages (SIMPER). Nematode composition was clustered using non-metric multi-dimensional scaling (nMDS). All techniques were based on a Bray–Curtis similarity using untransformed data. The analyses were carried out using the Primer 7 (PRIMER-E Ltd., 2015) software package.

Results

Organismal composition and predation effects

On average 955 (± 152, SD) individuals were collected from the sediment of the control aquariums (SC + DC). This benthic community was dominated by nematodes (69%), rotifers (11%), oligochaetes (7%) and tardigrades (6%), while copepods, dipterans (90% chironomids) and hydracarina accounted for < 3% each (Table 1). Fish had a significant impact on the total meiofaunal abundances in the SCs, reducing it by 62% (Fig. 2A). Looking at the individual taxa, abundances of nematodes, tardigrades, oligochaetes, copepods and hydracarina were significantly reduced by 41% to 74%. A comparison between the control aquariums and the fish aquariums (SC + DC) revealed the impact of predation. Except for rotifers, there was a trend towards a decrease in the number of individuals in the fish treatments for each taxon, but according to a Wilcoxon-test this was only significant for copepods (P = 0.034) and oligochaetes (P = 0.012). Overall, the meiofauna was reduced by 37% due to predation (P = 0.019; Wilcoxon-test).

Table 1 Percentage of the collected benthic invertebrates and their abundances (mean ± SD, n = 5) in the source chambers (SC) and dispersal chambers (DC) of the control and fish treatment
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Fig. 2
figure 2

Mean percentage change of the meiofaunal taxa (± SD, n = 5) after 24 h in (A) SCs of the fish treatments (fish SC/(control SC + DC)). To show statistical differences, the abundances (fish vs. control) were tested. (B) Grey columns show the percentage change by predation ((fish SC + DC)/(control SC + DC)), while white columns represent the change by dispersal effects (control SC + DC − fish DC/(control SC + DC)). n.s.  not significant, *P < 0.05, **P < 0.01 (Wilcoxon-test)

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Dispersal of benthic taxa

In both treatments only singly individuals of all seven taxa were found in the water column, accounting for < 0.05% of their respective density in the sediment (SC + DC). In the control treatment, no rotifers, tardigrades or dipterans were found in the DC after 24 h whereas in the fish treatment all taxa were recovered in the DCs after 24 h (Table 1). 50.4% (± 18.7%, mean ± SD) of the total nematodes, 49.2% of total rotifers and 39.7% of the total tardigrades (± 20.3%, mean ± SD) were found here (Fig. 3). In the oligochaetes and dipterans, we observed the lowest transfer into the DCs (< 5% of the individuals), which was not significant in both taxa. Along with the copepods, these taxa were not significantly transferred to the DCs of the fish treatment, whereas a significant effect was observed for all other taxa (Fig. 3). Finally, for copepods and oligochaetes, predation had a significantly greater impact on abundance than distribution (Fig. 2B). The initial populations of nematodes, tardigrades, dipterans and hydracarina were equally affected by predation and dispersal effects.

Fig. 3
figure 3

Mean percentage (± SD, n = 5) of meiofauna taxa found after 24 h in the DCs of the control (grey) and fish (white) treatments. n.s. not significant, *P < 0.05, **P < 0.01 (Wilcoxon-test)

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Effects on nematode composition

The abundance of nematodes with body lengths < 1 mm was significantly reduced by emigration from the SC of the fish treatment, however, this effect could not be detected for larger size classes (Fig. 4). Conversely, a significant top-down effect was only detectable in nematodes with a body length of 1 mm or more. 66% of the nematodes in the DC of the control belonged to the size class < 0.5 mm, which was a significantly higher proportion than in all other chambers or treatments (P < 0.05, pairwise Wilcoxon-tests).

Fig. 4
figure 4

Mean abundance (± SD, n = 5) of nematodes collected in the control (SC + DC) (black), in control minus predation (grey) and in the control minus dispersal (white), classified by size class. An ANAVO (DF = 2, F < 12.2, P < 0.002 for all tested groups) and pairwise T tests were used to analyse statistical differences between the data within a size class. For the post-hoc test, the significance levels are given above the columns: n.s.  not significant, *P < 0.05, **P < 0.01

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In total, 961 nematodes were identified and assigned to 58 species (see Online Resource). The number of species in the SCs was not affected by the activity of C. aeneus, however, the composition of species was affected by both predation and induced emigration when compared to the control. The nMDS plot showed a clear clustering of the three groups (Fig. 5). Moreover, the results of the ANOSIM showed a significant differentiation between these clusters (Table 2).

Fig. 5
figure 5

nMDS plot (stress 0.18) of nematode species composition in the control and fish group (SC + DC) as well as in the SCs of the fish treatment. The Bray–Curtis similarity was calculated without transformation.

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Table 2 Comparisons of the nematode species in the control and fish group (SC + DC) as well as in the SCs of the fish treatment
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Discussion

Our results demonstrate that the feeding behaviour of bottom-feeding fish has a major impact on benthic invertebrates, through predation but especially through physical disturbance. The latter results in an intense emigration of meiofaunal organisms across the water column, which can reduce the abundance of benthic organisms to the same extent as top-down effects.

The activity of the armored catfish was evident after 24 h from a strong turbidity of the water in the fish treatments, while the water in the control aquariums was clear. Consistent with H1, Corydoras aeneus reduced the meiofaunal abundances by predation, especially those of oligochaetes and copepods. In a laboratory study conducted by Majdi et al. (2018), who examined the predation on nematodes specifically by C. aeneus, just one juvenile catfish with a body length of 2–3 cm reduced the nematode density in the sediment by up to 50% and consumed up to 75,000 worms within 24 h. Unlike in our study, only nematodes were offered as a food source in a very high density (500 ind. cm−2 vs. 2 ind. cm−2 in our study) and coarse, defaunated sand without organic components was used as sediment. Accordingly, the variety of prey taxa was larger in our experiment but finding and retaining them was more difficult for the fish due to the fine sediment texture. We could only confirm a similar effect for larger nematodes (> 1 mm), which made up 37% of the nematode community. Additionally, in line with the study of Cook (1962), our investigation showed a strong top-down effect of C. aeneus on oligochaetes and microcrustaceans, which was also observed for other bottom-feeding fish like juvenile carps or gudgeons (reviewed by Ptatscheck et al., 2020). In contrast to these studies, dipteran larvae abundance was not reduced, possibly due to their low abundances in the aquariums (one larvae per 10 cm2). Furthermore, a predation of rotifers could not be shown by neither our study nor by various other studies that were carried out under more natural conditions (reviewed by Ptatscheck et al., 2020). These differing intensities of predation on each taxon can be attributed to both their size and nutritional value for juvenile C. aeneus. Previous studies have shown that an important reason for selective-feeding on meiofauna is prey size, as larger individuals may be easier to be found in the sediment and retained in the fish’s gill rakers (Ptatscheck et al., 2020). In addition, meiofaunal organisms, and in particular copepods, have generally been determined a highly nutritious food source for juvenile organisms due to their high amounts of amino acids and polyunsaturated fatty acids (Coull, 1990; Chesney, 2005; Ptatscheck et al., 2020). As shown in marine studies, the daily nutrient requirement of young fish can already be met with numbers of up to 7000 nematodes and 750 copepods, which can be contained within a few 10 cm2 plots (Ptatscheck et al., 2020; Feller & Coull, 1995; Street et al., 1998). Whilst the prey selectivity of C. aeneus cannot be determined without knowing the exact encounter rates between the fish and different prey, the presented findings suggest that the catfish were able to select their food items as oligochaetes and copepods were the only taxa that were significantly reduced by predation, despite not being the most abundant in this experiment.

When comparing our results with other studies, it must be noted that the starting abundances of meiofauna and specifically nematodes in our experiment were lower than the usually documented numbers, which may be due to the season in which the experiment was conducted as well as the experimental setup. As the meiofauna was collected from a stream in winter and afterwards acclimated at 20 °C, the individuals were subject to a strong increase in temperature, presumably leading to a higher mortality rate than usual in laboratory experiments. Thus, higher numbers of meiofauna are expected when conducting the experiment in summer, during which abundances are generally higher, and especially when performed as a field study.

A main goal of this study was to gain a better understanding of the influence of bottom-feeding fish on the dispersal of meiofauna. A look at the fish treatments reveals a large number of benthic organisms in the DCs. This concerns the rotifers, tardigrades, copepods and hydracarina with a maximum body length of ≤ 1 mm as well as nematodes in the same size range, whose proportion in the DCs of the fish treatment accounts for at least 28% of the total frequency (SC + DC). It can be assumed that these taxa were suspended by mechanical disturbance of the sediment as already pointed out by Palmer (1988b). Although an active emigration from the sediment cannot be ruled out as studies on nematodes demonstrate that certain environmental stimuli (e.g., food resources or the presence of predators) can trigger active emigration behaviour within the substrate (Traunspurger et al., 2006; Kreuzinger-Janik et al., 2022) and into the water column (Jensen, 1981), the few organisms in the DCs of the control treatments clearly indicate that C. aeneus is the general trigger for the dispersal, consistent with our initial hypothesis (H2). While Armonies (1988a) showed that without such stimuli copepods still actively migrate from the sediment into the water column in lentic environments, nematodes and oligochaetes do not, making it more likely that the organisms in the control aquariums were accidentally stirred up in the SCs and entered the DCs during the experimental setup. Since predominantly small nematodes and oligochaetes (< 0.5 mm) were found in the DCs of the controls, this assumption is supported as small nematodes sink more slowly than larger individuals (Crofton, 1966; Ullberg & Ólafsson, 2003b). As initially expected (H3), oligochaetes and dipteran larvae with main body lengths of > 1 mm did not get into the DCs of the fish treatments in large numbers. Both taxa can move directionally in the water column (Drewes & Fourtner, 1993; Brackenbury, 2000), but possibly due to their comparatively high biomass, they were not whirled up far enough by the fish to cross the barrier between the chambers, quickly sank or swam back to the sediment of the SCs.

The results of the nematode evaluation also correspond with these size-dependent distribution patterns and our assumption H4, since nematodes < 1 mm were primarily dispersed, and larger individuals rather reduced by predation as also pointed out by Weber & Traunspurger (2014a, 2014b, 2015). Thus, when looking at the impact on meiofauna communities, fish-induced disturbance leads to a decrease of individuals across all size classes in the disturbed area and an increase of small individuals in adjacent areas. Despite these changes in the size composition of communities, impact due to prey size selection does not necessarily affect the species diversity (Ptatscheck, 2021). At the end of the experiment, the predominant species were reduced the strongest in the SC’s due to predation and dispersal, increasing the relative abundance of less frequent species and thereby facilitating species coexistence (Weber & Traunspurger, 2015). Additionally, mainly the most frequent/abundant nematode species were found in the DC’s, leading to a change in their density composition as few species dominated. Comparable top-down effects have already been shown for the feeding behaviour of other fish species (Weber & Traunspurger, 2014b, 2015, 2016).

Surprisingly, despite this intensive transfer of benthic organisms in the fish treatments, we only found few individuals in the water column at the end of the experiment. This may be due to the continuous, but low transfer combined with a rapid sinking of the organisms into the sediment. On the other hand, certain activity periods of the fish may also have an influence. Corydoras catfish are diurnal, but their main phase of activity is at dusk (Paxton, 1997) and thus more than eight hours before the end of the experiment. In comparison, Palmer (1988b), who collected the drifting organisms with nets (63 µm) over a period of only 10 h, detected higher densities of nematodes (~ 140 ind.) and copepods (~ 490 ind.) in the water column.

What can we deduce from these results? Emigration triggered by mechanical disturbance of C. aeneus has a similarly large effect on the meiofaunal community as predation. The impact on individual taxa varies, with nematodes tending to emigrate more, while in oligochaetes and copepods the top-down effect was significantly greater than the dispersal effect. Numerous gut analyses of fish as well as laboratory investigations in closed systems (e.g., aquaria) and thus without the possibility of emigration have clearly shown that bottom-feeding fish regulate meiofaunal communities by predation. As our results show, the reduction of the local benthic community through a transfer into the water column also plays an important role in shaping meiofaunal communities. This fact must be taken into account in field studies (e.g., in enclosures), as a reduction of sediment-living organisms in the presence of fish may not necessarily be due to trophic interactions. It can therefore be assumed that the top-down effect in such investigations is overestimated. Our study shows that both factors can have a comparable impact. Especially under natural conditions the influence of these effects can be expected to be much greater than we have shown, given that fish such as armored catfish are often found in groups (Cook, 1962) and larger organisms such as rays (Thrush et al., 1991), walruses and whales (Nelson & Johnson, 1987) also cause strong physical disturbances to benthal, making a large number of meiofaunal organisms available for such a dispersal process. This effect may lead to another important ecological implication as the fish-induced dispersal also affects the meiofaunal community in adjacent, undisturbed habitats by large numbers of immigrating individuals. It is already known from earlier studies that immigration via the water column has a major influence on the composition of organisms in benthic systems and that habitats can thus be colonized within a few days or weeks (e.g., Widborn, 1983; Colangelo & Ceccherelli, 1994; Smith & Brown, 2006; Peters et al., 2007). The most important factor in this context is certainly the water flow, which determines the direction and distance of dispersal (Palmer, 1992). The success of such a colonization is also determined by the local environmental conditions and priority effects (Ptatscheck & Gansfort, 2021). Depending on whether arriving individuals can establish themselves in the new habitat, their immigration may increase the local species richness or, due to enhanced biotic interactions like predation and competition for resources, decrease local species richness over time. Finally, however, it is beyond the scope of our investigation to make a clear statement about this as information and available data on fish-induced colonization is lacking and should therefore be investigated in future studies. Field studies certainly represent a particular scientific challenge but should provide new insights into the effects of fish-induced meiofaunal dispersal on benthic communities, considering the environmental factors mentioned above.

In conclusion, the armored catfish C. aeneus has a massive influence on meiofaunal organisms through consumption and due to disturbance-induced emigration. The strength of the respective effect is taxon-specific but it leads to a reduced abundance in the affected area for most of the organism groups examined. Additionally, in the case of nematodes, the activity of the fish did not lead to a reduction in the number of species, but to a change in both size and species composition, with smaller taxa primarily being dispersed and larger taxa primarily being preyed on. Mainly the most frequent nematode species were affected, leading to more balanced species densities in disturbed communities. The increased transfer across the water column leads us to assume that not only the local meiofauna is affected by bottom-feeding fish, but also adjacent benthic communities by migrating organisms, which should be investigated in further studies.