Abstract
Lowland floodplains are complex ecosystems comprised of standing and flowing waters interacting with terrestrial habitats, and the main force creating, shaping and influencing, both habitats and biotic communities, is a hydrological regime and water supply from the parent river. In areas not much altered by anthropogenic influence, the Danube creates such floodplain areas, and temporary shallow water bodies within represent biodiversity important habitats. In the Kopački Rit Nature Park floodplain in Croatia, diversity based on Chironomidae (Diptera) in both benthic and epiphytic communities was studied in eight ponds (temporary shallow water body) and at two channel locations (permanent shallow water body). At each location samples of sediment and macrophytes were taken at three sites. The benthic chironomid community was comprised of 29 taxa, most abundant being representatives of the Chironomus genus and Tanypus kraatzi in ponds, and Polypedilum nubeculosum and Cladotanytarsus sp. in channel samples. Cricotopus gr. sylvestris, Paratanytarsus sp. and Endochironomus tendens were dominant epiphytic chironomids (18 taxa). Non-metric multidimensional scaling and analysis of similarity analyses showed there was a clear grouping of sampling locations based on their position in the park and the distance from each other, more evident in the case of benthic chironomid communities. Furthermore, when the water bodies were compared based on the community structure from different locations and substrates, there was also a statistically significant separation. Community composition indicates high productivity and organic matter production of studied water bodies, but moreover, the differences in substrate preferences evident in 16 common out of 31 recorded chironomid taxa, indicate the importance of habitat complexity preservation in a floodplain.
Introduction
Although river-floodplain ecosystems represent biodiversity hotspots and provide a broad range of ecosystem services, they are one of the most threatened aquatic habitats in Europe and worldwide.1–3 Conservation and restoration of these systems have become primary issues in environmental and water policy, facilitated by the data provided by the local experts. 1 One of the largest and most well-preserved natural floodplains in Europe, situated in the middle section of the Danube River, is Kopački Rit Nature Park in Croatia.4,5
Permanent water bodies (lakes and channels) and some of their biocenoses in the Kopački Rit floodplain have been well-studied6–13; however, shallow water bodies, such as temporary, and just ponds, were not included in previous research conducted in this area, only recently, and just at one location.14,15 Ponds, defined as small, shallow standing waters,16,17 occur naturally in floodplain areas. 18 Temporary ponds are shallow stagnations with annually alternating phases of drying and flooding, supporting the development of vegetation and animal communities.19–21 Ponds often support substantially higher biodiversity than other freshwater habitats including many rare and endangered species, thus contributing significantly to regional biodiversity.18,22,23 They are important habitats for many aquatic organisms, that is, macrophytes, plankton, macroinvertebrates, particularly insects, and fish,24,25 and represent a source of water, food and shelter for many terrestrial organisms.26,27 Therefore, ponds play an important role in the food web and energy flow between aquatic and terrestrial systems. 26 These findings indicate the importance of including small water bodies in the monitoring and conservation strategies,18,22,28–30 therefore research-based data are needed for the future direction of their conservation.
Although small in size, ponds provide many microhabitats suitable for many macroinvertebrate species, whereby microhabitat characteristics affect community composition. 31 Since they provide refuge from predators, a source of food, and influence the oxygen regime in the water, 32 macrophytes represent suitable substrates for organisms33,34 and increase the functional complexity and heterogeneity of aquatic ecosystems.35,36 In addition to epiphyton, periphyton developed on macrophytes, an important pond community is macrozoobenthos, which is sensitive to the changes in water conditions 37 and represents a key element in the aquatic food webs.
Within epiphytic and benthic communities in ponds, larval stages of Diptera are often dominant, 38 especially the Chironomidae family.39,40 Chironomids are good indicators of stable pond conditions and, based on their adaptive life strategy, colonize different habitat types.41,42 Bazzanti et al. 42 have found that the chironomid community differs between temporary and permanent ponds – some taxa are common for both pond types, while other taxa are exclusive to or more abundant in one pond type. Temporary water bodies are very unique and susceptible to immense change. Here, depending on the Danube water level, they can become connected shallow lakes or can dry out completely, which is why organisms in all biotic communities must be very adaptable. Since Chironomidae larvae can represent more than 50% in abundance and biomass in freshwater macroinvertebrate assemblages,14,41 changes in their community structure and composition can influence a cascade of changes in the aquatic ecosystem.43–45 Their characteristics enable them to occupy many ecological niches and belong to different functional groups. Consuming algae, detritus or other invertebrates, whilst being prey to other aquatic insects, fish, or waterfowl, they link different trophic levels.8,46 The whole community reflects the changes in the environment, some species more evident than others, proving them useful in the water quality assessment activities. 44 No studies were done on the diversity of Chironomidae in these temporal environments and, in general, the information on invertebrate diversity in ponds of Kopački Rit is lacking at present, so the information on biodiversity in marginal areas of the floodplain is scarce.
Accordingly, we aimed to provide new information on the diversity of the Chironomidae family in temporary shallow water bodies (ponds) across different flooding areas of a protected floodplain. Also, we wanted to assess if the differences in chironomid community composition between epiphyton and benthos reflect the differences in pond microhabitat complexity. Furthermore, we wanted to compare chironomid communities of ponds with communities in a permanent aquatic habitat (channel), with developed macrophyte stands, positioned in the vicinity of the sampled ponds, which represented a permanent shallow water body during our sampling campaign.
Materials and methods
Study area
Covering a surface area of 231 km2, Kopački Rit is one of the biggest preserved floodplains of the Danube, positioned along its course from 1383 to 1410 river km. The Danube creates a border on the east side of the floodplain while the river Drava is the southern border from 0 to 15 river km (Figure 1). The floodplain area is fed with water from the Danube through several pathways, Vemeljski Dunavac in the northern part and in the southern part (the Special Zoological Reserve) via Hulovo and Čonakut channels. Kopački Rit has had the status of a Nature Park since 1999, but primarily it was declared an ecologically important area in the 1960s. 4 The Park is also on the Important Bird Areas List and acknowledged as an important Ramsar and Natura 2000 area. It is part of the Mura-Drava-Danube Biosphere Reserve. Hydrological diversity of the Nature Park is visible in the diversity of water bodies created in the floodplain (e.g. lakes, ponds, channels), transforming under the influence of the Danube.4,47 On the other side of the embankment surrounding the main floodplain area, there is a network of channels and canals, ponds and fisheries that support diverse communities of flora and fauna. The landscape varies from meadows and willow groves to higher sandstone shores and oak woods.4,47
Research locations in the Kopački Rit Nature Park floodplain. Top right: the geographical position of Kopački Rit in Croatia; bottom left: rectangles and arrows: sampling areas enlarged on the bottom right; names of sampling locations are labelled in small rectangles and letters.
Eight research locations in ponds were positioned along the north-south transect of approximately 15 km alongside the embankment, in the flooding zone: Gredica (GR); Zlatna Greda 1 (ZG 1); Zlatna Greda 2 (ZG 2); Duga Bara (DB); Bara Ribnjak 1 (BR 1); Bara Ribnjak 2 (BR 2); Nasip 1 (NP 1) and Nasip 2 (NP 2), and two were located in the Čonakut channel (Čonakut 1 (ČN 1) and Čonakut 2 (ČN 2)), in the Special Zoological Reserve, at the (air) distance of approximately 1.6 km from the nearest pond location (Figure 1). During our research period, with the lower depth and developed macrophyte stands, Čonakut channel could be observed as a comparable shallow water body, albeit permanent.
Sampling procedure
Sampling was conducted in mid-September 2020. Chironomidae larvae (Diptera) were sampled at each location at three sites, from the shore towards the central part of the water body. On every site, samples were taken using a standard benthos hand net with a net mesh size 0.5 mm (frame size 25 × 25 cm), and preserved in 96% ethanol for transportation to the laboratory.
Epiphytic samples were taken using a plastic cylinder (ø 10.5 cm). Collected macrophytes were in situ inspected in a white tray and macroinvertebrates were removed and preserved in 96% ethanol. Water and macrophyte remains were drained over a 0.5 mm sieve, and what remained on the sieve was preserved in 96% ethanol for transportation to the laboratory.
Environmental parameters
Basic physico-chemical water parameters were measured on all locations using WTW 530i mini lab with conductivity probe and HACH HQ30d mini lab probe kit for oxygen concentration and saturation, pH and water temperature. Depth and transparency were measured using a Secchi disc.
Laboratory work
Sediment samples were rinsed on a sieve (0.5 mm) with tap water and the remaining material was sorted under a stereoscopic microscope Leica EZ4. Isolated macroinvertebrates were placed in separate vials with 70% ethanol. A similar procedure was applied for macrophytes with additional careful examination and cleaning of each macrophyte sample.
For the identification of chironomid larvae, native slides for immediate inspection were prepared using ethanol as a mounting medium, and for some specimens, it was necessary to prepare permanent slides in Berlese mounting medium. Identification keys: Caspers and Cranston, 48 Wiederholm, 49 Schmid, 50 Brooks et al., 51 Moller Pillot,52–54 Bitušík and Hamerlík, 55 and Vallenduuk 56 were used for identification to the lowest possible taxonomic level, under a Motic BA310 microscope.
Statistical analysis
PRIMER 6 software 57 was applied for univariate and multivariate statistical analyses. Principal component analysis was used for the analysis of environmental (water) parameters. Prior to the analysis, all variables were square root transformed to obtain normal distribution, and then normalised (for each variable, the mean was subtracted and divided by the standard deviation) in order to put variables on a common, dimensionless measurement scale.57,58 In order to compare benthic and epiphytic communities, we calculated for each sample relative abundances as the number of individuals of a given taxon divided by the total number of individuals collected in the sample. The diversity of chironomid communities was measured using two standard diversity indices, species richness (S) and Shannon index (H′). Non-metric multidimensional scaling (NMDS) was used to analyse and graphically present relations of chironomid communities in different locations within the floodplain, in sediment and on macrophytes. The same analysis was used to compare benthic and epiphytic Chironomidae. Analysis of similarity (ANOSIM) was applied to identify the significance of differences between the locations and habitat types. Both methods were applied to the Bray-Curtis similarity matrix based on the square root transformed relative abundance data. The contribution of Chironomidae taxa to the average dissimilarity between groups was assessed using the similarity percentages breakdown (SIMPER) analysis. A multivariate ordination method, redundancy analysis (RDA), was performed using CANOCO for Windows version 4.5 software package 59 to analyse the potential influence of environmental parameters on benthic and epiphytic chironomid communities across different locations. RDA was suggested by the initially performed detrended correspondence analysis, as the value of the longest gradient was lower than 4.0. 60 To obtain a normal distribution, all data were square root transformed prior to RDA. The manual forward selection option and Monte Carlo test with 499 unrestricted permutations were applied to identify the significant (p < 0.05) environmental parameters. Only dominant chironomid taxa (relative abundance > 5% at a certain location) were included in the analysis.
Results
Environmental parameters
Basic water parameters were measured at 10 locations where sampling was conducted (Table 1). Only 0.76 mg/L of oxygen was measured in Duga Bara and the maximal value for conductivity with 975 µS/cm was recorded in Bara Ribnjak 1 (Table 1). A substantial difference between some of the parameters was recorded in locations Duga Bara and Bara Ribnjak 1 and 2, in comparison to other locations; especially concerning oxygen, indicating anoxic conditions, and conductivity (Table 1, Figure 2). Both locations in the Čonakut channel, as well as Zlatna Greda 2 and Gredica, differed from the other locations by the highest values of depth and transparency and the lowest conductivity values. Together with Zlatna Greda 1 and Nasip 1 and 2, these locations were also characterized by the highest oxygen concentration and saturation (Table 1, Figure 2).
Principal component analysis (PCA) ordination plot of water parameters recorded at 10 sampling locations within floodplain area of Kopački Rit Nature Park in September 2020.
Water parameters recorded at 10 sampling locations within the floodplain area of Kopački Rit Nature Park in September 2020.
Abbreviations of sampling locations: GR: Gredica; ZG 1: Zlatna Greda 1; ZG 2: Zlatna Greda 2; DB: Duga Bara; BR 1: Bara Ribnjak 1; BR 2: Bara Ribnjak 2; NP 1: Nasip 1; NP 2: Nasip 2; ČN 1: Čonakut 1; ČN 2: Čonakut 2.
*Transparency to the bottom.
Benthic chironomid community
In benthos, 29 chironomid taxa from the subfamilies Tanypodinae, Orthocladiinae and Chironominae were recorded (Table 2). Community composition was similar in all ponds, though there were differences in overall diversity between the sampling locations (Figure 3). The highest number of taxa (19) was found in Zlatna Greda 2 and the minimal number of 4 chironomid taxa representatives was found in Bara Ribnjak 1 (Table 2). This locality also had the lowest recorded total abundance. The location Čonakut 1 was the location with the highest abundance of chironomid larvae. Both locations in the Čonakut channel differed from other locations supporting the highest abundance of Tanytarsini larvae, Polypedilum nubeculosum (Meigen, 1804), and Cladopelma gr. viridulum, and furthermore, it was the only water body where Cryptochironomus obreptans/supplicans and Dicrotendipes nervosus (Staeger, 1839) were found.
Non-metric multidimensional scaling plot of benthic chironomid communities from different site groups based on the relative abundance matrix data.
Benthic Chironomidae relative abundance average (N = 3) at all sampling locations within the floodplain area of Kopački Rit Nature Park in September 2020.
Abbreviations of sampling locations: GR: Gredica; ZG 1: Zlatna Greda 1; ZG 2: Zlatna Greda 2; DB: Duga Bara; BR 1: Bara Ribnjak 1; BR 2: Bara Ribnjak 2; NP 1: Nasip 1; NP 2: Nasip 2; ČN 1: Čonakut 1; ČN 2: Čonakut 2.
Tanypus kraatzi (Kieffer, 1912) (Tanypodinae) was recorded on all locations except Čonakut 2 and was especially abundant in Bara Ribnjak 1 and 2 where it made 62.5% and 40% of all recorded chironomids, respectively. Monopelopia tenuicalcar (Kieffer, 1918) was found only in Duga Bara. Orthocladiinae subfamily was represented by three species groups. Corynoneura gr. scutellata and Cricotopus gr. sylvestris were recorded at six different locations, whilst Hydrobaenus gr. pilipes larvae were present in only three ponds. All three Orthocladiinae taxa were recorded in Bara Ribnjak 2. Most taxa (17) belonged to the Chironomini tribe (Chironominae). Representatives of Chironomus genus were the most frequent and most abundant chironomid taxa, making 12.5% to 38% of recorded larvae per location, with exception of Čonakut 2, where it made only around 5%. Cladopelma gr. viridulum was dominant in Gredica and both locations in the Čonakut channel. Most frequent Tanytarsini taxa were larvae of Paratanytarsus genus, whilst Cladotanytarsus representatives were the most abundant, representing more than 20% at both locations in the Čonakut channel (Table 2).
Statistical analyses
Differences between benthic chironomid communities from different locations were indicated by NMDS analysis and ordinated on an NMDS plot (Figure 3). There was a clear grouping of locations from more distant areas in the floodplain, clustered in four site groups depending on their position: ZG – Gredica, Zlatna Greda 1 and 2; RB – Duga Bara, Bara Ribnjak 1 and 2; NP – Nasip 1 and 2; CH – Čonakut 1 and 2.
ANOSIM analysis confirmed the statistical significance of differences between communities (p < 0.001), Global R: 0.659. Results of Pairwise tests at the level of significance 0.1%, for each data group R are as follows: ZG, RB = 0.4; ZG, CH = 0.831; RB, CH = 0.821; at the level of significance 0.2% : RB, NP = 0.39; NP, CH = 1; and at the level of significance 0.3% : ZG, NP = 0.748. Taxa which contributed the most to the differences among the locations were indicated using SIMPER analysis (Table 3). A significant relationship between dominant benthic chironomid taxa and environmental parameters was indicated by the high species-environment correlations for axes 1 (0.960) and 2 (0.985) in the RDA. The two main axes explained 45.5% of the variance in the species data (Figure 4). Dominant chironomids in benthic communities from both locations in Čonakut channel were associated with high values of depth. In Gredica, Zlatna Greda 1 and Zlatna Greda 2, dominant benthic chironomids were related to high oxygen levels, while in Duga Bara, Bara Ribnjak 1 and Bara Ribnjak 2, low oxygen levels influenced the community (Figure 4).
Redundancy analysis (RDA) of benthic chironomid communities from different locations within the floodplain area of Kopački Rit Nature Park in September 2020. The plot shows the distribution of dominant chironomid taxa and sampling locations along the first and second RDA axes in relation to the statistically significant environmental parameters (DP: depth, OxS: oxygen saturation).
Results of the SIMPER analysis show the contribution of epiphytic and benthic chironomid taxa to dissimilarities between sampling locations from more distant areas in the floodplain.
Abbreviations of sampling location groups: ZG: Gredica, Zlatna Greda 1, Zlatna Greda 2; RB: Duga Bara, Bara Ribnjak 1, Bara Ribnjak 2; NP: Nasip 1, Nasip 2; CH: Čonakut 1, Čonakut 2.
Epiphytic chironomid community
Epiphyton of five macrophyte species was sampled (present on locations): Ceratophyllum demersum L. (GR, ZG 1, ZG 2), Myriophyllum spicatum L. (ZG 2, BR 2), Salvinia natans (L.) All. (all locations excl. GR), Utricularia vulgaris L. (DB), and Trapa natans L. (GR). In stands of those macrophytes, 18 chironomid taxa from the subfamilies of Tanypodinae, Orthocladiinae, and Chironominae (Chironomini and Tanytarsini tribes) were recorded (Table 4). The highest number of taxa (12) was found in Gredica, while the lowest number (6) was found in four different locations: Zlatna Greda 1, Nasip 1 and Bara Ribnjak 1 and 2. Bara Ribnjak 2 was the locality with the lowest number of sampled larvae (Table 4). Same as for the benthic chironomid community, diversity indices indicate differences in diversity between the water bodies (Figure 5). Monopelopia tenuicalcar, found in four locations, was the only recorded representative of Tanypodinae subfamily and was especially abundant in Duga Bara where it made 22.42% of all recorded chironomids. Orthocladiinae subfamily was represented by three species groups: Corynoneura gr. scutellata, Cricotopus gr. sylvestris and Psectrocladius gr. sordidellus/limbatellus. C. gr. scutellata was the most abundant in Čonakut 2, and C. gr. sylvestris in Zlatna Greda 2, where they made 20.79% and 57.58% of all recorded chironomids, respectively. Furthermore, Cricotopus larvae represented over 40% of the epiphytic communities in two more locations (Table 4). P. gr. sordidellus/limbatellus was recorded in Bara Ribnjak 2 and Čonakut 2. Most taxa (14) belonged to the Chironominae subfamily – 10 to the Chironomini tribe and 4 to the Tanytarsini tribe. Parachironomus gr. gracilior was recorded on all locations, except on Bara Ribnjak 2. Dicrotendipes nervosus was only found in the Čonakut channel. The most frequent and most abundant Tanytarsini taxa were larvae of Paratanytarsus genus, found in all locations except Zlatna Greda 1. The minimal relative abundance of this taxon was 2.38%, recorded at the Čonakut 1 location, whilst a maximal abundance of 63.90% was recorded in Bara Ribnjak 2. Larvae of Paratanytarsus genus were the most abundant of all recorded chironomid taxa in four locations as follows: Gredica, Duga Bara, Bara Ribnjak 1 and 2, with 23.94%, 48.57%, 42.82% and 63.90%, respectively (Table 4).
Comparison of diversity indices between benthic and epiphytic chironomid communities at all sampling locations (sample average per location ± standard deviation, N = 3). (a) Species richness; (b) Shannon diversity.
Epiphytic Chironomidae relative abundance average (N = 3) at all sampling locations within the floodplain area of Kopački Rit Nature Park in September 2020.
Abbreviations of sampling locations: GR: Gredica; ZG 1: Zlatna Greda 1; ZG 2: Zlatna Greda 2; DB: Duga Bara; BR 1: Bara Ribnjak 1; BR 2: Bara Ribnjak 2; NP 1: Nasip 1; NP 2: Nasip 2; ČN 1: Čonakut 1; ČN 2: Čonakut 2.
Statistical analyses
Differences between epiphytic chironomid communities from different locations were indicated by NMDS analysis and ordinated on NMDS plot (Figure 6). There was a clear grouping of locations from more distant areas in the floodplain.
Non-metric multidimensional scaling plot of epiphytic chironomid communities from different site groups based on the relative abundance matrix data.
ANOSIM analysis confirmed the statistical significance of differences between communities for given location groups (p < 0.001), Global R: 0.561. Results of pairwise tests are all at the level of significance 0.1%, for each data group R are as follows: ZG, RB = 0.607; RB, CH = 0.93; at the level of significance 0.2% : NP, CH = 0.711; at the level of significance 0.4% : ZG, CH = 0.388; and at the level of significance 0.2% : ZG, NP = 0.516. The exceptions were groups RB and NP, where the significance level was borderline 5.7%. Taxa which contributed the most to the differences among the locations were indicated using SIMPER analysis (Table 3). A significant relationship between dominant epiphytic chironomid taxa and environmental parameters was indicated by the high species-environment correlation for axis 1 (0.921) in the RDA. The two main axes explained 48.7% of the variance in the species data. Conductivity has been indicated as a significant parameter for the epiphytic chironomid community at locations Duga Bara, Bara Ribnjak 1 and Bara Ribnjak 2 (Figure 7).
Redundancy analysis (RDA) of epiphytic chironomid communities from different locations within the floodplain area of Kopački Rit Nature Park in September 2020. The plot shows the distribution of dominant chironomid taxa and sampling locations along the first and second RDA axes in relation to the statistically significant environmental parameter (EC: conductivity).
Benthic versus epiphytic communities
Out of the 31 recorded taxa, epiphyton and benthos samples shared 16 of them, whereby eight taxa (Cricotopus gr. sylvestris, Chironomus sp., Endochironomus albipennis (Meigen, 1830), Endochironomus tendens (Fabricius, 1775), Parachironomus gr. gracilior, Paratanytarsus dissimilis agg., Paratanytarsus sp. and Rheotanytarsus sp.) were present at the location Zlatna Greda 2, which makes it the most taxa diverse (Figure 5(a) and (b)) as well as the most abundant sampling location. The following locations with the highest number of shared taxa were Gredica and Duga Bara with six common taxa in both epiphyton and benthos. Locations Zlatna Greda 1 with C. gr. sylvestris and P. gr. gracilior, and Bara Ribnjak 1 with Corynoneura gr. scutellata and Chironomus sp. are two locations with only two common taxa which coincide with the lowest chironomid diversity in all of the samples.
When compared, it is visible that higher diversity in benthic and epiphytic communities was present at locations with higher depth, more oxygen and lower conductivity (Figure 5(a) and (b)). Interestingly, higher diversity was recorded in the sediment than on the macrophytes, with the exception of Bara Ribnjak 1 and Čonakut 2.
The species Monopelopia tenuicalcar was the only species of the Tanypodinae subfamily present in the epiphytic communities, and on the location Duga Bara it was present in both the epiphyton and benthos. Benthic chironomid communities included three additional Tanypodinae taxa belonging to Procladius and Tanypus genera. Species common in both communities from the Orthocladiinae subfamily were C. gr. scutellata and C. gr. sylvestris. The first one was present in both epiphyton and benthos on locations Gredica, Zlatna Greda 1 and 2, and Čonakut 2, while C. gr. scutellata was present at Gredica, Duga Bara, Nasip 2 and Bara Ribnjak 1 and 2 locations. The species of the Chironomus genus were the most frequent and abundant taxa in the benthic chironomid communities, and the same as Endochironomus tendens, were found in a total of six locations, whilst Parachironomus gr. gracilior and Paratanytarsus sp. were recorded in both epiphyton and benthos at five sampling locations. Even when recorded on both substrate types at the same location, the relative abundance of the taxa differed between the communities.
Statistical analyses
Differences between benthic and epiphytic chironomid communities, even at the same locations, were indicated by NMDS analysis and ordinated on the NMDS plot (Figure 8). ANOSIM analysis confirmed the statistical significance of differences between communities formed on a different substrate (p < 0.001), Global R: 0.464.
Non-metric multidimensional scaling plot of benthic and epiphytic chironomid communities from different location groups based on the relative abundance matrix data.
Discussion
This research gives insight into pond biodiversity, representing important new findings of invertebrate diversity in temporary shallow water bodies in a floodplain, which have so far been neglected in this protected area. We approached the question of differences in microhabitat preferences and diversity using one of the most ubiquitous, abundant and diverse groups of aquatic insects – Chironomidae (Diptera). As they are also used in the bioassessment,43,44,61 we can immediately have an indication of the system's health. However, most of the bioassessment strategies are based on (chironomid) benthic communities of the typical lentic or lotic water bodies,43,44,61–63 whilst ponds are overlooked, which was the same with chironomid diversity studies in this floodplain. Our research points out the importance of ponds as a specific environment in a floodplain aquatic network, and, furthermore, the macrophytes as another important microhabitat beside sediment, which harbours a specific chironomid community. The gathered data will provide additional information for the guidelines for planned future restoration activities. Since other project activities were focused mainly on bigger water bodies, these results could contribute to creating more precise management plans.
Restoration activities of floodplains are becoming vital since anthropogenic pressure increases over the last few decades, and vast changes in aquatic habitats are visible and even un-reversible.3,64–66 Large, non-wadeable rivers passing near towns and cities are poorly managed, channelized, losing their natural flow, water regimes are changing, and lastly, their surrounding natural habitats such as floodplains are disappearing; and the Danube is no exception.2,4 Deepening of the riverbed can lead to a lowering of the groundwater table, influencing and reducing the amount of water entering the floodplain, causing the disappearance of many shallow water bodies.2,67 To protect unique habitats and hydrology of those riverine areas, research of their biocenoses is crucial. Kopački Rit Nature Park, one of the largest preserved Danube floodplains, harbours great biodiversity, driving us to concentrate our efforts to study and protect it.4,6,68 Years with a constant drought and low precipitation in all seasons, occurring more often in the last years, clearly show how endangered these habitats are, and the number of flooded or inundated days is extremely low (pers. obser.). One of the ways to protect the area is to increase awareness of its uniqueness as a biodiversity hotspot.69,70
All macroinvertebrates in temporary aquatic habitats must be very adaptable. The ‘strategy’ to survive these intermittent conditions includes short life-cycles, good colonization abilities, dormant stasis, desiccation resistance, etc., adaptations all typical of chironomid larvae.41,54,56,71 These ecological traits are important factors influencing the composition and structure of all biocenoses in a floodplain.72,73 Chironomidae larvae have been defined as pioneer colonizers of different communities,41,68,74 and ecological engineers. 75 By burrowing in shallow sediments, they can also increase oxygenation of sediment, nitrification and denitrification. 76 Chironomids represent an important connection between trophic levels,41,46 and in summary, we could consider them to be important for functional network recuperation after disturbances in the aquatic ecosystems. These characteristics can also help in the recovery of pond ecosystems after rehydration, since chironomid larvae of some genera, for example, Chironomus, Polypedilum, Tanypus, Parachironomus, were recorded in the Paraná River floodplain to tolerate desiccation conditions. 77 Some authors have even included Chironomidae as their typical wetland group of organisms. 78 Given stated, this dipteran family is an important invertebrate group in floodplain ecosystems. During this research of shallow floodplain water bodies, in benthic chironomid communities 29 taxa were recorded, with Chironomus genus larvae being the most frequent and abundant. Some larvae were in the early developmental stages, preventing more precise identification, however, those that were present belonged to the Chironomus gr. plumosus and C. tentans Fabricius, 1805 taxa. In general, species of this genus are euryvalent, are very tolerant to a wide spectrum of habitat conditions and disturbances, 79 and are adapted to low oxygen concentration producing haemoglobin, 80 which was a useful trait on locations Duga Bara and Bara Ribnjak 1 and 2. Chironomus larvae were found to withstand extreme desiccation of its environment. 77 C. tentans can be abundant on Typha stems, but was also found to thrive on bottoms with decaying plant material and muddy substrate, 52 which coincides with its dominance in our study. The presence of these larvae in high abundances indicated high productivity in the system, which was previously recorded in many studies,43,80,81 and are thus used as indicator taxa in water quality monitoring systems applying macrozoobenthos as a biological element. 2 As well as Chironomus larvae, detritivorous Polypedilum taxa 82 have been recorded in the silty, muddy sediments with high organic content.83,84 Another species tolerant to very low oxygen concentration and saturation is Tanypus kraatzi, 85 which was most abundant in earlier-mentioned locations with hypoxic and anoxic conditions. This species is known to prefer silty bottoms and lush organic silt deposits 85 which we found in studied ponds. Diverse large macrophyte stands, standing water and very high production, as in these ponds, usually contribute to the high amount of organic matter in the bottom stratum.86,87 Cladopelma gr. viridulum, found in both temporary and permanent locations, is another species often abundant in organic-rich sediments, enduring low oxygen concentrations, also inhabiting mesotrophic waters.52,56 Permanent channel locations were characterised by a higher depth, which significantly influenced community structure in the sediment. During warm months and lower water levels, the Čonakut channel is comparable to standing water, especially when accompanied by the development of macrophyte stands. Regardless, the usual water flow most likely does change the substrate structure, and thus the composition of benthic chironomid communities. 88 Higuti and Takeda 89 recorded differences in Chironomidae community structure depending on the sediment composition and texture, between the river and lagoon sites.
The presence of macrophytes provides additional microhabitat, that is, shelter and substrate for feeding and oviposition,41,90,91 for most aquatic macroinvertebrates including chironomids,7,41,92 and influences oxygen concentration, nutrient cycling and sediment stability and structure.24,31,34 Some chironomid taxa, for example, Corynoneura gr. scutellata, Cricotopus gr. sylvestris and Paratanytarsus sp. preferred macrophytes as substrate. This was also recorded in Italian ponds and is concurrent with the species’ known ecology.31,54 In the previous research of epiphytic chironomid community, 93 C. gr. scutellata was recorded in high numbers in a floating dense macrophyte mat in the Čonakut channel. Furthermore, in the same research, percentage rate of Monopelopia tenuicalcar was up to 80%, however here it was not so abundant, representing approximately 20% at three locations. Although previously mentioned C. gr. sylvestris is a taxon known to be highly abundant in epiphyton, 94 it can differ in some ponds depending on the macrophyte diversity and development. 93 Nevertheless, as a ubiquitous and cosmopolitan species, it was in previous studies often recorded on different substrates in the floodplain, 14 and it was one of the taxa more frequently found in both microhabitats during this research. Some authors mention Psectrocladius sordidellus (Zetterstedt, 1838) to be abundant in the epiphyton since it is more impervious to water level drops.31,95 We recorded it only at two locations, and none in the sediment, but this might be due to the fact that we omitted emergent macrophytes from our research. Parachironomus gr. gracilior was one of the most frequent and abundant taxa in both microhabitat types. It does not show preference for particular habitat type or water quality, inhabiting various substrates and trophic conditions, but has been known to prosper in habitats with lush macrophytes which provide stable substrate and ample food, with no limiting oxygen supply.52,56
Differences in the preferences of chironomid taxa to either of the substrate/microhabitat types were evident in 16 common taxa, out of 31 recorded in total. Although it would be expected in shallow water bodies for larvae to migrate between different substrates, it was evident that some species did prefer specific microhabitats. These kinds of occurrences have been previously recorded in temporary and permanent shallow waterbodies.8,31,96,97 Interestingly, the location with the most shared taxa, Zlatna Greda 2, was the deepest of the ponds, with the highest oxygen concentration and pH. This could be explained by the presence of submerged macrophytes Myriophyllum spicatum and Ceratophyllum demersum which enabled mentioned movements of the larvae in search for the most suitable conditions between the microhabitats. The presence of macrophytes surely influences community structure, even in the sediment, contributing to its diversity and abundance in the aquatic systems. Habitat complexity can also have more influence on the structure and diversity of food webs than the water trophic state. 98 According to some studies,42,97,99,100 the abundance and diversity of Chironomidae can be lower in temporary or semi-permanent ponds, due to the bigger size, higher macrophyte diversity, better oxygen conditions, and lower nutrients in permanent ponds. If we compare these temporary ponds to Čonakut channel locations, representing permanent aquatic habitats, there is a notable difference. Even more, a clearer difference is established between benthic and epiphytic chironomid communities of the two water body types. This is due to the sediment characteristics in ponds with thick layers of organic matter, silty and finer sediment particles, which influences the community composition, in our, and other studies.101,102 On the other hand, this thick layer of organic matter with decaying macrophytes was not present in the Čonakut channel, which seemed to be a mixture of sand, silt and clay.
Chironomid communities, although different, were well developed in both habitat types in the studied shallow water bodies of the Kopački Rit floodplain. Since there is a network of waterways and water bodies in the vicinity of the ponds and a channel, they represent a refuge from predators, but also a colonization source of those habitats. Furthermore, with the flooding waters, a more complex trophic network is established, and chironomid larvae represent an important food source for fish and predatory invertebrates,46,103 which is very important for normal floodplain ecosystem functioning. Additionally, these shallow water bodies serve as migratory ‘stepping stones’ in the floodplain and its surrounding area, which adds to the list of their valuable ecosystem services 25 and the pertinence of their ecology research.
Some authors found that the ponds in close vicinity to the river have lower diversity since flooding can have a negative impact on the macrophytes, 104 as opposed to those filled with ground water, 105 whilst Brose, 106 and Bosiacka and Pieńkowski 107 recorded a negative influence of the isolation of the pond on its diversity. The connection of shallow water bodies to the Danube or the main delivery channels and the way they are supplied with water can be an important factor influencing their biodiversity 105 ; both benthic and epiphytic chironomid communities were influenced by water depth and conductivity and differed depending on their location in the floodplain area.
Different impact of anthropogenic influence on macroinvertebrates in epiphytic and benthic communities in ponds 15 indicates the importance of studying and protecting of habitat heterogeneity, which supports different diversity, which was evident in this research as well, and ensure higher total alpha and beta diversity. Although Kopački Rit is protected, it is surrounded by agricultural surfaces and woodland areas which are under different management regulations, and there is always a threat of habitat degradation unless the awareness of its biodiversity even in a more remote area of the floodplain is raised. Moreover, since macroinvertebrate diversity or community structure in natural floodplain ponds cannot be compensated by those present in rural or urban areas. 108
Additionally, this research contributes to the knowledge of chironomid diversity with the first record of species group Hydrobaenus gr. pillipes for Croatia. 93 We have not previously recorded it in the Kopački Rit floodplain, not even in the small pond Mali Sakadaš, where we made a preliminary pond diversity study in this area. 14
To conclude, Chironomidae larvae are diverse and abundant in different microhabitats of a floodplain, filling different ecological niches. Information on diversity, structure and adaptive traits of Chironomidae are essential for the conservation of floodplains, systems important from an ecological and biodiversity point of view. 109 Gathered data clearly show the value of these sensitive habitats, and with the future expanded research on the whole macroinvertebrate fauna and additional modelling approach, can provide a useful tool for the management of the protected area, being also applicable to other similar areas along the Danube watershed. The first step is to ensure that these temporary shallow water bodies do not become permanently dry.
Footnotes
Acknowledgements
We would like to thank our colleagues and students who helped in any way in the field or laboratory work. Also, we would like to thank the Reviewers and Editor for valuable comments and suggestions which improved this manuscript.
Declaration of conflicting interests
The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.
Funding
The authors disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This research was financed by the Institution's project No. 310513.
Author biographies
Dubravka Čerba graduated on the Department of Biology, Josip Juraj Strossmayer University of Osijek in 2004. She defended her PhD thesis in 2010 at the University of Zagreb. In the period 2005-2010 she worked as an assistant and 2010-2012 as a senior assistant/postdoctoral researcher at the Department of Biology, University of Osijek. At the same Department, 2012-2020, she was working as an Assistant Professor. In the period of 2016-2018 was Head of the Sub-department of Water ecology. Since 2020 she holds a position of Associate Professor. Her research focuses on functional role and ecology of Chironomidae (Diptera), application in bioassessment programs and palaeolimnology. Also, the trophic relations of plankton, macroinvertebrates and ichthyofauna. She was principal investigator and team-member on national and international projects and instructor/co-organizer of international workshops on chironomid identification. She has membership of the Croatian Association of Freshwater Ecologists, Croatian Biological Society and International Association for Danube Research.
Barbara Vlaičević, Senior Assistant, graduated from the Department of Biology, Josip Juraj Strossmayer University of Osijek in 2008. She defended her PhD in 2017 at the same institution. In 2008-2017 she worked as an assistant and since 2017 she has been working as a senior assistant at the Department of Biology, Josip Juraj Strossmayer University of Osijek. Her research focuses on ciliated phagotrophic protists (Ciliophora, Alveolata) in freshwater ecosystems, with the special emphasis on the ecology of periphytic ciliates in river-floodplain ecosystem. She also studies freshwater invertebrates, especially macrozoobenthos. She is a member of the Croatian Association of Freshwater Ecologists and the Croatian Biological Society.
Ramona-Ana Davidović graduated with a Master's degree in the studies of Nature and Environmental Protection at the Department of Biology, Josip Juraj Strossmayer University of Osijek in 2021. Since 2021 she has been working as an assistant for several Practice courses at the Department of Biology, Josip Juraj Strossmayer University of Osijek, Croatia.
Miran Koh, Professional Associate, graduated with a Master's degree in biology from the Department of Biology, Josip Juraj Strossmayer University of Osijek in 2016. Since 2016 he has been working as an expert (professional) associate at the Department of Biology, Josip Juraj Strossmayer University of Osijek. His research is centered around the biology and ecology of freshwater invertebrates, with an emphasis on the dipteran Chironomidae family, water quality assessment, in addition to his several forays in ichthyology.
Viktorija Ergović, Professional Associate, graduated with a Master's degree in biology from the Department of Biology, Josip Juraj Strossmayer University of Osijek in 2016. Since 2016 she has been working as an expert (professional) associate at the Department of Biology, Josip Juraj Strossmayer University of Osijek. She is working on the biology and ecology of freshwater invertebrates, with an emphasis on the Chironomidae family, and water quality assessment.
Ivana Turković Čakalić is a Senior Professional Associate. She graduated from the Department of Biology, Josip Juraj Strossmayer University of Osijek in 2008 and has been working there since 2009. Her research interests include the ecology of protozoans (testaceans) in periphytic communities and periphytic invertebrate fauna. She also works on the analysis of macrozoobenthos in freshwater ecosystems and ecological assessment of water quality.